Ethiopian airliner down in Africa
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FCeng84,
Given your familiarity with 737 systems I wanted to address a question to you:
As far as I understand, FCC-enabled MCAS receives single channel sensory data from the ADIRU, mainly AoA (so as to trigger MCAS in the first place) and speed (so as to regulate just how much trim input is required based on Mach number schedule).
My question is about the total control column forces with both SMYD stall protection features ANDhaving MCAS in its maximum setting. Allow me to explain further:
1. MCAS: assuming an AoA sensor failure where the data is relayed as equal or higher than the activation threshold of MCAS, then MCAS kicks in. Assuming a low speed readout by MCAS, it is then set to its maximum throughput (10 seconds of trim and 2.5 degrees nose down, since it works on a speed schedule based on Mach).
2. SMYD/EFS: As I understand, the SMYD computer enables the EFS (Elevator Feel Shift) and STS (Speed Trim System) to do the following during low airspeed conditions - EFS on one hand doubles Hyd A pressure to control column in its AFT axis, and STS trims the stab Nose Down (though respecting the column cutout switches, something MCAS ignores altogether).
The conclusion is the STS trimming you down (so as long as you don't pull aft), the EFS doubling aft forces, and MCAS also trimming at up to 2.5 degs nose down within 10 seconds (if uninterrupted). Essentially, you can end up in full Nose Down stab trim with doubled elevator control forces, thus severely affecting your authority.
Do you agree with my reasoning that SMYD/EFS interaction with MCAS (when low speed is relayed) can be additive in terms of struggling to keep the nose up and having limited authority?
Makes you wonder just how much attention Boeing engineering gave to both these systems' interaction at the design stage.
Given your familiarity with 737 systems I wanted to address a question to you:
As far as I understand, FCC-enabled MCAS receives single channel sensory data from the ADIRU, mainly AoA (so as to trigger MCAS in the first place) and speed (so as to regulate just how much trim input is required based on Mach number schedule).
My question is about the total control column forces with both SMYD stall protection features ANDhaving MCAS in its maximum setting. Allow me to explain further:
1. MCAS: assuming an AoA sensor failure where the data is relayed as equal or higher than the activation threshold of MCAS, then MCAS kicks in. Assuming a low speed readout by MCAS, it is then set to its maximum throughput (10 seconds of trim and 2.5 degrees nose down, since it works on a speed schedule based on Mach).
2. SMYD/EFS: As I understand, the SMYD computer enables the EFS (Elevator Feel Shift) and STS (Speed Trim System) to do the following during low airspeed conditions - EFS on one hand doubles Hyd A pressure to control column in its AFT axis, and STS trims the stab Nose Down (though respecting the column cutout switches, something MCAS ignores altogether).
The conclusion is the STS trimming you down (so as long as you don't pull aft), the EFS doubling aft forces, and MCAS also trimming at up to 2.5 degs nose down within 10 seconds (if uninterrupted). Essentially, you can end up in full Nose Down stab trim with doubled elevator control forces, thus severely affecting your authority.
Do you agree with my reasoning that SMYD/EFS interaction with MCAS (when low speed is relayed) can be additive in terms of struggling to keep the nose up and having limited authority?
Makes you wonder just how much attention Boeing engineering gave to both these systems' interaction at the design stage.
Last edited by jagema; 18th Mar 2019 at 23:52.
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Hi guys
A few points being missed here I think so far. Now some of these are of course just opinion but here goes.....
1 the 737 can fly with everything turned off. It’s a mechanical plane. That is it’s great virtue. So to crash one you have to forget that it’s actually a real ordinary plane and you are meant to be in total control! Always.
2 that being so, if STAB moves unexpectedly, turn off STAB SWITCHES as generations of pilots have done ,and fly straight and level. Why not take control of the plane? You can.
3 MACS only works with autopilot out at high AoA slow speed
4 the fact MACS malfunctions or maybe is not we’ll designed which we don’t know yet, is not the cause of the crash. although it would be desirable if it worked in a sensible manner. Eg two AoA inputs, doesn’t repeatedly incrementally trim nose down and a limit on the number of trim wheel turns etc etc as postulated here.
5 this nose down trim goes back to the 707 which had a stick pusher- it’s not really a new idea at all
6 the cause here? Looks to me like we’re forgetting the basics and the accumulated knowledge of 50 years of operating the 737.
as someone said, the MAX is like a cart (or perhaps a 707)with a couple of computers and glass screens, but underneath it’s a very simple uncomplicated plane. Fly it 6/60 and it will fly. 6 deg nose up. 60% power. Like any other plane.
