British Airways A321 at London on Apr 20th 2012, unreliable airspeeds
Last Update: September 29, 2013 / 08:58:35 GMT/Zulu time
The AAIB released their bulletin stating, that a subsequent event was included in the investigation, see Incident: British Airways A321 at Edinburgh on Jun 16th 2012, unreliable airspeeds and concluding:
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 that the captain requested maintenance to secure the flight data recorder after the flight but did not mention the cockpit voice recorder. In the time between the crew leaving the cockpit and the recorders being removed from the cockpit the cockpit voice recorder erase button had been pressed. Although the voice recording had been erased there are possibilities to recover the recording, though the recovery of the signal took longer than usual and delayed the investigation.
An examination of the aircraft could not clarify whether lightning strike marks above and below the cockpit windows were caused by a confirmed lightning strike on Apr 19th 2012, evident by the log book entries of the aircraft, or whether additional damage had been caused during the occurrence on Apr 20th. There was no damage outside the areas mentioned for Apr 19th. ADIRU3, both associated Air Data Modules and the captain's angle of attack sensor had recorded faults. ADIRU3 and its two Air Data Modules, the captain's AoA sensor and the TCAS computer were replaced.
A subsequent laboratory examination revealed no fault with any of the removed components.
The flight data recordings showed that beginning with the aircraft descending through 14,800 feet all three air data sources began to jump to unreasonable but valid or invalid data over a period of 27 seconds, at the same time there were jumps in the recorded air temperature and mach number. The #3 Air Data Computer latched a fault which remained active for the remainder of the flight, the other two air data computers did not show any faults. Following the 27 seconds period all data remained reasonable.
During the 27 seconds, although the aircraft was in a shallow constant descent, erroneous barometric data indicated a steep climb resulting in the TCAS resolution advisory, the TCAS stored a value of 3,250 fpm climb rate. The other aircraft did not receive a TCAS resolution advisory in accordance with the criteria for TCAS operation.
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 April 2012 incident began shortly after a bright flash of light, generally associated with lightning. There was no noise that is often associated with lightning strikes and identifiable damage was not found on the airframe. Existing aircraft skin damage may have masked any new lightning damage. Coincidence with the bright flash does not prove causation and it was impossible to be certain that a lightning strike occurred as there are other explanations for unreliable air data indication."
The AAIB analyzed that the spurios rapid vertical speed fluctions recorded by the Digial AIDS recorder caused the TCAS resolution advisory, though the crew did not notice any such indication, possibly due to their attention focussed elsewhere or these indications not being displayed on the primary flight display. The resolution advisory was "Monitor vertical speed", the aural alert was not detected by either crew member possibly because of "inattential deafness" which is normal within human performance and is why critical alerts should be provided to more than one sense.
The AAIB analyzed: "In this incident the crew reacted appropriately to a transient unreliable airspeed situation. They maintained the aircraft within known, safe datums which allowed its systems to recover from the initiating event. The crew then made a series of decisions which reduced consequential risk: they selected a hold in VMC, diverted to an aerodrome with better weather than the planned destination and, as the weather changed, prioritised the landing task over supplemental information gathering."
This article is published under license from Avherald.com. © of text by Avherald.com.
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