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April 1, 2004
Late in the afternoon of January 27, 2001, a Beechcraft Super King Air 200 twin turboprop departed Jeffco Airport in Broomfield, Colorado, for a flight to Stillwater Regional Airport in Stillwater, Oklahoma.
At the controls were two well-qualified pilots with extensive King Air experience. Their eight passengers were all members of Oklahoma State University's basketball team, which had just competed in a game against the University of Colorado at Boulder.
The IFR departure was uneventful and the aircraft began its climbout to the southeast. About eight minutes after takeoff, Denver Center cleared the flight direct to EPKEE Intersection. One of the pilots responded that they were making a 3-degree left turn in order to proceed direct. No further radio communications were received from the flight. The aircraft continued to climb, and at about 5:33 p.m. Mountain Standard Time (MST), Mode C transponder returns showed it had reached its assigned cruise altitude of Flight Level 230. Darkness had fallen and the flight was in instrument meteorological conditions (IMC).
Slightly more than three minutes later, at about 5:36 p.m., the last Mode C altitude return was received by Denver Center, indicating the aircraft had reached an altitude of 23,200 feet. The transponder continued to transmit Mode A information, which showed the aircraft on course. However, about 42 seconds after the loss of Mode C data, the flight began to turn gently to the right. Bank angle gradually increased while pitch decreased. By the time another 44 seconds had elapsed, the aircraft was spiraling through 10,000 feet with a greater than 80-degree nose-down-pitch angle and a greater than 100-degree right-bank angle. Its descent rate quickly reached more than 15,000 feet per minute.
The King Air completed a 360-degree turn before striking rolling terrain near Strasburg, Colorado, at an elevation of 5,223 feet. All aboard the aircraft died. Radar data, distribution of the wreckage, and other evidence led investigators to conclude that the pilot probably attempted to pull out of the dive at low altitude. The resulting extreme aerodynamic loading caused the aircraft to break apart several hundred feet above the ground. Cloud bases in the area were estimated at 600 feet, and it is possible that this last desperate maneuver was in response to the pilot's seeing some ground references after emerging from the clouds.
From start to finish the flight had lasted barely 19 minutes and traveled just 42 miles from the departure airport. Total time from loss of the Mode C signal to the accident was a little more than a minute and a half.
After eliminating icing and loss of pressurization as possible culprits, the NTSB's accident investigation focused on an electrical system malfunction as a likely factor in the accident. The loss of Mode C (altitude) returns and the fact that Mode A (position) continued to transmit suggested a partial loss of electrical power. Mode C information is generated from the airplane's alternating-current-powered air data computer, whereas Mode A receives power from a direct-current electrical source. Based on this and other physical evidence gathered at the accident site, investigators concluded that either a partial or complete loss of AC electrical power had occurred. They also determined that AC power was not restored at any time after it was lost.
A total loss of AC power would have rendered most of the flight instruments on the captain's side inoperative. Pitot-static and vacuum-system-powered instruments would still work, specifically the airspeed indicator and turn-and-slip indicator. Copilot-side flight instruments also would be available. These would have included an altimeter, airspeed and attitude indicators, and a turn-and-slip indicator. Ambient cockpit and instrument panel lighting are powered by DC sources, and so they too would be available.
The NTSB was unable to determine if the autopilot was engaged at the time AC power was lost. If it was, it would have disengaged because it receives its control signals from the AC-powered air data computer. At disconnect, flashing red master warning and autopilot disconnect lights would have illuminated.
The loss of AC power would not have been a subtle event in any case. Instrument flags would have appeared on the captain's attitude indicator, altimeter, horizontal situation indicator, radio magnetic indicator, radio altimeter, and altitude preselect window. The Super King Air 200 has dual inverters that produce AC power from a DC power source. Only one is used at a time, and had this component failed, an inverter annunciator light would have illuminated too.
Investigators concluded that there were multiple possible explanations for the loss of AC power. These ranged from single- or dual-inverter failures to failures of various other electrical system relays and components. But whatever the exact reason, they decided the malfunction was not a causal factor in the accident. It was merely a contributing factor, since the pilots could have safely operated the flight with the remaining flight instruments.
The Super King Air 200 pilot's operating handbook spells out what a pilot's response to an electrical failure should be. The procedure is fairly short, and it boils down to resetting essential circuit breakers one time only, and selecting the other AC inverter if the selected inverter has failed. The inverter selector switch is located on a small panel on the captain's side below the main instrument panel. The circuit-breaker panel is on the copilot's sidewall, at about knee level.
There was no cockpit voice recorder installed on this aircraft (nor was one required), so we can only speculate on the conversation that took place between the pilots following the electrical failure. But it's reasonable to presume that they were aware of the problem because of the various warning lights and flags they would have seen. And with or without reference to a checklist, popped circuit breakers would be high on their list of things to look for. So it's likely that the copilot would have spent some amount of time examining the circuit-breaker panel on his side.
In any abnormal or emergency situation, a pilot's first obligation is to fly the airplane. He or she needs to prioritize in such a way that control is maintained while the problem is handled. The adage "aviate, navigate, communicate" suggests a good priority order to follow. In the case of an electrical failure in IMC at night, "troubleshoot" could be added to the list, perhaps just before "communicate." With two pilots available, one needs to fly the airplane while the other works the problem. And as tempting as it may be for the flying pilot to jump into the fray to lend a hand, he or she needs to avoid this siren song at all costs.
The King Air captain would have had a choice of flying partial panel, or referencing the more complete set of working flight instruments on the copilot's side. Or he could have delegated flying duties to the copilot while he handled the troubleshooting. But again, we can only speculate as to what each pilot actually did. The condition of the wreckage precluded investigators from determining whether the second inverter had been selected, or whether any circuit breakers had been reset.
What we can surmise, however, is that the captain somehow allowed the distraction of the electrical failure to interfere with the primary task of letting someone fly the airplane. In fact, that is the essence of the NTSB findings. Specifically, investigators concluded that "the probable cause of this accident was the pilot's spatial disorientation resulting from his failure to maintain control of the airplane with the available flight instrumentation."
Spatial disorientation happens when a pilot lacks adequate visual- or instrument-supplied information about the aircraft's pitch and bank. Instead, he or she relies on cues from sensory organs in the inner ear that can be misleading. These organs detect angular acceleration in the pitch, yaw, and roll axes. It's important to note that these sensory organs are affected by head and body movement as well as the motion of the aircraft.
FAA Advisory Circular 60-4A points out that tests involving instrument-rated pilots show it can take as long as 35 seconds to establish aircraft control solely by instruments when visual references are lost.
Another study demonstrated that pilots can't easily detect roll rates less than about 2 degrees per second. So a pilot preoccupied with matters in the cockpit may not notice he or she has gradually entered a descending turn. In the classic graveyard-spiral scenario, a pilot incorrectly senses increasing bank angle as a descent rather than a turn. In response he or she pulls back, tightening the turn and losing even more altitude.
The best "cure" for pilot spatial disorientation is to reestablish a visual- or instrument-based picture of the aircraft's attitude and respond accordingly. But practicing preventive medicine is usually preferable to treating a problem. Remembering first and foremost to fly the airplane is the easiest way to avoid this kind of trap.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.
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