Incident Overview
During a Ground Proximity Warning System (GPWS) warning escape maneuver at 27,100 feet, four flight attendants (FAs) were injured, two of them seriously with fractured leg bones. The injured FAs were standing in the aft galley securing from the meal service when the event occurred, while passengers were seated with belts fastened.
Sequence of Events
In response to an ATC instruction, the flight was descending to maintain 27,000 feet. The controller had told the crew to maintain a good rate of descent (the airplane was descending about 4,000 feet per minute), and an indicated airspeed of 300 knots or greater. The flight was in instrument meteorological conditions at the time and had no outside visual reference. The controller advised the flight of opposite direction traffic at 26,000 feet, and a traffic alert symbol was displayed on the crew's TCAS indicator. As the flight leveled off at 27,000 feet, the opposite direction traffic passed almost directly beneath the flight. Immediately, the GPWS annunciated the warning, "terrain, whoop, whoop, pull-up."
GPWS Warning and Escape Maneuver
In accordance with the mandatory provisions of the company flight operations manual, the crew executed the prescribed escape maneuver, which calls for an aggressive application of thrust and a rapid nose pitch up to a 20-degree attitude. Later analysis of the Digital Flight Data Recorder (DFDR) data showed that the crew rotated the nose to only an 8-degree nose-up attitude during the maneuver. During the maneuver, the g-loads varied between +2.5 and +0.5 over a 2-second period, which caused the FAs' injuries. As part of the investigation, this flight's profile was flown in a Boeing 757 simulator four times. When flown according to operations manual instructions, the g-loads ranged from +2.0 to +4.0. The simulator computer computes g-loads and displays them to the instructor, but does not generate visceral feedback to the crew.
Maintenance History
As early as 1988, Boeing became aware that the dash number model GPWS computer installed in the airplane was subject to issuing false warnings when the airplane overflew another airplane. Boeing issued an all operators letter advising of the problem and a service letter in 1989 advising of an upgrade to prevent false nuisance warnings. Between 1987 and 1999, three service bulletins and 13 service letters were issued advising modifications to the GPWS and Radio Altitude (RA) systems. None of these improvements were accomplished by the airline, and the GPWS unit was three upgrades behind the current configuration. The company's decision to implement Service Bulletin upgrades was based on applicability, priority, and budget availability. Service Letters were not routinely reviewed when received but were filed for later review when discrepancy history patterns indicated a need.
For the Boeing 757, the GPWS, TCAS, and RA systems are interrelated, with the captain's RA unit providing data input to the GPWS and TCAS. Review of maintenance records disclosed that in the 16 months prior to the accident, the GPWS and/or RA systems on this airplane were written up as erratic, providing false warnings, or inoperable 45 times, with 18 discrepancies in the 60 days prior to the accident. On three occasions, the GPWS system provided terrain warnings at high altitudes when flying a profile similar to the accident flight. Corrective actions largely consisted of removal and replacement of affected units or subunits, as per maintenance manual procedures. No evidence was found of diagnostic troubleshooting procedures outside of those specified in the maintenance manuals. The airline's maintenance operations control system tracks problematic airplanes for special attention, requiring three write-ups in the same ATA code within 10 days to trigger an alert; the accident airplane's discrepancy pattern fell outside this trigger parameter. Flight crews have no ready access to this system and only see the last 10 log sheets.
Investigation Findings
In the 60 days following the accident, extensive examinations of the airplane and/or the GPWS/RA systems were conducted three times in response to continued problems, with no conclusive hard faults identified. At the end of this period, the RA units were examined at the manufacturer due to a failure that could not be replicated in testing. The manufacturer informed the airline that the system's central processors could become desynchronized during power transfers and cause erratic behavior. Another cause of earlier erratic behavior in the RA system (corrected before the accident flight) was the installation of antennas incompatible with the computer units, due to the airline's parts stocking system that carried all dash number models under the same part number.