When the captain of a Rocky Mountain Airways de Havilland DHC-6 “Twin Otter” showed up at Stapleton International Airport for his daily flights between Denver and the airports on the other side of the Colorado Front Range (Dec. 4, 1978), he was greeted by horrific winds as soon as he stepped outside. Winds in the Denver area were gusting 18-42 kt. Those of you who have lived along the Front Range know that it is Mother Nature’s fine example of a wind machine under conditions like these.
The weather forecast in the dispatch release package advised the captain that weather in north central Colorado at the time was under the influence of a strong pressure gradient between an area of low pressure centered over southeastern Wyoming and an area of high pressure centered over northeastern Arizona. This caused strong northwesterly winds over north central Colorado. A PIREP from a Convair 580 reported headwinds of about 100 kt. at 20,000 ft. near Hayden. Additionally, SIGMET “Charlie 2” was issued that afternoon for frequent strong updrafts and downdrafts along the eastern slopes of the Front Range. AIRMET “Echo 1” cautioned pilots over western Colorado for occasional moderate turbulence below 22,000 ft. with locally strong updrafts and downdrafts along eastern slopes.
You might think that the sturdy de Havilland Twin Otter with its well-known STOL capabilities would be very capable under such conditions, but Rocky Mountain Airways Flight 212 was unable to climb above the mountains west of Denver en route to Granby, Colorado, because of these adverse winds and was forced to return to Denver.
The aircraft was refueled and departed for Steamboat Springs. Those of us who have enjoyed many hours in the front of the Twin Otter have endured the jokes from others about its lack of stellar cruise speed, but the flight still took nearly 2 hr., considerably longer than it normally takes due to the high headwinds. The aircraft arrived safely on the ground at Routt County Airport. The captain arranged for an IFR flight plan to depart from Steamboat Springs to the Gill VOR via the V-101 airway.
The aircraft was turned around for a return leg to Denver as Flight 217. At 18:55, the Twin Otter reported to Denver Center that it had left Steamboat Springs and was cleared to climb to an altitude of 17,000 ft. on the published departure procedure. The aircraft crossed over the Steamboat NDB at 12,000 ft. and then intercepted V-101 eastward. However, as the aircraft approached 13,000 ft., it wouldn't climb any farther and reported to Denver Center, "...we're going to have to return to Steamboat." Twenty minutes later, the Twin Otter flight crew transmitted, "...want you to be aware that we're having a little problem here maintaining altitude...proceeding direct Steamboat beacon." The aircraft was unable to maintain altitude with maximum climb power applied. The pre-stall buffet was occasionally encountered. Despite climb power from the turboprop engines, 10 deg. of flaps extended and an airspeed 90-100 kt., the aircraft descended at a rate varying from 800-1,000 fpm. It was rapidly running out of altitude and out of airspeed, with nowhere else to go.
The first officer saw the ground 1.5 sec. before impact and saw a bright blue flash to his right just before impact. The aircraft first struck a high-voltage electrical transmission line tower, severing the outboard 10 ft. of the right wing. The nearly intact fuselage and left wing were found about 200 ft. from the base of the tower. Of the 22 persons on board, two died from injuries received in the crash.
The NTSB determined that a mountain wave existed over the mountains immediately east of Steamboat Springs. Severe downdrafts in the lee of Buffalo Pass prevented the aircraft from maintaining flight.
Downdrafts Can Exceed Aircraft Climb Capabilities
This is not the first time that a turbine aircraft has been placed in a very adverse condition due to the downdraft from a mountain wave, and some of these events have occurred to large, powerful aircraft. According to the NTSB, on March 31, 1993, a Boeing 747 operating as Japan Air Lines Flight 42E departed Anchorage and encountered mountain wave downdrafts. The 747’s rate of climb decreased to just 100-200 fpm at 3,000 ft. MSL. At 4,500 ft. MSL, the aircraft entered an area of even worse sink, causing the massive 747 to descend at 1,000 fpm, despite application of maximum climb power. The flight crew reported four stall warnings that did not stop until the aircraft exited the area of severe downdrafts. (Minutes later, another 747 would encounter extreme turbulence from the same mountain wave, which induced immense stress on an engine pylon, and the entire engine assembly completely separated from the aircraft.)
