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Mountain flying and Turbulence
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Mountain Flying Flying Mountains; ...General Advice; ....Flying the Plane; ...Climb; ...Descent; ...Airports; ...Mountain Airports; ...Speed; ...Altitude; Turns at High Altitudes; ...Performance; ...Location; ...Radios; ...Mountain Decisions; The Solution; ...Do Your Own Preflight; Wind; ...Mountain Winds; Turbulence; You and Turbulence; Wake turbulence; Five Stages of a Vortex; Vortices; Wake Avoidance Checklist; ...Density Altitude; ...Density Altitude Operations; Finding Density Altitude Bay Area to Reno/Tahoe in a C-172; Running out of Power; Saving Yourself from Hypoxia; Mountain/Route Checkout; ... When Terrain Raises, Know when to Fold; Mountain Flying Problems; Mountain Flying Revisited; Downdraft; ...
"How do I become a good mountain pilot?"
"Make good decisions."
"How do I learn to make good decisions?"
"How do I get experience?"
"By making bad decisions."
"What's wrong with bad decisions?"
"When I get away with them, I may try again."
I have never seen a mountain fly but it behooves the inexperienced to believe they do fly. Be extra careful not to fly mountains when the conditions will limit your aircraft's ability to climb above terrain and turbulence. Bad weather and high terrain breed flying problems like your wife meeting your girl friend.
The altitude that will avoid terrain will not provide protection from the effects of terrain on the weather/wind. Terrain/weather turbulence can extend to several times the height of the terrain. The first warning is when your aircraft begins a climb with pilot input. If this should happen take all the altitude you can get while making a 180-degree return-turn. The 180 that your were able to make at sea level is not possible at altitude. The aerodynamic characteristics of you aircraft may differ significantly.
The thin air changes the way an airplane flies. The reduced weight of the air has less effect on control surfaces. Lift is reduced. Drag is reduced giving a higher true airspeed even though indicated airspeed remains constant. Horsepower is reduced because of fuel burn being a 1 to 16 factor by weight the air in the engine. At absolute altitude an aircraft becomes uncontrollable to hand flying and sluggish. Rate of climb decreases with altitude. All high altitude flights are on autopilot.
The existence of a low or front makes mountain weather and wind certain. Moisture will give low ceilings and visibilities. Such summer weather gives thunderstorms and winter gives snow and ice. When flying westward you want to fly away from the prevailing jet stream. Flying eastward you should do what you can to make use of any jet stream benefits. Making ground speed checks is a practical consideration. GPS is taking the 'fun' out of flying. Flew to Oshkosh just before the Reagan controllers strike. Never flew any higher than I had to. Had a great time until my wife said to come home. If she hadn't I'd still be out there.
--Every mountain flight will be a lesson.
--The 'good judgment' of the experienced pilot is acquired by mistakes, caution and good luck over many years of flying.
-- Get and take any local advice that may be available.
--The hotter it is the earlier you want to arrive at your destination.
--Don't let others (passengers) interfere with safe decisions.
-- Night mountain flying lowers all available options.
--Don't compromise safety for the enjoyment of mountain flying.
--The safest route is not always the shortest. Safety has no price.
--Certain flight operations have inherent risks and do not get safer or less risky by counting the times flown.
--When you need advanced mountain flying and survival skills they are really needed.
--Don't make your first long cross-country in an aircraft newly checked out. Be comfortable and proficient before you go. You don't need speed you need hours.
--Plan a very early morning departure and arrival by 11 a.m. Any later, any time except winter, will likely result in a most unpleasant arrival.
-- You will get a thousand foot a minute up or down draft for every ten knots of wind over a ridge..
--Mark your MC (magnetic course) for each leg on the sectional and ETE (estimated time en route) as well.
--Fly small i small f small r. i follow roads. Fly an airport vicinity route or at least within gliding distance of flat ground.
--Procure a set of en route low-altitude charts (IFR) for your flight as well as approach plates where you might land. These have frequencies and valuable altitudes not readily available to the VFR pilot.
--Fly with WAC charts. They may not have the frequencies but they do have all the airports. I have flown off the sectional chart edge only once and it was not a happy experience.
--Flying high is not always the most efficient. The basic rule should be to fly fast when you are heavy; fly slow when you are light. The savings in miles-per-gallon could be 30%. If the fuel weight is a small part of the gross it is better for fly POH range data. The data are for no wind and may not include reserve, taxi, takeoff, climb and descent allowances. Maximum range is not greatly affected by altitude unless favorable winds can be reached.
--Use the 'spot' landing technique of getting your constants of speed, power, trim and flaps into order so that the landing spot stays in one place.
--Avoid flying down the middle of a valley due to wind shear and turbulence. If you can't get an updraft on one side, try the other.
--If you must fly a canyon, fly to the highest end and fly downhill not uphill.
--When following along a ridge or valley fly the upwind side if it is on the right. This gives you both the potential speed increase available from the wind uplift and obeys the mountain/road flying rule of flying to the right side.
--Approach ridges at a 45 degree angle and 2000' higher to maximize your ability to turn away from the ridge prior to crossing.
-- After crossing the ridge fly at 90 degrees from it to maximize your distance from possible down draft effects.
--Maintain high wind-cross wind landing skills.
--Fly the valleys when given a choice. This reduces density altitude and gives improved aircraft performance. Valleys tend to have less wind and friendlier terrain.
--Always keep the back door open to lower terrain.
-- If you need to make a steep turn get as close to the edge of your turning space as you can. Slow to Vy with some flaps, make a 45 degree bank and maintain level or a slight descent during the turn. This will get you around in both the least time and space.
--The use of flaps may assist the liftoff but not the climb. You can determine the flap setting by lowering them to the same angle as can be achieved with full down deflection of the ailerons. This is the angle the manufacturer found to be the most lift for the least drag.
--Don't press your climb rate. Take what you can get and leave a back door open.
--Take advantage of all rising thermals. The altitude you gain may be just enough but don't plan on it.
