The causes of accidents are often not found because of inadequate flight-data recorders.



The United Airlines Boeing 737-200 which rolled on its back and crashed for no apparent reason on the approach to Colorado Springs in March 1991 carried a six-parameter flight-data recorder (FDR) which provided no clues as to the cause. The crash remains a mystery today.

Now, with the crash of a similarly equipped USAir 737-300 near Pittsburgh in 1994 still unsolved, the US National Transportation Safety Board (NTSB) has called for flight-data recorders on every 737 in US airline service to be urgently upgraded by the end of 1995, and recommends that every jet-powered airliner in the USA should be similarly upgraded by January 1998 (Flight International, 1-7 March).

More than 4,000 jet-powered airliners, including more than 1,000 737s, will be affected by the change, should the Federal Aviation Administration accept the NTSB recommendations.

"Both the Colorado Springs and Pittsburgh aircraft were equipped with a flight-data recorder. In each case, however, the FDR did not provide needed information about aircraft motion and flight-control surface position during the accident sequence," says the NTSB.

Had both been upgraded the NTSB would have been able to "...quickly identify any abnormal control-surface movements, configuration changes, or autopilot status changes that may have involved in the loss of aircraft control", it says.

The FAA, which has 90 days to respond to the NTSB, says that it "...fully supports the desire for the right amount of data to quickly and more definitely identify the cause of aircraft accidents." It also says it has "...responded favourably to over 90% of the 293 urgent recommendations issued by the Safety Board since 1967".

Explaining its urgency on the 737, the NTSB says that enhanced FDRs on the aircraft involved in incidents will " a pivotal role in preventing additional unresolved accidents". It notes that 187 flight-control incidents have been recorded on 737s between 1970 and 1994. Of these, 35 happened in 1993 and 1994.

"Because the Boeing 737 will be used for years to come, it is essential that the aircraft involved in future incidents be equipped with enhanced FDRs," it adds.

The NTSB recommends that, by December 1995, 739 of the older 737s should have FDRs which record, in addition to current data requirements, lateral acceleration, the position of primary flight-control surfaces in pitch, roll and yaw and crew-control inputs in pitch, roll and yaw.

Aircraft manufactured before 1989, such as the 737-291 in the Colorado Springs accident, are already being upgraded from six to 11 parameters under a pre-set deadline of May 1995. These are now being recommended for upgrade by 31 December to record up to 24 parameters. The six parameters now required are time, altitude, airspeed, heading, vertical acceleration and microphone keying. After May they will also record pitch and roll attitude, longitudinal acceleration, thrust of each engine and control-column, or pitch control-surface position.

Another 13 parameters would be added by the end of 1995 if the FAA concurs with the NTSB.

The FDRs on more than 140 737-300, 400 and -500s built between May 1989 and October 1991 already record 17 parameters but are also recommended for upgrade. New parameters will include outside air-temperature, angle of attack, ground-spoiler position, lateral acceleration, autopilot engagement status, automatic flight-control system modes and engagement. Primary flight-control positions are also specified as enhancements.

Some 130 of the most recent 737s, built after November 1991, have the most sophisticated FDRs which record parameters such as glideslope and localiser deviation, radio altitude, and the passage of marker beacons.

They are now only required to record the control input or surface position of primary flight controls, not both, as is now recommended by the NTSB.

More than 3,000 Boeing 727s, 747s, early 757s and 767s, McDonnell Douglas DC-9s, DC-10s, MD-80s, Lockheed L-1011s, Airbus A300s and early A310s, British Aerospace BAe 146s and Fokker 28s are recommended for fitting with enhanced 24-parameter FDRs by 1 January. 1998.

In addition to the most recent 737s, FDRs on some 590 747-400s, 757s, 767s, MD-80s, MD-11s, A300s and Fokker 100s made after November 1991 are being recommended for FDR improvement to record primary flight-control, surface position and crew input.

Principal FDR manufacturers, AlliedSignal Commercial Avionics Systems (formerly Sundstrand) and Loral Fairchild are now working out how to manage the mammoth upgrade job.

"For some of the aircraft, particularly the older ones with the first-generation digital-tape FDRs, it would be quite a job. We're still trying to work out what sort of tack to take on that," says AlliedSignal senior project engineer Duncan Schofield.

These convert analogue signals to digital format for inputting into the FDR and, in the case of early DC-9s with little avionics space, may even have to be "...bolted to a wing box, or an engine cowling". The NTSB estimates that update costs could average between $20,000 and $70,000 per aircraft, depending on age.

The NTSB says that operators of turboprop regional airliners should voluntarily modify FDRs to record, at the minimum, the 24 parameters recommended for jet aircraft. It is not recommending regulatory enforcement of the enhancement to the regionals.

Source: Flight International