Boeing 737 MAX

In some ways, I find this reminiscent of the Challenger O rings. Probably, in the final analysis, many individuals with pertinent knowledge at different points in the production/training process, but not piped to the operational level.

I've never flown for a paycheck and never piloted anything heavier than an Aerostar, but I get the sense that the aircrews (and pax) were victims, not screwups.

Seevee said:

...I'm a little disappointed that Boeing produced such an automated product and left the pilots out of the equation. That's not like them. They have produced a lot of automated products, but have always included the crew. ....

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You have to accept that the airlines that bought the MAX were all at least a little bit complicit in the whole MCAS thing. They wanted an airplane with the advantages of new technology but also was the same type rating as the old 737 aircraft. Essentially wanting their cake and eating it too.

At the very least they didn't look the gift horse too closely in the mouth.

Finally, almost all advances in aviation safety are the direct result of an accident.

soin2la said:

Perhaps the mother of all dumb questions after 330 posts: could the MCAS have not simply have been turned off before takeoff and the plane flown manually? Boeing knew there was a problem, why not issue that directive?

Or, are either the planes or pilots, incapable of flying by hand?

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Not sure I fully understand your question. The only way the pilots can turn MCAS off is by the trim cutout switches. As pilots, we don't just deactivate critical systems because we don't like them....BEFORE takeoff. If there is a problem, you write it up and get it fixed. These guys just didn't know anything was wrong BEFORE take off.

Planes and pilots are perfectly capable of flying by hand. Flying without electric trim can certainly be done but it is neither normal nor peferred.

Ski in NC said:

I agree that it needs to be fixed. What that fix entails will be left to the experts (and I am not one!).

The question I have now is why the fleet remains grounded. I guess the answer to that exists in the political realm as this thing has blown up so huge. And that is explainable considering the large loss of life.

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I don't think it is political at all...but you did answer your own question in that last sentence.

From Aviation Week:
(note: they use the term "manual trim" when referring to the yoke mounted electric trim switches)

LOS ANGELES—As the investigation continues into the causes of the Mar. 10 Ethiopian Airlines Boeing 737 MAX accident, sources close to the probe say flight data recorder (FDR) data firmly supports the supposition that the aircraft’s left angle-of-attack (AOA) sensor vane detached seconds after take-off and that, contrary to statements from the airline, suggests the crew did not follow all the steps for the correct procedure for a runaway stabilizer.
Detailed analysis of the FDR trace data shows that approximately six seconds after liftoff was signaled by the weight-on-wheels switch data, the data indicate the divergence in angle-of-attack (AOA) and the onset of the captain’s stick-shaker, or stall warning. Almost simultaneously, data shows the AOA sensor vane pivoted to an extreme nose-high position.
This, says one source, is a clear indication that the AOA’s external vane was sheared off—most likely by a bird impact. The vane is counter-balanced by a weight located inside the AOA sensor mounting unit, and without aerodynamic forces acting on the vane, the counterweight drops down. The AOA sensor, however, interpreted the position of the alpha vane balance as being at an extreme nose-high angle-of-attack.
With the stick shaker active, the trace indicates the crew pushed forward on the column to counteract what they believed were indications of potential approach to stall. The aircraft, now in level flight, also accelerated rapidly as its power setting remained at 94% N1 thrust used for take-off. This was followed by some manual trim inputs using the thumb switches on the control column.
Seconds after speed advisories were heard, the crew raised the flaps. With the autopilot turned off, flaps up and erroneous AOA data being fed to the flight control computer (FCC), the stage was set for the MAX’s maneuvering characteristics augmentation system (MCAS) to activate. This is indicated by approximately 8-sec of nose-down stabilizer movement, which was followed by the use of manual trim on the control column. However, with the MCAS having moved the stabilizer trim by 2.5 units, the amount of manual nose-up trim applied to counteract the movement was around 0.5 units, or roughly only 20% of the amount required to correctly re-trim the aircraft.
Because of the way the aircraft’s flight control computer P11.1 software worked, the use of manual trim also reset the MCAS timer, and 5 sec. later, its logic having not sensed any correction to an appropriate AOA, the MCAS activated again. The second input was enough to put in the full nose-down trim amount. The crew again manually counteracted with nose-up trim, this time offsetting the full amount of mis-trim applied by the latest MCAS activation.
By then, some 80% of the initial MCAS-applied nose down trim was still in place, leaving the aircraft incorrectly trimmed. The crew then activated the stabilizer trim cutoff switches, a fact the flight data recorder indicates by showing that, despite the MCAS issuing a further command, there was no corresponding stabilizer motion. The aircraft was flying at about 2,000 ft. above ground level, and climbing.
The crew apparently attempted to manually trim the aircraft, using the center-console mounted control trim wheels, but could not. The cut-out switches were then turned back on, and manual trim briefly applied twice in quick succession. This reset the MCAS and resulted in the triggering of a third nose-down trim activation lasting around 6 sec.
The source says the residual forces from the mis-trim would be locked into the control system when the stabilizer cut-off switches were thrown. This would have resulted in column forces of up to around 50 lb. when the system was switched back on.
Although this could have been reduced by manually trimming the aircraft, this did not occur, and the third MCAS activation placed the aircraft in a steep nose-down attitude. This occurred with the aircraft near its peak altitude on the flight—about 6,000 ft. The engines remained at full take-off power throughout the flight, imposing high aerodynamic loads on the elevators as the crew attempted to pull back on the columns.
Vertical acceleration data also indicates momentary negative g during which the AOA sensor on the left side unwinds. This is seen as further validation of the theory that the external part of the alpha vane was detached as the apparent change in angle indication could only be explained by the effect of negative g on the counterbalance weight, forcing it to float up inside the sensor housing. In addition, the captain’s stick shaker also comes off twice in this final phase, further reinforcing the severed vane notion.
The source indicates the crew appeared to be overwhelmed and, in a high workload environment, may not have followed the recommended procedures for re-trimming. Boeing’s stabilizer runaway checklist’s second step directs pilots to “control aircraft pitch attitude manually with control column and main electric trim as needed,” according to one U.S. airline’s manual reviewed by Aviation Week. If the runaway condition persists, the cut-out switches should be toggled, the checklist says.