--Fly through downdrafts at cruise or higher turbulence permitting.
--Leave yourself room to turn and descend without power.
--Overfly airports to determine terrain, wind, and go-around options.
--Never waste an inch of runway in the mountains. Hit the threshold on landing and use any overrun for takeoff. Full power runup with leaning for takeoff.
--Lean for takeoff with full power to max EGT + 100 degrees
--If you are going to a 'one-way airport. Try to land uphill and takeoff downhill. Get some local advice before going. Each degree of slope adds 10% to your takeoff/landing requirement. A 10-kt adverse wind doubles your takeoff and landing requirement.
Due to increased ground speeds you will be covering more ground per minute at mountain airports. This means you will be running out of runway much sooner than you would at sea level. Not only will your ground roll be longer so will your climb be shallower with a gain of fewer feet per unit of time and distance.
Mountain airports tend to always have a slope. Either their approach or departure end will face higher terrain. It is best to take off downhill and land uphill. Uphill landings cause an illusion that makes you think you are too high on final. Get all the help from gravity you can this way. Where possible, consult with locals to find out how to do what. Usually you add ten percent per degree of upslope to the takeoff distance. When a runway has three degrees of upslope, takeoff downslope. A downslope shortens your takeoff by about 5% per degree.
If you need to get extra lift on takeoff, consider putting the flaps down to the maximum deflection angle of the ailerons. This will be close to the maximum lift/minimum drag flap angle for a given aircraft. On takeoff, if you haven't reached 75% of your liftoff speed by the runway half-way point, abort the takeoff.
1. Figure the 70% of speed required for takeoff. Mark the half-way point of the runway. If you don't reach 70% of required speed by the halfway point of the runway, abort. Use this method regardless of the surface involved.
2. Abort your takeoff if you have not reached 40-kts by the half-way point of the runway. Many short runways have this spot marked.
3. Know your best-rate-of-climb speed for weight and altitude then fly it.
4. Never carry excess speed for landing.
5. True airspeed will be considerably faster than indicated airspeed at high density altitudes.
6. Always fly indicated airspeeds as though at sea level. Accept the fact that every thing will move faster on the ground when at mountain airports.
7. Uphill landings give the illusion of being high on approach.
8. Use all the runway for both takeoff and landing.
Regardless of the altitude or mode of flight, FAR 91.113(b) places the avoidance of traffic on the pilot. FAR 91.159 tells you the altitudes you should fly above 3000' according to the hemispheric rule. Below 3000' this rule does not apply. FAR 91.121 says that you must have your sensitive altimeter set to the nearest available setting source.
--Above 5000' do not takeoff or land with a full rich mixture.
--Due to the pressure gradients existing at valley passes plan your passage so as to be 2,000' above the terrain.
--Since the venturi effect of wind through passes can cause lower pressure, assume that your altimeter will be reading higher ......than you actually are.
--You can tell if you are higher than terrain in front of you if you see more and more terrain by looking over the top.
--You can gain altitude while flying to your destination by making stretched S - turns with the horizontal part of the S parallel to ....ridge or mountains.
Turns at High Altitudes
When making turns at altitudes close to the service ceiling, you must be careful to keep your banks shallow. Otherwise, you stand a probability of getting behind the 'banked excess power curve'. This is a condition where on making the turn you begin descending and in trying to prevent the descent you pull back on the yoke instead of leveling the wings. Without the wings level the resulting loss of altitude will be far more than you have ever before experienced.
1. Use maximum performance techniques for takeoff.
2. No flaps until reaching 50 knots, then 10 degrees
3. Pre-plan an abort speed and point. Use it.
4. Adjust mixture for every 2000' of climb or descent.
5. Fly at 90% of gross weight to improve performance-you will need it.
6. Keep the aircraft as light as you can. Minimum fuel plus reserve.
You can always find the density altitude by seeing how the airplane performs.
--Keep your checkpoints close(er) together.
--Select easy-to recognize checkpoints, fly to them and identify.
--Unexpected weather may make reliance on your pilotage skills essential. In mountains you won't know what the weather is ....until you get there.
--Mark your chart with pattern altitudes of airports along the way, Put in runway numbers, pattern direction and CTAF ....frequencies.
--Set up your communications options. This should include 'what if'' both at your departure point, your destination, an alternate, a filed and opened flight plan and ATC coverage. Get advice from locals. On your chart mark the points where you plan to make position reports. Put them on your flight plan, too.
--Monitor flight watch, EFAWS, on 122.0. Give reports to help other pilots.
--Use VORs or NDBs for cross-checking checkpoints if available but don't base your flight on either availability or usability.
--AWOS (Automated Weather Observing Systems) exist at an ever increasing number of mountain airports. They have both radio and telephone capability. Use the latest A/FD or call an FSS to get the latest information. ASOS gives precipitation information.
--Be sure to write down the ARTCC frequencies for your flight. You may be too low to communicate but you will be able to listen in on other pilots. If you should ever go down, knowing the frequency will give you a direct line to invaluable assistance. Your cellular phone may not have a cell you can reach.
You cannot learn mountain flying by reading about it. Flying in density altitude is different than reading about it. You must get training and instruction. Several times!! Personally, I did not cross the Sierras as PIC until I had three hundred hours. Prior to that I made four or five trips with instructors or more experienced pilots. Mountain flying is different because mountains limit your flight options. The effects of route, wind, weather, density altitude, emergency preparation, and aircraft performance are different and require a different pilot perspective. A pilot who views mountain flying as a routine flight is heading for a trap. There is always an alternative to making a dangerous flight, no matter how inconvenient. Have an alternate plan; be flexible. The ultimate alternate plan is cancellation.
Pilots control their judgment and decision making. The dangerous transition from VFR to MVFR to IMC requires decisions that allow little room for error. Entering a situation where conditions are controlled by the weather means that the VFR pilot is beyond his ability, skill and knowledge level.
--Exercise cautious judgment.
--Upgrade your capability.
--Improve your preparation.
--Adjust your attitude.
