Re: [SystemSafety] Boeing 787 position flaw

From: GRAZEBROOK, Alvery N < >
Date: Tue, 15 Dec 2015 17:52:19 +0000


Hi Martyn,

[] I can only guess - the guess is based on what I have seen in integrated avionics elsewhere

Can anyone on this list explain how this works? Presumably the Lat/Long are provided by GPS, so why would one of them be missing in a data packet that contains the other? And why would the transponder software have been designed to handle such an occurrence and to extrapolate the missing value, rather than rejecting the packet and requesting another?

Martyn

[] Reading between the lines in the report, the missing data could be the root cause of the bug. I'm sure you can imagine bugs that would fail to update output data correctly. The other part of this is

  1. Transmitters and receivers are typically asynchronous processes passing their data through cyclically updated labels. It is reasonable for the communications layer to assume that transmitted data will miss an occasional transmission slot.
  2. For cyclically transmitted data, a missing label will have its value held at the receiver and not marked as invalid for a fixed time interval.
  3. If this is a velocity vector, the effect would be to extrapolate location.
  4. Even if it is the location value is transmitted perhaps someone felt that the correct behaviour was to extrapolate.

The part that most surprises me is that the extrapolation was permitted to continue for such a long period. It takes some time for an aircraft to travel 70km and I would expect missing data to only be tolerated for a few transmission cycles, not for several minutes. Therefore perhaps my guess about the failure mechanism is wrong.

Cheers,

            Alvery

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