Dutch Safety Board: Malfunctioning altimeter caused THY 737 autopilot to decelerate
From ATW this morning:
A malfunctioning altimeter caused the autopilot system on the Turkish Airlines 737-800 that crashed last week on approach to Amsterdam to reduce power prematurely, and by the time the pilots reacted it was "too late to recover the flight," Dutch Safety Board Chairman Pieter van Vollenhoven said yesterday.
DSB's "initial findings" provided a strikingly clear explanation of the crash's likely cause less than a week after the Feb. 25 accident that killed nine (ATWOnline, Feb. 27) and led Boeing to issue a statement warning 737NG pilots to "carefully monitor primary flight instruments during critical phases of flight."
Vollenhoven explained that the aircraft's "automatic throttle system. . . received incorrect information because of a malfunction in the left radio altimeter." The cockpit voice and flight data recorders show that at 1,950 ft. the "left radio altimeter suddenly indicated a change in altitude, from 1,950 ft. to -8 ft., and passed this on to the automatic pilot. . .It seems that the automatic system, with its engines at reduced power, assumed it was in the final stages of the flight. As a result, the aircraft lost speed. Initially, the crew did not react to the issues at hand."
The crew, which included a captain, a first officer on a training flight and a third pilot in the cockpit, "were notified that the left radio altimeter was not working correctly" by a "landing gear must go down" warning signal, Vollenhoven said, adding that "provisional data indicates that this signal was not regarded [by the crew] to be a problem."
But the autopilot's deceleration "reduced [the 737-800] to minimum flying speed," setting off an alarm. The crew "immediately" attempted to apply full power, he said. "However, this was too late to recover the flight, the aircraft was too low and, consequently, the [aircraft] crashed 1 km. short of the runway."
DSB issued a warning to Boeing regarding 737-800 altimeters, advising the manufacturer to amend its manual to say that autopilot should be switched off if an altimeter malfunctions during flight. Boeing, which confirmed that three passengers who died in the accident were company employees, said it would "look closely" at the recommendation.
Vollenhoven noted that the aircraft's tail hit the ground first, followed by the undercarriage, and its forward speed "was about 175 km. per hr. upon impact. . .An aircraft of this weight should normally have a speed of 260 km. per hr. for landing." The three pilots died in the crash and 28 surviving passengers remain hospitalized.
"The board's investigation will now focus fully on the workings of the radio altimeters and the connection to the automatic throttle," Vollenhoven said. He conceded in a press conference that DSB was issuing highly detailed findings at an early stage in the investigation. "The reason to go public now. . .is to warn Boeing and all users of this plane type that vigilance is required with regards to the altimeter," he said.
Weather conditions at the time of the accident included a "low cloud base and. . .mist [that] probably meant that the [AMS runway] was not yet visible at the height at which the descent was commenced," which may have been a contributing factor, he said. Regarding the aircraft, there have been no "irregularities" found except for the altimeter malfunction, he said.