An instrument rating and an aircraft with altitude performance do not make all mountain flying either safe or practical. 300-fpm climb capability is a minimum of required performance after reaching cruise altitude. Mountain flying becomes relatively safer and more practical when preparation determines that a flight can be made. Weather is the primary deterrent. Every pilot should have personal weather and wind conditions under which a flight will or will not be made. Allow the real possibility of rapidly changing or unstable weather. A C-172 is not a mountain aircraft. I have had pilots return and thank me for not checking them out in a C-172 for a family trip over the Sierras.
If your aircraft lacks the performance to out climb a downdraft at Vy, don't hesitate to increase your airspeed. The stronger the downdraft greater the airspeed needed to reduce the angle of descent. Va is the recommended speed to use for downdraft penetration in turbulence. This is counter-intuitive but must be done. Again, if the aircraft is being flown at Vy and is sinking faster than it should be climbing, accelerate to maximum-cruise airspeed. Do not attempt to out-climb a downdraft. but speed up in a downdraft. Flying at cruise speed through a downdraft will give a lower net loss of altitude over distance than will any attempt to climb. Downdrafts can extend from 1 to 12 miles to the lee side of mountains. Under such conditions you could lose 65% more altitude per nautical mile at Vy than at cruise speed.
As a mountain pilot you must continuously position your aircraft to give the best selection of options available. If the wind is within 30 degrees of perpendicular at more than 15 knots increasing with altitude with a stable air mass or inversion below 15,000 you can expect a mountain wave to exist. Orthographic lifting forces air up the windward side and will form a mountain cap if instability exists the rise continues to form cumulus and no mountain wave. Altocumulus, rotor clouds or standing lenticular (ACSL) clouds are indicative of a mountain wave but if moisture is not present the turbulence may be there without the visible warning. Winds aloft weather information doesn't always apply to mountains. In mountains fly whatever wind correction needed to maintain course.
Density altitude is pressure altitude corrected for non-standard temperature and humidity. The most serious negative effects on aircraft performance occur when landing and takeoff occurs in conjunction with a high-density altitude. Higher altitude, hot air, and humidity reduce power, thrust, and lift. There is a 3-factor times 3-factor reduction in the ability of the aircraft to perform.
Due to these factors I have taken up to thirty minutes with two people in a four passenger aircraft to climb to a safe crossing altitude of local terrain. I have chosen to take the time to climb when the preceding aircraft has requested a straight out departure. The next day I get to read about the other aircraft in the newspaper. It seems that high density can affect brain operations as well as aircraft operation.
Once you opt to climb for altitude before crossing, you should put into practice any glider experience you may (read 'should') have acquired. If you note any wind prior to takeoff you can use that knowledge to locate a local mountain that may offer ridge lift. A brief conversation with a local pilot may be helpful. The best rate of climb speed decreases with density altitude.
Even after a successful takeoff the three factors times three factors work (3 to the third power = 27) against the airplane's ability to gain altitude. Use the POH to figure the new-plane performance figures. Figure in a 1-% safety factor every year of your aircraft's age. Average age of U. S. aircraft is 28 years. POH figures on rate of climb are figured as feet per minute. In the mountains you are moving further per minutes than you would at sea level because the ground speed is necessarily faster to acquire the needed lift. Early morning or late afternoon takeoffs is one way around much of the problem.
The pilot who has developed a sense of when the aircraft is prepared to takeoff at sea level is in for a surprise when this 'sense' fails him at a mountain airport. Speed over the ground at sea level in no wind conditions usually agrees with indicated airspeed. At high altitudes the ground speed will be considerably greater before the indicated airspeed required to takeoff is reached. The illusion is likely to cause the inexperienced pilot to rotate too soon and too much. Once out of ground effect the aircraft, behind the power curve, will either stall or fly into the ground in a nose high attitude. How many times have you head of an aircraft crashing at Lake Tahoe two or three miles from the runway during takeoff. It happens nearly every summer on the first really warm weekend.
For much the same reason, the winter landing techniques that made for near perfect landings will result in 'carrier-like' controlled crashes. The cold dense air, even in the mountains, deceives the pilot into believing that the same density exists in the warmer air of spring. It doesn't. The high flare in winter conditions will not be cushioned by the warmer air of spring and the aircraft will fall right through any existing ground effect. Watch the big twins fall at CCR on the first really hot day to see what I mean. It happens to the best of us.
Gross weight performance from the POH is less than indicated at high-density altitudes. A 20% reduction in weight will at best result in only a 10% improvement in performance. It is usually easier to leave luggage and fuel behind than passengers. An intermediate fuel stop is never a waste of time.
--Personal restrictions should be related to your fatigue factor, weather limitations, eating requirements, and kidney limits.
--Takeoff restrictions should be related to the length of the runway, density altitude, aircraft capability, visibility, and enroute ceilings.
-- T-storm avoidance must be guaranteed. Icing avoidance must be guaranteed, destination weather must be above personal minimums with a nearby VFR alternate.
-- No night circling approaches, no contact approaches at unfamiliar airports, stabilized within 500' AGL or call go-around to missed.
--60' minimum runway width and runway figures 50% over POH figures.
--Vectors through a localizer are likely to be disorienting as is a tight close in vector. Advise ATC that you would prefer another option.
--A crew-member will always observe fueling. Discrepancies to be recorded.
--The more professional you are the more closely you will adhere to your personal limits.
Headwinds, tailwinds, heading corrections, runway selection, pattern adjustments, weather-vaning, light, variable, strong, light, swirling, turbulence, gusts, wake, relative, calm, variable all of these exert a powerful and fundamental influence on our flying.
--Learn to read the winds and their signs of high velocity.
Lenticular, rotor, and cap clouds advise against flying.
-- Headwinds can greatly alter "no wind" figures from the POH.
--Consider it a "rule" that you will always be flying into headwinds.
--Keep wings level and ride the altitude wave at Va speed to avoid bending the airplane. Avoid turns in turbulence.
--Plan to land more often for fuel. Keep a two hour reserve so you can get back when you can't go on. In the mountains you will have head winds no matter which way you fly. Be conservative.
