No-Fly Zones

Large weather systems and local geography can combine to create new hazards


You are flying a well-appointed, 180-hp Cessna 172 and with a friend have just taken off from Aspen, Colo., on a trip back to Wichita. You make a right climbing turn over the hills north of the airport and, at 80 knots, continue climbing to the southeast. The air has a gentle texture, not even light turbulence.

As you gain altitude in the valley of the Roaring Fork River you are below the tops of the mountains. The September gold of the aspens is phenomenal. Theres a cloudless, blue sky. You are feeling good because you flew in and out of Aspen successfully and didnt foul up in the busy mix of aircraft types and opposite direction traffic on runway 15-33.

Your airplane rises steadily at 300 feet per minute. Using careful pilotage as you ascend in the center of the valley, you avoid turning southeast into the deceptive Grizzly Peak drainage that local pilots had warned you about. Ahead you can see the stripe of the road over Independence Pass, which crosses the Continental Divide at 12,093 feet msl. You aim for the Pass.

Suddenly at 11,700 feet you feel a sharp tremor in the yoke. The airplane gives a shake, sort of like a dog slinging off rain. Instantly the airspeed reads 40 knots. The needle in the vertical speed indicator swings to 2,000 fpm down. The wing is stalled. The engines already at full throttle. You are too close to the ground to lower the nose much. Your hand slams the flap lever full down. Rocks and snow loom over the cowling.

All you can do is kick rudder to parallel the ridge – and you crash. A motorist on the Pass road sees the crash and initiates rescue. You and your passenger survive but suffer serious back injuries.

This account is a composite sketched from four windshear-related mountain-flying crashes near Aspen. In each case the airplane was climbing out of the Aspen area to the east, in good weather, up rising terrain, toward the Continental Divide, with westerly or northwesterly winds aloft.

At an altitude close to pass level, on the windward side of the Divide, each aircraft penetrated a powerful downdraft in which the aircraft was stalled followed by a high rate of descent from which no recovery was possible. In each case, the entire event took about 12 seconds.

The pilots in two of the accidents had more than 5,000 hours and held an ATP. The other two pilots had learned to fly in the Aspen area. In each case, despite experience in mountain flying, the pilot did not anticipate vertical windshear.

The windshear encounters and crashes occurred in a very narrow horizon west of and below the altitude of the Continental Divide. And in each instance the altimeter setting at Leadville, 30 miles to the east across the Divide, was much higher than that at Aspen (from 0.10 to 0.17 inches higher). These surface pressure reversals are very unusual.

Microscale Weather
These windward-side windshear crashes exemplify one kind of mountain microscale weather accident. Microscale means a weather event involving a time scale of less than one hour and a geographic scale of less than three or four miles. Microscale weather mishaps are all too common in the Colorado Rockies. Sometimes they result from previously unrecognized phenomena.

Ordinarily, pilots envision mountain weather in large scales. They pay careful attention to weather phenomena such as fronts, airmass thunderstorms and mountain waves.

Recently recognized is that all of these larger-scale weather systems are modified by mountain terrain and can spawn extremely powerful microscale systems that include downdrafts stronger than the climb rates of most general aviation aircraft. Meteorologists know a lot about the dynamics of mountain weather as related to topography, but pilots seem to miss the knowledge thats out there.

The four windshear cases sketched above probably resulted from mountain drainage winds or from some kind of thermally driven flow reversal.

Mountain Drainage Winds
Mountain drainage winds result when a mass of very cold air accumulates over an elevated area and then flows downhill toward lower, warmer areas. The air mass is still cold when it flows out into valleys or on to a plain.

Mountain drainage flow has been documented as having a leading edge like a thunderstorm gust front. It is around 600 feet thick and has a curling type of instability at the top.

One reason that mountain drainage winds may be implicated in the Continental Divide crashes is that the area around the Divide and east of Leadville includes hundreds of square miles of very high, cold terrain and is studded with 14,000-foot peaks. Cold air above that area, at the times of the four crashes, could account for Leadvilles higher surface barometric pressure, since cold air is heavy.

Thermally Driven Airflow Reversals
Another potential explanation is some type of thermally driven flow reversal in which the wind system from one valley crosses a pass and enters a higher, shallow valley. Clark King, a Boulder meteorologist, conducted a year-long study in 1997 that centered on Rollins Pass in Colorado. King measured strong easterly flows rising up-valley to the west against prevailing westerly winds aloft.

