Quote:
Originally Posted by Hayek Reading the dry bulletin does not bring out what a complex situation the pilots were faced with. Even after reading the bulletin and watching the video, I wouldn’t know what to do if this happens (and more importantly how do you know this is what has happened and not something else). We find it hard enough to manage sensors on cars. Also tells you why human pilots will be needed for the foreseeable future. |
As I said before, I don’t know what happened, in fact nobody does. All we have is the Boeing bulletin. Based on what it says and just knowledge of the B738 system it is not that difficult to piece together what a likely scenario could look like, if this particular problem with the AoA sensor occurs. Again, nobody knows for sure whether this is what happened.
Check out the real pilot forums and you will find much more eloborate en depth stories how this scenario is likely to play out in the cockpit. But again, we don’t know if this happened, although from what we know it is definitely a very likely possibility. But let’s not speculate.
At the heart of the Boeing bulletin is how Boeing implemented some of it’s control system. For those that like some more details that might not be so apparent from the Boeing bulletin. This is from one of my pilot forums:
Quote:
737 MAX8 Emergency Airworthiness Directive
The recently released Emergency Airworthiness Directive directs pilots how to deal with a known issue, but it does nothing to address the systems issues with the AOA system, which may be the causal system in the Lion Air accident. This is significant. The positive takeaway is that we are advised, as pilots, that once we recognize the issue, we can stop the negative impacts by taking the trim system out of the loop.
At the heart of this investigation is the MCAS system (description from Boeing):
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
|
If pilots are faced with this problem of a wonky AoA it might be very difficult and counterintuitive for them to respond. Very likely the wonky AoA also means the stall warning is going and the stick shaker is telling you the plane is in a stall.
The standard recovery procedure is push the nose down, firewall the throttles and build up air speed. But in this particular scenario they would need to haul back on the stick, trim back the stab to a normal position and throw the cut out switches.
All (commercial) pilots train regularly on their simulator check rides AoA faults. But nobody knew of this particular problem apparently.
We will just have to wait for more information from the accident investigation to see whether such a scenario was indeed the cause of this tragedy.
I am not sure your conclusion is correct. There were two human pilots up front. Did not make a difference, they still crashed.
Automation in aviation has played an incredible important role in the ever increasing safety. To a large extend you can say that a lot of aviation automation addressed all sorts of human issues. So automation has done away with a lot of the pilot error of the past. But as always, it also introduces some new problems and error. If you don’t have a angle of attack sensor, you can’t have a false angle of attack reading obviously! Still, having incorporated AoA sensor and automation in the flight controls has saved thousands and thousand of lives.
Loss of control (i.e. stalling a plane) as it is called by the FAA is still the number one cause for fatal accidents in General Aviation. And guess what General Aviation has a lot less of automation than your typical commercial jet liner. Few GA aircrafts have AoA sensors. Thus they often fall out of the sky close to airport.
None of the planes I pilot have AoA sensors. So they can’t go wonky on my. But then again, statistically I do have a much higher chance of stalling it and falling out of the sky.
Jeroen