I am a private pilot and Aircraft Mechanic. I have worked on several different commercial airline aircraft. Including 737-700 and -800. I don't understand why Boeing thought the 737 Max needed an automatic stall prevention system. Pilots flying this aircraft surely know how to recognize and recover from an impending stall. This is taught in the first 10 hours of flight training. Is there a pilot out there with 1000's of hours that flys a 737 or other airline aircraft that can answer this?

Thanks
Mike

All large airliners have some form of automatic stall prevention systems because of the stall characteristics of swept wings. This will take the form of some mechanism which introduces automatic pitch down at high angles of attack, such as a stick pusher.

The straight wing of a Cessna 172 or Piper Cherokee has very benign stall characteristics and the pilot sits at the center of small mass. Easy to sense the stall, and easy to recover. Basically all you have to do is let go of the controls. However, such a wing is very inefficient at higher speeds, which is why heavy jets have swept wings, and these swept wings introduce some "interesting" aerodynamic issues in the stall regime.

In a heavy jet, it can be hard to "feel" what the aircraft is doing because of the sheer size and the lack of visual clues. You don't fly by feel. You fly by instruments. Stall characteristics are typically somewhat nasty, and one wing often stalls before the other so you don't get a straight down pitch motion but a dropped wing with lots of roll moment. You really never want to get into a stall, which is why we practice "approach to stall" and not "stall".

I’d say at low altitudes, it’s quite possible to fly accurately enough by looking out the windows. I had a couple of references to hold level flight, bank 30* and climb and descend. Its the way I was taught.

That said, identifying a stall and being wrong has some severe penalties, so we are always monitoring speed as sensing speed is hard other than known pitch and power relationships. I could hear the engines but had no idea on actual speed without looking.


GF

I'll add that compared to light aircraft, airliners have relaxed stability in pitch for efficiency reasons. The center of gravity is further back in relation to the center of lift. Added to the swept wings, this makes stall recovery more difficult than in a light aircraft.

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall


In the past stick pushers provided a nose down input to the elevators, why Boeing designed it this way makes me wonder if their flight test results showed that the elevators lacked the necessary authority in that scenario


Problem is you can’t override the stab with elevators, it is much more powerful

GalaxyFlyer wrote:

I’d say at low altitudes, it’s quite possible to fly accurately enough by looking out the windows. I had a couple of references to hold level flight, bank 30* and climb and descend. Its the way I was taught.

That said, identifying a stall and being wrong has some severe penalties, so we are always monitoring speed as sensing speed is hard other than known pitch and power relationships. I could hear the engines but had no idea on actual speed without looking.


GF

Fair point on visual at low level.

We are encouraged to stay far away from the ground from climbout and approach so I haven't really had a chance to try this low level stuff in an Airbus.

Well, traffic patterns, low level cruise levels, once above about 10,000’ it gets harder, I’ll admit.

GF

Biggest difference, and the reason why stall training / upset recovery has been reintrudouced:

While flying a small prop airplane, once you are in a stall you always add power right away. The prop wash alone adds a lot air flow across the wings and elevator (as long as it is mounted at the front). Most jets on the other hand have their engines under the wings und below the center of gravity. When adding power there, no airflow is added across the wing, and in addition those engines add a further nose-up input increasing the angle of attack, worsening the stall.

AmtMike wrote:

I am a private pilot and Aircraft Mechanic. I have worked on several different commercial airline aircraft. Including 737-700 and -800. I don't understand why Boeing thought the 737 Max needed an automatic stall prevention system. Pilots flying this aircraft surely know how to recognize and recover from an impending stall. This is taught in the first 10 hours of flight training. Is there a pilot out there with 1000's of hours that flys a 737 or other airline aircraft that can answer this?

Thanks
Mike

Read all about it here.

https://theaircurrent.com/aviation-safe ... cas-jt610/

A thread based on "automatic stall prevention system" might be doomed to failure. But you already knew that.

Kind of a lame article. Why could the MCAS system have not been a Master caution and an EICAS message instead of the MCAS system mysteriously auto trimming the plane? Since a pilot was already manually flying the aircraft such a warning and message would have got their immediate attention.

It certainly would have solved the mystery of what the Lion Air plane was doing. Or even if the system was auto trimming, an aural warning and EICAS message would have alerted them to what the aircraft was doing.

Trim provides a visual indication right next to both pilots as the trim wheels move, and trim indication is displayed.

It's not really a fast process. Less than a third of a degree per second, and it doesn't have far to go.

