3 ms·

Your basic point is spot on, it isn't simple. The challenge I think is to keep two things separated, one is the flight control laws that the system is implemen

Your basic point is spot on, it isn't simple. The challenge I think is to keep two things separated, one is the flight control laws that the system is implementing to keep the plane in the air (to the best of its ability), and the other is the situational awareness indicators for the pilots so that they can tell what what the plane is "thinking" about how it is flying (or not). The closest analogy I can come up with is the SQL explain command. That command will generate the complete decision tree for how records are included or excluded from a SQL query. The air equivalent might be display that shows the flight status, and the state of the instruments that are being used to determine that status. And then it is up to the pilots (or DBA :-) to figure out if what is happening is what they think should be happening. To use this particular example, it is, in my opinion, negligent on Boeing's part not to include an indicator for every change in the flight control laws. If MCAS activates to avoid a stall it should always indicate that it is, and why it is activated. It has been reported that this was an "extra price" option for the jet, and it is that choice that makes it feel negligent to me. Generally, there seems to be enough indications with backups in the cockpit so that the pilot can ascertain what is going on with the aircraft reliably (backups and such), but what was missing here was, again in my opinion, was the rationale the plane was using for the flight laws it was implementing being available as well.

chupa-chups 8y ago
This is not the problem. MCAS in itself sadly is the problem. They had to solve the issue that the pitch-up moment shouldn't be accelerating on its own just depending on AoA, which was (aerodynamically) inevitable without automated adjustments due to the placement of the engines. They had to resort to the worst possible cludge, since they weren't even allowed to add new electrical systems (which would have caused a recertification and/or a retraining), so they resorted to an already existing system (assisted trimming by autopilot). The effect of MCAS is only "fixed" by manually adjusting the trim, which involves moving a jackscrew which holds the last section of the elevator at a fixed angle. Unfortunately the required force of moving this jackscrew increases with the airspeed. There is no easy way out. A bit more background can be found here: https://www.satcom.guru/2018/11/stabilizer-trim.html https://www.satcom.guru/2018/11/stabilizer-trim.html Btw, the extra price item was an AoA disagree warning. This is related to MCAS in a way like an odometer failure to traction control. It wouldn't have helped, if pilots stuck to their memory items and checklist (which they have to: they recognize runaway trim and have to react accordingly). To get a slight feeling what it means to have runaway trim (without assistance which they had to disable in concert with MCAS) please have a look at this video: https://vimeo.com/329558134 https://vimeo.com/329558134
- et2o 8y ago
  Thank you for sharing that video! Incredible.
- johnp_ 8y ago
  The video is gone (censored?), does someone have a backup copy?
weaksauce 8y ago
> It has been reported that this was an "extra price" option for the jet, and it is that choice that makes it feel negligent to me. not quite. the things they offered for extra were the AoA sensor readouts and an annunciator for AoA disagreement. the mcas was not part of that and the mcas was not briefed in the difference training from the other 737 models. the mcas taking input from a single sensor was to blame. it should not have activated when there was substantial disagreement from the two sensors. it also should have 3 sensors and vote on agreement and not just difference.
- tuco86 8y ago
  That's what I wonder since I heard of the two sensor thing. Why wheren't there 3 sensors or even more. We omit those kinds of failures for the simplest blog style websites. Feed that info into 3 flight controllers for that matter and fall back to Manual flight in case less than two agree.
mannykannot 8y ago
The problem with this approach is that it does not generalize. You might think that if pilots just had this one piece of information, it would be obvious what's wrong, so let's provide that information. But if you apply that consistently to every system, the crew will be buried in notifications all the time, most of which are perfectly normal, and most of the rest are normal responses to the abnormal conditions produced by the actual fault. Even in a space shuttle, the flight crew did not see most of the data, and NASA had a room full of engineers following the telemetry. When you have less than a minute to save everybody, the last thing you need is something like an SQL explain result.
teyc 8y ago
It wasn't due to the cost. It is the whole business of avoid pilot retraining - which is apparently "must-have" for airlines. At some point, Boeing prioritised papering over the fundamental difference in the flight characteristics of MAX _above_ the pilots' needs to be informed of what is actually happening with the plane.

chupa-chups 8y ago

This is not the problem. MCAS in itself sadly is the problem. They had to solve the issue that the pitch-up moment shouldn't be accelerating on its own just depending on AoA, which was (aerodynamically) inevitable without automated adjustments due to the placement of the engines. They had to resort to the worst possible cludge, since they weren't even allowed to add new electrical systems (which would have caused a recertification and/or a retraining), so they resorted to an already existing system (assisted trimming by autopilot). The effect of MCAS is only "fixed" by manually adjusting the trim, which involves moving a jackscrew which holds the last section of the elevator at a fixed angle. Unfortunately the required force of moving this jackscrew increases with the airspeed. There is no easy way out. A bit more background can be found here: https://www.satcom.guru/2018/11/stabilizer-trim.html https://www.satcom.guru/2018/11/stabilizer-trim.html Btw, the extra price item was an AoA disagree warning. This is related to MCAS in a way like an odometer failure to traction control. It wouldn't have helped, if pilots stuck to their memory items and checklist (which they have to: they recognize runaway trim and have to react accordingly). To get a slight feeling what it means to have runaway trim (without assistance which they had to disable in concert with MCAS) please have a look at this video: https://vimeo.com/329558134 https://vimeo.com/329558134

et2o 8y ago
Thank you for sharing that video! Incredible.
johnp_ 8y ago
The video is gone (censored?), does someone have a backup copy?

weaksauce 8y ago

> It has been reported that this was an "extra price" option for the jet, and it is that choice that makes it feel negligent to me. not quite. the things they offered for extra were the AoA sensor readouts and an annunciator for AoA disagreement. the mcas was not part of that and the mcas was not briefed in the difference training from the other 737 models. the mcas taking input from a single sensor was to blame. it should not have activated when there was substantial disagreement from the two sensors. it also should have 3 sensors and vote on agreement and not just difference.

tuco86 8y ago
That's what I wonder since I heard of the two sensor thing. Why wheren't there 3 sensors or even more. We omit those kinds of failures for the simplest blog style websites. Feed that info into 3 flight controllers for that matter and fall back to Manual flight in case less than two agree.

mannykannot 8y ago

The problem with this approach is that it does not generalize. You might think that if pilots just had this one piece of information, it would be obvious what's wrong, so let's provide that information. But if you apply that consistently to every system, the crew will be buried in notifications all the time, most of which are perfectly normal, and most of the rest are normal responses to the abnormal conditions produced by the actual fault. Even in a space shuttle, the flight crew did not see most of the data, and NASA had a room full of engineers following the telemetry. When you have less than a minute to save everybody, the last thing you need is something like an SQL explain result.

teyc 8y ago

It wasn't due to the cost. It is the whole business of avoid pilot retraining - which is apparently "must-have" for airlines. At some point, Boeing prioritised papering over the fundamental difference in the flight characteristics of MAX _above_ the pilots' needs to be informed of what is actually happening with the plane.