7 the stabilizer trim switches on the yoke-the ones you use all the time, cannot apparently override (apply automatic brake) against MACS. Now- That’s something very different from the previous models and if true an important issue. But not the cause.
Thats my ten pence worth for today
i would fly a Max in a heartbeat. Provided the pilots
A few points being missed here I think so far. Now some of these are of course just opinion but here goes.....
1 the 737 can fly with everything turned off. It’s a mechanical plane. That is it’s great virtue. So to crash one you have to forget that it’s actually a real ordinary plane and you are meant to be in total control! Always.
2 that being so, if STAB moves unexpectedly, turn off STAB SWITCHES as generations of pilots have done ,and fly straight and level. Why not take control of the plane? You can.
3 MACS only works with autopilot out at high AoA slow speed
4 the fact MACS malfunctions or maybe is not we’ll designed which we don’t know yet, is not the cause of the crash. although it would be desirable if it worked in a sensible manner. Eg two AoA inputs, doesn’t repeatedly incrementally trim nose down and a limit on the number of trim wheel turns etc etc as postulated here.
5 this nose down trim goes back to the 707 which had a stick pusher- it’s not really a new idea at all
6 the cause here? Looks to me like we’re forgetting the basics and the accumulated knowledge of 50 years of operating the 737.
as someone said, the MAX is like a cart (or perhaps a 707)with a couple of computers and glass screens, but underneath it’s a very simple uncomplicated plane. Fly it 6/60 and it will fly. 6 deg nose up. 60% power. Like any other plane.
7 the stabilizer trim switches on the yoke-the ones you use all the time, cannot apparently override (apply automatic brake) against MACS. Now- That’s something very different from the previous models and if true an important issue. But not the cause.
Thats my ten pence worth for today
i would fly a Max in a heartbeat. Provided the pilots
nose down trim by a stick pusher a very different thing than motoring the whole stab 'in the background' without so much as a 'by your leave' or whisper to the pilot !!!
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You are stretching my 737 knowledge! I have never read or heard anything definitive about whether or not horizontal tail control (either electric or manual via the trim wheel) would be stalled out if the elevator were at blowdown and thus generating maximum force on the jack screw. I would hope that someone on this forum with that knowledge would share it with the rest of us. Not wanting anyone to find out the hard way my recommendation is to keep up with the pitch trim so that the column forces don't build up to the point where you become the one to find out in flight.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"
(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.
(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.
(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.
Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.
We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.
An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
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As a long-time 737 driver I'll just chime-in a few points.
[Long-ish, totally on-point observations, clearly based upon long experience, here.]
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
[Long-ish, totally on-point observations, clearly based upon long experience, here.]
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.
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This, to me, a once-upon-a time mech and long-time engineer, is the "truthiest" post of this long thread. From the moment, in the Lion Air thread that we began to understand the existence and potential reality of MCAS, my image of being on the flight deck of one of these accident aircraft has been eerily like Aloha_KSA's depiction.
I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.
I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.
Reducing the speed (or not letting it grow) is an obvious remedy but easier to say afterwards. If they have the stick shaker since the take-off, with a normal airspeed, they may think the fault is on airspeed and apply the procedure to keep N1 at a relatively high value that will accelerate the aircraft beyond controlability limits...
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10 rotations for one unit of trim.
(Source: Boeing FCOM Bulletin FLR-6(FR) dated August 10, 2012. Subject: Runaway Stabilizer Procedure. From Background '.. the stabilizer trim wheel coasted up to 40 turns (four units of trim).')
(Source: Boeing FCOM Bulletin FLR-6(FR) dated August 10, 2012. Subject: Runaway Stabilizer Procedure. From Background '.. the stabilizer trim wheel coasted up to 40 turns (four units of trim).')
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As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"
(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.
(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.
(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.
Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.
We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.
An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"
(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.
(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.
(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.
Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.
We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.
An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
absolutely nailed it. Thanks for being a realist instead of one of the many 5 stripe armchair pilots out there.
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As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"...
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"...
