Recent reports of a possible autothrottle malfunction on the Asiana Boeing 777 that crashed in San Francisco on 6 July do not explain the accident or exonerate the crew, say aviation safety experts.
Media outlets including the Wall Street Journal reported in recent days that Asiana and the pilots of the doomed 777 have raised the possibility that the aircraft’s speed control systems may have failed prior to the crash, which led to the deaths of three passengers and injuries to hundreds.
The reports, which cite an unnamed aviation expert familiar with the investigation, say the pilots and airline have suggested that the autothrottle disengaged on its own.
The US National Transportation Safety Board (NTSB) has not commented on the accident since mid-July, when it said it found no indication of a mechanical problem with the aircraft, which slammed into a seawall at the edge of the runway 28 Left.
The NTSB did say, however, that one of the pilots assumed the aircraft’s speed, which deteriorated during the final seconds of flight, was being maintained by the autothrottles.
Asiana also reportedly said shortly after the crash that there was no evidence of mechanical error with the aircraft or its Pratt & Whitney PW4090 engines.
Former NTSB member John Goglia, tells Flightglobal that suggestions of a malfunction are likely “smoke” intended to “mitigate” blame.
Even if there was an autothrottle malfunction, that does not excuse the pilots because the pilot not flying the aircraft should have been monitoring the systems, watching for problems, says Goglia, a board member from 1995 to 2004.
“If they are saying [the autothrottle] tripped off... and they didn’t know it, they are impeaching themselves,” Goglia says.
The pilot not flying should have his head down and be monitoring “100% of the time” during the approach, he adds.
Asiana could not immediately be reached for comment. The Air Line Pilots Association, which criticised the NSTB for releasing too much information during the investigation's infancy, did not respond to a request for comment.
Hans Weber, president of aviation consultancy TECOP International, says its possible that the pilots set the autothrottle — intentionally or unintentionally — to a mode that deactivates it during approach.
Pilots sometimes select that mode for certain landings, such as when they must approach low or slow, adds Weber, who consulted the FAA for 21 years on aviation safety issues.
As the 777 descended through 500ft, flight instructor Lee Jeong-min noted the aircraft was too low and told the pilot to pull up, according to NTSB comments made shortly after the crash. At the same time, the aircraft drifted off the runway centreline, forcing the crew to make corrections, the NTSB said.
Crewmembers then lost track of the aircraft’s slowing speed, and at least one crewmember, Lee Jeong-min, believed the crew had set the autothrottles to maintain a speed of 137kt (254km/h), the NTSB said.
"They had set speed at 137kt and he assumed the autothrottles were maintaining speed," NTSB chair Deborah Hersman said during a press conference.
The aircraft slowed to 103kt, then accelerated to 112kt as it crossed too low over the runway threshold.
The landing gear struck the seawall, causing the tail to strike the runway and separate. The aircraft careened down the runway before coming to a stop.