An interesting tid bit of infromation.....

You notice when I talk about the "yoke mounted trim switches" I use the plural of switcheS ???? I am not talking about a swtich on each side. There are actually two switches on each control column. Why you ask? Because Boeing thought it was necessary to provide two seperate electrical pathways with seperate grounds so a simple short would not cause a runaway trim. Pretty ****ing brilliant, right??? Not really. One must activate BOTH switches to complete the circuit to move the trim. This is the kind of redundancy we have come to expect of ANY manufacturer of airliners and it is the reason why people like me are a little "put off" by the fact that there was no such redundnacy with this MCAS system...having only one data input to a system that can drive the flight controls.

One thing (at least) that I don't understand from the Aviation Week excerpt above is that it says the plane was persistently (and erroneously) in a nose-down trim, with pilot thumb trim counteracting some of the MCAS trim action, but never all of it.


That's not what I recall from the initial report, but maybe I don't recall it correctly. I though the pilots were counteracting the MCAS trim each time it occurred, right up until the end.


Also, the plane continued to climb, which seems consistent with corrected trim.


Can one of our 737 pilots help clear this up? I'm probably misunderstanding what's being said in one or more articles. I'm also highly suspicious of magazine technical articles which are typically written by people who barely understand the material.

Thanks, Baker. Good stuff from you as usual.

And yes, I did answer my own question. Thinking while typing, I guess.

The Av Week bit seems to fit the scenario that was gelling in my mind.

I agree that the single point failure presented by having only one AOA sensor is poor design. In nuke power, critical sensors were often arranged in triples. Three things monitoring the same parameter. If one fails (like short to ground, open circuit, otherwise implausible signal, etc), it sets an alarm, but system keeps working. If two fail, system will not act on the apparent parameter, but alarm and just sit there. (I was not a control engr, so the above is a loose generalization).

I see a few big flaws in the MCAS:

One is relying on a single AOA sensor as discussed.

Second is when AOA went WAY off (like 74deg nose up, right?) that in itself means signal is implausible. 74deg nose up means the AC is charging through the air nose up to the sky and belly first. It is not flying, and not in a stall. To put AC in nose down trim in this instance is crazy.

Third is the repeated application of nose down trim. Even if you are approaching a stall, if the MCAS keeps applying downward trim (as it apparently did), is there not a point where the trim setting is so nose down that configuration is basically not fly-able?? (an actual question from this non aviator). So the system can automatically put the AC into a configuration that would be known to not fly?

In a true stall situation (MCAS's purpose) is there not some point of nose down trim that beyond which the system should say "I've done my bit, any further nose down trim is stupid)?

Fourth: There really should be a way to simply turn off MCAS without disabling electric trim. And the pilots should have some clear indication that nose down trim is due to MCAS and not some other cause of runaway trim where the cutout switches are the correct fix.

As configured, the only way to kill an erroneous MCAS trim command is to kill all trim control. Except for the trim wheels, but they were not usable due to aero loads.

It seems you could turn the cutout switches back on, trim nose up electrically, then when in proper trim go back into cutout, as the trim switches would over ride MCAS. (is that correct?) But the pilots did not know this, apparently. And you would have to be quick with the cutout as any delay would let MCAS dive you again.

More I read about this, the more it seems Boeing really screwed the pooch in system design (barrier one preventing an accident fails). Then the pilots screwed up (barrier two fails). And the pilots were not trained or otherwise did not know the system (barrier three fails).

Somewhere in this system development there should have been a room full of engrs and pilots going methodically through the system asking questions like these:

OK, left AOA sensor poops on take off. What happens? What should the pilot do?

OK, MCAS goes nuts for whatever reason. Again, what happens and what should pilot do?

OK, pilot is in task overload, what will AC do?

OK, let's apply all possible failure modes in all possible flight scenarios and pick through the possibilities.

OK, lets repeat the exercise and assume this is a third world pilot that might not even be fluent in english.

Etc, etc..

Seems this was not done very well. Barrier four fails.

Just some rambling thoughts from a non-aviator. Not trying be an expert here at all, just trying to educate myself.

twistedtree said:

One thing (at least) that I don't understand from the Aviation Week excerpt above is that it says the plane was persistently (and erroneously) in a nose-down trim, with pilot thumb trim counteracting some of the MCAS trim action, but never all of it.