747Whale wrote:

Trim provides a visual indication right next to both pilots as the trim wheels move, and trim indication is displayed.

It's not really a fast process. Less than a third of a degree per second, and it doesn't have far to go.

Side note, there are no trim wheels on the A350.

If we are talking 737s, the stall prevention is the human, stick shaker for attention getting and a resistance to up elevator via the elevator feel and centering unit from a output from the SMYD which guess what relies on AOA.

This MCAS is something that comes in at certain flight conditions, not all. So shouldn’t be confused with a all flight conditions stall prevention system.

Most aircraft that have active stick pushers are T Tails due to the inherent inability to recover if the stall condition is able to exist for to long and they enter a deep stall.

The TLDR answer is that it is difficult to ascertain when an airliner is stalled.

Until recently, “stall training” in an airliner was just an exercise in slow flight. The aircraft is not stalled even with the stall warning/stick shaker and until recently there was no training for recovering from an aircraft that is fully stalled because the flight simulator could not demonstrate how the airplane actually behaves when it is stalled.

For most airline pilots the only time they demonstrated recovery from a fully stalled airplane is when they did it for their private pilot check ride in the Cessna 172.

As a result of stall accidents in the last couple years, aircraft manufacturers have been gathering data on the handling characteristics of their aircraft in the stalled regime and simulators are now getting certified with an enhanced flight envelope, covering 4 degrees AOA past stall so that stall training can be done. At least in the US airlines are now doing stall demonstrations and training from full stalls in these simulators with the enhanced flight envelope.

A Cessna 172 when stalled, the nose pitches down and the stall warning indicator sounds. This doesn’t happen in an Airbus 320. When the 320 stalls there is no nose pitch down moment indicating a full stall. It just mushes and starts “falling out of the sky” in a nose level wings level attitude. The aural stall warning is not Mach compensated, so at high altitude (FL350 for example) with the aircraft fully stalled, there is no aural stall warning.

The hard part of stall recognition and recovery training is recognizing that the aircraft is stalled. It becomes harder when you have issues with the flight instruments, i.e. erroneous airspeed indicator, trying to figure out which of the three airspeed indicators in the cockpit is telling you the correct airspeed. Once pilots realize they’re stalled though, pilots are recovering. Delays in recognizing the stall can add to the amount of altitude required to recover from the stall.

It is easier to recognize a stall at low altitudes. It’s quite different at FL350, when your airspeed indicator is not showing anything, the other pilot airspeed indicating an over speed, the standby indicating something normal and the overspeed aural going off in the cockpit, nose +2.5 nose up and wings are level engines at full thrust but the FPA is indicating -15 and VSI pegged full down. Yeah you’re fully stalled.

stratclub wrote:

Kind of a lame article. Why could the MCAS system have not been a Master caution and an EICAS message instead of the MCAS system mysteriously auto trimming the plane? Since a pilot was already manually flying the aircraft such a warning and message would have got their immediate attention.

It certainly would have solved the mystery of what the Lion Air plane was doing. Or even if the system was auto trimming, an aural warning and EICAS message would have alerted them to what the aircraft was doing.

I’m thinking the system is designed to
operate automatically without notifying the pilots to keep the aircrafts handling characteristics the same as the NG, Boeing’s constant quest to convince the market that all versions of the 737 are the same

Woodreau wrote:

As a result of stall accidents in the last couple years, aircraft manufacturers have been gathering data on the handling characteristics of their aircraft in the stalled regime and simulators are now getting certified with an enhanced flight envelope, covering 4 degrees AOA past stall so that stall training can be done. At least in the US airlines are now doing stall demonstrations and training from full stalls in these simulators with the enhanced flight envelope.

The aircraft aren't certificated with altitude loss of other stall recovery performance requirements; some transport category aircraft, and indeed many turbojet aircraft, can take thousands of feet to recover.

Pete Reynolds, a test pilot who did a lot of work on the Learjet among other, noted that the Lear held in a "deep stall" took up to 15,000' to recover, and it's very possible that an aircraft may be placed in a condition that's not recoverable. Hence the reason that stall recognition and prevention is emphasized.

There was a time that stalls, including single engine stalls, were emphasized in light multi engine training aircraft, but it was soon realized that more fatalities and mishaps were occurring in training than in the "real world," especially with multi-engine aircraft. We don't teach or train in single engine stalls, or control departures; we focus on preventing the departures and the stalls. The standard is recovery at the first sign of the stall.