Wind is a flying variable that remains constant in its variability. As you descend both velocity and direction may change. The amount of change may be a matter of degree but often it is significant. A headwind can become a tailwind. A steady wind ceases or gusts.
Proximity to mountains, buildings, and terrain cause orthographic wind changes in direction, speed and drafts. Virga is indicative of violent wind shifts. Dramatic wind changes occur where thunderstorms exist. The most likely wind change will be due to surface friction, which will reduce wind velocity and change its direction. Holding a tight yoke grip during gusty conditions reduces your ability to react to wind changes. In high wind conditions add at least 1/3 of anticipated gust velocity to your approach speed.
Against a headwind, speed increase should be sooner than later. With a tailwind do not give up on Vy unless the sink exceeds the Vy climb capability by three times. Headwinds increase descent angles relative to the ground; tailwinds decrease the descent angle. This effect can be best noticed by making practice downwind landings before going to a strange place where the skills acquired will be essential for survival.
Wind speeds can easily double through mountain passes. The
Bernoulli effect of lower pressures in a pass can give altimeter
errors of a 1000' or more. Occasionally the pressure gradient
through a pass can cause a reverse flow of the wind. Mountain
winds can become overwhelmingly strong in very short order. With
the strength will come turbulence and runway cross winds. Waiting
twenty minutes one way or the other can make a difference. The
wind at ground level is sure to be stronger higher up. Mountain
surface winds are accompanied by up and down drafts, which make
holding altitude impossible. Accept the changes and try to fly
in areas of upslope winds along upwind ridges. Turbulence is
an unavoidable function of strong winds that can only be reduced
by getting as high as possible.
If you can determine that a forecast headwind is stronger than predicted, keep careful record of time and fuel. If in doubt, land and refuel. Many isolated airports have 24-hour credit card automated fuel pumps.
Cross-country flying into single runway airports requires that the pilot be proficient in crosswind landing procedures and taxiing techniques. Proficiency in reading winds and ground reference airport patterns is an additional requirement. Always fly so that you are in positing to turn toward lower terrain.
Allow one thousand feet of ridge clearance for every ten knots of wind speed
In a sink, go to full power with nose slightly down below Vne
Use GPS to get highest ground speed
In lifting air slow down, fly into the wind and go for the ride.
Airplanes dislike stress as much as humans. Like humans stress can cause an airplane to break. Stress for airplanes is defined as load factor. Load factor is the ration of the total air load acting on the gross weight. Level flight produces one times the force of gravity or 1 G. The aircraft is designed to carry 3.8 G's positive load before stress causes folding, spindling, or mutilation. Excess loads may be causes by flight maneuvers, turbulence, wind, or excess weight.
Aircraft stall instead of breaking. As the load factor increases so do the stall speeds. What were previously safe flying speeds now use load factor to create stalling speeds. A stall is a type of aerodynamic safety valve. The aircraft has an airspeed called Va or maneuvering speed. At this speed in rough air and level flight conditions an aircraft is able to withstand the excess stress. Additionally the aircraft is designed to withstand full control defections at this speed without breaking. These structural speeds are determined in power off conditions. Any use of power becomes an experiment when maneuvering above Va. To avoid becoming a pilot of an experimental aircraft you should begin by taking ten knots off the Va and two additional knots for every 100 pounds of weight below gross allowable. Oddly, a lighter aircraft has a lower Va than does a heavy aircraft. Repeated stress above design load can cause structural failures. Structures likely to fail are tail surfaces and wing ribs. Failure can occur during normal operations when prior operations have exceeded design capability.
Sometimes turbulence is a nuisance. Less often it is a hazard. The pilot's first option is to slow down. Quickly get to Va -10 knots. Structural breakup is most likely with an abrupt pull-up effort to regain altitude lost. Keeping a light touch will prevent the two-for-one bumps you get by holding tight. Do not slow below the previous recommendations since you will be likely to stall when a vertical gust strikes. 180-degree turns are not recommended since they increase the load factor and risk of exceeding structural limits. Ride the altitude changes of turbulence without striving to hold altitude. The use of flaps reduce the amount of stress wing structures are capable of withstanding. Turn off the autopilot.
Any mountain flying in strong winds or after 10 a.m. should be flown in anticipation of turbulence. Cumulus clouds mean some turbulence exists. Turbulence at lower levels is caused by hot air thermals or by wind in motion. Clear air turbulence (CAT) is usually a high altitude phenomena but in cold weather can occur as low as 5000'. Wind shear is caused by two adjoining airflows moving in different directions and speeds. The most dangerous wind shear is a decreasing headwind on approach. Winds usually lose velocity at lower altitudes. Full power is the only correction. This is an emergency.
Know your Va speed before reaching turbulence. Prepare the aircraft and passengers. Turning adds to structural stress. Stay level and accept altitude changes. Change power only to remain at Va. Turn off autopilot. Keep a light touch; accept any altitude gain you get.
Turbulence affects you physically by adding stress to your body. Of greater import will be your emotional stress. As the pilot you do have some ability to control or reduce the effects of turbulence. Use the rudder to counter yaw. Rudder will raise the low wing and steady the nose. Avoid reactive aileron and elevator movements. Gentle and smooth will average out the gyrations into a less stressful flight.
Instructor Opinion on Turbulence
Two aspects about turbulence (and stalls, for that matter) are particularly disconcerting to many pilots:
--First, we often cannot "see" the turbulence coming (unless you've got standing lenticulars, strong surface
winds, and other clear tell-tale signs);
--Second, we often feel a sense of "loss of control" over the situation as it's happening -- pilots are, after all, control
As many have replied already, experience does help with all of this. But so too does a better understanding of weather, aerodynamics, and the design of the airplane (all of which I hope will become clearer to you as you gain experience and advance in your training with your instructor).
