In comments filed on the FAA’s proposed directive, United says upgrades will likely cost $1,700 to $3,300 “per unit.” The FAA says 497 U.S.-registered 737s need the work, putting the total cost for U.S. operators at between $845,000 and $1.6 million.
The agency’s proposed 737NG autothrottle update requires swapping existing GE-made computers with new boxes containing updated software to prevent an anomaly tied to several in-service incidents and one fatal accident,service information detailing the change shows.
FAA’s draft directive, published March 3, explained the change in general terms, referring operators to a November 2013 Boeing alert service bulletin (SB) for details. Like the SB, FAA would give operators 36 months to make the changes.
“As a result of in-service incidents, it was found that an erroneous unflagged radio altimeter output would cause the autothrottle to enter landing Flare retard mode prematurely on approach,” Boeing explains in a November 2013 alert service bulletin (ASB) just made public as part of the FAA’s rulemaking. “If this change is not done, an erroneous unflagged radio altimeter output on a manual or single-channel autopilot approach could result in premature autothrottle landing flare retard and the loss of automatic speed control.”
The scenario contributed to the February 2009 crash of a737-800, triggering the retard flare mode and reducing engine thrust to idle when the aircraft was at about 750 ft. on final approach to Amsterdam Schiphol Airport. The crew did not realize the aircraft was stalling until it was too late to recover. The aircraft crashed about one mi. short of its assigned runway, killing nine passengers and crewmembers, including the three pilots onboard.
While the Dutch Safety Board acknowledged the crew could have recovered from the near-stall condition, it cited the altimeter error as a significant link in the accident sequence chain.
The November 2013 SB is the latest in a series of changes Boeing made to address the issue identified in FAA’s draft directive. Others include operations manual updates and flight control computer software changes in April 2010, and incorporating an “airspeed low” aural alert on the 737NG production line in July 2010.
Boeing tested the new autothrottle configuration on a737-700 as part of certifying the fix, the SB says.
The public has until April 18 to comment on FAA’s proposal.