That's not what I recall from the initial report, but maybe I don't recall it correctly. I though the pilots were counteracting the MCAS trim each time it occurred, right up until the end.


Also, the plane continued to climb, which seems consistent with corrected trim.


Can one of our 737 pilots help clear this up? I'm probably misunderstanding what's being said in one or more articles. I'm also highly suspicious of magazine technical articles which are typically written by people who barely understand the material.

Click to expand...

The Aviation Week report was correct and the initial report was correct as well. You did not read it right or you did not remember it correctly. And just because a plane is climbing does not mean it is in trim...it just means they are overpowering the trim with muscle. At some point, they were unable to do even that.

Aviation week is a good technical resource....about as good as it gets reference Aviation.

Ski, you have a VERY good understanding of the incident. I will answer your questions the best I can. And since I don't know how to do multiple quotes,, I will answer like this....<<answer>>



Thanks, Baker. Good stuff from you as usual.

And yes, I did answer my own question. Thinking while typing, I guess.

The Av Week bit seems to fit the scenario that was gelling in my mind.

I agree that the single point failure presented by having only one AOA sensor is poor design. In nuke power, critical sensors were often arranged in triples. Three things monitoring the same parameter. If one fails (like short to ground, open circuit, otherwise implausible signal, etc), it sets an alarm, but system keeps working. If two fail, system will not act on the apparent parameter, but alarm and just sit there. (I was not a control engr, so the above is a loose generalization).

I see a few big flaws in the MCAS:

One is relying on a single AOA sensor as discussed.

Second is when AOA went WAY off (like 74deg nose up, right?) that in itself means signal is implausible. 74deg nose up means the AC is charging through the air nose up to the sky and belly first. It is not flying, and not in a stall. To put AC in nose down trim in this instance is crazy.

<<You can be flying or stalled in any configuration at any pitch angle. But yes, there should be a "reasonability algorithm"...which there will be on the correction which I will post on this thread....another Av Week article>>

Third is the repeated application of nose down trim. Even if you are approaching a stall, if the MCAS keeps applying downward trim (as it apparently did), is there not a point where the trim setting is so nose down that configuration is basically not fly-able?? (an actual question from this non aviator). So the system can automatically put the AC into a configuration that would be known to not fly?

<<That is pretty much what happened. Due to full nose down and increasing airspeed, the control "weighting" just kept getting heavier and heavier to the point they could not overpower the trim of the airplane...via the yoke or the manual trim wheels>>

In a true stall situation (MCAS's purpose) is there not some point of nose down trim that beyond which the system should say "I've done my bit, any further nose down trim is stupid)?

<First off, as I said in one of my initial posts on this thread, the purpose of MCAS is NOT stall avoidance. It does that indirectly. But its purpose is to make the airplane "feel" like the previous versions of 737s. The MAX had a tendency to pitch up in a turn under certain "rare" conditons. They put MCAS in to counter this pitch up tendency. It is weird in almost any aircraft to have to push the nose down in a turn. When you turn, you lose some of the vertical component of lift and you have to apply back pressure to maintain altitude. And no, there is no point in the current software that pimits the amount of nose down trim...it can go full nose down and did in this incident>>

Fourth: There really should be a way to simply turn off MCAS without disabling electric trim. And the pilots should have some clear indication that nose down trim is due to MCAS and not some other cause of runaway trim where the cutout switches are the correct fix.

<<That one simply is not going to happen>>

As configured, the only way to kill an erroneous MCAS trim command is to kill all trim control. Except for the trim wheels, but they were not usable due to aero loads.

<<True. They really aggravated the situation by never touching the power. In this article, it says had they left the switches off, the airplane was still flyable and "only" 50lbs of pressure was needed to control it. But they turned the switches back on and 2 quick flicks of the trim switches allowed MCAS to trim all the way to nose down and that was the end>>

It seems you could turn the cutout switches back on, trim nose up electrically, then when in proper trim go back into cutout, as the trim switches would over ride MCAS. (is that correct?) But the pilots did not know this, apparently. And you would have to be quick with the cutout as any delay would let MCAS dive you again.

<<They did exactly that. The only problem is they did not recover all of the trim they lost. In fact, they just bumped the trim enabling MCAS to do its thing one more time.>>

More I read about this, the more it seems Boeing really screwed the pooch in system design (barrier one preventing an accident fails). Then the pilots screwed up (barrier two fails). And the pilots were not trained or otherwise did not know the system (barrier three fails).

<<I agree fully>>

Somewhere in this system development there should have been a room full of engrs and pilots going methodically through the system asking questions like these:

OK, left AOA sensor poops on take off. What happens? What should the pilot do?

OK, MCAS goes nuts for whatever reason. Again, what happens and what should pilot do?

OK, pilot is in task overload, what will AC do?

OK, let's apply all possible failure modes in all possible flight scenarios and pick through the possibilities.

OK, lets repeat the exercise and assume this is a third world pilot that might not even be fluent in english.

Etc, etc..

Seems this was not done very well. Barrier four fails.

