Air safety probes to focus on sensors and automation
Air safety investigators are now probing several serious Airbus A330 incidents that have chilling similarities to the apparent system failures that brought down Air France Flight 447 over the Atlantic on June 1.
Following another alert last week the USA’s National Transportation Safety Board (NTSB) is now investigating events aboard a Northwest Airlines A330 on a flight from Hong Kong to Tokyo on June 25.
The flight was at its 39,000 ft cruising level as it flew into Japanese airspace with an outside air temperature of -50C. Entering the cloud tops the crew experienced light to moderate turbulence, and about 15 seconds later encountered moderate rain.
Five seconds later, the pilots’ dual airspeed indicators and a standby unit all “rolled back” to indicate 65 knots airspeed. The autopilot automatically disengaged, as did the autothrottles and the rudder limit protection system which prevents over-stressing the rudder at high airspeeds. The flight management system automatically reverted to “Alternate Law”, which removes some of the computerised protections when incoming data is lost, placing more reliance on direct crew input.
The master warning and master caution systems flashed numerous messages along with a series of audible alerts. As Captain Jerry Staab hand-flew the aeroplane on the shortest possible route to clear the rain, the airspeed indicators briefly functioned correctly but then failed again for a further three minutes. With airspeed indicators disabled the computers operating the controls and navigating the aircraft lack reliable data and the aircraft is hand-flown using recommended power settings and (in some cases) speed data from the GPS satellite navigation systems.
Airspeed indicator failure in these situations is symptomatic of ice formation in the airspeed-sensing “pitot tube” probes which is now widely believed to be the triggering causal factor in the Air France and similar other recent events.
On May 21 2009, a TAM Airlines A330, flight 8091 enroute from Miami to Sao Paulo, lost primary airspeed and altitude information while in cruise flight. Initial reports indicate the crew noted a sudden drop in indicated outside air temperature, an alert to the loss of the air data reference system, disconnection of the autopilot and autothrust systems, the loss of speed and altitude information, and reversion to Alternate Law. As in the Northwest event, the crew used backup instruments and recommended procedures, and airspeed/altitude data was restored in about five minutes.
Yet another incident was reported in 2008 after an Air Caraibe A330 flight in tropical convective activity north of the inter-tropic convergence zone – a meteorological term for what is also known as the ‘monsoonal trough, the moving boundary that seasonally divides the northern and southern hemispheres’ weather patterns.
Icing over of the pitot and temperature probes occurred at 35,000 feet and automatic messages, flags and system reversions were consistent with those reported on the AF447 A330’s aircraft communications addressing and reporting system (ACARS) report. (The automated transmission, along with recovered wreckage, may be all that investigators have to guide them if the flight data and cockpit voice recorders are not recovered.)
The described total air temperature increase from minus 14 to minus 5 degrees is typical of an iced-up probe measuring ice temperature instead of ram air. At one point in this event the indications of calibrated airspeeds, Mach numbers and altitudes moved respectively from:
- 274Kts, Mach 0.80 and FL350; to
- 85 kts , Mach 0.26 and FL347.
Simultaneously a cascade of audible and visual warnings and cautions included: navigation systems/air data reference disagree; flight control in Alternate Law; flight control rudder travel limited; engine pressure ratio mode faults; speed flags on primary flight displays; loss of flight directors, auto thrust etc….Including at some point STALL STALL audio warning (no protection in Alternate Law.)
The handling pilot flew “pitch and power” with the support pilot providing information from the quick-reference handbook on procedures for unreliable speed indication, disregarding the false stall warning and using backup information of GPS-derived ground speed and altitude and data from the navigation display and flight management guidance computer.
“Air Caraibe has now modified all its A330 probes earlier this year,” says an experienced air safety commentator: “Again, the Air Caraibe report is very similar to what is now known of the AF447 troubles – weather, turbulence, and airspeed data problem.”
The recent Qantas A330 event over WA in November 2008, although it shared similar symptoms, was not due to pitot icing, but to the failure of one of the triplicated air data inertial reference units (ADIRU) and the system’s slow reaction in excluding spurious data and restoring normal flight.
Safety specialists who have spoken to AviationAdvertiser believe airline pilots, engineers and managers, and perhaps millions of air travellers, may sleep more soundly when these worrying issues are resolved.
AviationAdvertiser will be publishing a more detailed feature on these and related incidents later this week.