According to the World Meteorological Organization’s Aviation Aspects of Mountain Waves, “One of the more important aspects of mountain waves from the viewpoint of aviation is the vertical currents associated with these waves…. These low-level downdrafts, which are of most importance, constitute one of the principal threats to aircraft safety.” Aviation Aspects of Mountain Waves documents numerous cases of turbine-powered aircraft losing significant amounts of altitude during encounters with mountain wave downdrafts.
In the NTSB investigation of a fatal Cessna TR-182 accident near Eagleville, California, on April 25, 2003, a safety board weather analysis determined the cause of the accident was a mountain-wave-induced downdraft greater than 5,000 fpm.
The Australian Transport Safety Bureau’s Flight Safety Australia magazine (January-February 2002) warns pilots, “Many dangers lie in the effects of mountain waves and rotors on aircraft performance and control. In addition to generating turbulence that has sufficient ferocity to significantly damage aircraft or lead to loss of aircraft control, the more prevailing danger to aircraft in the lower levels in Australia seems to be the effect on the climb rate of an aircraft. Aircraft rarely have the performance capability sufficient to enable the pilot to overcome the effects of a severe downdraft generated by a mountain wave….” Flight Safety Australia reports that “The vertical airflow component of a standing wave may exceed 8,000 fpm.”
Aviation Aspects of Mountain Waves documented one particularly eye-opening atmospheric research experiment in the famous “Bishop Wave” (near Bishop, California). A P-38 aircraft encountered updrafts of such intensity that the pilot was able to feather the propellers of the fighter aircraft and use it as a glider. The estimated vertical updrafts were on the order of 40 m/s (approximately 7,872 fpm). One can only imagine the strength of the downdraft, which would have been roughly equivalent.
While these are extreme values found in the atmospheric research, even “average” downdrafts are cause for considerable concern. Research cited in Aviation Aspects of Mountain Waves found that vertical speeds of 2,000 fpm were very typical in the lee waves of the Sierra Nevadas. Other atmospheric research has found that downdrafts in excess of 3,000 fpm are not uncommon, and 7,000 fpm has been reported in the U.S.
Atmospheric research in Europe has found that vertical air currents of 5 m/s (984 fpm) are not uncommon, and in some incidents, flight crews of turbine aircraft executing an “escape maneuver” at full power have lost significant amounts of altitude in downdrafts that exceeded 12 m/s (2,363 fpm). Aviation Aspects of Mountain Waves states the clearly significant result of this threat. “The danger to aircraft of downdrafts of the above magnitude can be easily appreciated…in such circumstances catastrophic loss of height might occur.”
BCA examined 125 NASA Aviation Safety Reporting System (ASRS) reports in which downdrafts caused by lee wave encounters created a threat to a business aircraft’s safety. Eighty-six percent occurred when the aircraft was in the takeoff or landing configuration, features which would lessen its excess power and thus lessen its rate-of-climb capability.
What type of “undesired aircraft states” occurred in the ASRS sample? All of these narratives stated the inability of the aircraft to attain or maintain a safe minimum IFR altitude. This was sometimes made worse by the aircraft needing anti-ice equipment due to IMC conditions. Seventy-one percent experienced a significant loss of airspeed during an attempt to arrest the aircraft’s descent rate; 63% indicated a threatening loss of terrain separation; 19% indicated a descent below the minimum IFR altitude; 13.5% descended in the mountain wave rotor, which is an area of extreme turbulence.
Note: The next part of this three-part series will examine the conditions that create mountain waves.
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