British Airways A321 at Edinburgh on Jun 16th 2012, unreliable airspeeds
Last Update: September 28, 2013 / 21:56:24 GMT/Zulu time
On two occasions the aircraft encountered atmospheric conditions that resulted temporarily in unreliable air data.
The first event occurred within the boundary of current icing certification standards, which only consider supercooled water droplets. The second occurred outside the proposed revised boundaries and may have involved an encounter with ice crystals. Icing certification standards are being reviewed by the manufacturer and EASA.
The hazard of such events persists. However, the safe outcome of these incidents indicates that training to deal with unreliable air data can be effective.
The AAIB reported flight BA-1445 from Edinburgh to London Heathrow on Jun 16th 2012 was the second of the two events which was handled very briefly by the AAIB. The Digital AIDS Recorder showed the static air temperature rose to -41 degrees Centrigrade leading to the event. The cloud top temperatures were at about -50 degrees C.
The aircraft was climbing in visual meteorogic conditions. At FL265, while flying through the top of a "dome" of clouds, the captain's airspeed dropped to 0, returned to normal and reduced again, the first officer's indications were similiar, a SPD caption appeared. The autopilot disconnected, the crew commenced the checklists for unreliable airspeed, disconnected autothrust and disengaged the flight directors. After the initial items had been completed the airspeed appeared to have returned to normal. The fly by wire had changed to Alternate Law however meaning the aircraft would revert to Direct Law on landing. The crew therefore decided to divert to Stansted which had more favourable weather conditions with winds from 210 degrees at 22 knots than Heathrow Airport, which showed winds from 230 degrees at 24 knots gusting up to 39 knots.
A post flight examination of the aircraft revealed no evidence of a lightning strike, the pitot system was serviceable and all air data and flight control systems were working normally. As a precaution all three pitot tubes were removed and dispatched to the AAIB for further examination.
The AAIB reported that during both events the cockpit area microphones (CAMs) recorded "a number of periods during which large audio pulses were recorded, often resulting in a recorded waveform using the full amplitude capability of the recording. The time between pulses varied during the affected periods."
The AAIB analysed that an unrelated but similiar event, see Incident: British Airways A319 at London on Dec 17th 2010, unreliable airspeed on short final, showed such recordings of the CAMs as well, unknown to the crew with no such recordings on other CVR channels. The AAIB stated about the results of the investigation of 2010: "The investigation found that the effect on the CAM could be replicated with an electrostatic discharge applied to the connector of the CAM control panel."
The AAIB analyzed: "The June incident occurred as the aircraft transited the top of developing cloud at a temperature of approximately -50 degrees C. An ice crystal encounter in those conditions seems likely and would have been outside the certification standards for the pitot system.", further "Problems with the pitot or static probes would affect system Mach calculations, which are used to calculate corrections to other parameters. However, given the altitude errors were small compared to the speed errors, it is likely that the problems were associated with the pitot probes. The location and time of the problems correspond to weather likely to be associated with ice-crystals, so it is probable that air data errors were due to the affect of ice crystals on the pitot probes temporarily defeating the pitot heat system."
The AAIB analyzed further: "At the time of writing, the aircraft has been flying without a recurrence. It is feasible that component removals associated with this investigation resolved an undetected problem, such as component bonding."
This article is published under license from Avherald.com. © of text by Avherald.com.
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