Aviation safety issues and actions

Air data inertial reference unit (ADIRU) fault detection

Issue number: AO-2008-070-SI-03
Who it affects: All operators of aircraft fitted with LTN-101 ADIRUs
Issue owner: Northrop Grumman Corporation
Transport function: Aviation: Air transport
Background: Investigation Report AO-2008-070
Issue release date: 19 December 2011
Current issue status: Partially addressed
Issue status justification:

The modification to the BITE processes will enhance the potential for the ADIRU to cease data output and provide appropriate fault indications in the event of another instance of the data-spike failure mode.

Safety issue description

For the data-spike failure mode, the built-in test equipment of the LTN 101 air data inertial reference unit was not effective, for air data parameters, in detecting the problem, communicating appropriate fault information, and flagging affected data as invalid.

Proactive Action

Action number: AO-2008-070-NSA-125
Action organisation: Northrop Grumman Corporation
Date: 19 December 2011
Action status: Closed

The ADIRU manufacturer conducted a detailed review of options for improving the LTN-101 BITE so that another instance of the data-spike failure mode would be detected. The options included comparisons of the data and/or label fields that were expected to be outputted on certain parameters with the data and/or label fields actually outputted. However, during testing it was found that these modification options adversely affected the performance of high-frequency tasks.

Subsequently, at the aircraft manufacturer’s request, the ADIRU manufacturer enhancedthe wraparound monitor test on the dummy label to improve the ADIRU’s ability to detect data transmission failures. More specifically:

  • The filter was reduced from three consecutive failures to two.
  • A class 1 maintenance fault message will be declared if two output databuses fail from either the IR part or the ADR part (previously all databuses had to fail from one part to reach a class 1 level).
  • In the event of a class 1 message, all data transmissions from the failed part will be terminated.
  • A class 1 failure message will also be reported from the surviving partition to aid in fault identification.

The ADIRU manufacturer advised in October 2011 that the enhancements had passed testing, with certification expected by the end of 2011.

Last update 11 February 2014