These easterly flows actually climbed over the Continental Divide and descended into west-facing valleys. The flows caused an abrupt wind shift (from prevailing westerlies) at Divide altitude.

The study gathered and evaluated a huge quantity of data, including valley and Divide climates, wind speed and direction on the Divide and in and above the two valleys, temperature profiles, radiation budgets, and measurements of heat fluxes on tundra, forest and rangeland. Interestingly, the presence of clouds reduced the frequency and structure of thermally forced circulations.

The flows are described as thermally driven because different daily temperature ranges within and outside of the valley produce the varying pressure gradients that drive the valley wind systems. The study concluded that west-facing valleys should experience frequent thermally forced circulations.

Warnings From Surface Pressure
Measuring unusual winds is one thing, but pilots making the trip need to know the warning signs that hazardous vertical windshear may exist to the west of the Continental Divide.

As part of preflight planning, in addition to analyzing forecast winds and temperatures aloft, the pilot should note the surface barometric pressures for Grand Junction (GJT), Aspen (ASE), Leadville (LXV) and Denver (DEN). Normally, Colorado surface pressures decrease from west to east. For example, a typical pressure gradient from January was: GJT 30.25, ASE 30.18, LXV 30.13 and DEN 29.85.

If you are planning a trip east from the Aspen area at a time with west or northwesterly winds aloft and Leadvilles barometric pressure is even slightly higher than Aspens, be wary. If Leadvilles surface pressure is 0.08″ or more higher than that at Aspen, select a different route. Unless you are flying an aircraft capable of a 3,000 fpm climb at a density altitude of 12,000 feet, you should not be climbing out east toward the Continental Divide.

Sea Level Pressure Reductions
Some pilots wonder about the validity of a windshear warning based upon barometric pressures corrected to sea level. Aspens airport elevation is 7,815 feet msl and Leadville sits at 9,927 feet. Therefore Leadvilles actual station pressure is always lower than Aspens. Remember learning that atmospheric pressure decreases approximately one inch per 1,000 feet of altitude gained? Standard atmosphere at sea level weighs 29.92 inches of mercury, Standard at Aspen is approximately 22 inches and at Leadville, 20 inches.

What the sea level pressure comparison does tell us is that the air mass over Leadville is relatively heavier than that to the west.

Windward-Side Windshear Summary
Pilots are trained that when flying uphill to the east toward the Continental Divide they ordinarily can expect westerly updrafts forced by rising terrain, with westerly winds aloft conditions. However it appears that an Aspen/Leadville barometric pressure discontinuity may offer a good, if simple, warning of downdrafts west of the Divide.

Windshear on the windward side of a mountain barrier has not previously been identified as a factor in Colorado mountain flying crashes. Investigators have tended to blame the pilot for being too low, for not recognizing density altitude effects. Those factors are relevant.

But its also true that climbing up rising terrain on the windward side of a mountain pass is something that pilots do successfully all the time. They use up-valley winds to enhance minimal rates of climb in underpowered aircraft. In the four crashes summarized, something very unusual occurred – something small scale and extremely powerful.

As part of preflight planning, compare surface pressures for mountain airports, and select a good route accordingly.

A time-honored rule of thumb is to fly 2,000 feet above the mountains at all times. Of course, you can only do that if you have an aircraft that will perform that well. Since no windward side downdraft cases from that height have been reported, maintaining 2,000 feet agl may be a safe procedure traveling downwind across mountains.

It doesnt always work trying to cross mountains upwind because lee side downdrafts are often thicker than 2,000 feet. In addition all sorts of severe turbulence producers, such as rotors, hydraulic jumps, breaking waves, gap winds, wakes and low level jets exist on the lee sides of mountain barriers.

Other Microscale Systems
Many other better-known microscale hazards are generated by area-wide weather. Most include dangerous turbulence or various sorts of windshear. Three of the most familiar are up-valley winds, thunderstorm virga and lee-side downdrafts.

Up-Valley Winds
At 8:40 on a July morning in 1979, a hiker was resting on the northeast side of 11,910-ft. Weston Pass (southeast of Leadville). He noticed a single engine Cessna 177 rising up the valley below, flying southeast to northwest.

