Investigation and final report SilkAir Flight 185

1 investigation , final report

1.1 potential motives
1.2 pa announcement
1.3 cvr , fdr deactivation
1.4 servo valve issue

investigation , final report

the accident investigated indonesian ntsc, assisted expert groups us, singapore, , australia, , american ntsb.

around 73% of wreckage (by weight) recovered, partially reconstructed, , examined. both of aircraft recorders, cockpit voice recorder (cvr) , fdr (flight data recorder) retrieved river , data extracted , analyzed.

at 16:00, cvr showed captain tsu left cockpit; 5 seconds later, cvr stopped recording. tests indicated click heard on cvr recording if cvr circuit breaker had tripped normally, not if had been pulled out manually. there no click, captain tsu pulled out cvr circuit breaker before leaving cockpit. ntsc , ntsb investigators thought if captain tsu responsible crash, must have made excuse first officer leave flightdeck prior disabling fdr (which have triggered master caution on both pilots control panels), actions not noticed. several minutes later, recorded indonesian ground radar, aircraft entered rapid descent, disintegrated, , crashed musi river.

on 14 december 2000, after 3 years of investigation, indonesian ntsc issued final report, in stated evidence inconclusive , cause of accident not determined:

the ntsb, participated in investigation, concluded evidence consistent deliberate manipulation of flight controls, captain.

in letter ntsc dated 11 december 2000, ntsb wrote:

the examination of of factual evidence consistent conclusions that: 1) no airplane-related mechanical malfunctions or failures caused or contributed accident, , 2) accident can explained intentional pilot action. specifically, a) accident airplane’s flight profile consistent sustained manual nose-down flight control inputs; b) evidence suggests cockpit voice recorder (cvr) intentionally disconnected; c) recovery of airplane possible not attempted; , d) more nose-down flight control inputs made captain first officer.

geoffrey thomas of sydney morning herald said secret report confirmed indonesian authorities not issue public verdict because feared make own people frightened fly. santoso sayogo, ntsc investigator worked on silkair 185 case, said ntsb opinion shared indonesian investigators, over-ruled boss.

potential motives

in aftermath of crash, several potential motives captain s alleged suicide/homicide suggested, including recent financial losses of $1.2 million (his share-trading showed trading of more 1 million shares , securities-trading privileges had been suspended 10 days before accident due nonpayment), obtaining $600,000 life insurance policy previous week have gone effect on day of accident (though later emerged routine policy taken out part of mortgage requirement), receipt of several recent disciplinary actions on part of airline (including 1 related improper manipulation of cvr circuit breaker), , loss of 4 squadron mates during military flight training, 18 years earlier on exact date of crash. had had several conflicts ward , other copilots had questioned command suitability. investigations later revealed total assets greater liabilities, although liquid assets not cover immediate debts; monthly income less family s monthly expenditure; , had outstanding credit card debts.

an official investigation singapore police force evidence of criminal offence leading crash found no evidence pilot, copilot or crew member had suicidal tendencies or motive deliberately cause crash of [the aircraft].

tsu formerly republic of singapore air force pilot , had on 20 years of flying experience in older t/a-4s skyhawks, newer t/a-4su super skyhawks. last appointment instructor pilot of skyhawk squadron.

pa announcement

captain tsu made appeared routine public address announcement flight @ 15:44:37, 7 min after takeoff, recorded cvr , transcribed ntsc: tsu s announcement ended @ 15:46. @ 16:05, 19 minutes later, cvr stopped recording. 6 minutes later, @ 16:11, fdr stopped recording, , @ 16:12 aircraft plunged fatal dive.

cvr , fdr deactivation

the cockpit voice recorder , flight data recorder stopped recording minutes before abrupt descent, not @ same time. cvr stopped functioning approximately 6 minutes before dive captain leaving cockpit short break. fdr recorder deactivated 5 minutes later approximately 1 minute before dive. overload , short circuit tests show distinctive 400-hz tone recorded cvr when cvr circuit breaker trips. investigators not find sound on flight 185 s cvr, made them conclude cvr circuit breaker manually pulled out. radio continued work after failure of cvr, indicates power failure not cause. subsequent investigations, including national geographic channel documentary, revealed fdr had failed, periods lasting between ten seconds , ten minutes. testing of unit ntsc found no evidence malfunction or failure caused either recorder stop recording data.

servo valve issue

starting in 1991, several accidents , incidents involving boeing 737 result of uncommanded movement of rudders. on 3 march 1991, united airlines flight 585, 737-200, crashed in colorado springs, colorado, killing 25 people. on 8 september 1994, usair flight 427, 737-300, crashed near pittsburgh, pennsylvania, killing 132 people. there 4 more incidents 737 rudder power control unit (pcu) malfunction suspected.

the seattle times devoted series of 37 articles boeing 737 loss of control malfunctions. accident occurred in middle of controversy on ntsb s role in accidents caused rudder control unit.

during investigation of flight 427, ntsb discovered pcu s dual servo valve jam, well, , deflect rudder in opposite direction of pilots input, due thermal shock, caused when cold pcus injected hot hydraulic fluid. result of finding, federal aviation administration (faa) ordered servo valves replaced , new training protocol pilots handle unexpected movement of flight controls developed. faa ordered upgrade of boeing 737 rudder control systems 12 november 2002.

according series mayday, rudder issue had been corrected before construction started on accident aircraft. nevertheless, theory of rudder malfunction investigated possibility of corrosion of and/or debris getting stuck in power control unit, , disproved.