Abrupt Wind Changes Elevate Mountain Flying Risk

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Mountain flying is susceptible to the elevated risk of severe, unexpected weather changes. Adverse winds caused by mountain wave, diurnal “canyon” winds or convective activity can create downdrafts of significant strength. Weather in mountainous areas can change rapidly and is affected not only by large-scale weather systems but also “micro-scale” systems. A classic example of a “micro-scale” weather system is the cliff adjacent to the threshold of Telluride Regional Airport’s (KTEX) Runway 9. As the sun’s angle moves across the sky and begins to heat that slanted terrain, the air immediately adjacent to that cliff begins to heat and rises rapidly in a thermal. For those without the benefit of a soaring background, thermals are rising parcels of air that continue to rise as long as the surrounding air is cooler. It is not uncommon for the strength of the cores of these thermals to exceed 2,000 fpm in the western states. (Anecdotally, I’ve been soaring in these thermals, attempting to climb in the core, and the strength of the gusty bubbles rising in these cores has completely “tossed” my sailplane into a nearly vertical [opposite!] bank angle despite full aileron to oppose the abrupt rolling motion.)

Conversely, the outer portion of these rising bubbles (imagine the shape of a doughnut, with the middle rising and the outside descending) can be nearly as strong. Aircraft control and flight path maintenance can instantly be compromised by the sudden and surprising strong air currents. Incidentally, there is a warning for pilots that KTEX sits on a 1,000-ft. mesa, with the precaution of strong vertical turbulence along the mesa’s edge. A study by pilot-researchers at the Johns Hopkins School of Public Health concluded that these micro-scale weather systems might be the root cause of many mountain accidents.

This type of rapidly changing adverse wind close to the approach end of the runway was a contributing factor in the crash of a Socata TBM 700 on Feb. 15, 2003, in Aspen. The approach was stabilized at 100 kt. with landing gear and flaps in the landing position. The approach was normal until approximately 100 ft. above the runway, at which time the airplane encountered a turbulence condition causing rapid-roll tendencies right and left. As the pilot began his landing flare at about 15 ft. above the runway, the left wing dropped rapidly, combined with a sudden high sink rate, and struck the runway. Fortunately, none of the four aircraft occupants were injured. Winds at the time of the accident were reported as 310 deg. at 6 kt. Records suggest that the winds were variable throughout the day. The NTSB determined that the pilots failed to maintain aircraft control. Contributing factors included the tailwind and the turbulence.

Confined Maneuvering Space

Maneuvering within the tight confines of adverse and high terrain during landing and takeoff for business jets was examined in the Flight Safety Foundation’s study on business jet safety conducted by this author (“Business Jet Approach and Landing Safety,” Fight Safety Digest, May 2004). The in-depth study, which examined 251 accident reports involving business jets from January 1991 through December 2002, found that constrained maneuvering room was a contributing factor in 22 (69%) of the 32 accidents that occurred during approach or departure from mountain bowl airports.

Rapidly rising terrain near airports in mountainous regions limits a pilot’s ability to maneuver during takeoffs and landings. At Aspen/Pitkin County Airport (KASE), the terrain rises 780 ft. within 0.5 mi. of the runway. A commercial approach plate for the airport warns pilots to be aware that “Terrain will not allow for normal traffic patterns. High rates of descent may be required due to terrain and local procedures. Airport located in high mountain valley with mountainous terrain from 12,500 feet to 14,000 feet MSL in proximity to airport; numerous unlighted obstructions. All adverse weather situations magnified in mountains. Unless ceilings are at least 2,000 feet above highest terrain and visibility is 15 miles or more, mountain flying is not recommended.” (Source: Jeppesen Sanderson, Aspen, Colorado, approach plate, 13-1, Oct. 9, 1998, Amendment 6.)

