Since 2009, 42 Air Force MC-12Ws have been deployed to war zones in the Middle East. The MC-12W is a modified Beech King Air 350, externally similar except for a plethora of antennas and several unsightly bulges, including a huge belly pack. Communications and surveillance equipment and stations for two technicians replace the usual executive amenities.
Last April, an MC-12W crashed during a surveillance mission 110 miles northeast of its base in Kandahar, Afghanistan, killing its crew of four.
The terrain elevation in the planned working area was between 6,000 and 7,000 feet. On arriving, the crew began to orbit in left turns at FL 200. A large cumulus was building; the airplane circled into and out of it, encountering some precipitation and moderate turbulence but no lightning or icing.
The airplane was not equipped with a flight data recorder, and so investigators reconstructed the accident sequence from the cockpit voice recorder. Thirty-seven minutes after takeoff, the pilot requested and received clearance to climb to FL 230 to get out of the clouds. He started the climb on autopilot, evidently using the vertical speed, or V/S, mode. Significantly, he did not use the flight level change, or FLCH, mode. In FLCH mode, the autopilot holds airspeed; the rate of climb to a target altitude depends on the power setting. In V/S mode, the pilot selects a climb rate and the autopilot adjusts speed — that is, angle of attack — to maintain it. According to the Air Force accident report, V/S is habitually used by “half the pilots” — presumably meaning MC-12W pilots — for altitude changes even though there is a slight risk that with insufficient power the autopilot will maintain the climb rate by raising the nose until the wing stalls.
And that is what happened.
Not long after the start of the climb, the pilot noticed his airspeed decaying and said, “A little slow, correcting.” Seven seconds later, the mission commander, who was the senior pilot but occupied the right seat, said, “All right, firewall.” A second later, the autopilot was disengaged, and a warning tone indicated the angle of bank had exceeded 50 degrees.
The mission commander again called for full power and ordered, “Eyes inside, eyes inside,” meaning that the pilot should rely on the instruments rather than outside visual cues. The CVR recorded the stall warning horn and then the clatter of objects flying around, suggesting the temporary weightlessness of a spin departure. The bank angle warning stopped, but that did not mean the airplane was upright. Instead, it meant the PFD had shifted to its “declutter mode,” in which distracting information is removed and red chevrons appear pointing toward the horizon.
Twenty-two seconds after the start of the upset, the mission commander took control of the airplane. It was then descending at more than 11,000 fpm. Two seconds later, the overspeed warning sounded; the airspeed had surpassed 245 kias.
The King Air crashed less than a mile horizontally from the location where the stall occurred. Debris was confined to an area 100 yards in diameter with the exception of the right wingtip and winglet, which separated before impact and came to earth a third of a mile away.
The investigation suggested that the accident was initially precipitated by loss of airspeed due to pilot distraction — the pilot was changing orbit position at the same time as the autopilot was taking care of the altitude change — and that rapid application of power at a high angle of attack caused the nose to slice to the left and the airplane to enter an incipient spin.
Although the pilot’s reaction to the stall appears to have been inappropriate — at one point he exclaimed, “Whoa, pull up!” — the wing seems somehow to have gotten unstalled and the spin to have been replaced by a rapidly rotating spiral. It’s likely that neither pilot fully understood what was happening, even after the airplane emerged below the clouds.
Although this accident was of an unusual kind, several things may be learned from it.
First, vertical-speed hold is a potentially risky autopilot function for climbing and should be used only when ample excess power is available to ensure that speed will not be lost. It does not relieve the pilot of the need to monitor airspeed.
Second, the leftward pull of noncounter-rotating propellers is most severe at low forward speed and maximum power and will cause the nose to swing to the left if the pilot does not anticipate it. Sudden application of power also commonly causes a pitch-up, as does the gyroscopic moment of the propellers with left yaw. Everything, in other words, conspired to pull the King Air into a spin departure to the left.
Third, the appropriate pitch command in both a spin and a developed spiral is nose-down, not nose-up, regardless of what the PFD chevrons may say. The mechanics of stall recovery are familiar enough to all pilots, but those of recovery from a “graveyard spiral” are less so because the characteristics of a fully developed spiral cannot be simulated in training.
In the final moments of its dive, the King Air was pulling sufficient G — probably at least five, implying a bank angle of around 80 degrees — to break off one wingtip. Obviously, a further pitch-up command, as recommended by the horizon-seeking PFD, wasn’t going to help. What was needed was right roll to level the wings and a pitch-down command to ease the G-loading during the pullout. Lowering the landing gear, regardless of gear speed limitations, would also have helped slow the airplane.
In a spiral, part of the pitch-up correction from the horizontal stabilizer is directed inward, toward the center of the turn, rather than upward, and so it does not tend to reduce speed as much as it would if the wings were level. At the same time, the curved flight path tends to overcome what lateral stability the airplane has — most have little or none to start with — and to steepen the bank. The hallmarks of the spiral are ever-increasing speed, ever-increasing bank angle and ever-increasing rate of descent.
The Air Force investigation of the accident found its cause to be “a stall due to insufficient airspeed.” A nonpilot reader or newspaper reporter might conclude that such stalls occur of their own accord and have nothing to do with pilot actions. Three factors were said to have contributed to the stall. First, impeded visibility because of IMC weather; this, however, ought not to have been a factor for an instrument-rated King Air pilot. Second, the pilot’s inexperience in the MC-12W; he had only 21 hours as PIC in type, but, again, he was an Air Force-trained pilot, and the basic elements of speed control and autopilot use are not substantially different in a King Air from those in other airplanes.
Finally, the report cited “known MC-12W program risks associated with sustaining required combat capability in theater.” The risks arose from the brief preparation time available for pilots entering the program and the lack of qualified instructors at the operating bases. The implication was that things like this are bound to happen now and then in a war zone. The same could be said of aviation in general, but that does not prevent us from mentioning, from time to time, the importance of attention to airspeed.
_This article is based on the U.S. Air Force’s report of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory. _
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