4 ms·

Because there was no available extra sensor. Trevor gets a few other things wrong too. The elevator feel system (EFS) uses multiple sensors, in fact it has it

Because there was no available extra sensor. Trevor gets a few other things wrong too. The elevator feel system (EFS) uses multiple sensors, in fact it has its own pitot probes[1] (two of them) on the elevator. It's not clear if it uses both, but I suspect it also uses both alpha vanes up front like MCAS should be doing. Input from the alpha vane (AoA sensor) is new on the MAX, IIRC. His tweets were a bit ambiguous, but the elevator feel system does not change the trim at all (also EFS was non-op on the Lion Air bird). It's almost certainly NOT a sensor problem. Look at the graphs from the black box on the Lion Air flight. The angle-of-attack was almost exactly offset by twenty degrees left to right[2]. That sensor was working just fine, but something else on the path was fucked up (bad wiring? bad ADIRU? both have happened before). The fact that the alpha vane was replaced and that didn't fix the angle-of-attack readings supports this. Also, the 737 NG and 737 MAX use identical alpha vanes. While the angle-of-attack data is less important on an NG, we haven't seen people come out of the woodwork talking about how common alpha vane failures are on an NG. I'm inclined to believe that they're not all that common. The Lion Air flight crew did write up the sensor problems[3]. I saw another one of these "I'm a software engineer so I've got great insight" posts[4] and chuckled. It's a bit of swing and a miss as well. MCAS is hardly silent or subtle. The same trim wheels his Cessna have are present on the 737 and they're noisy[5]. MCAS doesn't quite override the pilot either, and you can see that on the black box graphs from the Lion Air flight[2]. Twenty times the pilot flying entered opposite trim to counteract MCAS and twenty times MCAS paused for five seconds. On the twenty-first time something changed and the plane crashed. It's not quite that simple and that's why there are active investigations. 1: https://4.bp.blogspot.com/-ZsL1asAUBRU/W-pSdWs2dYI/AAAAAAAAFBg/RlymmfQCny8tUpNqZW6deaS-6GXWPL5KQCLcBGAs/s1600/Screen%2BShot%2B2018-11-12%2Bat%2B8.26.08%2BPM.png https://4.bp.blogspot.com/-ZsL1asAUBRU/W-pSdWs2dYI/AAAAAAAAF... 2: https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-LQP_PRELIMINARY.pdf https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-... 3: https://theaircurrent.com/wp-content/uploads/2018/10/jt43-jt610-lionair-mx-log-annotated.jpeg https://theaircurrent.com/wp-content/uploads/2018/10/jt43-jt... 4: https://medium.com/@jpaulreed/the-737max-and-why-software-engineers-should-pay-attention-a041290994bd https://medium.com/@jpaulreed/the-737max-and-why-software-en... 5: https://www.youtube.com/watch?v=3pPRuFHR1co https://www.youtube.com/watch?v=3pPRuFHR1co

bjowen 8y ago
One thing I’ve seen kicking around is that MCAS alternates input between the left FCC and right FCC, which might help understand why the JT43 only had to fight the stick shaker the whole flight (!) but JT610 had the automated trim input as well.
cameldrv 8y ago
I’m the OP Dave BTW. Trevor copied a Facebook post I made a couple of days ago into those tweets. I don’t know all of the details about the EFS system but I’d be pretty sure it doesn’t use data from both sides for anything, because that would be against the brick wall isolation concept in the air data and flight control computers. You say that the MCAS is neither silent or subtle, but there is no light or master caution or anything, just the trim wheel turning and clicking. I’m not sure you’d notice it if you were wearing a headset and were being distracted by the stick shaker. The reason they died on activation #21 and not #1 is that the effects are cumulative. Every time it activates it dials in another 2.5 degrees of nose down trim. Eventually you can’t pull on the yoke hard enough to hold altitude. I agree that the jury is still out as to whether it’s a sensor issue per se or some sort of ADC or data transmission issue.
- inferiorhuman 8y ago
  The reason they died on activation #21 and not #1 is that the effects are cumulative. The stabilizer position was graphed on that preliminary report (see: pitch trim position). There was a slight trend towards nose down pitch, but no, at the time they lost control there's a sharp decrease in pitch that doesn't correlate with MCAS. If you look at the "trim manual" line you'll see that the pilot flying was continuing to input nose up pitch on the switches to no effect. Something changed and it wasn't simply that the pilot was overridden by MCAS. After 21 even MCAS continued to counter the pitch up inputs as the stabilizer continued to contribute additional nose down pitch. I agree that the jury is still out as to whether it’s a sensor issue per se or some sort of ADC or data transmission issue. To be clear I highly doubt the alpha vane was defective. It's a possibility, but look at that graph. The left "angle of attack indicated" mirrors the right one almost exactly except that there's a near constant twenty degree offset. If that's a sensor problem in two separate sensors I'll pull a Werner Herzog and eat my freaking shoe.

bjowen 8y ago

One thing I’ve seen kicking around is that MCAS alternates input between the left FCC and right FCC, which might help understand why the JT43 only had to fight the stick shaker the whole flight (!) but JT610 had the automated trim input as well.

cameldrv 8y ago

I’m the OP Dave BTW. Trevor copied a Facebook post I made a couple of days ago into those tweets. I don’t know all of the details about the EFS system but I’d be pretty sure it doesn’t use data from both sides for anything, because that would be against the brick wall isolation concept in the air data and flight control computers. You say that the MCAS is neither silent or subtle, but there is no light or master caution or anything, just the trim wheel turning and clicking. I’m not sure you’d notice it if you were wearing a headset and were being distracted by the stick shaker. The reason they died on activation #21 and not #1 is that the effects are cumulative. Every time it activates it dials in another 2.5 degrees of nose down trim. Eventually you can’t pull on the yoke hard enough to hold altitude. I agree that the jury is still out as to whether it’s a sensor issue per se or some sort of ADC or data transmission issue.

inferiorhuman 8y ago
The reason they died on activation #21 and not #1 is that the effects are cumulative. The stabilizer position was graphed on that preliminary report (see: pitch trim position). There was a slight trend towards nose down pitch, but no, at the time they lost control there's a sharp decrease in pitch that doesn't correlate with MCAS. If you look at the "trim manual" line you'll see that the pilot flying was continuing to input nose up pitch on the switches to no effect. Something changed and it wasn't simply that the pilot was overridden by MCAS. After 21 even MCAS continued to counter the pitch up inputs as the stabilizer continued to contribute additional nose down pitch. I agree that the jury is still out as to whether it’s a sensor issue per se or some sort of ADC or data transmission issue. To be clear I highly doubt the alpha vane was defective. It's a possibility, but look at that graph. The left "angle of attack indicated" mirrors the right one almost exactly except that there's a near constant twenty degree offset. If that's a sensor problem in two separate sensors I'll pull a Werner Herzog and eat my freaking shoe.