One particular case I had was a private pilot who routinely flew through the Gorman Pass (connects SoCal with the San Joaquin valley), which typically can have lumpy-to-down-right-nasty air roiling in the vicinity. The pilot was very nervous about flying his 172 through there -- so much so that it was becoming incapacitating to him (not to mention squelching his desire to fly).
So he and I planned a one-hour sortie flying 'round and 'round the rim of the valley surrounding Gorman. But here was the catch: I had him trim the airplane for a comfortable, slow-cruise setting. Then I made him sit on his hands. The point was to get him to relax and learn how to absorb the turbulence with his feet, using small, quick rudder inputs to maintain a general heading and approximately wings-level.
We found one particularly lumpy patch of sky, so I had him go hands on, bank to 30 degrees, trim hands off, and sit on his hands again. He maintained the turn hands-off, just using quick rudder actions to cancel turbulence-induced bank excursions from the established 30-degree bank.
I think this was quite instructive for him. The other aspect
of turbulence, flying in wind, and stalls is that many pilots
approach these with a defeatist attitude from the start ("oh
know, the wind is blowing," or "oh no, stalls.")
Instead, I advocate a different approach -- treat these as a game, a contest! Pilot on one side, wind or stall break on the other. Will you let the wind or the stall kick your butt, or will you kick back? Go in looking to "win" the battle -- don't resign yourself to losing before the contest ever begins!
Of course, we all have to know our limitations, too -- some days the wind IS clearly the superior force. On such days, it's best not to take the field in the first place...
Every aircraft generates wake turbulence when producing lift. As air flows over the wing it creates low pressure. Higher pressure air below the wing tries to fill the vacuum. The low pressure of the vortex is formed above the wing; the high pressure below rises over the wing tip and rolls the vortex inward while the rest of the wing flow holds it away from the aircraft path about a wingspan apart. This filling is easiest at the wing tips so a spiraling swirl of air forms at the tips much like water down a drain. Wing tip vortices are horizontal with the left tip forming a clockwise twist and the right wing a counterclockwise twist. The center of the core is very low pressure which maintains the life of the whole by speeding up the winds of the outer edges. The longer the low pressure lasts the longer the existence of the vortex.
This swirling caused by differing relative pressure creates a pair of counter-rotating vortices that in good conditions will carry for up to ten wingspans behind and below the aircraft. Strong wake turbulence dissipates less and sinks further. When the vortices are blown close together they tend to destroy each other. Wakes far apart are the longest lasting. Anything that keeps the wake from sinking will destroy it such as the ground. 200 mph turbulence peels off a B-757 as a 12 inch horizontal tornado. The wake strength is weaker if the aircraft is going fast and has a long wing. Wake strength is greater at altitude (high density) slow speeds and short wings. Helicopters can create strong wakes of short life span while being nearly immune to wake effects.
The vortex diameter may reach up to 40 feet for large aircraft.
The wind velocity may exceed 130 knots. The two vortices will
remain a wingspan apart and not dissipate until other forces
such as friction or turbulence has an effect. The vortices sink
about 4-500' per minutes for two minutes before breaking up.
On reaching the ground the vortices will spread apart and may
reach parallel runways.
Wake turbulence is insidious. It will strike when you least expect it and will not exist where you think it should. Wake vortices are not as simple as the AIM makes them seem. They are a hazard any time the aircraft in front is the larger aircraft. Turbulence can extend significantly greater than FAA standards would indicate and persist longer than would be expected. FAA minimum standards if extended would decrease the risk and possibly the severity. It is not until the last 10% of a vortex's life span that the power of the vortex disperses abruptly. Wake turbulence causes nearly one accident a month and a fatality once a year usually to small aircraft and their passengers.
We are still learning about the dissipation of wakes as causes by ground friction, their blending together at a distance of six wingspans, and a bursting of the vortex tube. Perpendicular flight into a vortex is a strong abrupt bank as though by a sledge hammer. These are less dangerous than where the vortex roll exceeds the aircraft control authority. Unstable air will cause a wake to dissipate. An inversion that prevents sinking will cause a wake to dissipate. Neutral air stability will prolong vortex life. Certain wind velocities at right angle to the vortices can cause one to dissipate while the other gains power. IFR separation seems to provide adequate takeoff and landing safety. VFR separation seems to rely upon a pilot's judgment and concern.
Over 90-percent of all wake turbulence occurrences is evenly
divided between two places in aviation airspace:
1. 200 feet to ground level on approach to landings
2. 1500 feet to 5000 feet when leveling off at final approach fix.
--Chicago delays cost 20 million dollars a year with
an average of 10 minutes per flight.
--1993 had 51 accidents with 27 killed, 8 injured and 40 aircraft destroyed.
--50% of all wake turbulence events occurred between aircarriers.
--Separation only decreases the rate of occurrences not severity.
Takeoff early, land late
Turn early and fly above to avoid
Stages of a Vortex.
--First, is the formation of the vortex, which grows over the wing as a series of vertices. An aircraft has a dominating pair of vortices that roll up other vortices into a trailing edge vortex sheet. This roll-up occurs two to four wing spans behind the aircraft. The dominant vortices are at 80% of the wing span from the fuselage.
-- The second stage is the mutual effect the dominant vortices begin to have one on the other. A vortex has a wind flow velocity field causes the other vortex to descend. Simplified, the vortices would descend but this does not happen every time. The vortex cores have an axial movement parallel to the flight path but spin in the opposite direction.
-- The third stage is when atmospheric turbulence and temperatures, which lead to their dissipation, are influencing the vortices.
--The fourth stage is an enlargement of the core and a change of orientation. The fourth stage is not well understood, yet.
--The fifth stage is composed of vortex rings and are considered non-hazardous.
While vortices usually descend, near the ground they may rise or even bounce. Wind has a critical effect; the downwind vortex tends to climb. The stronger the winds and unstable the air, the shorter the life of a vortex. Certain combinations of wind and stable temperature can cause a vortex to remain stationary. Atmospheric effects determine the vortex strength. The strength of a 'heavy's' vortex can extend for five miles.
