Originally Posted by
Dan Winterland
There's something fundamentally wrong here. The AoA sensors on the Max are failing. They are normally very reliable - I've never seen one fail in about 10,000 hours of flying aircraft with the same sensors. What you do notice while doing the walk around check is that they are fully deflected up if reverse thrust was used on the previous landing. The Max has engines moved far forward of the original position. Perhaps the sensors are being damaged by use of reverse thrust.
Let me offer a couple of thoughts about 737MAX AOA failure rates.
First, we know that the Lion Air accident airplane had an errant AOA sensor signal feeding MCAS for the accident flight and the flight the day earlier. We also know that this sensor had been swapped out just prior to these two flights so it appears to have been reading incorrectly the whole time it was fitted on that airplane. We don’t know yet what caused it to have the bias it exhibited. It is possible that the erroneous operation was the result of improper installation or the associated test/checkout. I don’t think we can yet say with certainty that this sensor had failed. Hopefully time and investigation efforts underway now will shed more light on this.
Second, the data for the Ethiopian accident has not yet been made public so we don’t yet know if errant AOA played a role there. Has anyone learn the history of that sensor on that airplane? Hopefully the data will tell us if it was reading wrong on the accident flight and how it behaved on recent previous flights.
Third, my conservative rough calculation of 737MAX fleet hours to date is half a million. Given that most individual airplane components have failure rates in the neighborhood of 1x10^5 hours one would have expected to see about 10 failed AOA across the MAX fleet by now. (Remember that there are two per airplane). I have not seen or heard anything to suggest a failure rate significantly higher than that.
The bottom line line here is that we don’t yet have clear indication of an abnormally high 737MAX AOA sensor failure rate. If that is an issue let’s see the supporting data.