I am personally shocked at how many posters here truly believe that the pilots should be clairvoyant when presented with a cacophony or warnings and little time to react. It has been covered in other posts but both the Lion Air and Ethiopian crashes presented the pilots with a series of stimuli that did not match the stimuli presented in (most carrier's) training for stabilizer runaway; it is thus not expected for the pilots to immediately reach for the cutout switches. Many of the other proposed actions by the crew(s), which some posters seem to consider common sense, again fail to consider human performance limitations in cognitively demanding situations.
Boeing may have a lot to answer for in terms of design philosophy, flight manual omissions, training deficiencies, etc.. However, I think another critical realization the professional aviation community should draw from these recent tragedies is that basic human-automation interaction principles are still not being given adequate consideration by manufacturers.
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Sums it up beautifully from my perspective. Very easy to look at the issue out of context and say things like "they should have simply flicked the stab trim cutout switches", but when you have two seemingly very similar instances of that not happening, you need to look a bit deeper at the why, rather than reverting to the 1970s and citing 'pilot error' as alarmingly, some are still quick to jump to.
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As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"
(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.
(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.
(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.
Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.
We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.
An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"
(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.
(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.
(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.
Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.
We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.
An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
As a professional pilot we all hope it isn't our day. A wise aviator once remarked "no pilot assumes at the start of the day it is their last day"
It is part of the human condition to assume we are not that guy.
A cacophony of noise, spurious alarms, warnings, surges in power, vestibular disturbance. A growing awareness that this is getting really bad and eventually a windshield full of earth approaching at a breakneck speed with a windshield and radar dome between you and it is not something many want to think about. That four pilots found themselves in that situation is something we reluctantly accept. There but for His grace goes I.
However, that a regulatory process is compromised by a cost focused regulator and manufacturer that knew potentially of the problem (at least after Lion Air) is inexcusable.
That the CEO of Boeing calls the President begging for the aircraft to continue flying when they knew there was a probability it was the same problem that occurred last year is inexcusable.
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Trim wheel
For context here is the trim wheel on the throttle quadrant. The "Green Band" is the normal takeoff range. On the NG it is usually around 5 units for takeoff Don't know on the Max. You can see the handle folded in to the wheel across from the white stripe, which is there to indicate rotation visually. If, for example, one were to go from 0 back to 5 units, that would apparently take 50 rotations. Lower your hand next to your knee in a sitting position and make 50 X 25cm (10inch) rotations to simulate full deflection.
The cutout switches are on the bottom right front.
The cutout switches are on the bottom right front.
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The Miami Herald, in an editorial, suggests four questions for the Boeing CEO:
1. What, precisely, caused the crashes of these two jets within the space of five months?
2. Did any element of the design or certification process for the 737 Max introduce a risk factor?
3. Was pilot training adequate?
4. How will Boeing assure that the 737 Max is safe to fly?
Their reasoning is at the link below. Number 3 seems particularly important. Could Boeing have actually "ordered" simulator training to explain the new "feature" and how to handle it malfunctioning? Because if they didn't, that could (and should) have serious consequences.
".... news reports after the Lion Air crash quoted some pilots suggesting Boeing didn’t adequately explain to pilots how the plane could commandeer flight controls. Boeing and regulators also apparently agreed that pilots previously cleared to fly the 737 didn’t require additional time training in a flight simulator."
https://www.miamiherald.com/opinion/...228112599.html
1. What, precisely, caused the crashes of these two jets within the space of five months?
2. Did any element of the design or certification process for the 737 Max introduce a risk factor?
3. Was pilot training adequate?
4. How will Boeing assure that the 737 Max is safe to fly?
Their reasoning is at the link below. Number 3 seems particularly important. Could Boeing have actually "ordered" simulator training to explain the new "feature" and how to handle it malfunctioning? Because if they didn't, that could (and should) have serious consequences.
".... news reports after the Lion Air crash quoted some pilots suggesting Boeing didn’t adequately explain to pilots how the plane could commandeer flight controls. Boeing and regulators also apparently agreed that pilots previously cleared to fly the 737 didn’t require additional time training in a flight simulator."
https://www.miamiherald.com/opinion/...228112599.html
A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
There was a test flight with a Boeing callsign yesterday, from Renton to Moses Lake. At the time it was the only B38M airborne worldwide. (FR24 data)
Clearly doing approach to stall tests. I watched as it let its speed slowly decay at +/- 15000 feet, and then accellerated again.
Clearly doing approach to stall tests. I watched as it let its speed slowly decay at +/- 15000 feet, and then accellerated again.