Improved instrumentation has shown that vortices retain 90% of their power through 85% of its duration. Aircraft speed, angle of attack, small wing size, clean configuration, and weight all produce strong vertices. Gear down, flaps and spoilers change the spanwise lift and turbulence to reduce vortex strength. Newer aircraft such as the large 757 are 'slicker' than older aircraft. They have fewer protrusions to disrupt the vortex so the vortices tend to last longer. The 757 has an exceptionally steep climb, and produces 50% more vortex for its size than other aircraft. A light aircraft will be unable to climb above the flight path of a departing 757 as it might other aircraft. The wake turbulence of the 757 is greater than many heavier aircraft.
The heavier the aircraft and the slower it is flying, the stronger the vortex. Vortices last about 80 seconds and decay suddenly. The roll forces of a vortex from a heavy aircraft will exceed the power of your ailerons. Entry at right angles will cause pitch and airspeed displacement. Years ago I flew through a pair of such vortices and it felt as though a sledgehammer had hit the bottom of the aircraft. An oblique entry will have symptoms of both. The vortices move apart at 4-5 knots. A breeze of 4-5 knots is capable of keeping a vortex stationary for its life of 80 seconds. Pilots of small aircraft should avoid operating within three-rotor diameter of any helicopter in slow hover taxi or stationary hover.
The 757 is an aircraft that in a clean configuration can produce wake turbulence in excess to that produced when dirty. (Gear and flaps extended) The 757 does not produce the multiple vortices typical of other aircraft. 757 vortices are very focused and are at least twice that of similar aircraft in its class.
--last longer in calm or light winds
--- are most dangerous close to the ground
----are stronger when made by heavy aircraft
-----affect light aircraft the most
------can be avoided by flying above and upwind
-------of large helicopters are deadly...avoid
--------Climb rates of new jets make it necessary to wait.
You will experience wake turbulence something in your flying life. You may never hit the center but you will never avoid it completely. Read all you can about wake turbulence. If you are on approach following a larger aircraft, plan that your touchdown will be well past the large aircraft's touchdown. If a larger aircraft is departing prior to your landing make your landing point well short of his rotation. A take off behind a larger aircraft must break ground before his rotation and climb at a steeper rate or a turn upwind away from his flight path.
The latest information suggests that you stay at least 1000' above and below the possible turbulence path. Although stronger at slower speeds the wake turbulence will exist at all speeds. The more turbulence in the atmosphere the more quickly will any turbulence be dispersed. With ATC separation standards both old and new, there have been no reported wake turbulence accidents in nearly thirty years.
--When aircraft is close to your altitude avoid flight below. Execute 360 and advise ATC.
--Allow three minutes space before takeoff. Rotate and climb to avoid prior flight path and headings. Windy conditions can reduce time considerably.
--Make landing approach above prior approach path and land beyond touchdown point.
-- prior aircraft is on parallel or intersecting runway consider your ability to estimate location of wake. Better part of valor may be a go-around.
Hot, high, humid weather will change mountain operations proportionately more than flatland operations. Under density altitude conditions the engine, propeller and wing become less efficient and effective. Even a turbo engine flies with a less efficient and effective propeller and wing. Add an additional 10% to your operational parameters under humid conditions. Engine power can be cut up to 12% when humidity is high. It does this by displacing air with water in the engine.
The decreased efficiency of the aircraft at density altitude results in longer takeoffs, reduced climb, higher landing speeds/roll and longer takeoff distance and ground roll. Leaning (not turbos) the mixture to adjust the weight of the fuel in proportion to the weight of the air will improve the engine operation but power will be affected by lack of oxygen. Any temperatures above standard will affect all parameters of operation and performance negatively from the pilot's viewpoint. A 6000' airport such as Tahoe has a standard temperature of 31F. At 90 degrees the density altitude is almost 12,000'. At 80 degrees it is over 11,000'.
Density altitude goes up about 100 ft per degree of C rise. You can estimate density altitude by knowing "standard temperatures". Standard at 5000' = 5-degrees C. For any temperature increase or decrease from standard add/subtract 100 feet per degree of change.
Humidity may not be used because of unimportance. Error less than 200 feet.
--Not a height reference
--Used as an index of aircraft performance
--An approximate value of density altitude is all that is needed
--Density altitude can rise from sea level to 3000 feet at 100-degrees F.
--High density altitude reduces:
--High density altitude increases:
--Time to climb
--Actual required ground speeds to takeoff, landing and flight
1. Always check density altitude.
2. Lean the mixture for takeoff.
3. Taking two trips to nearby airport with longer runway is always an option.
4. Know your service ceiling. (100 fpm climb altitude)
Most density altitude accidents do not occur because the aircraft was too heavy, out of C.G. limits, on too short a runway, winds, runway surface or because the computed density altitude exceeded the performance capability of the aircraft. The accidents happen because of improper aircraft operation. Tire inflation, flaps, and leaning are pilot induced difficulties.
In a density altitude situation it is advisable to adjust the mixture at full cruise on the downwind pattern altitude and leave it there. This is especially true if you are planning to make an immediate departure. This is the only time when I can truly recommend using the mag key to stop the engine. Having the mixture set will make a hot start relatively easy. If the aircraft is going to be down for a day, use the mixture to kill the engine.
In an 'engine cold' high-density start keep the mixture in idle cut off. Set in a little throttle. Give a minimal prime and slowly advance the mixture while cranking the propeller. Don't try a full rich start. You will probably flood the engine. When you do your runup for takeoff. Do it at full power and adjust the mixture for best operation. Remember if you takeoff at full rich the engine will become even more rich as you climb to the point of choking itself 'dead'. Watch the EGT during takeoff and climb and make appropriate adjustments.
Don't assume that an airplane can fly anywhere and at any time. The temperature of the runway environment (asphalt pavement is likely to be far different (higher/worse) than that of the grass parking area. The difference can be as much as 20 degrees. A short walk might make a difference in your density altitude computations and your departure plans.
