Mountain Wave

Going from smooth VMC to turbulent IMC in a matter of seconds can lead to spatial disorientation.


Learning to fly as I did in the relatively flat Eastern U.S., I never really had to deal much with windy, mountainous terrain until after I earned my instrument rating and started using a personal airplane for transportation. My most memorable encounter involved flogging a Skyhawk north out of Asheville, N.C., at 9000 feet. Improbably, I had a tailwind. All was well until passing Mt. Mitchell, the highest point in North Carolina. While I enjoyed the tailwind, I did not enjoy the mountain wave it generated.

I soon found my relatively heavy 160-hp Skyhawk needed to be in a climb attitude with full power-which aint much-to maintain a descent rate of less than 300 fpm. In this encounter, there was very little “up” movement in the wave, along with intermittent moderate turbulence. I told ATC I wasnt able to maintain altitude and ended up with a block to 7000. The minimum en route altitude for that airway was 6000 feet. I was getting there.

After a few minutes of this and a descent well below my cruising altitude, I needed to do something. The only real solution-which, thankfully, I dredged up from my training-was to turn an approximate right angle to the wave and get out of it. It wasnt long after settling on the new heading-still unable to do much more than maintain altitude-that I finally flew out of the wave. By this point, I was away from the airway and continued the descent-with greater control over its rate-down to 5000 feet, chose an alternate routing away from the mountain wave and motored on home.

Since then, Ive encountered quite enough turbulence, thank you, but few mountain waves. On that particular day, I was in VMC, something for which I was glad. Even so, Ive also seen days when abrupt transitions from visual conditions to instruments and back were profoundly unsettling. On those days, Im glad I didnt have to deal with a mountain wave in the bargain. Perhaps things might not have turned out as well.


On August 1, 2008, at about 1433 Pacific time, an amateur-built Lancair ES lost radar and radio contact 40 miles northwest of Yakima, Wash. The wreckage was found nine miles northwest of Cliffdell, Wash. The instrument-rated private pilot and single passenger were killed; the airplane was destroyed. Visual conditions prevailed on the ground, but instrument conditions existed at the flights 11,000-foot cruising altitude. The flight was operating on an IFR clearance.

At 1431:30, the aircrafts track started a descending right-hand turn. The final radar return was at 1432:06, at 9700 feet msl. The airplane wreckage was located about one mile west of the final radar return at an elevation of 3830 feet and was spread over a distance of 0.5 miles on a densely wooded mountain slope. No initial point of impact was identified.


The accident site was located behind a cold front. The NWS Radar Summary Chart for 1421 PDT (2121Z) depicted several areas of light-to-strong-intensity echoes over central Washington, with rain shower tops to 29,000 feet. At the accident airplanes cruising level of 11,000 feet, winds were determined to be from 245 degrees at 27 knots with a temperature of -1 degree C.

The flights track from 2129 to 2132 PDT was overlaid with weather radar imagery and depicted the airplane heading in a southeasterly direction. The image depicted an area of very light to light intensity echoes extending from Mt. Rainier east-northeastward through the accident site. The accident airplane was then observed encountering the band of echoes and making a right turn towards the west when the upset occurs.

Numerous photos of Mt. Rainer taken by the occupants minutes before the accident sequence were recovered from a personal camera that survived the accident. The accident airplane was operating immediately above a broken to overcast cloud layer at the time. A band of higher clouds extended immediately downwind from Mt. Rainier with tops as high as the mountain. The last known picture taken shows signs of undulation motions or wave action at the cloud top, with a higher band of cirriform cloud layer further south along the track.

While researching the wreckage, inspectors discovered a rod-end bearing in the primary pitch control system was assembled with only 2-3 threads exposed from the end. At the opposite end of the push rod, all of the rod-end bearings threads were used, with its locknut up against the components shoulder. The NTSB did not mention this maintenance issue in its probable cause finding.

Probable Cause

The National Transportation Safety Board determined the probable cause of this accident to include “The pilots failure to maintain aircraft control while in cruise flight due to spatial disorientation. Contributing to the accident was turbulence and clouds.”

The Instrument Flying Handbook (FAA-H-083-15A) defines spatial disorientation as “the lack of orientation with regard to position in space and to other objects.” The Handbook goes on to note the false horizon visual illusion can lead to spatial disorientation: “A sloping cloud formation, an obscured horizon, an aurora borealis, a dark scene spread with ground lights and stars, and certain geometric patterns of ground lights can provide inaccurate visual information, or false horizon, for aligning the aircraft correctly with the actual horizon. The disoriented pilot may place the aircraft in a dangerous attitude.” Based on the images recovered from the onboard camera, the pilot may have encountered a false horizon illusion.

Spatial disorientation, of course, is something of a catch-all term to describe when inner-ear and other sensations do not match physical reality. Its something all aspirants to the instrument rating learn about but rarely get to experience outside of the training environment.

In this instance, the pilot had been cruising above an undercast-presumably in smooth air-and soon encountered both IMC and the mountain wave. Unless one anticipates entering IMC and initiates an early transition to flight by reference to the instruments, the first few moments can be exciting.

Couple simultaneously entering mountain-wave turbulence with an abrupt transition to instruments and its easy to see the difficulties facing the pilot. In such an instance, theres an overwhelming urge to follow what your body tells you instead of the flight instruments. As weve seen, thats the wrong reaction.


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