Just some rambling thoughts from a non-aviator. Not trying be an expert here at all, just trying to educate myself.

<<A very excellent assessment. Read my next post. It is on the fixes that they are incorporating. You should have been a Boeing engineer because they are doing everything you said!>>

From Aviation Week:


Boeing has demonstrated the old and new versions of the MAX’s Maneuvering Characteristics Augmentation System (MCAS) to pilots and regulators in its 737 MAX engineering cab simulator in Seattle. The MCAS is a new flight-control-computer (FCC) function added to the MAX to enable it to meet longitudinal stability requirements for certification.
However, the system is only needed to enhance stability with slats and flaps retracted at very light weights and full aft center of gravity (CG). The aircraft exhibits sufficient natural longitudinal stability in all other parts of the flight envelope without the MCAS to meet the rules. Boeing emphasizes that the MCAS is not an anti-stall or stall-prevention system, as it often has been portrayed in news reports.
The new software load [P12.1] has triple-redundant filters that prevent one or both angle-of-attack (AOA) systems from sending erroneous data to the FCCs that could falsely trigger the MCAS. It also has design protections that prevent runaway horizontal stabilizer trim from ever overpowering the elevators. Boeing showed pilots that they can always retain positive pitch control with the elevators, even if they don’t use the left and right manual trim wheels on the sides of the center console to trim out control pressures after turning off the trim cut-out switches.
Most important, the MCAS now uses both left and right AOA sensors for redundancy, instead of relying on just one. The FCC P12.1’s triple AOA validity checks include an average value reasonability filter, a catastrophic failure low-to-high transition filter and a left versus right AOA deviation filter. If any of these abnormal conditions are detected, the MCAS is inhibited.
Three secondary protections are built into the new software load. First, the MCAS cannot trim the stabilizer so that it overpowers elevator pitch control authority. The MCAS nose-down stab trim is limited so that the elevator always can provide at least 1.2g of nose-up pitch authority to enable the flight crew to recover from a nose-low attitude. Second, if the pilots make electric pitch trim inputs to counter the MCAS, it won’t reset after 5 sec. and repeat subsequent nose-down stab trim commands. And third, if the MCAS nose-down stab trim input exceeds limits programmed into the new FCC software, it triggers a maintenance message in the onboard diagnostics system.
According to a pilot who was shown the changes in a simulator session, the demonstration begins with the original MCAS software load. During a normal takeoff, at rotation, the left AOA indication moves to its maximum reading—as seen from the flight data recorder in the Ethiopian Airlines accident. Pilots currently do not experience this during initial or recurrent simulator training. The stickshaker fires continuously, using loud sound and control wheel vibration to focus the pilot’s attention on the critically high AOA indication. The erroneous AOA reading also creates large-scale indicated airspeed (IAS) and altitude errors on the primary flight display (PFD) which can be both distracting and disorienting.

AOA is used by the aircraft’s air data computers to correct pitot and static pressure variations induced by changes in nose attitude in relation to the relative wind. Large errors in AOA can cause 20-40-kt. errors in IAS and 200-400-ft. errors in indicated altitude. This is accompanied by the illumination of annunciators on both PFDs that warn of disparities in the IAS and altitude between the left and right displays. As part of the MCAS redesign, Boeing also is upgrading the MAX with AOA dial indicator displays and AOA disagree warning annunciators on the PFDs.

After the high-AOA indication, pilots then follow the checklist for “airspeed unreliable,” which assures that auto-pilot, auto-throttles and flight directors are turned off. They then pull back power to 80% fan speed, set 10-deg. nose-up pitch attitude and climb to 1,000 ft. above ground level. At that point, they lower the nose, start accelerating and begin retracting slats and flaps at 210 kt. indicated airspeed. When the slats and flaps are fully retracted—the MCAS kicks in.

“It’s a good thing we knew what to expect. Otherwise tunnel vision from the ‘airspeed unreliable’ event could have blinded us to the subsequent MCAS nose-down trim input. When I noticed the trim wheels racing, I grabbed the left wheel. It was easy to stop the trim with hand pressure, but I knew in advance what was happening,” says the pilot flying. “We followed the checklist for runaway stabilizer, checking again for auto-pilot off and auto-throttle off. We turned off both trim cut-out switches and cranked the ‘frisbees’ [manual trim wheels on both sides of the center console] to relieve control pressures. We used manual trim for the remainder of the flight to landing touchdown and rollout. That was quite an eye-opener, as I had never been exposed to that during sim training,” he notes.

It is critical to follow the checklist memory items: Pull back thrust to 75% after retracting slats and flaps and set attitude at 4 deg., nose up. If speed builds up beyond 220-250 kt., controllability becomes increasingly difficult, he adds.

Pilots for three U.S. air carriers tell Aviation Week that during their sim training they had never been exposed to extreme and continuous AOA indication errors, they’ve not experienced AOA induced airspeed and altitude deviations on PFDs and have not had to deal with continuous stall-warning stickshaker distractions. They also note that they have never been required to fly the aircraft from the point at which a runaway stab trim incident occurred all the way to landing using only the manual trim wheels. “We’re just checking boxes for the FAA,” says one Seattle-based pilot.

