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Old 8th Jul 2010, 23:36
  #1716 (permalink)  
Join Date: Jun 2009
Location: NNW of Antipodes
Age: 77
Posts: 1,330
Reference has earlier been made to an A330/AF447 blog by Matthew Squair, an Australian Engineering Systems Analysist, who has a Marine Engineering background. He has questioned the viability of the Airbus triple redundancy systems, and has used the QF72 faulty air data incident as an example of how asynchronous as opposed to synchronous sampling of data can lead to momentary spikes skewing the resultant output. The case he has made relates in particular to the polling method used in determining valid AoA, a special case, as it is possible that in certain sideslip type situations AoA vanes #1 & #2 on the port-side may disagree with the single vane #3 on the starboard-side.

Issue was taken with the Airbus statement that spikes occur in air data on many flights, but there is little evidence of them ever having caused any untoward affects. To which Matthew commented, "... for high integrity systems a lack of evidence cannot be used as evidence of lack". At face value, a valid point.

Without knowing precisly the algorithms used by Airbus in determining what is "valid" and what is "not valid" sensor input, it is not easy to confirm nor deny that momentary spikes have been or could be responsible for delivering erroneous data to to the AIRDU's and PRIMs. My initial reaction is that time domain analysis of raw data needs to be made, but time is of the essence and delaying data by a few milliseconds could be critical. Why not use higher sample rates? Well, some noise spikes could appear over a number of cycles, but higher rates do lend themselves to determining a trend quickly. Comparison of multiple sensor trends should enable determining if the trend is valid or not. Different sensors may have different trend rates, but if their directions of trend are the same and fall within allowable rates of change, then it should be possible to sort out "good" from "bad" data. Common mode rejection techniques are often used to sort out noise problems that are potentially localised, e.g. earth loops as in audio equipment and other power supply induced noise. For instance, a lightning strike may cause a momemtary blip in otherwise clean power, but the common mode rejection techniques ensure that a similar out of phase blip cancels out any rogue input/output.

This is drifting a bit, but situational awareness is nothing more than an overview of current trends that leads you to believe that the bulls-eye is where it should be. In the example I gave back in post #1690, i.e.
There is something about this whole situation that doesn't quite fit, i.e. a sophisticated FBW aircraft is being held by AP in an increasing nose up attitude and decreasing A/THR to maintain a BARO-ALT. The available inertia is disappearing fast, and somehow I think that that scenario would have been covered in the design algorithms as indicative of unreliable airspeed.
- is actually a relatively long-term trend which if put to the bulls-eye test would fail.

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