Even where a stall in an unaccelerated state, or a slightly accelerated state such as a 20 degree turn is performed, it's hardly indicative of what can be experienced at high-alpha conditions, especially accelerated ones (steep bank, for example), tip stalls, and other aspects of stalling. Entire books have been written on the subject.

The emphasis with MCAS is to prevent the stall from progressing by decreasing angle of attack early in the event.

Max Q wrote:

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall


In the past stick pushers provided a nose down input to the elevators, why Boeing designed it this way makes me wonder if their flight test results showed that the elevators lacked the necessary authority in that scenario


Problem is you can’t override the stab with elevators, it is much more powerful

Aerodynamically, that may be true, but on every Boeing I've worked on (B727/747/757/767, note the conspicuous absence of the B737), an elevator (column) input in the opposite direction will disable the stabilizer trim system. I can't imagine the B737 doesn't have the same set-up.

Each one of the AMM's I looked at has this statement:

The switches employ a pilot's natural reaction to correct a runaway condition.

Now, tell me that the pilot can't override if the MCAS is trimming, I'm concerned.

fr8mech wrote:

i]

Now, tell me that the pilot can't override if the MCAS is trimming, I'm concerned.

MCAS can be overridden, and stopped; it's stopped by application in the opposite direction of the yoke trim switch, and can be shut off with the cutoff switches.

One of the issues with MCAS is that it was a change that wasn't taught with differences training, and that wasn't really covered or addressed. Unlike other trim systems, if the control wheel trim switch is used to interrupt the trim, it will start trimming again a short time after the trim switch is released. Something crew should really know.

Presently domestically this isn't seen as a particularly large issue. it's known, it's taught, and the procedures are there, and there's software coming to mitigate and alter this action. Presently, the trim action can be interrupted and cut off, as needed.

That's the problem. MCAS. Even if the pilot could over ride the auto feature of MCAS it will start trimming nose down again if the AOA input to it senses a stall. That is why I suggested a Master Caution and EICAS message when MCAS is auto trimming. Pilot awareness to something the aircraft is automatically doing goes a long was in understand when the aircraft is doing something that could exasperate a stall.

stratclub wrote:

That's the problem. MCAS. Even if the pilot could over ride the auto feature of MCAS it will start trimming nose down again if the AOA input to it senses a stall. That is why I suggested a Master Caution and EICAS message when MCAS is auto trimming. Pilot awareness to something the aircraft is automatically doing goes a long was in understand when the aircraft is doing something that could exasperate a stall.

Is the clacking trim wheel gone on the -max? While I don't disagree with you, isn't there already an aural and visual indication of stab movement (trim wheel)?

fr8mech wrote:

stratclub wrote:

That's the problem. MCAS. Even if the pilot could over ride the auto feature of MCAS it will start trimming nose down again if the AOA input to it senses a stall. That is why I suggested a Master Caution and EICAS message when MCAS is auto trimming. Pilot awareness to something the aircraft is automatically doing goes a long was in understand when the aircraft is doing something that could exasperate a stall.

Is the clacking trim wheel gone on the -max? While I don't disagree with you, isn't there already an aural and visual indication of stab movement (trim wheel)?

I don't know if the clack, but they're definitely still there.

I think I can see the stick shakers in the pic. Do they use the same AOA input as MCAS?

Starlionblue wrote:

I don't know if the clack, but they're definitely still there.

Yeah, I found a few images, but you know the internet doesn't always tell the truth.

I like eyeball confirmation when I'm trying to figure things out.

stratclub wrote:

I think I can see the stick shakers in the pic. Do they use the same AOA input as MCAS?

There are definitely stick shakers there. Why wouldn't the MCAS use the same inputs? Question: We know, on other Boeing aircraft that the stick-shaker activates before a stall. Does the MCAS activate before the stick-shaker?

stratclub wrote:

Kind of a lame article. Why could the MCAS system have not been a Master caution and an EICAS message instead of the MCAS system mysteriously auto trimming the plane? Since a pilot was already manually flying the aircraft such a warning and message would have got their immediate attention.

It certainly would have solved the mystery of what the Lion Air plane was doing. Or even if the system was auto trimming, an aural warning and EICAS message would have alerted them to what the aircraft was doing.

EICAS? In a 737? Why does anybody waste time here looking for answers from children in high school?

Laughably poor understanding and uneducated opinion.

pikachu wrote:

EICAS? In a 737? Why does anybody waste time here looking for answers from children in high school?

So, instead of being a dick, why don't you just tell us what the system is called on the aircraft.