A full aerodynamic stall with the MCAS inoperative is another exercise pilots experience in the MAX engineering cab simulator. “We reduced thrust at 5,000 ft. and slowed the aircraft at about 1 kt. per sec. We were at a midrange cg [center of gravity] with gear, slats and flats up. We trimmed until we reached 30% above stall speed and then just continued to ease back on the control wheel,” one of the pilots says.

“Pitch feel was natural, progressively increasing as airspeed decayed. Somewhere between the audible low airspeed warning and stickshaker, I felt the slightest lightening on control pressure in my fingertips. Quite candidly, if I had not been watching for it, I don’t think I would have noticed any difference between the MAX and the Next Gen [NG] models. I kept pulling back through stickshaker, then buffet, then elevator feel shift [a function that doubles the artificial control feel forces near stall] and finally until the yoke was buried in my lap. The nose just flopped down gently at the stall, and I initiated recovery as I would in most other airplanes I’ve flown,” he adds.

During design of the MAX, Boeing added two more leading-edge vortilons [generating vortices over the top of the wing at high AOA] in 2018, for a total of six per side and also lengthened and raised the inboard leading-edge stall strips to assure stall behavior would be as docile as that of the NG.

Repeating many of the same maneuvers in the engineering cab simulator with the new software load would have been academic at best, as the triple-redundant AOA validity checks all but assure that the MCAS will not be triggered by erroneous AOA inputs in the future. But, FCC P12.1 changes do not protect against erroneous AOA causing stickshaker or large-scale distortions in indicated airspeed and altitude values. Those malfunctions still can cause distraction and disorientation, especially when flying at night and/or in instrument conditions.

The new MCAS protections built into the P12.1 software load preserve its essential role in enhancing the MAX’s longitudinal stability, while virtually guaranteeing that it won’t be triggered by erroneous AOA. And when it does activate, its nose-down stabilizer trim command authority will be limited to assure the pilots always can control aircraft pitch with the elevators.

However, the FCC software upgrades are not the only critical changes needed to boost safety margins for operators. Pilots who underwent the demonstration also say the sessions underscored the need for additional simulator training for dealing with compound emergencies involving AOA and runaway trim failures.

You can stall an aircraft in any attitude. Granted nose high is the most plausible case for a stall especially in transport category aircraft.

The stabilizer has much more control authority than the elevators (and why a mis-trim can be impossible to recover from). In some cases you could have a situation where the elevators do not have enough authority to overcome the nose high attitude and stab trim is necessary to recover.

A fully stalled aircraft could have a very high AOA. The AF447 crash held a high AOA for about 3 minutes while falling pretty much flat into the ocean (this was mostly pilot error).

Relying on a single sensor was poor design. However, with a system that relies only on sensor input even if you allowed for two sensors, if one sensor is different from the other, which one does the system believe?

If you look at a theoretical example, suppose that the MAX incident had happened a little differently. Instead of a (possible) bird strike knocking the AOA vane off, suppose it jammed the sensor somewhere in the normal range and then somewhere along the way the aircraft came close to a stall. The pilots then exacerbated the stall by applying power in a low speed condition and the aircraft pitched up and stalled completely and crashed. There would be a lot of questions about why there was a failure of the system that was supposed to put the nose down when the aircraft is close to a stall and why couldn't the pilots activate it manually when needed.

Both cases described are the result of poor system design.

Baker- Thanks again. Also, now I know why the airspeed and alt readings were off on the left side. High AOA readings cause computer to tweak readings from pitot and static ports.

SSO- Is it not well known that most all AC with low mounted engines, if near stall, a sudden application of power can force a stall? So pilots are trained to gradually apply more power? Due to engine thrust vector being below AC CG and center of drag. So add power and it naturally pitches up. Did not think this was unique to the Max.

ssobol said:

A fully stalled aircraft could have a very high AOA. The AF447 crash held a high AOA for about 3 minutes while falling pretty much flat into the ocean (this was mostly pilot error).

Click to expand...

I know you know this but to just to clarify for others reading. A fully stalled aircraft ALWAYS has a high AOA...that is the reason why wings stall...the critical angle of attack has been exceeded. Just be careful we don't confuse AOA with pitch or deck angle.

I was in the 767-400 simulator and one of our little exercises is unusual attitudes. We are told to look down and away...IOW, don't watch what the instructor is doing. The IP puts the aircraft in an unusual attitude and we are told to look up and recover. Well this particular day, as soon as he said to look up, we were pointed straight at the ground in an accelerated stall. Do you know how confusing it is to be pointed straight down at the earth with a windscreen full of countryside and be in a stall?? Obviously the natural inclination is to pitch up to arrest the descent....WRONG!!!! All you are doing then is aggravating the condition. Anyway, just an illustraion that a stall can happen at any deck angle....but it always occurs when the critical AOA is exceeded.

