Boeing 787 pilots are being warned not to make sudden control inputs in response to a “sudden, unrealistic” drop in airspeed shown on cockpit displays.
The US Federal Aviation Administration (FAA) will adopt an airworthiness directive on 1 April requiring 787-8 and 787-9 operators to update the flight manual with the warning message.
The FAA accelerated the release of the airworthiness directive, bypassing the normal rulemaking process to make operators adopt the change as quickly as possible.
Boeing made an identical recommendation to 787 operators on 4 March, which the FAA directive will make mandatory.
The fleet has made three reports of displayed airspeed plunging significantly below actual airspeed, the FAA says. In each case, the 787 was flying in conditions involving significant water ingestion and possibly icing of two of the three pitot tubes feeding speed and altitude information to the air data system.
The FAA and Boeing are continuing to investigate the cause of the erroneous displayed speed changes.
In one case, the pilot reacted to the inaccurate data by commanding a “significant” nose-down dive, over-riding the auto-pilot in the process.
Boeing and the FAA are concerned that a pilot might command a dive that exceeds the structural limits of the 787, as a response to erroneous information from the air data system.
While the cause of the erroneous data is being investigated, 787 operators must update the manual to instruct pilots to not apply “large, abrupt control column inputs” in response to an “unrealistic” drop in displayed airspeed.