Maneuvering Risks at Mountain Bowl Airports
Business aviation’s most scenic locations are often the preferred destinations of Hollywood’s A-list and business elite. On an average day the ramps at popular destinations such as Aspen, Eagle, Telluride, Truckee, Sun Valley and South Lake Tahoe are filled with rows of business aircraft.
Unfortunately, these mountain airports are also some of the most challenging for flight crews because the departures and approaches into them require maneuvering among the tight confines of forbidding terrain. There are additional threats created by localized wind patterns that can shift suddenly without warning, as well as the many effects on an aircraft’s performance caused by high density altitude. These combine synergistically to perilously diminish an aircraft’s safety margins.
One example of a rapid deterioration in an aircraft’s safety margins in a high mountain-bowl setting occurred on Aug. 4, 2018, when a 79-year-old Junkers Ju 52 departed from Locarno Airport (LSZL), Switzerland, en route to Dübendorf Airport (LSMD) near Zurich. The Ju 52 was operated by Ju-Air, a branch of the Association of the Friends of the Swiss Air Force. In the previous 10 years it had conducted about 900 sightseeing flights annually. The organization was staffed by eight employees and 96 volunteers, which included 27 pilots and 30 flight attendants who doubled as tour guides.
At the controls were two highly experienced pilots. Both had extensive experience as fighter pilots with instructor duties in the Swiss Air Force and later as commercial airline pilots for Swissair, where they retired as captains flying long-haul international flights. The captain, who was the pilot flying (PF), had a total of 20,714 flight hours and five years specific experience in the Ju 52. In the previous two months he had flown 33 Ju 52 flights. The pilot monitoring (PM) had 19,751 total flight hours, with 41 flights in the Ju 52 in the previous two months.
After takeoff, the aircraft proceeded at altitudes below those of the surrounding terrain. Contrary to Swiss aviation regulations, the pilots flew over a designated “countryside preservation quiet zone” at heights between 400 ft. AGL and 1,000 ft. AGL, often with minimal lateral separation from terrain. The Ju 52 made a relatively tight left turn into a high mountain bowl with the intention to exit the basin over a narrow pass as they had done previously on a similar flight. The flight crew flew up the middle of the basin instead of flying up one side of it. The latter is considered a standard practice in mountain flying to provide more room for an emergency turn. The flight path through the center of the basin left the flight crew with only one option to exit the basin--by flying straight ahead into abruptly rising terrain. The Swiss accident investigation report emphasizes “One of the basic principles of flying in mountainous areas is that there must always be the option of an alternative flight path or to turn back.”
The aircraft began to enter downdrafts, including a 1,200-fpm descent that lasted 4 sec. Rapid changes from downdrafts to updrafts are common when in close proximity to mountainous terrain. Normal terrain-induced turbulence had already occurred earlier in the flight, which should have alerted the pilots. The aircraft’s angle of attack increased to 15 deg., putting it precipitously close to its stall angle of attack, but the flight crew responded by increasing pitch. The aircraft then abruptly encountered a significant updraft. The updrafts rapidly increased the aircraft’s angle of attack to the point of stall. Shortly thereafter the left bank reached nearly 33 deg. and continued increasing despite significant aileron deflection. The pitch continued toward a vertical down trajectory. The flight crew attempted to recover, but since the aircraft was being flown too close to terrain, there was insufficient room for recovery. Approximately 1 min. after entering the basin, the Ju 52 impacted terrain at an estimated 108 kt., fatally injuring all 20 occupants and destroying the aircraft.
The in-depth investigation stated, “With the prevailing minimal height above terrain and no other escape options, the aircraft was placed in an irrecoverable condition.” Two direct causal factors were also identified. The pilots operated the aircraft in a very high-risk manner by navigating it into a narrow valley at low altitude and with no possibility of an alternative flight path, and they chose a dangerously low airspeed in regard to their flight path. Both these factors meant the turbulence that was to be expected led not only to a stall resulting in loss of control but also to an unrecoverable situation.
The investigation also uncovered a disturbing number of other contributing factors. The Ju 52 was operating marginally outside its flight envelope due to a CG position that was slightly beyond the rear limit, rendering the aircraft less stable around the pitch axis.
A few days prior to the accident flight, the PM on the ill-fated flight made three Ju-Air sightseeing flights as pilot in command. On all three of these flights, it was determined that the aircraft had been flown significantly below 1,000 ft. AGL in mountainous areas, “disregarding essential principles for safe mountain flying.”
The investigation evaluated radar data for 216 flights carried out in the five months prior to the accident, paying particular attention to: flight phases with no possibility of turning back or alternative flight path; flight over terrain when significantly below the recommended safety margin or the minimum required flight altitude prescribed for non-commercial VFR flights.
More than one-third of the flights analyzed contained significant violations of elementary principles of safe flight management in mountainous areas, with almost half of them judged to have a very high potential risk. Sixteen of the operator’s 27 pilots had been initially trained as Air Force pilots and “It was these pilots who were the ones mainly violating elementary safety rules.”
The accident investigation report determined that Ju-Air’s pilots exhibited a tendency toward systemic reckless violation of generally recognized aviation rules. Furthermore, its pilots were found to have insufficient up-to-date knowledge of basic flying principles such as the structure of airspace, flight preparations, calculation of the mass and center of gravity, and knowledge of aviation regulations.
Higher Accident Rates at Mountain Airports
The similarity of this accident to many in business aviation illustrates the high risks associated with approaches and departures at airports in confined mountainous terrain. A 1993 Government Accountability Office (GAO) report on mountain flying risks examined NTSB accident records from October 1983 through September 1992. The report, “Aviation Safety: FAA Can Better Prepare General Aviation Pilots for Mountain Flying Risks,” determined that the accident rates at mountain airports, especially those with confined maneuvering space, were significantly higher than non-mountain airports. For example, Telluride Regional Airport (KTEX), Colorado, had 13.71 accidents per 100,000 operations, and Aspen/Pitkin County Airport (KASE) had 4.10 (per 100,000 operations). For the sake of comparison, the GAO compared this accident rate to airports in flatlands like Devils Lake, North Dakota (KDVL), which had 1.86 accidents (per 100,000 operations).
The GAO report found numerous factors associated with mountain flying caused or contributed to 1,879 of the 22,352 general aviation accidents (over 8%) from October 1983 through September 1992. An aircraft’s performance decreases in the high, mountainous environment, which causes a reduction in engine output, negatively affecting takeoff, landing and rate of climb. This is exacerbated as the temperature increases.