ADIRU-computers cause trouble on an Air Canada Rouge Airbus
An Air Canada Rouge plane ran into trouble on May 25th, after its Air Data Inertial Reference System (ADIRS) broke down on a flight from Miami to Toronto. When the Airbus 320 of the low-cost carrier started to descend, it encountered turbulence, after which the fly-by-wire reverted from Normal law to Alternate Law. The crew received a “NAV ADR DISAGREE”-message on their monitor, which means the three redundant ADIRUS (Air Data Inertial Reference Units that collect external data like speed and angle of attack) were in disagreement. The crew declared a pan-pan (emergency) and proceeded towards Toronto airport, where it landed safely.
Back on the ground, not only the three ADIRUS were replaced by maintenance, but also the Angle of Attack meters. These are vane-like devices that are located outside of the fuselage. On a fly-by-wire aircraft, the ADIRU’s collect information from pitot tubes, static ports and angle of attack meters and transmit that information to the Flight Control Computers. Sometimes these ADIRUS break down. Since these failures, can lead to a loss of control in flight (LOC-I), there are three redundant ADIRUS.
Qantas Flight 72
An infamous case of a malfunctioning ADIRS was Flight 72 of Qantas, which flew from Singapore to Perth on October 2008. The aircraft was cruising at 37,000 feet when it suffered two uncommanded pitch-down maneuvers, causing the plane to plunge in a 650-foot nosedive. The incident caused injuries to 74 passengers and crew, from broken bones to spinal injuries.
An investigation of the Australian Transportation Safety Board (ATSB) concluded that there had been a failure within the ADIRS. “These very high, random and incorrect values of the Angle of Attack, led the Flight Control computers to order a nose-down aircraft movement, which resulted in the aircraft pitching down to a maximum of about 8.5 degrees,” according to ABC-news, quoting the ATSB-report. “Even with the Auto Pilot off, the plane’s Flight Control computers still command key controls in order to protect the jet from dangerous conditions, such as stalling.” In other words, even when the Auto Pilot is switched off, the plane will still fly within its protective envelope (Normal Law). And when a failing ADIRU transmit erroneous information, the Flight Control Computers will react to this non-existing flight condition. In the case of Flight 72, the Flight Computers interpreted that the plane was stalling (which was not the case) and automatically ordered a pitch-down (against the will of the pilots) to counter this fictitious stall.
These ADIRU’s breakdowns are more common than is publicly known. It is not an exclusive problem of Airbus-aircrafts either. The Boeing 777, also a Fly-by-wire plane, has experienced similar glitches with its ADIRUs, the most known case being the one of Flight 124 of Malaysian Airlines from Perth to Kuala Lumpur (2005).
The case of Flight 1641 of Air Canada Rouge was disturbing since the same plane with serial number C-FYIY had already experienced ADIRU-failures in 2016: on October 27, when flying to Orlando and a week later, near Las Vegas. On that flight of November 2th, the ADIRU#1 failed, while the aircraft had taken off with the ADIRU#3 inoperative.
The normal procedure after these occurrences is to replace the ADIRU-units on the plane. After the Las Vegas-event in November 2016, Air Rouge pledged to replace all three ADIRU’s and inspect the wiring. It is not clear whether the airline complied with that or not and put the lives of crew and passengers needlessly at risk.