As an aside, my previous sim check was all about upset training and recovery. Basically stalls in all flight regimes at all altitudes. I will say this. The 737 has some of the most docile stall characteristics of any airplane I have ever flown...including something like a C172. It is truly amazing how a swept wing airplane can fly at such a low airspeed and fly through a stall and recovery. I will bore you with one more sim scenario we did on this day. A high altitude stall. What many people don't realize that even during normal operations when we are high and heavy, the high speed buffet margins and low speed buffet margins are very close together. You might have a 20KIAS window. Let's say you are in the middle of that window. What that means is if you go 10 knots faster, you will approach a high speed stall. 10 knots slower and you will approach a low speed stall. This regime is often referred to as "coffin corner". Well on this day, the instructor put us in a stall situation while in coffin corner. Guess what, we were stalled BOTH high speed and low speed. The red buffett margins on the display overlapped...leaving just an entire red bar on the airspeed tape. Now what do you do??? Do you speed up or slow down???? Again, just another example of how things aren't always as they seem and things can get confusing very quickly. In this case, physics and gravity do the work for you. There really is nothing you can do at that altitude. You just have to keep the wings level and let the aircraft descend to the point where high speed and low speed no longer overlap and then recover from the low speed stall.

Ski in NC said:

Baker- Thanks again. Also, now I know why the airspeed and alt readings were off on the left side. High AOA readings cause computer to tweak readings from pitot and static ports.

SSO- Is it not well known that most all AC with low mounted engines, if near stall, a sudden application of power can force a stall? So pilots are trained to gradually apply more power? Due to engine thrust vector being below AC CG and center of drag. So add power and it naturally pitches up. Did not think this was unique to the Max.

Click to expand...

It is well known. Generally, almost all aircraft(except FBW Airbuses) will eventually pitch up due to increase of airspeed. But yes, the underwing mounted engines do cause a pitch up moment when power is applied. It is not unmanageable...it is fairly slight but you feel it and you anticipate it. BTW, FBW Airbus aircraft trim everything off of the stick. IOW, you pitch down...the nose stays exactly where you put it. It has completely neutral stability on all axes....synthetic of course. A conventional airplane would pitch back up due to the trimmed airspeed it was at. It is why pilots going from Airbus to Boeing have a little extra "relearning" to do.

soin2la said:

...could the MCAS have not simply have been turned off before takeoff and the plane flown manually? Boeing knew there was a problem, why not issue that directive?

Click to expand...

Baker said:

Not sure I fully understand your question. The only way the pilots can turn MCAS off is by the trim cutout switches. As pilots, we don't just deactivate critical systems because we don't like them....BEFORE takeoff. If there is a problem, you write it up and get it fixed. These guys just didn't know anything was wrong BEFORE take off.

Click to expand...

It seems there was an indication of these problems even before the LION AIR crash Oct. 29-18.

https://www.bbc.com/news/world-asia-46373125

"There were serious technical problems on previous flights
The air flight maintenance log showed six problems had been identified on the plane since 26 October, including errors with its airspeed and altitude information displays.
The plane's angle-of-attack sensor - that measures the angle between the wings and the flow of air - encountered problems and was replaced the day before the crash."

I’m asking, if it wouldn’t have been prudent for BOEING to issue a notice stating; “MCAS is suspect, don’t use it until we figure it out?"

soin2la said:

...
I’m asking, if it wouldn’t have been prudent for BOEING to issue a notice stating; “MCAS is suspect, don’t use it until we figure it out?"

Click to expand...

There may not be a way in the MAX to just turn off the MCAS without also turning something else important off too. Apparently the only way to disable it is to turn off the electric powered trim which probably means you can't dispatch the aircraft (at least with passengers in it).

BTW, on Airbus aircraft, the plane will only stay where you point it if the computer thinks it is a reasonable idea (in normal law). Excessive nose down angles will be corrected right away (or prohibited altogether). Reasonable nose down attitudes will be corrected if the aircraft approaches or exceeds max speed. In normal law on the Airbus, the computer always wins.

There was an interesting comment from Boeing in the Aviator weekly article that Baker posted. They emphasized that MCAS is not a stall prevention device, but rather a normal flight "compensating" device. But here we have MCAS acting as a stall-prevention device, and an aggressive one at that. So it does seem to have gotten pretty far out of the box that it was intended to operate in.

Now American and Southwest announce the 737 Max will not return to service until August. Aviation Week doesn't mention it in an April 11 article on Boeing's MCAS software improvements. How do you fellas in the industry interpret this?

I'm thinking the airlines aren't planning to use equipment that may not be recerted by the FAA anytime soon. Or maybe they don't trust the Boeing fix? Or legal advice to let other entities fly it first to see if it's effective?

It is simply a marketing thing. Marketing drives the entire airline. They can't very well plan for airplanes that MIGHT be available. So they just put a time stamp so they have a known amount of aircraft to cover their (now reduced) flying. In one of the articles I read they will be used as spares if they are re certified before then. If you want the headline to read something differently but the same it would be something like "American Airlines cancels all planned 737MAX flying through August 19". That is what I get out of it anyway.... So if they are gonna use them for spares when available before that date, I would not exactly call that "grounded".

Apr 10, 2019Guy Norris | Aviation Daily

Jemal Countess/Getty Images

LOS ANGELES—As the investigation continues into the causes of the Mar. 10 Ethiopian Airlines Boeing 737 MAX accident, sources close to the probe say flight data recorder (FDR) data firmly supports the supposition that the aircraft’s left angle-of-attack (AOA) sensor vane detached seconds after take-off and that, contrary to statements from the airline, suggests the crew did not follow all the steps for the correct procedure for a runaway stabilizer.