The hiker observed the aircraft climbing gradually to Pass elevation, then heard the engine laboring, saw the aircraft attempt to turn around and watched it cartwheel into terrain. All three occupants were killed. The hiker commented that a breeze had been blowing up the valley from the southeast. Yet the forecast winds aloft at Pass altitude were northwesterly.

From the hikers wind report, it is evident that the aircraft rose on an up-valley wind, encountered wind-shear at the boundary layer between valley wind and prevailing winds aloft, and did not have the power to overcome the windshear.

Up-valley winds are accorded a brief description in FAA AC 00-6A, Aviation Weather. On a sunny morning, southeast mountain slopes are heated by solar radiation. Then the air next to the slope is heated by the ground. The warm air rises up the mountain slopes and valley.

An unknowing aviator flying on these warm, rising currents will perceive the airplanes good climb rate as produced by horsepower, rather than by nature. Up-valley winds are strongest around nine oclock in the morning. They are microscale. They dont last long, and they are confined to a very small area.

Virga Near Thunderstorms
At 5:15 p.m. on a July afternoon in 1983, a pilot flying eastward from Buena Vista, Colo., to work in Denver in a 160-hp Cessna 172 crossed a 9,900-foot ridge. He then reported experiencing light rain and 500 fpm downdrafts. He had flown into rotor winds. He had difficulty keeping the wings level and pancaked into a rocky meadow.

Most pilots will recognize this description of an encounter with virga. Thunderstorms and unusual easterly winds were reported by another pilot flying 15 miles south of the crash site. Virga indicate extreme turbulence and should be avoided. Virga, too, are microscale.

Lee-Side Downdrafts
A typical kind of Colorado mountain crash is that of an aircraft trying to climb up-hill against prevailing winds aloft. The winds, after funneling through passes, plummet down the lee slopes. The east (usually lee) side of Monarch Pass – another Continental Divide pass – collects airplanes in failure to outclimb terrain crashes. For example, in May 1985, at 7:25 in the morning, three people in a 180-hp Cessna 172 were attempting to fly east-to-west across the 11,312-foot pass into leeside downdrafts. They crashed east of the Divide. Density altitude was 12,600 feet. The pilot later said, At 10,300 the plane did not climb anymore. We had a big problem. Suddenly the plane just dropped. I dont know why.

Monarch Pass popularity as a place to crash may have something to do with the fact that V95 crosses the Pass. VFR pilots may imagine they will have navigational help on the airway. However, the minimum enroute altitude is 16,200 feet.

MEA is the lowest published altitude between radio fixes which assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes. V95 may have been a factor in a 1997 Monarch Pass Navion crash, particularly since the pilot had filed and flown IFR for the first part of his journey.

Two other mountain passes with a high rate of salmon swimming upstream crashes are La Veta Pass, across the Sangre de Cristo Mountains in southern Colorado, and Rollins Pass, northwest of Denver. La Veta and Rollins Passes, too, are crossed by Victor airways.

Many pilots flying east-to-west find that lee-side downdrafts on these passes are impossible to outclimb. The answer: go somewhere else. Try again another day.

Identifying Hazards
Most pilots intent on flying in the mountains recognize the large-scale warnings that are expressed in different types of cumulus clouds, lenticulars and roll clouds (rotating cylinders of air in cloud parallel to the axis of a mountain ridge). However, microscale warnings are very subtle and occur most often over highly complex terrain.

A good example is the relatively tiny rotor cloud whose swirling tendrils must be examined carefully in order to recognize the turbulence which it exemplifies. The rotor cloud photograph on this page was taken looking north near Monarch Pass. The clouds were rotating clockwise – from left to right – under mountain wave conditions. They marked an area of severe turbulence. The clouds disappeared within seconds.

Another rapidly-changing cloud is the unsteady lenticular. These are small lumpy lenticulars whose lee sides are torn up by convective or mechanical turbulence. They, too, reflect severe turbulence. If possible, go around the area they mark on their upwind sides.

