Archive for June, 2009

Firmware Disasters

Tuesday, June 23rd, 2009 Michael Barr

First, an Airbus A330 fell out of the sky. Then two D.C. Metro trains collided. Several hundred people have been killed and injured in these disastrous system failures. Did bugs in embedded software play a role in either or both disasters?

An incident on an earlier (October 2006) Airbus A330 flight may offer clues to the crash of Air France 447:

Qantas Flight 72 had been airborne for three hours, flying uneventfully on autopilot from Singapore to Perth, Australia. But as the in-flight dinner service wrapped up, the aircraft’s flight-control computer went crazy. The plane abruptly entered a smooth 650-ft. dive (which the crew sensed was not being caused by turbulence) that sent dozens of people smashing into the airplane’s luggage bins and ceiling. More than 100 of the 300 people on board were hurt, with broken bones, neck and spinal injuries, and severe lacerations splattering blood throughout the cabin. (Article, Time Magazine, June 3, 2009)

Authorities have blamed a pair of simultaneous computer failures for that event in the fly-by-wire A330. First, one of three redundant air data inertial reference units (ADIRUs) began giving bad angle of attack (AOA) data. Simultaneously, a voting algorithm intended to handle precisely such a failure in 1 of the 3 units by relying only on the other matching data failed to work as designed; the flight computer instead made decisions only on the basis of the one failed ADIRU!

(A later analysis by Airbus “found data fingerprints suggesting similar ADIRU problems had occurred on a total of four flights. One of the earlier instances, in fact, included a September 2006 event on the same [equipment] that entered the uncommanded dive in October [2006].” Ibid.)

Much of the attention in the publicly disclosed details of the Air France 447 crash has focused on the failure of one of several air speed indicators. Were there three of those as well? If so, was the same flight computer to blame for failing to recognize which to trust and which was unreliable?

It is very early in the investigation of yesterday’s collision between two D.C. Metro red line trains, in which a stopped train was rear-ended and heavily damaged by a moving train on the same track, to place blame. But a article headlined “Collision was Supposed to be Impossible” says it all:

Metro was designed with a fail-safe computerized signal system that is supposed to prevent trains from colliding.


During morning and afternoon rush hours, all trains except longer eight-car trains typically operate in automatic mode, meaning their movements are controlled by computerized systems and the central Operations Control Center. Both trains in yesterday’s crash [about 5pm] were six-car trains. (Article, Washington Post, June 23, 2009)

Are bugs in embedded software to blame for these two disasters? You can bet the lawyers are already looking into it.