- Air Inter A320 at Strasbourg
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Air Inter A320 at Strasbourg
Photo of Accident Airplane
Photo copyright Andy Pope - used with permission
History of the Flight
On January 20, 1992, an Airbus A320 airplane registered F-GGED and operated by Air Inter was on a scheduled night flight between Lyon-Satolas airport (now called Lyon-Saint Exupery) and Strasbourg-Entzheim airport, both in France. The flight departed Lyon at 1720 local time with 90 passengers and six crewmembers aboard. The flight was uneventful en route to Strasbourg. The captain in the left seat was the pilot flying (PF), and the first officer in the right seat was the pilot not flying (PNF).
The airplane's autopilot and autothrottle were engaged during the entire approach phase. The Automatic Terminal Information Service (ATIS), which gives the flight crew a description of weather conditions and runway status, reported winds of 30 knots and layers of scattered to broken clouds at 1,100 and 2,600 feet above the ground.
Although ATIS announced that the active runway was 05, the flight crew discussed the winds and weather conditions and planned to fly an Instrument Landing System (ILS) approach to runway 23 at Strasbourg, followed by a visual landing on runway 05. At 1809 local time, upon entering the terminal area for Strasbourg, the flight was cleared by the Approach Air Traffic Controller (ATC) to descend to 5,000 ft. The airplane was approximately 25 nm southwest of its destination at this point, and approximately 12 nm from intersection ANDLO, part of the approach procedure for the VOR DME runway 05 approach.
After the airplane passed ANDLO intersection, the approach air traffic controller cleared the flight for the VOR-DME approach to runway 05. At that time, the airplane was almost over the Strasbourg VOR (STR), approximately 2 ½ nm to the southwest of the airport. The flight crew made a request to the controller to fly the ILS to runway 23, followed by a visual approach to runway 05. The controller responded by informing the crew that this would result in a delay, as there were three aircraft waiting to take off from runway 05. The flight crew then agreed to accept the complete VOR-DME 05 approach. The approach controller suggested radar guidance to bring the airplane back towards ANDLO intersection in order to shorten the approach procedure, and the flight crew accepted. At this point the aircraft was essentially over the Strasbourg VOR and still descending towards its assigned altitude of 5,000 ft. The controller issued instructions to turn left to a heading of 230 degrees magnetic, putting the airplane on an outbound track parallel to the VOR DME runway 05 inbound final approach course. Shortly after beginning this left-hand turn, the airplane was leveled at 5,000 feet.
Aerial photo of Runway 05 and Runway 23
While proceeding on the outbound track, the captain, adjusting from the planned approach to runway 23 to the assigned approach to runway 05, briefed elements of the approach procedure, including the glide path angle, which the captain noted was equivalent to 3.3 degrees. At 1818 local time, air traffic control (ATC) issued a left turn to a heading of 090 degrees and the captain used the autopilot's heading mode to comply with the ATC vectors. This would set the airplane up towards intercepting the final approach course, inbound to the airport. Approximately 35 seconds later, the airplane's flaps were selected to "Flaps 1." After 20 more seconds had elapsed, ATC issued a further left turn to 051 degrees, but noted that the airplane was "askew" to the left of ANDLO intersection. The first officer, seeing that the airplane would not be overflying ANDLO as expected, commented to the captain that "we're going inside," and suggested a small turn back to the right (to a heading of 070) to intercept the final approach course.
At 1819, still under the control of ATC and level at 5,000 ft, the flight was cleared for final approach. Shortly thereafter the flaps were selected to "Flaps 2," the landing gear was lowered, and approximately at a position allowed by the approach procedure, the aircraft initiated its descent. As the descent began, the controller instructed the flight crew to report when the airplane was over the VOR, and the flight crew acknowledged. The acknowledgment was the last transmission from the flight. The flight began its descent 11 nm from the Strasbourg VOR. The investigation concluded that, based on conversation on the cockpit voice recorder, the descent was intended to be performed at a normal negative 3.3 degrees flight path angle. However, the investigators noted that the actual commanded descent was 3,300 ft/min, and while not conclusively established, may have been the result of inadvertently selecting the vertical speed mode. In this case, "-33" would have appeared on the autopilot display, implying 3,300 ft/min descent, while in the flight path angle mode, "-3.3" would have appeared in the same window.
Photo of crash site - Accident Docket photo
Approximately 30 seconds after beginning the descent of 3,300 ft/min and at 1,000 feet above the local terrain, the captain, who was flying the approach, remarked, "need to watch...descent." Virtually simultaneously, the first officer noted that the airplane was almost on the final approach course. Fifteen seconds later, with the airplane still in a rapid descent, the radar altimeter aural announced, "TWO HUNDRED," indicating that the airplane was 200 feet from the ground. Approximately 45 seconds after having initiated the descent, the airplane struck the ground.