Detailed analysis of the FDR trace data shows that approximately six seconds after liftoff was signaled by the weight-on-wheels switch data, the data indicate the divergence in angle-of-attack (AOA) and the onset of the captain’s stick-shaker, or stall warning. Almost simultaneously, data shows the AOA sensor vane pivoted to an extreme nose-high position.

This, says one source, is a clear indication that the AOA’s external vane was sheared off—most likely by a bird impact. The vane is counter-balanced by a weight located inside the AOA sensor mounting unit, and without aerodynamic forces acting on the vane, the counterweight drops down. The AOA sensor, however, interpreted the position of the alpha vane balance as being at an extreme nose-high angle-of-attack.

With the stick shaker active, the trace indicates the crew pushed forward on the column to counteract what they believed were indications of potential approach to stall. The aircraft, now in level flight, also accelerated rapidly as its power setting remained at 94% N1 thrust used for take-off. This was followed by some manual trim inputs using the thumb switches on the control column.

Seconds after speed advisories were heard, the crew raised the flaps. With the autopilot turned off, flaps up and erroneous AOA data being fed to the flight control computer (FCC), the stage was set for the MAX’s maneuvering characteristics augmentation system (MCAS) to activate. This is indicated by approximately 8-sec of nose-down stabilizer movement, which was followed by the use of manual trim on the control column. However, with the MCAS having moved the stabilizer trim by 2.5 units, the amount of manual nose-up trim applied to counteract the movement was around 0.5 units, or roughly only 20% of the amount required to correctly re-trim the aircraft.

Because of the way the aircraft’s flight control computer P11.1 software worked, the use of manual trim also reset the MCAS timer, and 5 sec. later, its logic having not sensed any correction to an appropriate AOA, the MCAS activated again. The second input was enough to put in the full nose-down trim amount. The crew again manually counteracted with nose-up trim, this time offsetting the full amount of mis-trim applied by the latest MCAS activation.

By then, some 80% of the initial MCAS-applied nose down trim was still in place, leaving the aircraft incorrectly trimmed. The crew then activated the stabilizer trim cutoff switches, a fact the flight data recorder indicates by showing that, despite the MCAS issuing a further command, there was no corresponding stabilizer motion. The aircraft was flying at about 2,000 ft. above ground level, and climbing.

The crew apparently attempted to manually trim the aircraft, using the center-console mounted control trim wheels, but could not. The cut-out switches were then turned back on, and manual trim briefly applied twice in quick succession. This reset the MCAS and resulted in the triggering of a third nose-down trim activation lasting around 6 sec.

The source says the residual forces from the mis-trim would be locked into the control system when the stabilizer cut-off switches were thrown. This would have resulted in column forces of up to around 50 lb. when the system was switched back on.

Although this could have been reduced by manually trimming the aircraft, this did not occur, and the third MCAS activation placed the aircraft in a steep nose-down attitude. This occurred with the aircraft near its peak altitude on the flight—about 6,000 ft. The engines remained at full take-off power throughout the flight, imposing high aerodynamic loads on the elevators as the crew attempted to pull back on the columns.

Vertical acceleration data also indicates momentary negative g during which the AOA sensor on the left side unwinds. This is seen as further validation of the theory that the external part of the alpha vane was detached as the apparent change in angle indication could only be explained by the effect of negative g on the counterbalance weight, forcing it to float up inside the sensor housing. In addition, the captain’s stick shaker also comes off twice in this final phase, further reinforcing the severed vane notion.

The source indicates the crew appeared to be overwhelmed and, in a high workload environment, may not have followed the recommended procedures for re-trimming. Boeing’s stabilizer runaway checklist’s second step directs pilots to “control aircraft pitch attitude manually with control column and main electric trim as needed,” according to one U.S. airline’s manual reviewed by Aviation Week. If the runaway condition persists, the cut-out switches should be toggled, the checklist says.

Interesting practical thinking layman`s interpretation, thanks Don.
It must be tough running an airline which had a number of the grounded aircraft in service. I don`t imagine airlines have easy access to replacements, especially for an anticipated short term disruption,when putting a lot of $ into replacement is not economic. Especially as the Max has bigger pax load than older versions. It must be a huge headache if not a mess. Fiji`s airline had 2,a small airline, I can only imagine the difficulty,it could have a serious affect on tourism which Fiji relies on for its economy.

