What happened
On March 12, 1999, an Iberworld Airlines Airbus A320-214, registration EC-GZD, was taxiing at Groningen Airport Eelde. The aircraft was performing a scheduled stop to pick up passengers for a flight destined for Tenerife. As the aircraft moved toward the runway, the pilot attempted to slow down to avoid a light aircraft parked on the taxiway. However, the aircraft failed to respond to the brake pedals.
In an attempt to restore function, the pilot cycled the anti-skid and nose wheel steering selector off and on, but the brakes remained ineffective. To prevent a collision with the stationary aircraft, the pilot steered the Airbus A320-214 off the taxiway and into the adjacent grass. The aircraft came to a halt after the landing gear sank into the soft ground. There were no injuries among the 183 passengers and 6 crew members on board.
The investigation
The investigation, conducted by the Dutch Safety Board in collaboration with French and Spanish authorities, focused on why the braking system failed without any cockpit warnings. Investigators examined the Braking and Steering Control Unit (BSCU) and found that while the system appeared to be online, it was not executing braking laws.
Technical analysis revealed that a previously unknown hexadecimal code had been stored in the EEPROM memory of the BSCU. This corrupted code prevented the system from functioning correctly while simultaneously suppressing any visual or aural alerts on the ECAM. Further inspection of other units discovered that several other braking computers contained similar unidentified code fragments.
Findings
- The primary cause of the loss of braking was a faulty braking and steering control unit containing an unknown hexadecimal code.
- The flight crew did not correctly implement the established "loss of braking" procedures, which would have allowed for the use of the backup braking system.
- The aircraft's monitoring systems failed to provide any warning or indication of the inhibited braking status to the crew.
- The presence of the unknown code in the memory of the BSCU prevented the system from switching to a secondary channel or deactivating itself.
Safety action
Following the incident, Airbus and the manufacturer of the braking computer, Messier-Bugatti, developed a software modification known as Standard 9 to prevent such latent errors from causing a loss of braking. Additionally, Airbus revised the standard operating procedures for taxiing to ensure that brake functionality is verified immediately after the aircraft begins moving.