Sriwijaya Air Accident Probe Prompts Mandatory 737 Classic Tests
The probe into the Jan. 9 crash of a Sriwijaya Air Boeing 737-500 has uncovered a potential latent autothrottle system failure that the FAA determined needs immediate attention on the in-service fleet, but the U.S. regulatory agency cautions that the issue did not likely play a role in the accident sequence.
An immediately effective airworthiness directive (AD) set for publication May 18 will require operators of U.S.-registered 737 Classics to run a specific test to detect failure of a flap synchro wire. Checks must be done within 250 flying hours or two months, whichever comes first.
The FAA said the Boeing-prescribed test “is currently not required to be performed repetitively, leading to a potential latent failure if the test is not performed regularly.” The AD, which is based on a Boeing multi-operator message issued March 30, calls for repetitive tests.
During the Sriwijaya Flight 182 (SJ182) accident probe, Boeing discovered that the most recent version of the 737 Classic autothrottle (a/t) computer “does not properly account for a possible latent failure of the flap position sensor,” the FAA AD explained. This can cause the a/t system’s asymmetric cruise thrust monitor to malfunction, creating a thrust imbalance between the aircraft’s two engines.
Investigators probing SJ182 are evaluating autopilot and the a/t computer’s actions as part of the accident sequence, a preliminary report from Indonesian National Transportation Safety Committee (NTSC) suggests. SJ182 went down in the Java Sea about 5 min. after departing Jakarta. An unexplained decrease in the 737-500’s left thrust lever while the autopilot and a/t were engaged set up an asymmetric thrust scenario. The autopilot disengaged, and an upset ensued, sending the aircraft into a hard-left bank and nose-down attitude. The pilots did not recover. All 62 passengers and crew onboard were killed.
Investigators are looking at several issues beyond the a/t system’s performance, including how mechanics handled faults reported during earlier flights, how SJ182’s crew were trained to handle upsets, and how they reacted during the accident flight, the NTSC report said. Flap synchro-wire failure does not appear to be on the list.
“At this time, the preliminary data of the ongoing accident investigation shows that it is highly unlikely that the accident resulted from the latent failure of the flap synchro wire,” the FAA said in its AD. “However, the FAA has determined that the unsafe condition identified in this AD could exist or develop in Model 737-300, -400, and -500 series airplanes, and that this AD is therefore necessary to address the identified unsafe condition.”
The FAA said its directive affects 143 U.S.-registered aircraft. Aviation Week Intelligence Network Fleet and Data Services shows about 680 737 Classics are in service worldwide, with another 125 in storage.