AGUA CALIENTE, AZ, USA
N904AW
Boeing 757-2S7
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.
1.1 HISTORY OF FLIGHT On September 20, 1999, at 1623 mountain standard time, the flight crew of America West Flight 2208, a Boeing 757-2S7, N904AW, executed an escape maneuver when the ground proximity warning system (GPWS) activated near Agua Caliente, Arizona. The aircraft was not damaged; however, 2 flight attendants received serious injuries while 2 others received minor injuries. The remaining flight attendant, the flight crew of 2, and the 170 passengers onboard were not injured. The aircraft was being operated by America West Airlines, Inc., under 14 CFR Part 121 as a scheduled domestic passenger flight when the accident occurred. The flight originated from the Los Angeles International Airport, Los Angeles, California, at 1535 Pacific daylight time that afternoon. Instrument meteorological conditions prevailed at the in-flight altitude of the event and an instrument flight plan was filed. A review was conducted of the recorded air-ground communications transcripts between the airplane and Albuquerque Air Route Traffic Control Center (ARTCC) and Phoenix Terminal Radar Approach Control. Data plots from the Digital Flight Data Recorder (DFDR) were also compared to the transcript and recorded radar data to reconstruct the events surrounding the GPWS warning and subsequent escape maneuver. In addition, pertinent conversations recorded on the Cockpit Voice Recorder were reviewed. The flight checked in with ARTCC Sector 91 at 1618 at Flight Level (FL) 300 (30,000 feet msl) with a previous sector's clearance to fly direct to ARLIN intersection. The sector 91 controller acknowledged the flight and assigned a 120-degree heading for traffic separation. Shortly thereafter the controller asked the flight what their indicated airspeed was and the crew replied "265 indicated." The controller turned the flight further right to a 125-degree heading, instructed the flight to descend and maintain FL270, and to increase their indicated airspeed to 300 knots or greater. At 1621, the controller advised the flight to expedite their descent to FL270 and to expect a lower altitude in 2 minutes due to opposite direction traffic below FL270. The controller pointed out the traffic as "11 o'clock 10 miles westbound at flight level 260 [FL260]." The flight responded that they were unable to visually acquire the traffic because they were in instrument meteorological conditions. At 1623, the controller told the flight to descend and maintain FL240 at a "good rate of descent." The flight responded, "We just got a terrain warning and had to climb out of it." The controller queried, "You mean aircraft warning [because of] that guy below you?" The flight responded, "Negative, must have been a nuisance warning." According to the crew's written and oral statements, the first officer was the flying pilot. The airplane had been cleared to descend to FL270 and was descending at a rate of about 4,000 feet per minute in instrument meteorological conditions. Albuquerque center issued a traffic advisory for opposite direction traffic at 12 o'clock and passing 1,000 feet below. The traffic was being displayed on the traffic alert and collision avoidance system (TCAS) indicator as an amber traffic alert (T/A) symbol showing "-1700 warning." The TCAS also provided the aural alert, "Traffic, Traffic." As the traffic disappeared from the TCAS indicator, a GWPS "terrain, terrain, whoop, whoop, pull up" warning was received. The first officer said that he looked briefly at the captain who said, "Let's do it." The first officer said he then executed the GPWS escape maneuver as the aural warnings sounded continuously. He increased power to maximum thrust and pitched the nose up to about 8 degrees. The aircraft began a climb at 3,400 fpm. As he reached FL 280, the aural warning ceased and the aircraft entered visual meteorological conditions. As he leveled the aircraft, the captain advised Albuquerque Center that they had received a GPWS warning and had responded. Albuquerque Center asked if was a TCAS alert to which they answered "no." [The four injured flight attendants were standing in the rear galley when the escape maneuver was performed. The events in the cabin are contained in summary form in the SURVIVAL FACTORS section of this narrative; complete statements from all cabin crewmembers can be found in the Survival Factors Group Factual Report, which is appended to this document.] The crew stated that Albuquerque Center told them to descend and maintain FL 240. As the aircraft began its descent with a vertical speed of about 300 to 500 fpm, the GWPS resumed the same aural warning with an occasional "sink rate." The volume level of the warning made communications in the cockpit difficult, and it was silenced only after the first officer pulled the circuit breaker. Examination of the radar data and mode C altitude reports form both aircraft obtained from Albuquerque ARTCC disclosed that the true ground track of the flight was 131 degrees in a descent while the opposite direction aircraft's true ground track was 280 degrees while level at FL260. At 1623:27, the opposite direction aircraft was 1.52 nautical miles and on a relative bearing of 350 to America West flight 2208; the radar target returns merged at 1623:34. Based upon calculations of the ground tracks and the timing, the opposite