Tuesday, August 12, 2014

NTSB disagrees on Asiana Flt 214 Crash last year

Asiana Flt 214, a Boeing 777, hit short of the runway coming into San Francisco last year.  The pilots attempted to go around at the last minute, but it was too late.  They hit the seawall at the approach end of the runway, ripping off the landing gear, and sending the aircraft skittering down the runway on its belly.  Due to good luck and a very rugged airframe, the plane did not catch fire, and nearly all the passengers and crew survived the accident.
   The accident was caused on the approach, when the crew assumed the autothrottle system would maintain a commanded airspeed, hands off.  The autothrottle decided it had been turned off, and allowed airspeed to decay, leading to a higher sink rate, and coming in too low.  The three man crew failed to notice the loss of airspeed (no one looked at the airspeed indicator).  This was kinda surprising.  Back in the 1960's, the first autothrottle system in the C-141 transport was hardly ever used.  The crew preferred to hand fly the C-141 into a landing.  After an exciting six hour flight on autopilot, the pilot and co-pilot used to squabble over the only bit of real flying they got to do on the whole trip,  making the landing.  They weren't about the give the autothrottle a share of the fun.
   Boeing, maintained that the autothrottle was supposed to go off line because of some very complicated mode changes that had taken place a few minutes before.  They pointed to obscure instructions in the autothrottle manual.  It's not a bug, it's a feature, saith Boeing.  I read the explanation and it made little sense to me.  It's too complicated to repeat here.
   NTSB staff wrote the accident investigation report, which just got published.   Staff wanted an investigation of how the autothrottles were supposed to work, and a complete redesign to make it more goof proof.  One board member supported this viewpoint.  Another NTSB member dissented, arguing that the existing 777 autothrottle has been flying for 30 years, with an excellent safety record. The entire board voted 3-1 not to require Boeing to redesign the autothrottle, over ruling staff recommendations, a very unusual event.
   The majority clearly felt that the crew was on the flight deck to land the plane especially if the autothrottle broke, and the real cause of the accident was a crew that placed too muich reliance on automation.  The crew essentially let the autopilot make the landing while they supervised.  They supervision wasn't good enough to keep the plane safe when the autothrottle dropped off line. 

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