Even visual approach slopes can be steep. The PAPI for the visual approach to Telluride’s Runway 9 is 3.55 deg. Such abnormally steep approaches make a stabilized approach very difficult to comply with. In addition, there is a visual illusion created by the down-sloping runway threshold.

Rapidly rising terrain and high density altitude can also create dangerous conditions in which the terrain rises at a greater rate than an aircraft can climb. As a result, pilots can become trapped in mountain basins and canyons. The airport qualification instrument approach plate for Jackson Hole Airport (KJAC), Wyoming, states, “Rejected landing, missed approach and engine inoperative procedures may require special attention due to high climb gradients required for obstacle clearance.” (Source: Jeppesen Sanderson, Jackson Hole, Wyoming, airport qualification instrument approach plate, 19-02, March 20, 1998.)

Confined maneuvering room was a factor in the fatal accident of an aircraft during approach to Aspen-Pitkin County Airport. On February 13, 1991, a Learjet 35A was executing a VOR/DME approach to Runway 15. The aircraft was seen below the clouds on the downwind leg of the approach to the west of the airfield. However, when turning onto final approach, the Learjet appeared to continue farther to the east than would be expected, with the turn then being tightened as if the crew was attempting to bring the aircraft back onto the extended centerline of the runway. This turn became very steep, described by some witnesses as almost a 90-deg. bank, before the aircraft began to lose height. The Learjet impacted the ground about a mile north of the airfield. Just before impact, someone on the aircraft was reportedly heard to scream “Oh no [a] stall” over an open microphone. The NTSB’s probable causal statement attributed the accident to the flight crew's failure to maintain airspeed and control of the airplane while maneuvering to land. Contributing factors were the flight crew's execution of an unstabilized approach and the surrounding snow-covered mountainous terrain.

Confined maneuvering room is bad enough in VMC. The threat is magnified when an aircraft has to execute an instrument approach to these mountain bowl airports in limited visibility. Instrument approaches into mountainous airports are affected by the steep descent gradients, the complexity of the instrument approaches when available, and particularly by the numerous step-down fixes. For example, the LOC/DME-E approach at Aspen requires four step-down fixes from the initial approach fix. The missed approach point is 2,383 ft. above the aerodrome but only 2.6 mi. from the runway threshold.

Winter-time conditions prevailed at the popular Truckee Tahoe Airport (KTRK), California, on Dec. 28, 2005, when a Learjet 35A was cleared for the GPS-A (circling) approach. The published weather minimums for Category C and D airplanes at the 5,900-ft. MSL airport was 3 mi. visibility, and the minimum descent altitude was 8,200 ft. MSL. Airport weather observers noted that when the accident occurred, the visibility was between 1.5 and 5 mi. Scattered clouds existed at 1,200 ft. AGL, a broken ceiling existed at 1,500 ft. MSL, and an overcast condition existed at 2,400 ft. AGL. The airplane overflew the airport in a southerly direction, turned east, and entered a left downwind pattern toward Runway 28. A 20- to 30-kt. gusty surface wind existed from 220 deg., and the pilot inadequately compensated for the wind during his base-leg-to-final-approach turning maneuver.

A witness, located in the airport's administration building, made the following statement regarding his observations: "I saw the aircraft in and out of the clouds in a close base for [Runway] 28. I then saw the aircraft emerge from a cloud in a base to final turn [and] it appeared to be approximately 300-400 feet above the ground. The left wing was down nearly 90 degrees. The aircraft appeared north of the [Runway 28] centerline. The aircraft pitched nose down approximately 30-40 degrees and appeared to do a half cartwheel on the ground before exploding." The airplane collided with the ground during a low-altitude, steep-banked, base-to-final left turn, impacting terrain one-third mi. from the approach end of Runway 28, and north of its extended centerline. Both pilots (the only occupants) were fatally injured. The NTSB determined the probable cause to be the flight crew's inadequate compensation for the gusty crosswind condition and failure to maintain an adequate airspeed while maneuvering in a steep turn close to the ground.