Photo of Mont La Bloss
The wreckage was found at 2235 local time on La Bloss Mountain at an elevation of 2,620 ft, 0.8 nm left of the final approach course and 10.5 nm from the end of the runway. Eighty two passengers and five crew members died in the accident. The aircraft was completely destroyed.
A320 Flight Deck and Flight Control Unit--Display and Function
Photo of Flight Deck of A320 (left) - Photo copyright Laszlo Gyori - used with permission, Diagram Flight Deck arrangement (right)
The A320 cockpit is equipped with a number of displays. The Primary Flight Display (PFD) depicts aircraft flight parameters such as attitude, airspeed, and altitude, while the Navigation Display (ND) displays navigational information. The A320 is equipped with an autopilot, which responds to pilot input via the Flight Control Unit (FCU) mounted centrally on the flight deck glare shield.
Diagram of Vertical Speed/Flight Path Angle Mode
Display difference - Flight Path Angle vs. Vertical Speed
The autopilot system allows for control of the lateral path (through control of heading or lateral navigation path) and, germane to the Air Inter accident, the vertical path. Vertical path control includes the vertical speed, or "VS" mode, whereby the autopilot responds to pilot input of a desired vertical speed in units of feet per minute climb or descent, and the flight path angle, or "FPA" mode, in which the pilot inputs the desired flight path angle in degrees. Control and display of VS or FPA commands are made via the FCU. These two modes share a common location for pilot control of VS or FPA as well as display of selected input. A pushbutton toggles the selected mode between VS and FPA. Rotating the toggle pushbutton allows selection of the target VS or FPA.
At the time of the accident, the displays for the two modes were very similar. Vertical speed was indicated by two digits representing the VS in hundreds of feet per minute, and FPA was indicated by two digits, separated by a decimal point. Investigators concluded that in the minutes preceding the accident, while configuring the airplane for approach, the flight crew may have intended to select a -3.3 degree flight path angle (which would have been depicted as "-3.3"), but may have inadvertently selected a vertical speed of -3300 feet/min (which would have been indicated as "-33"). The active mode (VS or FPA) would also have been annunciated near the numerical indications. The investigation did not establish conclusively that the flight crew made this error, but did state that it was highly probable.
Subsequent to the accident, the displays were updated such that if vertical speed were the selected mode, two additional zeros would be displayed. Thus, a VS of -3300 ft/min would be displayed as "-3300". In addition to control of VS and FPA via the FCU, the PFD displays the modes selected and specific VS rate or FPA angle. An animation of the flight path during the final minutes of the flight, and a comparison of the flight path effects of the two autopilot modes, is available at the following link: Click here to view Air Inter Flight 148 Flight Path Accident Animation.
Magnified photo of (post-accident) modified Flight Control Unit showing zeros added to Vertical Speed mode
Ground Proximity Warning Systems (GPWS)
Ground Proximity Warning Systems (GPWS) are designed to reduce the risk of controlled flight into terrain (CFIT) accidents. A GPWS alerts a crew to impending collision with terrain by using a variety of available on-board information such as position, speed, and radio altimeter height above the ground. Both visual and aural alerts are provided to the flight crew. In 1975, after a December 1974 Boeing 727 accident near Dulles Airport, Washington D.C., FAA regulations were modified to require large commercial U.S. registered aircraft to be equipped with GPWS. Despite some initial problems (including erroneous alerts and/or alerts occurring during normal operations where airplane safety was not in jeopardy), studies conducted at the time in the United States indicated a reduction in the number of CFIT accidents had been achieved and was attributable to GPWS. More specifically, over the five years prior to 1975, an average of 2.8 CFIT accidents per year had occurred in the United States. The period during which GPWS was installed in the U.S. transport airplane fleet extended from mid-1974 to the end of 1976. The number of CFIT accidents then fell appreciably: 0 in 1975, 2 in 1976 and 1977, 1 in 1978, and 0 from then on with the exception of 1985 (1) and 1989 (2).