To stay out of trouble, learn all you can about local conditions
and the weather. Always have an escape route. If in doubt, stay
on the ground. Don't fly into convective clouds. You can only
avoid all thunderstorms by maintaining VFR at all times. When
winter comes it will rain on your airplane.
Finding Density Altitude
--Over twice as many per hour accidents occur in the mountains
--The ground in the mountains is closer and the slope of the terrain is important
--Aircraft performance is less
--The 'sea-level mind-set' results in the poor planning that ends in an accident.
--The climb capability can cease to exist
--True airspeed increases 2 percent per 1000 feet
--Density altitude increases 2 percent per 1000 feet
--Engine power decreases 3 percent per 1000 feet
--temperature drops 2 degrees Celsius per 1000 feet.
--By the time you reach 10,000 feet your engine power has decreased by 30 percent
--At altitude water vapor takes the place of oxygen
Area to Reno/Tahoe in a C-172
After SAC just stay to the right side of the freeway.
Travis Approach 119.9
Sac Approach 125.25 Handoff to 127.4 or 119.1
Oakland Center 127.95
Reno: get ATIS for Approach frequency abeam Truckee and contact Approach.
Expect right base entry...request the left, taxi to Mercury unless you know a better place.
FSS is close to Mercury.
Mercury will give you a ride to the other side of the airport. Catch a shuttle...cheaper than cab.
Flying the Route
Use a cruise climb of about 90/95 knots until Auburn.
Lean even in climb.
At 10,000+ you can see the freeway on the other side of the hills just past Blue Canyon Airport. Take the shortcut. Fly about 090 degrees.
I would recommend that you plan to arrive at either airport before 10 o'clock a.m. In fully loaded 172 fly as high as comfortable for your oldest person. Don't go if winds are close to 20 knots unless you can get extra altitude and enjoy bumps. Approach Verdi ridge at an angle so you can turn away, if you must. Stay out of the pass if windy.
It is hot and the least bit windy you will not be able to out climb even the least of downdrafts. VERY uncomfortable. Always leave yourself an escape route. Let down after crossing ridge. You have lots of altitude to lose.
Direct to Placerville VOR and follow highway. Getting in is the easy part. Getting out in a loaded C-172 will be a problem. Do a full power leaned check for best power before takeoff. Do not climb directly toward the West. You'll never make it.
Depart either early morning (best) or at dusk. Expect the flight home to take a while if into headwinds. Check fuel. Flying into a setting sun hurts. Just plan to be out of the mountains before dark.
I hope you have done some gliding. There is a golf course to the right. Go over there and do switchback turns to stay as close to the nearby ridge as you can. If you get ridge lift keep climbing until you are well above 9000' In a C-172 it is hard to have too much altitude. It is not a good mountain plane.
Alternate climb is to stay in the pattern until you get altitude sufficient to head west. Rental car may be best transportation. Tiedown is expensive. Don't take any more gas than you need to get to SAC or such.
Departed Tahoe in mid-afternoon in PA-28 180 with two aboard and half tanks. Took nearly 30 minutes to get to safe crossing altitude
Departed Tahoe in PA 28 181 and went to golf course and gained altitude. Aircraft before me departed straight out to the pass. Read about his fatal accident in C-182 the next day.
Had pilot ask to be checked out in C-172 for trip over to Nevada. Refused and suggested PA-28. After making flight in PA-28 the pilot called to thank me.
I have made the trip to Nevada on average 5 times a year for
thirty years. I even flew in a C-150 once. I few back at night
'once'. I flew to Reno for the first time in my own C-172
last month with just my wife. That is my first trip in C-172. By picking time and weather carefully it is no problem BUT conditions must be right.
Out of Power
--The one time we deliberately run out of power is in the flare.
There are four places where you can expect to run out of power:
--When approaching to land you have mistakenly added small bits of power in an effort to stretch your approach
to the runway only to go lower and lower and slower and slower.
--When slowing behind slower traffic you have gradually added power in order to fly slower and slower while maintaining altitude until you are so slow and without additional power so that the only option is to lose altitude.
--You are in a high-density altitude situation and have applied full power but are still unable to out-climb the rising terrain. Your option is to turn away to lower terrain. The loss of altitude is the only remaining option.
--You are in a slow-flight and have slowed to a speed that requires full power just to maintain altitude. You
would like to either gain more altitude or more airspeed. Your only option is to sacrifice altitude first.
--Whenever you use power to fly slower you are in the region of reverse command. Once you are using full power a descent in flying attitude will commence and continue unless you lower the nose or find a way to get
--Induced drag will vary inversely to the square of the airspeed. For every 2-knots lost in airspeed, the induced drag will increase the effective loss by four-knots with additional drag.
--Now the elevator works backwards. Raising the nose with the elevator will cause the aircraft to descend if power is constant or unavailable. Lowering the nose will cause the aircraft to climb.
--Reducing the power will increase the airspeed and increasing power will reduce airspeed.
--Every aircraft must be retrimmed for ANY change in airspeed, power or flap.
--We can safely fly behind the power curve in the flare because we benefit from ground effect.
--Light aircraft have a very narrow range of operation as determined by power, weight, loading, lift, altitude, and pilot performance as outlined in the POH.
Yourself from Hypoxia
--Hypoxia is insidious in its combination of functional impairment and denial.
--Four hours at 8,000 are the same as 1/-hour at 16,000 feet.
--17-hours without sleep gives you the same functional impairment and mental capability as a drunk
--Hypoxia is a form of stupidity.
--Symptoms are headache, nausea, tingling skin, fatigue, dizziness, visual impairment and euphoria
--the sequence and degree of these symptoms varies with the individual and within the individual.
--Effects of hypoxia are accumulative. Brain cells die from lack of oxygen.
--TUC means 'time of useful consciousness'. At 25,000'it is less than three minutes.
--Night vision begins to deteriorate at 4000 feet.
--Hypoxia is made worse, much worse, in the presence of carbon monoxide.
Took a pilot to Lake Tahoe from the North Bay Area and gave a series of 'lessons' that might be suitable for your situation. Pilot had planned flight via VOR which was o.k. but the weather was too good. I talked to him that this was a perfect day to get a 'read' of what the route would need to be in marginal conditions.
