British Airways A319 at London on Dec 17th 2010, unreliable airspeed on short final

Last Update: April 12, 2012 / 13:45:31 GMT/Zulu time

Bookmark this article

Incident Facts

Date of incident
Dec 17, 2010

Aircraft Registration
HL7734

Aircraft Type
Airbus A319

ICAO Type Designator
A319

The British Air Accident Investigation Branch (AAIB) released their bulletin reporting the first officer was pilot flying, the captain (ATPL, 16,080 hours total, 1,221 hours on type) pilot monitoring, when the aircraft descended towards London. After overflying BIG VOR on approach to runway 27L the aircraft entered icing conditions in instrument meteorologic conditions, ECAM messages "ANTI ICE CAPT R STAT" and "ANTI ICE CAPT TAT" appeared. The crew actioned the relevant checklists which required the air data source to be switched to CAPT 3 and to select the probe heat on. A cross check of indicated airspeed with the standby airspeed indicator showed speeds were in agreement. Seven minutes later, the aircraft descended through 7000 feet MSL, a "ANTI ICE STBY R STAT" message appeared. Because of the many anti ice messages the crew decided to review the procedures for unreliable airspeed. The captain assumed control of the aircraft, the first officer was to review the procedure. Just after the first officer had started to review the procedure, the aircraft was already on final approach, the captain's airspeed indication dropped rapidly to lowest selectable speed. The commander selected the autoflight system to use the FCU's speed rather than the FMGS supplied target speed and increased the FCU speed, the indicated airspeed however continued to reduce to 50-60 knots, the standby airspeed indicator fell to 0 knots. The aircraft was descending through 800 feet AGL, fully configured for landing with full flaps. The captain announced "unreliable airspeed" and called for a go-around, autoflight and autothrust systems were disengaged, TOGA thrust applied and the nose rotated up to 15 degrees nose up. The flaps and gear remained fully extended according to the memory checklist for unreliable airspeed indication. The captain declared Mayday and advised ATC they'd climb straight ahead. The aircraft levelled off at 4000 feet MSL, the gear was retracted, subsequently the crew climbed to 5000 feet which took the aircraft clear of icing conditions. Working the checklists and cross checking air data with GPS derived data the crew determined data off air data reference (ADR) #2 were reliable, while ADR1 and ADR3 were unreliable. ADR1 and ADR3 were turned off according to the checklist for flying with only one reliable air data source, the aircraft switched to alternate control law. The crew decided to divert to Luton where the weather was clear and no icing conditions existed. The aircraft touched down on runway 26 with flaps at position 3, control law had further reduced to direct law, and rolled out safely, during roll out ECAM showed a message indicating the nose wheel steering had become inoperative. The captain therefore requested the aircraft to be towed off the runway. Post flight tests were unable to reproduce the faults, all tests showed all systems fully serviceable. Flight data and cockpit voice recorder however confirmed the sequence of events, the fault and warning messages were recorded. The AAIB analysed that when the first ECAM messages were generated prompting the crew to switch air data source to CAPT 3 according to checklists, the flight data recorder showed all three air data and inertial reference units (ADIRU) produced indicated airspeed within 2 knots from each other (all indications reliable), hence there was no indication of the pitot system #1 being obstructed and indication that communication with the air data unit was unaffected. Seven minutes later, when the "ANTI ICE STBY R STAT" message appeared, the system also generated a message "NO PHC3 DATA" indicating that the probe heating computer (PHC) could not communicate with the probe's heating, there was no fault message recorded by that system however. The AAIB stated: "At this time, there was no degradation of the airspeed outputs from ADIRU 1, ADIRU 2 and ADIRU 3 recorded on the DAR, indicating that there was no ice accumulation on the probes and the air data system was serviceable." Another 6 minutes later maintenance status messages "F/CTL" and "ADR3" were generated, the flight data recorder showed ADIRU 3's airspeed output now reducing, while ADIRU 1 and ADIRU 2 remained within 2 knots of each at about 140 knots, while the captain's airspeed fell to 50 and the standby airspeed fell to 0 knots. At that point the stand by instruments as well as ADIRU 3 were both supplied from the stand by pitot and static probes. The AAIB analysed: "The recording on the DAR and the indications reported on the Capt PFD and stby ASI were consistent with possible icing of the stby pitot probe." The AAIB continued: "The crew determined, using the UNRELIABLE SPEED INDIC/ADR CHEC procedure from the QRH, that ADR 2 was the only reliable source of air data and, therefore, turned ADR 1 and ADR 3 off . In fact, the data from ADR1 was still reliable but the earlier anti ice capt r stat and anti ice capt tat caution messages and associated ECAM actions had meant that the commander had selected CAPT 3, so ADR1 data was no longer being presented to the crew. Therefore, they were not able to determine its serviceability without reversing the previous ECAM actions." The AAIB further analysed that in fact the aircraft suffered two independent faults: icing of the stand by pitot and/or static ports as well as loss of communication between the probe heating computers (PHC) 1 and 3 and the centralized fault display interface unit (CFDIU). At the time, when communication between PHC1, PHC3 and CFDIU was lost, sounds identified as electrical interference were recorded on the cockpit voice recorder's cockpit area microphone. A number of similiar events were recorded in the maintenance history of the aircraft suggesting the loss of communication between a single PHC and CFDIU. Subsequent maintenance actions showed the system serviceable however. The AAIB commented: "However, during the event flight it would appear that this intermittent fault occurred at the same time as the icing of the stby pitot probe. This then resulted in the loss of airspeed information on the commanderÂ’s and standby flight instruments at a late stage of an instrument approach under demanding weather conditions. This led to an increased crew workload and a declaration of a MAYDAY by the commander. The ECAM and QRH procedures, as carried out, also resulted in a reconfiguration of the flight controls system during the diversion. The safe flight path of the aircraft was maintained at all times under challenging circumstances and a diversion and uneventful landing were carried out at the alternate airport." The aircraft manufacturer advised they were unaware of any other occurrence of the loss of communication between two PHC and the CFDIU during one flight. The PHCs are powered by different busses and use different communication pathes which never meet at the same connectors. Further laboratory tests are to be conducted on PHC and CFDIU, the entire fleet is being monitored for further occurrences of Anti-Ice ECAM messages together with "PHC NO DATA" messages.