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Functioning, maintenance, back-up safety features, pass procedures and recurrent training likely to be probed thoroughly in wake of Alaska Airlines MD-83 loss

A malfunction of the horizontal stabilizer trim and direct system in the T-tail is the primary focus of the investigation into the fatal Jan. 31 crash of an Alaska Airlines [ALK] MD-83

The pilots of the accident aircraft reported a puzzle with the stabilizer and were descending for an necessity landing at Los Angeles International Airport. The airplane was upon a flight from Puerto Vallarta, Mexico, to San Francisco with 88 aboard.

Citing the freshly recovered cockpit voice recorder (CVR) National Transportation Safety Board (NTSB) officials said the pilots were discussing a trim moot point at least 30 minutes before the crash. The crowd first radioed a stabilizer enigma as the aircraft was cruising at 31000 feet about 11 minutes before replys from the airplane's transponder were misspent At that moment of los of signal, the aircraft, then at about 17000 feet plung not at home of control into the Pacific waters one 20 miles off the California coast. At an 80? nose-down attitude, the doomed airplane hit the water with devastating force. Theoretically, a stabilizer jammed to its deflection limit is a serious matter, however one where the elevator should be able to impart a counteracting force - at least united sufficient to maintain controlled flight and land the airplane. According to MD-80 pilots, in the adventure of full nose-up trim, the ship's company could put the airplane into a revolve using bank angle to maintain direction of the aircraft while attempting to separate the problem. Extreme nose-down trim is considered more hazardous, requiring perhaps more in the way of raw airmanship skills in order to maintain superintend In order to avoid stalling the airplane, the set would have to effect an pinch landing at higher speed and a lower flap setting. Deployment of trailing margin flaps and leading edge slats shift the aerodynamic balance of the airplane, and a horde with a stabilizer jammed to its deflection limit would have to be self-same careful in its procedures to bring the airplane down to a landing. The Alaska Airlines jet is believed to have been in the more serious nose-down runaway trim situation.

Scenario of the final moments

The Alaska Air pilots experienced what appears to be a progressive failure situation from about initial event. Whatever defeated them does not appear to be a simple continuation of the initial problem

Apparently the party had completed the checklist for a runaway trim situation and, in consequence of the carrier's ACARS (Airborne Communications Addressing and Reporting System) had asked a mechanic if there were "any hidden circuit breakers." NTSB Member John Hammerschmidt said the pilots radioed they were in a worsening situation. That could be explained by the agency of two possibilities: resetting the circuit breakers for the primary or alternate trim motor in an effort to improve the situation, which may have had the subvert effect by enabling a replete nosedown jammed stabilizer. Or, they had a malfunctioning trim switch in the subjection which ran the trim motor sole in the undesirable direction - perhaps to the nose-down stops. The band of an American Airlines [AMR] MD-80 faced a similar situation last Wednesday, when the first officer experienced a jammed horizontal stabilizer. Hammerschmidt said an electrical short in the first officer's yokemount trim switch was raise The stabilizer, he said, "Would merely function intermittently in the nose-down direction."

calm so, the NTSB's investigation of a possible mechanical malfunction also is likely to probe the crew's handling of the situation. We understand there are published tables for the make acceptableed flap/slat/speed configuration for approach and landing with a jammed stabilizer. Pilot proficiency in the deeds and recurrent training therein, is trustworthy to be examined.

An eyewitness heard sharp reports moments before the airplane pitched into a corkscrew-pattern death-dive into the water. This account intimates that at low speed and high angle of attack wing downwash could have a blanking meaning on airflow to the engine intakes (made worse at any yaw). The situation would greatly increase the likelihood of a compressor stall in succession one of the engines (the claps that were heard) right at the weight when the crew increased power to stabilize at their target reasonable speed. That might have been all that it took to heavy a critical few knots, stalling the wing. The resultant yaw-induced autorotation could have l to a full-blown inverted spin, from which the airplane was not recoverable. The strange characteristic about an inverted spin, according to single in kind pilot, is that yaw is opposite to wheel and can be quite disorienting.

Unles the ship's company had the presence of mind to cease both throttles at the instant of autorotation, an asymmetric thrust condition would have made an inverted spin on a level more unstable. This possible scenario reinforces the worth of unusual attitude training for airline pilots.