As soon as we left the Class D we dropped down to 600' AGL since the beginning route was a 700' transition area. Along the route were some 400+ power poles adjacent to the river. Just when the transition area lifted to 1200' we were crossing relatively close to some 2000' TV antennae. At 900' our departure VOR became useless. We tuned in Hangtown VOR and got a good signal.
As we approached the Sierras we could see some cloud buildups.
We initiated a climb and with some deviations found that 500-fpm
was never going to catch up with the rising clouds. At 13,000
we opted to descend and have a go underneath. We were in continuous
contact with radar facilities and overheard a PA 28 making initial
contact with ZOA (Oakland Center) leaving Tahoe westbound. He
was at 8500 so we stayed at 9000 (legal because we were within
3000' AGL) Flight below clouds was a bit bumpy, as it usually
is, but gave pilot a taste of how bad it could be.
Descent and landing into Tahoe required loss of several thousand feet in less than five miles. Pilot experienced his first landing on a runway with a 2000' displaced threshold. Told him to ask for a 180 on the runway to save taxi time. Most interesting part, to me, was the difference in pilot attitude and confidence that occurred when we flew
between Hangtown and Tahoe. He had been driving to Tahoe since age four and knew every hamlet and place
along the highway. Flying low over such familiar territory became an enjoyable sightseeing tour.
The initial departure, over the central valley, was off road and filled with unfamiliar obstacles and landmarks.
Used the density altitude of Tahoe to show how to lean for climb, descent, arrival, and departure. We climbed
in the pattern to 9000' before heading over Echo Summit. When unable to contact RNO FSS by radio I had him have the tower open our flight plan. He also learned the safety aspect of flying on the right side of valleys and roads. I took a very straight forward VFR cross-country day and turned it into a learning experience based upon learning the options, limits, procedures and frequencies that would be required or tried in less than favorable conditions.
When Terrain Raises, Know when to Fold.
--A gradual slope can out climb an aircraft at high density altitudes
--When climb rate is low make S-turns and 8's along a ridge to get a thousand or more feet above highest terrain.
--Do not rely on POH numbers, make flight test to determine Vref climb performance at density altitudes.
--Use GPS, LORAN or DME to simulate flight to cross a pre-selected altitude (obstacle).
--Above 10,000 feet there is little excess power to accelerate or climb in light aircraft.
--A high angle of attack the flight path is likely to have a very slow rate of climb.
--If you are unable to climb at Vx for the altitude and Vref you will not climb at Vy either. Turn back.
--Fly on the right side of valleys and turn downhill before running out of room to turn
--Don't takeoff into a situation where you will be unable to climb.
--Take some ridge-soaring lessons in a sailplane to learn how to use ridge thermals for climb.
--Approach mountains and ridges at an angle that will allow you turn away in a downdraft.
--Weight reduction improves performance in airplanes much as it does in people.
--A ten knot wind can produce a 1000-fpm downdraft over a ridge.
--There are no longer any new causes of mountain accidents.
--Lessons not learned are doomed to be repeated
--Mountain takeoffs should be short field but not the rolling short field.
--Abort the takeoff if ever you should over-rotate.
--A l2 knot tailwind increases required distance l10 percent.
--There are13 common causes of takeoff difficulty beginning with tire inflation, slope, maintenance, leaning, prop damage, airspeed calibration, and MORE
--By unloading 300 pounds off the aircraft you reduce needed runway by 1000 feet.
--Use boost pump and make two power checks before releasing brakes.
--Essential that a normal rotation take place.
--No night IFR in the mountains because of high accident rate
--No night VFR in the mountains.
Mountain Flying Revisited
--Mountain flying is not always fun:
--Low performance aircraft
--High peaks and low ceilings
--Poor weather reports and under forecast wind velocities.
--Have a plan and know where you are. (Nothing muddles the brain as much as being lost)
--Always keep track of where the low land lies.
--Even IFR pilots are required to have current sectionals
--Know the terrain altitudes and your planned safe altitude
--Night mountain flying reduces your emergency options to zero
--Terrain Awareness Warning Systems now exist for small aircraft.
--Turbulence causes about 10% of accidents
--10% of mountain accidents occur during mountain checkouts
--Don't cross ridges having winds above 25 knots.
--Mountain winds, as forecast are inaccurate insofar as ridge winds are concerned.
--Downwind side of mountains are always most turbulent
--Absolutely worst turbulence is indicated by roll clouds on downwind side of mountains
--Cold front passage means that there will be severe up and downdrafts.
--Wait several hours till cold front is past before flying the mountain route
--In summer, early morning flying is always best
--It is the upper level air that causes the weather east of the Rockies
--Eastern lows differ from Western lows in that there is more moisture available.
--Additional moisture causes weather to remain bad longer.
--You need instruction to fly the mountains.
--Careful route selection and good navigation makes mountain flying safer.
--The downdraft occurs as strong winds cross a ridge crest as an updraft that becomes a turbulent downdraft.
--Flying directly into a headwind, an airspeed indicator will not tell the difference wind speed and airspeed.
--Fly a downdraft at Va since the higher speed will get you out of the downdraft sooner with less loss.
--Use maximum speed required clearing the highest point and still remaining above descending terrain.
--Cross a ridge at a 45-degree angle to reduce the amount you need to turn to escape a downdraft.
--When crossing a ridge in a downwind direction the 45-degree crossing angle does not apply.
--On regaining climb capability, ASAP gain 2-3000' agl before attempting to cross ridge.
--A downdraft at night where an MEA altitude exists can become a crisis situation.
--The lighter you are below gross weight the slower you want to fly in turbulence
--These conditions have occurred at altitudes as high a flight level 40.
--In turbulence reduce to the Va maneuvering speed for your weight.
--Use full throttle and maximum rpm and lean for maximum power.
--Fly the attitude required and ignore the loss of altitude,
--Once in a downdraft head for lower terrain.
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