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Posted on 5th June 2019
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In my previous post about the Boeing 737 Max I wrote about the rules that apply to safety critical systems.
As made clear in this article on CNN, Boeing neatly sidestepped these rules, by simply deciding that the Angle Of Attack (AOA) system was not safety critical.
The AOA system being treated as not safety critical meant that there was no requirement for redundant systems or sensors. The AOA system relies on only one sensor, even though two are fitted to the 737 Max. Even two sensors would not have been enough, because, in the case where one fails, it is not possible to decide which is correct and which has failed; three sensors are needed to build a proper redundant system.
Without a third sensor, the only option is to do what Boeing is now planning to do: disable the AOA system when readings from the two sensors disagree. I have to ask, why only now, after two crashes and many deaths? The FAA has received at least 216 reports of AOA sensors failing or having to be repaired, replaced or adjusted since 2004, so the failure mode behind the two crashes should have been noticed by Boeing and the FAA.
That is, however, not really the key issue here. More important is how on earth did Boeing get away with declaring a system which can crash a plan when it fails as not safety critical? Not only are Boeing to blame for this, but so are the FAA, for failed oversight.
Due to all the press attention on Boeing and the FAA in the wake of the crashes and subsequent investigations, more safety issues have come to light with then 737 Max, including faulty parts related to the leading edge slats. If these do not deploy when they should, the plane is at risk of stalling during take-off and landing.
Some people, including some airlines which own 737 Max aircraft, are hoping and even planning on the basis that the planes will be cleared to fly again in June or July this year. That seems to be extremely premature, given that the investigations are not yet concluded, and probably won't be until the end of 2019 or later.
I think that this debacle will mean that, in future, other aircraft regulators will be less eager to accept certification by the FAA as a basis for certification in other jurisdictions. I see that as a healthy development, although it will increase costs and delays in certifying aircraft, pushing up the costs of air travel.