Aircraft Accident/Incident Report

Agua Caliente, Arizona 85333
Monday, September 20, 1999 16:23 MST

During a Ground Proximity Warning System (GPWS) warning escape maneuver at 27,100 feet, 4 flight attendants (FA's) were injured, 2 of them seriously with fractured leg bones. The injured FA's were standing in the aft galley securing from the meal service when the event occurred and the passengers were seated with belts fastened. 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 being displayed on the crew's TCAS indicator. As the flight was leveling off at 27,000, the opposite direction traffic passed almost directly beneath the flight. Immediately, the GPWS annunciated the warning, "terrain, whoop, whoop, pull-up." In accordance with the mandatory provisions of the company flight operations manual, the crew executed the proscribed escape maneuver, which, in part, calls for an aggressive application of thrust and a rapid nose pitch up to a 20-degree attitude. Later analysis of the DFDR data showed that the crew only rotated the nose to an 8-degree nose up attitude during the maneuver. During the maneuver, the g-loads generated on the airplane varied between +2.5 and +0.5 over a 2-second time period, which was caused the FA's 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 generated ranged from a low of +2.0 to a high of +4.0. It should be noted that the simulator computer computes the g-loads and can display them to the instructor; however, the simulator does not generate visceral feedback to the crew of the amount of g's being experienced. 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. In response, Boeing issued an all operators letter advising of the problem and issued a service letter in 1989 advising of an upgrade to the GPWS computer to prevent false nuisance warnings. Between 1987 and 1999, 3 service bulletins and 13 service letters were issued advising of modifications to the GPWS and Radio Altitude systems to prevent false warnings. None of these improvements were accomplished by the airline, and the GPWS unit installed in the accident airplane was three upgrades behind the current configuration. The company decision to do Service Bulletin upgrades was based 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 Radio Altitude (RA) systems are all interrelated, with the captain's (left side) RA unit providing data input to the GPWS and the TCAS. Review of the 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 written up in the 60 days prior to the accident. On three occasions, the GPWS system provided terrain warnings at high altitudes when flying a profile very similar to the accident flight. In accordance with the maintenance manual procedures, the corrective actions largely consisted of removal and replacement of the affected units or sub units. No evidence was found of any diagnostic trouble shooting procedures outside of those specified in the maintenance manuals undertaken by the airline. The airline's maintenance operations control has a system to track problematic airplanes for special maintenance attention, which requires three write-ups in the same ATA code within 10 days to trigger an alert; the accident airplane's discrepancy history pattern fell outside of this "3 in 10" trigger parameter. Flight crews have no ready access to this system and only see on a routine basis the last 10 log sheets where discrepancies are entered. In the 60 days following the accident, extensive examinations of the airplane and/or the GPWS/RA systems was conducted three times in response to continued problems with these systems, with no conclusive hard faults identified. At the end of this 60-day period the RA units were being examined at the system's manufacturer due to a failure that could not be replicated in testing, the manufacturer told the airline that it was aware that the system's central processors could become desynchronized during power transfers and cause erratic behavior in the units. Another cause of earlier (problem corrected before accident flight) erratic behavior in the airplane's RA system was the installation by maintenance of antennas that were incompatible with the computer units; this was due to the airline parts stocking system that carried all dash number models under the same part number.

the systemic failure of the airline's maintenance department to identify and correct the long standing history of intermittent faults, nuisance warnings, and erratic behavior in this airplane's GPWS system. Also causal is the airline's failure to perform the service bulletins and service letter upgrades to the system, which would have eliminated or greatly reduced the likelihood of this particular nuisance warning, a condition that was identified and corrected by the manufacturers 11 years prior to the accident, and was the subject of one or more of the SB/SL upgrades.