Date: 20 January, 1992
Operator: Air Inter
Flight number: 148
Aircraft type: Airbus A320-100
Location: 10nm short of runway 05, at Strasbourg Airport [SXB], France
Fatalities: 87 of 96
Accident type: controlled flight into terrain
Primary causes: Pilot error, organisational failings, design faults, air traffic control errors
(Aviation safety network, 20081)
The following is a human factors analysis of Flight 148 which will highlight the chain of errors that took place, resulting in the crash; factual information of the sequence of events is drawn primarily from the official accident report composed by the BEA (BEA, 19932).
Sequence of events
Both pilots on Flight 148 had only limited experience on the relatively advanced A320 aircraft, with little over 200 hours between them. Furthermore, both pilots had received comparatively little training from Air Inter on the A320's advanced systems. In particular, training on non-precision approaches in the A320 was quite lacking. The captain had the most A320 time, but had never made a non-precision approach into SXB. Air Inter also strongly encouraged pilots to avoid delays, and went as far as promoting flight below 10,000 feet at 350 knots instead of the standard 250 knots.
Change of duty runway
The events germane to the crash began as the aircraft was approaching SXB, and the pilots discovered that the duty runway had changed from runway 23 to runway 05 due to a wind change. The pilots had anticipated using runway 23, which was equipped with an ILS. The reciprocal runway, 05, did not have a precision approach system installed due to intervening terrain blocking radio signals between the airport, and aircraft on approach.
Initial communication with ATC
The captain asked Strasbourg air traffic control [ATC] whether the beginning of an ILS approach to runway 23 followed by a visual circle around to runway 05 would be a possibility. Unfortunately, due to several aircraft about to takeoff from runway 05, and the traffic conflict this would create, the controller advised the captain that this would result in a significant delay. ATC then suggested that he could guide Flight 148, by radar vectoring, to a radio beacon on the approach to runway 05, from where the pilots could more easily complete the approach. The pilots agreed, and followed the instructions from ATC to continue the approach.
By accepting the radar vectoring from ATC, the pilots' workload was significantly decreased as they simply had to follow instructions. Unfortunately, as Flight 148 neared the radar beacon where the vectoring would cease, the controller read out an incorrect vector to the pilots. This meant that Flight 148 turned too early onto finals and undershot the required turning path. It resulted in an incorrect lateral positioning on finals and meant the aircraft started heading towards a mountain range.
Cessation of radar vectoring
After a reasonably low workload following ATC radar vectoring, several events occurred about the same time, resulting in a workload spike. After the controller gave the last (incorrect) vector, the pilots once again became responsible for navigation. Shortly thereafter, the controller noticed that Flight 148 was undershooting the turn onto finals (as a result of the incorrect vector) and attempted to warn the pilots, but used non-standard and confusing phraseology.
At the same time that the captain calculated the required descent rate (correctly) at -3.3 degrees, Flight 148 was struck by turbulence resulting in the attitude increasing several degrees, and the aircraft climbing slightly. The captain then entered the descent rate into the autopilot, but the aircraft began to descend at over three times the required rate, at an angle of -11 degrees and vertical speed of -3,300 feet per minute. It is highly likely, although not conclusively determined, that the captain entered -33 in rate of descent mode, instead of -3.3 in angle of descent mode, resulting in the rapid descent.
Rapid pitch change
Because the descent was initiated in the few seconds while Flight 148 was in the turbulence-induced climb, the autopilot's rapid pitch change mode was activated. This is a system which results in a rapid pitch change from a climb to descent, or vice versa, the flight computer reading the situation as an emergency maneuverer, such as traffic avoidance. As a result, Flight 148 descended even faster than the -11 degree descent erroneously requested by the captain. Had the rapid pitch change mode not been activated, Flight 148 would almost certainly have cleared the mountains.
As Flight 148 continued on finals towards runway 05, still descending at over three times the required rate and half a dot from centreline, the pilots were still not fully aware of the aircraft's positioning, both vertically and horizontally. Then, just moments after a radio altimeter callout of "200" (feet AGL), Flight 148 plowed into the Vosges mountains. The pilots did not have any time to react, and did not receive any ground proximity warning system [GPWS] warnings, because Air Inter had not fitted its aircraft with this system as the company policy of flying at 350kts below 10,000 feet caused too many false alarms.
Incorrect and inadequate guidance from ATC
The incorrect vector, and subsequent inadequate communication resulted in an increased workload, which likely assisted in saturating the pilots' cognitive abilities.
Likely incorrect handling of the autopilot
The captain calculated that a descent rate of 3.3 degrees was required, but instead likely entered a descent rate of 3,300 feet per minute, or 11 degrees. This meant that Flight 148 was descending too rapidly. This has not been conclusively determined but remains as the most likely explanation of the accident, by far.
The pilots were "behind the aircraft", and were not fully aware of its position on the approach.
Air Inter pilots received poor training, and in particular, little if any training on nonprecision approaches in the A320.
Poor pilot pairing choices
Air Inter did not pair pilots effectively, in this case both the captain and first officer had little flight time in the A320.
Poor culture at Air Inter
Air Inter management's procedures of encouraging pilots to avoids delays at all costs, and encouraging flight at up to 350 knots below 10,000 feet, instead of the standard 250 knots, displayed an obvious lack of safety culture.
Poor autopilot design ergonomics
Both 'flight angle' and 'vertical speed' are shown in the same window on the autopilot display, and controlled by the same knob. This likely resulted in the captain inputting an excessive descent rate. There has been several occasions where pilots became confused by this ambiguous design. See also ICAO on ergonomics here.
No legislation for mandatory GPWS
At the time of the accident, there were no rules in place mandating the use of GPWS in the A320.
Air Inter decided to not fit its aircraft with GPWS.
The A320 autopilot's rapid pitch change mode is not an error, but did contribute significantly to the crash.
This accident would likely have been prevented by the use of synthetic vision systems.
The crash of Flight 148 resulted from a series of events, most notably severe failings on behalf of Air Inter, and also poor autopilot design, inadequate guidance from ATC. This resulted in a small data-input error, which when combined with a chance event, and poor situational awareness, resulted in the crash of one of the most advanced airliners, and 87 deaths.