From: firstname.lastname@example.org (Brian A. Reynolds) Organization: Rockwell Avionics - Collins Date: 14 Nov 95 14:46:20 References: 1 2 3
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AvWeek 17 April 1995 The Romanian team investigating the probable cause of the March 31 crash of a Tarom A310 transport is focusing on the combination of an engine autothrottle system failure that generated asymetrical power setting and on the pilots' apparent failure to react quickly to the developing emergency. When the autothrottle was set to climb thrust, the Digiatl Flight Data Recorder (DFDR) showed the right engine remaining at takeoff thrust, while the left engine's thust reduced to idle over a 42-second period. Investigators speculated that the right throttle may have been mechanically jammed and the authrottle electric motor continued applying force through a slipping clutch to the jammed right throttle, while the left clutch was drawing its throttle to idle. "We are amazed. We do not understand (what happened); the roots of the accident could be somewhere else. The asymmetrical thrust situation developed slowly, giving the pilots enough time to identify the failure and act accordinly" an A310 French pilot said. The pilot added the A310 is not equipped with fly-by-wire controls and that the aircraft has a conventional yoke - two features that contribute to giving the pilots the tactile feel of developing asymetcial engine power. --------------------------------------------------------------- Commentary. I have in this forum and others used throttle movement as an example of good human factors design as it provides a clear indication of what the automatic system is attempting to do. But alas, here is a case of where, apparently, the left throttle was at flight idle but the right remained at takeoff thrust setting. Because of other factors, the flight crew failed (or was not provided the correct information) to recognize what was happening in time to break the chain. (The weather was also a factor, "due to poor visibility and heavy snow, the pilots had no visual horizon reference.") This accident could have happened in anyones flight deck (assuing that the mechanics of how throttle lever position is conveyed to the engines is similar). I appologize for making the generalization from an A310 incident to the later fully automated flight deck design of the modern Airbus aircraft. Brian p.s. In the Douglas implementation of autothrottles (and I think also for Boeing), it is the actual position of the throttle lever which is used as the throttle command. A slip clutch is provided so that the flight crew can override a run-away servo motor. In addition, the autothrottle system also has to release a brake in order to drive the throttle levers. The drawing show in the AvWeek article is generally consistent with this scheme. So increase friction in the mechanical connection between the throttle levers and the engines could have been overridden by the action of the flight crew, but still been too high for the servo system to drive against. "Seconds before the aircraft impacted the ground, both engines were at idle thrust, indicating the pilots then were acting to eliminate asymmetrical poser condition and were trying to restore a normal flight pattern." I hope that this has cleared up any confusion my original posting may have caused.