direction aircraft passed laterally behind flight 2208 by about 1,300 feet with a vertical separation of 1,200 feet. The direct distance between the two aircraft at the closest point of approach was about 1,770 feet. The radar data and mode C altitude reports showed that flight 2208's vertical profile changed from a descent to a climb at 1623:38; the change occurred at FL272 and the airplane climbed to FL282 before leveling briefly and then resuming a descent. Data readout from the DFDR disclosed that the GPWS escape maneuver began at subframe reference 47924 (subframe reference is the number of seconds from beginning of the recording) at an altitude of 27,100 feet. The data traces for the vertical acceleration (g's) oscillated during the escape maneuver from 1 to 2.5 to 0.5 over a 2-second period. Nose-up and down elevator inputs consistent with the vertical acceleration oscillations were recorded during this same 2-second interval. The data plot shows that coincident with engine spool-up, and within about a 2-second interval, the right elevator moved to a 5-degree deflection and the nose pitch attitude achieved 8 degrees. The right elevator then moved to a zero or slightly minus deflection as the nose pitch attitude decreased to 5 degrees. The pitch attitude then increased back to 8 degrees over a 6-second interval, where it remained until the airplane leveled off at 28,250 feet. The plot traces for the Radio Altitude parameter were noted to be erratic from the time of liftoff at Los Angeles until touchdown at Phoenix, with jumps every so often from 1,700 feet to 500. About 5 seconds before the initiation of the escape maneuver, the Radio Altitude jumped from 1,700 to 500, then back to 1,200, then down to 200 before resuming a 1,700 reading. Following that, the Radio Altitude parameter oscillated in 6-second cycles from 1,700 to 200 and back again; this oscillation continued until touchdown at Phoenix. After the flight resumed a normal descent, it was handed off to ARTCC sector 42, and that controller gave the flight further descent instructions down to FL110 and cleared the flight for the ARLIN 1 arrival procedure into Phoenix. Subsequently, sector 42 handed the flight off to Phoenix Terminal Radar Approach Control (TRACON) at 1628. The flight declared an emergency at 1637 with Phoenix TRACON, announcing, "We need to get to [runway] 26 right and expedite to gate A14…we have injured flight attendants and a couple are going into shock." The TRACON controller gave the flight priority direct to the runway and the airplane landed at 1641. The CVR was transcribed and is appended to this report. The recording begins at some point after the aircraft has parked at the gate with the captain noting that the Auxiliary Power Unit (APU) is running and that the "secure checklist" is complete. During the next 5 minutes, the captain and first officer are discussing the event. The first officer remarked, "you know when you're strapped in, and doing that kind of thing in the sim [simulator], you don't realize what kind of an effect [unintelligible word]. I was totally surprised when she said what happened back there." He then went on to note that a status message, "ground prox byte" was on the EICAS in flight after the event. The crew then had a discussion on the radio with maintenance control informing them of the nature of the event and that the EICAS is now displaying the "ground proximity byte message." Shortly after that, the assistant chief pilot entered the cockpit and the crew discussed the event with him. Comparison of this conversation with the crews later written and oral statements revealed that they are consistent. The conversation then turns toward the need to secure the CVR for readout and how to write up the event on the log page. A maintenance technician then entered the cockpit and had a discussion first on pulling the CVR, then on the event. After the crew described the GPWS warning and subsequent events, the technician states "This aircraft had a tremendous history on radio altimeter problems…did you have any associated radio altimeter messages or warnings or anything like that?" The first officer responded that they did not. 1.2 PERSONNEL INFORMATION The America West personnel, flight and training records for both of the flight crew were reviewed. In addition, the FAA Airman and Medical Certification files were examined. 1.2.1 Captain The captain held an Airline Transport Pilot certificate with a class rating for multiengine land airplanes, with commercial privileges in single engine land airplanes. The most recent issuance of the certificate was dated October 2, 1989. His certificate was endorsed for type ratings in the Boeing 737, 757, and 767. His most recent first-class medical certificate was issued on March 10, 1999, with the limitation that he have available glasses for near vision. Hired by America West in April 1984, the captain served as a first officer on Boeing 737's and was upgraded to captain in August of that year. He completed transition training and initial operating experience as captain on the Boeing 757 in October 1989. His most recent recurrent proficiency training session was completed in March 1999. A line check was accomplished in August 1999, and the most recent recurrent proficiency check was completed on September 14, 1999. According to flight department records, the captain had accrued a total flight time of 18,000 hours, with 4,000 as captain on the Boeing 757. In the 90 and 30 days prior to the accident, he flew 186 and 71 hours, respectively. 