David Learmount/LONDON
THE COCKPIT-VOICE recorder (CVR) of the Birgenair Boeing 757-200 which crashed in February reveals a picture of pilots who, faced with minor problems, became confused and lost control of a flyable aircraft, according to an interim report by the Dominican Republic's accident-investigation commission.
The aircraft crashed into the sea some 10km (18nm) north of Puerto Plata, Dominican Republic, on 6 February, killing all 189 people on board. The 70-parameter digital flight-data recorder (FDR) and CVR were recovered (Flight International, 6-12 March).
Released by chief accident investigator Emmanuel Tamayo, the transcript shows that the crew knew from the time of the co-pilot's "80kt" call during the take-off run that the captain's airspeed indicator (ASI) was not working, but elected to continue the take-off using the co-pilot's ASI.
The crew did not consult checklists or manuals and, points out the report, neither did they compare the main instruments with the standby ASI. The 757 had been at Puerto Plata's airport since its previous flight on 23 January. "During that time the [three] pitot heads [dynamic pressure sensors for the ASIs] were not protected with the appropriate covers," the report says.
The captain flew the take-off. Almost 2min later, the flaps had been retracted and the captain ordered the centre autopilot engaged. This is normal procedure - but the centre autopilot is slaved to the air-data computer, which senses airspeed, from the captain's ASI. The captain's ASI, as height increased, began to read a speed of 350kt which was far faster than the real airspeed. As a result, the autopilot/autothrottle acted to increase pitch-up attitude and reduce power, to lower the speed.
At this point two "advisory" messages [rudder ratio and Mach/airspeed] appeared on the engine-indicating and crew-alerting system. 757 pilots say that no response was essential for safety, but the crew's reaction indicated confusion.
Meanwhile, the co-pilot stated that his ASI was reading 200kt "decreasing". The captain's response indicated that he thought the co-pilot's ASI was now also wrong. Stall speed at that configuration is close to 160kt. The over-speed warning, slaved to the captain's faulty ASI, sounded, but, within 22s, the stall-warning alert started as the captain brought the speed back, and the sound of the stick-shaker (stall indication) continued until impact.
The report says that the FDR shows the aircraft out of control for 1min 41s, but that the engine and systems recordings "...were normal and consistent with the different manoeuvres indicated". Investigators have "flown", the FDR profile, in a 757 simulator and conclude that, "...the anomalies in the FD recording, correspond to ... an obstruction in one of the airspeed sensors".
Source: Flight International