Photo of GPWS Visual Warnings on A320
These positive safety results led the International Civil Aviation Organization (ICAO) in 1979 to include among its standards the requirement to equip all turbine-powered aircraft with a maximum takeoff weight exceeding 15,000 kilograms, or authorized to carry more than 30 passengers, with a GPWS. ICAO stipulated that aircraft receiving an Airworthiness Certificate on or after July 1, 1979 be equipped with a GPWS. For those aircraft that had received an Airworthiness Certificate before July 1, 1979, a GPWS was only recommended. According to a study by the GPWS manufacturer at the time of the Air Inter accident, approximately 95% of the world fleet meeting the criteria of aircraft required to carry a GPWS as defined by the ICAO were, in fact, so equipped.
From January 1983 to April 1991, the confidential Aviation Safety Reporting System (ASRS) noted 64 reports of crews which had experienced a GPWS alert. In at least 35 cases, flight crews reported that the GPWS warning had prevented an accident.
Diagram of GPWS Mode associated with Air Inter accident
The situation in France, as documented in the accident report, with regards to adopting GPWS as a system required by regulation, was different than ICAO at large. The following factors were noted: A test program to evaluate the system had been carried out by the CEV (Centre de Essais en Vol - the French Flight Test Centre) in 1975. Taking into account the level of spurious alarms encountered during this program, the French government concluded that the GPWS afforded safety gains which were less positive than had been indicated by the U.S. studies. A letter from the French government sent to the ICAO in 1977 stated that the carrying of a GPWS should have "at most the status of a recommended practice," and not of a standard. However, the ICAO decided to include the installation of GPWS among its standards. In 1978 ICAO was notified that France would be making an exception concerning GPWS standards.
Photo of crash site - Accident Docket photo
Photo of crash site - trees severed by airplane
Following a subsequent inquiry by the ICAO into national variations, as noted in the accident report, France replied, in error, that its regulations did not contain any exceptions and, therefore, the note indicating a French derogation on this point was removed from ICAO documents. In 1990 the European Joint Aviation Authorities (JAA) began the task of developing common European operational regulations. The draft regulations required, beginning March 1, 1993, that GPWS systems be installed. At the time of the Air Inter accident (January 1992), French regulations did not make the installation of a GPWS mandatory. Even so, as of December 31, 1991, approximately 75% of the aircraft operated by French carriers, and meeting the ICAO criteria for carrying a GPWS, were equipped with such a system. Air Inter, based on the results of the CEV tests, elected not to install GPWS systems on its aircraft.
Air Inter company had participated in the CEV trials carried out in November 1975. The trials were conducted on Mercure and A300 aircraft, and included flights by Air Inter crews over the company route structure in 1976 and 1977. In view of the fact that a high number of "nuisance", or inappropriate alarms was observed during these evaluations of the initial GPWS versions, and given the absence of any statutory requirement, Air Inter chose not to equip its fleet with GPWS. However, following technical improvements to the GPWS, at the end of the 1970's, Air Inter began procuring equipment to provision their A300 and Mercure aircraft.They had also begun acquiring equipment for the A320, but had not begun installations on their A320 fleet.
Flight Crew Interaction
Photo of Flight Data Recorder at crash site
The accident report devoted significant comment to the personal interaction of the two pilots, and an apparent lack of cooperation, leading to a breakdown in good crew resource management (CRM) practices. The accident report stated that the relationship between the two pilots was subdued, and though not hostile, was strained. Prior to the flight, they had never met nor flown together. The investigation established that there was a "serious lack of communication" between the crew members, based most probably on personality differences, and a "lack of affinity." The result was a breakdown in flight deck protocol and procedures, and a lack of attention to the airplane position and progress of the approach. The investigation established that a number of procedurally required callouts were omitted during the approach, and both crew members focused on the lateral flight path to the detriment of the vertical path monitoring. Standard procedures and crew communication seemed to have completely broken down, leading to a loss of situational awareness and the development of the high descent rate prior to impact. The investigation concluded that if proper flight deck discipline and adequate CRM procedures had been followed, the arrival approach options would have been discussed, and an approach plan agreed upon by the two pilots. Cross-crew communication would have been established and maintained, and the flight path would have been properly monitored, potentially avoiding what appeared to have been an error or oversight in configuring and verifying the vertical flight path.
Photo of crash site
Human Factors-Federal Aviation Administration Human Factors Team Report on: The Interfaces Between Flight Crews and Modern Fight Deck Systems, dated June 18, 1996
Many accidents have highlighted difficulties in flight crew interactions with flight deck automation. The FAA, along with U.S. and foreign authorities, manufacturers and operators, launched a study to evaluate the flight crew/flight deck automation interfaces of current generation transport category airplanes. This report is a culmination of that study. The purpose of this study was to identify opportunities for improvements which could be applied in future regulatory and policy development associated with flight crew/flight deck automation interfaces. The report can be viewed at the following link: (Human Factors Report)