Mountain Winds
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.
Mountain Winds
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.
Turbulence
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.
You and Turbulence
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
freaks ;)
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...
Rich Stowell
Wake turbulence
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.
Continue To Next Page