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The A350 has 4 AoA sensors: I suspect it is a lesson learned from an A320 that had 2 AoA sensors froze in the same position, outvoting the only AoA sensor that
by Xixi 7y ago
The A350 has 4 AoA sensors: I suspect it is a lesson learned from an A320 that had 2 AoA sensors froze in the same position, outvoting the only AoA sensor that was still working. In other words: failures are not always independent.
- __m 7y agoWouldn’t they eventually all freeze at roughly the same position when the AoA doesn’t change for a while? Can’t they be heated?
- rurban 7y agoThey are heated now, after the Air France loss. This incident was also the reason for the MCAS system in the first hand: "Don't trust the pilot, regulate it into safety by your own".
- presto8 7y agoAre you referring to AF447? According to the BEA incident report, the AF447 plane was already equipped with heating systems for the pitot probes and AoA sensors. "1.6.9.1 Probe heating The probes that are installed on the aircraft are heated electrically to remove water by vaporisation when the aeroplane is on the ground and to protect them from icing in flight." Reference: https://www.bea.aero/docspa/2009/f-cp090601.en/pdf/f-cp090601.en.pdf https://www.bea.aero/docspa/2009/f-cp090601.en/pdf/f-cp09060...
- rurban 7y agoRight, I mixed that up. With AF447 the initial problem was not the AOA but the very common pitot tube icing, which renders the airspeed indicator useless, thus leading to wrong stall warnings. Even if the pitot tube is heated (mandatory), very often airframe icing leads to a blocked static port, rendering even more indicators useless. https://en.wikipedia.org/wiki/Pitot-static_system https://en.wikipedia.org/wiki/Pitot-static_system
- madarcho 7y agoThey definitely are now. If you want to see what they look like, the channel AvE on YouTube tears one down (Search for BOLTR AoA) and discusses the mechanics. You can see what they suspect to be small cables for heating go up into the element.
- silvestrov 7y agohttps://www.youtube.com/watch?v=NhZ0D-JRtz0 https://www.youtube.com/watch?v=NhZ0D-JRtz0
- cardinalfang 7y agoThe A350 has two types of AoA sensor - vanes and multifunction probes - which should reduce the chance of everything failing together.
- magicalhippo 7y agoI still don't see how a frozen AoA sensor cannot be detected by the reduction in noise or correlation with other sensors (such as gyroscope or accelerometer).
- tempestn 7y agoIt probably could, but you'd have to code for that, rather than expecting the majority of 3 sensors to always be right.
- foldr 7y agoThe trick is to detect conditions like this with virtually zero false positives. Say the sensor fails one in 100,000 times and your algorithm has a 1/100 change of flagging a false positive in the case where the sensor is in fact working properly. Then the chance of a real failure successfully detected is: 0.00001 * 0.99 = 0.0000099 The chance of a false alarm is: 0.99999 * 0.01 ~= 0.01 Thus: P(failure|alarm) = P(alarm|failure)*P(failure) --------------------------- P(alarm) = 0.99 * 0.00001 ---------------- 0.0000099 + 0.01 ~= 0.001 So alerts from your seemingly ok-ish algorithm will be false alarms 99.9% of the time!
- pdpi 7y agoMajority vote is a very simple system to reason about. What you’re describing adds a whole bunch more complexity. For the noise sensor scenario, is the sensor failing or is the ancillary (and single point of failure) noise sensor failing? What happens when you have a false positive like that and a false negative on another sensor? For the correlation scenario, when you have two systems that disagree, how do you determine which one is failing? Are their precisions and tolerances sufficiently close that you’ll get a warning at a useful point in time?
- 7952 7y agoMaybe you could have a pair of vanes. Each vane is physically limited to a portion of possible angles. Vane A would only work above say -10 degrees AOA and vane B below +10 degrees AOA. This would give you a regular test of both sensors when the AoA changes in normal flight. You have more bounding of correct values.