In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

Means of recording parameter invalidityThe FDR obtained its ADIRU data through the flight data interface unit (FDIU)(Figure B1). The FDIU extracted the data bits from the ARINC 429 data words thathad been programmed to be recorded by the FDR. Only the data bits were sent tothe FDR for recording, and not the sign/status matrix (SSM) bits, which indicatedthe validity of the data word. To provide some indication of when the data bits for aparticular word had been flagged by the ADIRU as invalid, or if the word wasmissing (or not refreshed), the FDIU cycled the data bits depending on the error.A typical cycle for a parameter that was recorded once per second, when the FDIUhad detected an error, was:• 1 st second: data• 2nd second: error code (1,536 for a parity error, 2,048 for a missing word,2,560 for a failure warning, and 3,072 for no computed data)• 3rd second: data• 4th second: zero.This cycle was then repeated until the reason for the error code disappeared.The FDR processing software scaled these error codes as if they were real data. Forexample, in the case of angle of attack (AOA), an error code of 3072 (no computeddata) was scaled as -45°. A routine example of this behaviour can be seen duringtakeoff and landing at low airspeeds (Figure B2).As noted during the 7 October 2008 flight, there were some corrected AOA valuesof -45°. These values were brief indications of no computed data rather than of dataspikes with a -45° value.Figure B2: Parameter cycling due to FDIU error code- 228 -