1.2.2 First Officer The first officer held an Airline Transport Pilot certificate with a class rating for multiengine land airplanes, with commercial privileges in single engine land airplanes. The most recent issuance of the certificate was dated July 3, 1996. His certificate was endorsed for type ratings in the BA-3100 and BA-4100. His most recent second-class medical certificate was issued on November 11, 1998, without limitations. Hired by America West in July 1998, the first officer completed his initial training and operating experience for the Boeing 757 in October of that year. His most recent recurrent proficiency training session was completed in April 1999. The most recent recurrent proficiency check was completed on August 24, 1999. According to flight department records, the first officer had accrued a total flight time of 9,800 hours, with 500 in the Boeing 757. In the 90 and 30 days prior to the accident, he flew 179 and 44 hours, respectively. 1.2.3 GPWS Escape Procedures Training According to America West's Senior Check Airman for the Boeing 757, GPWS escape procedures are given in both the Proficiency Training and Proficiency Check simulator sessions. The procedure is inserted in the session at random point(s) with no forewarning. The pilot is expected to immediately respond to the warning and fly the procedure specified in the America West Boeing 757-200 Operations Manual. On recognition of the GPWS warning, the pilot is to take the following actions: -Disengage the autopilot -Disconnect the autothrottle -Apply maximum thrust (aggressively) -Rotate to a 20-degree nose pitch-up attitude (roll wings level if in a bank) -Retract speedbrakes -If the terrain threat remains, the nose is to be rotated to either the pitch limit indicator if installed, or stick shaker/stall buffet. The 757 Operations Manual page covering the procedure has a bold warning notice that states, "Do not delay pull-up for diagnosis." The only exception allowed to the immediate initiation of the escape maneuver is if the flight is in day visual meteorological conditions and positive visual verification can be made that no threat exists. 1.3 AIRCRAFT INFORMATION 1.3.1 Fleet Information, General At the time of the accident, America West Airlines had its headquarters in Phoenix, Arizona, and operated 104 airplanes, including 1 Boeing 737-100, 17 Boeing 737-200, 44 Boeing 737-300, 13 Boeing 757-200, 21 Airbus A-320-231, and 8 A-320-232 airplanes. The company employed 1,425 pilots and 298 certificated mechanics, and holds Air Carrier Operating Certificate Number AWXA420A. 1.3.2 Boeing 757-200 Series Aircraft The FAA approved the Model 757-200 (Type Certification Data Sheet A2NM, revision 8, June 1, 1992) for production on December 21, 1982. The airplane is still in production, and 1,001 model 757 airplanes have been built, which includes the 300 series. The airframe maneuvering load factor limits (flaps up) are +2.5 g's positive and -1.0 g's negative. 1.3.3 Accident Airplane Information The airplane, a Boeing 757-2S7, serial number 23566 (line number 96) was manufactured on May 1, 1986, for Republic Airlines. America West acquired it on June 3, 1987, and at the time of delivery it had accrued 3,377 hours and 1,595 cycles. At the time of the accident, the airframe total time was 55,129 hours and 21,308 cycles. Two Rolls-Royce RB211-535E4 engines were installed, serial number 30515 on the No. 1 position and serial number 30558 on the No. 2 position. 1.3.4 Maintenance Check Intervals America West Airlines uses a maintenance system that comprises five scheduled maintenance checks. Throughout the following narrative, reference is made to ATA codes. Air Transport Association (ATA) system designations are used in aviation maintenance to create a standardized format. ATA Code sections 31, 33, and 34 refer to instruments, lights, and navigation components, respectively. RON Check: Accomplished any time the airplane is scheduled for 4 hours or more on the ground where maintenance personnel are assigned and on duty. The last check was completed on September 17, 1999, in Phoenix (log 7357025). There were no ATA 31, 33, or 34 related issues for work activity. No non-routine discrepancies were generated in these areas. Weekly Check: Accomplished at a maintenance base or approved contract vendor at an interval not to exceed 7 calendar days since the last Weekly Check, "A" Check, or "C" Check. The last check was completed with the "A" Check on September 14, 1999. "A" Check: Accomplished at intervals not to exceed 30 calendar days in service since the last "A" Check. They are numbered "A1" through "A12" and will be accomplished in numerical sequence from the last check performed. The last check (A10) was completed on September 14, 1999, in Phoenix (log 7357009). There were no ATA 31, 33, or 34 system related issues for work activity. No non-routine discrepancies were generated in these areas. "C" Check: Accomplished at 18 calendar month intervals since release from the last "C" Check. They are numbered "C1" through "C12" and will be accomplished in numerical sequence. A "C4" check was completed on September 17, 1996, in PHX, and included a work card f
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.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
