Investigation Summary
The investigation focused on the design of the Airbus A320's Flight Control Unit (FCU). The pilots intended to descend at a 3.3 degree flight path angle (FPA). However, the FCU was in Vertical Speed (V/S) mode rather than FPA mode. In V/S mode, the input '33' commanded a descent of 3,300 feet per minute instead of the intended 3.3 degree angle (which would have been approximately 800 feet per minute). The display for both modes was nearly identical, differing only in the presence of a decimal point and the specific mode indicator. This 'mode confusion' was exacerbated by a high workload, late changes to the approach strategy by ATC, and the crew's relative inexperience with the highly automated A320. Furthermore, Air Inter had opted not to install GPWS on their A320 fleet, believing the aircraft's advanced systems made it unnecessary, which deprived the crew of a final warning before impact.
Final Conclusions
The BEA concluded that the crew failed to notice the abnormally high descent rate and nose-down attitude. Contributing factors included the ergonomic design of the FCU, which allowed for easy confusion between vertical modes, and the lack of a GPWS. The investigation also cited poor cockpit resource management (CRM) and a lack of cross-checks between the captain and first officer.
Photographic Evidence (1)
Video Analysis
Gripping Survival Story of Air Inter Flight #148's Ill-Fated Journey | Mayday: Air Disaster — A detailed look at the crash of Air Inter Flight 148, exploring the mode confusion that led to a fatal descent into the Vosges Mountains and the harrowing survival story of the nine passengers who lived through the impact and the freezing night that followed.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Flight Control Unit (FCU)
The interface used by pilots to input autopilot commands; its design allowed for confusion between Vertical Speed and Flight Path Angle modes.
Ground Proximity Warning System (GPWS)
A safety system designed to alert pilots of imminent contact with the ground; its absence was a critical factor in the lack of warning.
VOR/DME
A non-precision navigation system used for the approach to Runway 05, requiring manual vertical profile management.
Interesting Facts
- 01The aircraft was not equipped with a Ground Proximity Warning System (GPWS) because Air Inter believed the A320's technology made it redundant.
- 02The pilots had a combined total of only 223 flight hours on the Airbus A320 at the time of the accident.
- 03The descent rate of 3,300 feet per minute was more than four times the intended rate for the approach.
- 04The aircraft struck a ridge on La Bloss Mountain, which is part of the Mont Sainte-Odile massif.
- 05The emergency locator transmitter (ELT) was destroyed in the impact, significantly delaying the rescue operation.
Safety Actions & Advisories
FCU Interface Redesign
Airbus modified the Flight Control Unit (FCU) to use a four-digit display for Vertical Speed (e.g., -3300) and a two-digit display with a decimal for Flight Path Angle (e.g., -3.3) to prevent mode confusion.
GPWS Mandate
Following the accident, the French government mandated the installation of Ground Proximity Warning Systems (GPWS) on all public transport aircraft, a requirement Air Inter had previously bypassed.
Enhanced CRM Training
Training programs were updated to emphasize cross-checking automated inputs and monitoring vertical flight path parameters during non-precision approaches.
Known Controversies & Unanswered Questions
Rescue Delay
Reference ↗There was significant public and official criticism regarding the four-hour delay in locating the wreckage, which was attributed to the failure of the ELT and poor coordination between civil and military search authorities.