Virga are columns or strands of falling water or snow particles that evaporate before reaching ground. In Colorado they appear almost always with thunderstorms but also in many other kinds of violent weather. Virga almost always indicates severe to extreme turbulence. It is very dangerous to fly under a line of virga. Retreat to an airport and wait for conditions to improve. If no airport is available and it is absolutely necessary to go under virga, sometimes there is less turbulent air a few hundred feet above the ground.

Flying Turbulence
Mountain flying occasionally can be exceptionally smooth on calm, early mornings, but crossing mountains usually involves turbulence. There can be plenty of jolts at night, too; and unless theres moonlight the pilot cannot tell exactly what terrain or cloud feature is producing them. Microscale systems often provide the worst turbulence encounters.

The right technique can not only make the trip more comfortable, it can mean the difference between staying aloft or landing on boulders.

First, use maneuvering speed in turbulence. The aircraft flight manual specifies different maneuvering speeds for different aircraft weights. For example, maneuvering speed in a Cessna 206 at gross weight is 120 knots. But when flown 650 lbs. lighter maneuvering speed is only 108 knots.

If you only know maneuvering speed at maximum gross weight, there are several rules of thumb for calculating Va at lower weights. Some instructors say to figure the percentage that you are under the airplanes maximum weight, divide by three and subtract that from the published max-weight Va. For example, if youre flying at 85 percent of maximum gross, then youre 15 percent below gross. Divide by three to get five. Subtract five knots from the published Va. Other instructors suggest dividing by two instead of three.

When anticipating turbulence, tighten seat belts and shoulder harnesses and make sure that nothing on the floor can roll under the rudder pedals.

Many pilots arrive in the Colorado Rockies without being very aware of the importance of the rudder in aircraft control. Pilots who have not encountered strong crosswinds are not accustomed to pushing full rudder on a crosswind landing, or to employing constant subtle corrections in cruise to keep the ball in the center.

Because pilots are automobile drivers first, pilots tend to respond to a banking jolt with hard aileron, which creates a lurching skid back to level flight. Cloud types indicate the kind of turbulence to be expected. In a sky with a rotor cloud or two the aircraft is apt to be rolled suddenly. If rotor turbulence hits make a gentle coordinated turn back to level flight.

Fly light to moderate turbulence with rudder. Keep a balancing hand on the yoke, but dont correct for a bumpy wing down by yanking at the ailerons. Use gentle coordination with rudder and yoke. In choppy turbulence dance lightly on the rudder pedals. The instant the nose swings to one side, correct with rudder. After practicing this you will find that your passengers in the back seat will hardly feel any yaw at all.

Then there is pitching turbulence, usually encountered when one is flying on the lee side of a group of peaks. One can envision the airflow around the summits being tumbled as it swirls around and over the sharp contours.

Flying pitching turbulence is like being in a canoe on an open bay, where first you see the wave lift the bow, then suddenly you slide forward into the next wave. In pitching turbulence you sense the tail rising. Then you can feel a plunge into a small wave, then the nose lifts again. You may need to make power adjustments in pitching turbulence but recover from altitude excursions with gentle elevator pressures.

The most difficult turbulence, often found in the vicinity of severe rotors, virga and thunderstorms, is a combination of rolls and pitches: sunfishing bronc turbulence. This compares to a rodeo ride where the horse is bucking and also twists in the leap. The best way to fly this is to have a steady hand on the yoke and be very active on the rudder pedals. When thrown into a steep bank, use coordinated controls to bring the aircraft back into level flight. And dont count on any rodeo clowns to remind you to keep the ball in the center.

Mountain flying
Mountain flying is exhilarating. To make it a satisfying experience, know your airplane and be proficient in the commercial certificate flight test maneuvers. Review aircraft performance figures – including maneuvering speed and best rate of climb – before entering mountainous areas. Remember that best angle speed increases slightly with altitude while best rate decreases.

Learn everything you can about mountain meteorology. Anticipate turbulence by reading clouds. If no clouds exist, anticipate the turbulence anyway by evaluating wind direction, surface barometric pressures and topography.

In uncomfortable circumstances a 180 to a nearby airport is always preferable to frightening your passengers.

Handle your flight controls gently in turbulence. Lead with rudder. And have confidence in the inherent stability of your aircraft.


-by Margaret Lamb

Margaret Lamb is a mountain flying instructor whose Navion is based at Alamosa, Colo.


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