A side comment about the 737 max effect and a kinda rant against airline.
Today we took a plane for Martinique on Air Canada. As I am lucky to afford it, last December we booked two seats in business class from Montreal to Fort-de-France. Of course with the 737 max being grounded, thing changed so Air Canada switched to an Airbus 319, no problem in itself, not really care about the plane model. They also change the schedule, again not a problem I am on vacation, I don't care about a delay of 2 or 3 hours.
Yesterday I start my check-in online as usually, at the end both boarding pass shows me GTE as the seat instead of the 2D and 2F originally choose.
As I was a bit surprised, I call customer service to check in case of an issue, the guy tells me no problem, as the plane changed sea map changed so the seat number does not appear and will be given a the gate. As I am not totally dumb I tells the guy that usually GTE means that you don't have a seat, he replies that I should not worry my original booking did not change, fine.
This morning we came to the airport, the lady at the priority desk repeat us the same no worry your seat is booked, my wife asked her twice "so we still have our seats in business class", twice the answer is yes no worry.
Then we came at the boarding gate, where we are told the plane is full so we do not have a seat. The guy, as much concern as would have been a letuce salad, checked the plane after seeing start to be embarrassed (little word to say just pissed of). He tells us ok we have 1 seat left in business and 1 in eco and tells me that my ticket must be bough with travelers point or a complimentary offering so I cannot have a seat. I replied that no I paid big bucks in cash for this, I have the original booking and bill on my tablet, moment of confusion and shame, he gives me a eco boarding pass telling that this is the only thing he can do, of course I should be happy to be able to board the plane.
Long story short, boarding the plane my wife insisted to the man cabin officer that I should at least have the service even if I do not have he seat.
For me this is a shame, and totally unacceptable to see an airline acting like this. Don't misunderstand me, I merely understand hat this 737 grounding created a big mess, and do not have any issue travelling in eco, but how is it possible for a serious company to merely lie to a customer, twice in two days. They would have send me an email saying sorry the plane change created a situation where you lost your seat so you have the option to go in eco and we refund you the difference or you change your booking for another flight I would have evaluated the situation and make my choice. What happened today is just the obvious sign that these airlines just do not care at least a bit about their customers and they jus consider hem as granted knowing that the are few options to them.
I am very disappointed to see this from a company like air canada.
At the end my wife was in business, me in eco and a lot of people were complaining in both class as the plane assigned to this trip was configured for low cost airlines with very very tight space and not for the regular flight they originally booked at big price.

L

Lou,I`d be furious too,if I paid for Business and got downgraded to Economy. The propensity of airlines to lie to customers may in your case have a connection to the grounded plane being replaced with something smaller, in my experience it`s common practice.
In your case they had a choice, fess up or lie, they chose lie, to keep the balls in the air a day or a minute longer,until the burden of truth fell to a less fortunate co worker.
Qantas/Jetstar is just as bad. Long haul we use Asian airlines,good value and good service.

Don(Moonstruck)....see post #335....I hope you are well my frined!!! Very regretful that we missed you in Feb!!!!

Lou
Sympathies. But for Air Canada this is standard practice. They have no understanding of the words 'customer service', 'honesty', 'integrity' and other similar concepts.

They caught me once with that 'seat at the gate' trick. Some years ago I was supposed to go Toronto to Vancouver, business. There was no crisis then, they just decided to cancel a flight that was not fully loaded. They offered me an economy seat just before boarding finished. I refused. They said I could take business on a later flight. I refused as that would have meant missing Vancouver to Los Angeles connection. So instead they put me on a partner airline's flight from Toronto to LA. But initially only offered economy. Again I refused, so after they apologised yet again 'for the error', I was on the US airline in first. I got to LA a couple of hours before I would have on the original flight booking, and no problem with getting on my flight back to Australia. Of course, no luggage when I landed back home. I knew that would happen, and told the one guy at Toronto who had a bit of empathy that I did not expect to ever see my luggage again. He said I should have more faith! I did get it, intact, 3 days later. Not so bad coming home, but outbound it is unpleasant at best. Why could they not say 'your booked flight/seats area cancelled, sorry'. And then work through your options well before you get to the boarding gate. Why give the staff at the gate a 'hand grenade'? They ought to sack the top 20 layers of management and give the cabin crew a break.

I vowed to never use Air Canada again, but if travelling in Canada there is not a lot of choice! My next experience with them was a Vancouver to Calgary flight. They left my skis in Vancouver. The skis caught up with me 24 hours later. By their standards that service was probably OK. But, I was not impressed and as they never offered a genuine apology for any of the problems I encountered I decided not to forget, and to tell the stories when I get the chance.

Rant over, back to topic. But one last comment - hope your vacation goes well!

Lou_tribal said:

A side comment about the 737 max effect and a kinda rant against airline.
L

Click to expand...

You're not the first to rant against Air Canada!

To be fair though I think that Canadian airlines have been hit disproportionally hard by the groundings. Out of a worldwide total of < 400 grounded planes 41 belong to Canadian carriers. Air Canada has 24 grounded planes in its fleet of < 200.

There's going to be disruption and inconvenience. Sorry to hear it caught you.

Genecop said:

https://youtu.be/zGM0V7zEKEQ

Click to expand...

I couldn't handle the delivery. And am always suspect when someone is staged next to an airplane for significance. I would be much more impressed if he was sitting at a desk and told me right off the bat, why I should give his opinion weight or why I should spend my time listening. Obviously, he did not do that and I lasted less than a minute....

Based on this (the YouTube), it sounds like the computer weenies didn't bother checking in with the aviators in designing the system parameters. All the mods seem directed to curing pretty obvious, from an operational perspective, flaws.

Looks like the buyers who cheaped out on the optional second AOA sensor will have to retrofit #2 to accommodate the new software protocol.

I'm still disturbed that Boeing and at least some operators didn't see fit to prominently emphasise a new system that could directly affect flight controls in manuals and training.