Woodreau wrote:

A Cessna 172 when stalled, the nose pitches down and the stall warning indicator sounds. This doesn’t happen in an Airbus 320. When the 320 stalls there is no nose pitch down moment indicating a full stall. It just mushes and starts “falling out of the sky” in a nose level wings level attitude. The aural stall warning is not Mach compensated, so at high altitude (FL350 for example) with the aircraft fully stalled, there is no aural stall warning.

The hard part of stall recognition and recovery training is recognizing that the aircraft is stalled. It becomes harder when you have issues with the flight instruments, i.e. erroneous airspeed indicator, trying to figure out which of the three airspeed indicators in the cockpit is telling you the correct airspeed. Once pilots realize they’re stalled though, pilots are recovering. Delays in recognizing the stall can add to the amount of altitude required to recover from the stall.

It is easier to recognize a stall at low altitudes. It’s quite different at FL350, when your airspeed indicator is not showing anything, the other pilot airspeed indicating an over speed, the standby indicating something normal and the overspeed aural going off in the cockpit, nose +2.5 nose up and wings are level engines at full thrust but the FPA is indicating -15 and VSI pegged full down. Yeah you’re fully stalled.

Dear Woodreau,

I would invite you to have a look at the following video:
https://www.youtube.com/watch?v=4WuPoVjOXLY
Admittedly, this is about the A330, but look how vigorously the aircraft is shaking at 8:30. I doubt that such a flight condition could go unnoticed / unidentified by a flight crew. Okay, there was AF447...

Best regards,
Hendric

One of the many things I don’t understand about MCAS is why it’s necessary at all


Normally, if a jet transport is approaching a stall the stick shaker will activate, if the pilots still take no action and the aircraft is still slowing a stick pusher (on certain types) will activate, aggressively pushing the nose over using the elevators


Why were these tried and true devices, well known and understood by pilots worldwide not considered adequate for the Max ?


If youre close to a stall or stalling who cares what configuration you’re in or why it’s happening, a clear, unambiguous warning and / or preventive system is what’s most important



With the Max, apparently the trim can be running forward for up to ten seconds if it senses proximity to a stall, that is an eternity and a very significant control input, all the time it’s running it’s not telling you why, so as a pilot you’re wondering, am I close to a stall ? do I have a trim runaway or other malfunction ? Perhaps a CG issue ?


Way too many variables to have to figure out in the heat of the moment, I also find it incredible that, unlike other types opposite movement of the yoke apparently will not brake the stab and stop the aircraft from doing what the pilots dont want it to do


No greater Boeing fan than I but this is a really poor design

Looks like they were trying to avoid installing a pusher which the earlier 7373s didn’t have as I understand it.

fr8mech wrote:

Max Q wrote:

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall


In the past stick pushers provided a nose down input to the elevators, why Boeing designed it this way makes me wonder if their flight test results showed that the elevators lacked the necessary authority in that scenario


Problem is you can’t override the stab with elevators, it is much more powerful

Aerodynamically, that may be true, but on every Boeing I've worked on (B727/747/757/767, note the conspicuous absence of the B737), an elevator (column) input in the opposite direction will disable the stabilizer trim system. I can't imagine the B737 doesn't have the same set-up.

Each one of the AMM's I looked at has this statement:

The switches employ a pilot's natural reaction to correct a runaway condition.

Now, tell me that the pilot can't override if the MCAS is trimming, I'm concerned.

The 737 has the same set up for all other trimming other than MCAS, and that is also being changed with the new MCAS software. An opposite column input on the 737 will stop stab trimming. It will stop MCAS trimming also with the forthcoming design.

Max Q wrote:

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall.

I was watching the "Mayday" episode on QF72 last night and the A330/A340's envelope protection has (had?) a high-AOA protection mode which commanded the elevators to pitch the nose down at up to a 10 degree angle. There was an undiscovered issue where the air data inertial reference units could send corrupted data to the flight control computer making the FPC think the aircraft had too high of an angle of attack and trigger the high-AOA protection mode. QF71 ran into the same issue two months later, but their crew was aware of the issue and revised procedures issued by Airbus so they were more easily able to recover.

Stitch wrote:

Max Q wrote:

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall.

I was watching the "Mayday" episode on QF72 last night and the A330/A340's envelope protection has (had?) a high-AOA protection mode which commanded the elevators to pitch the nose down at up to a 10 degree angle. There was an undiscovered issue where the air data inertial reference units could send corrupted data to the flight control computer making the FPC think the aircraft had too high of an angle of attack and trigger the high-AOA protection mode. QF71 ran into the same issue two months later, but their crew was aware of the issue and revised procedures issued by Airbus so they were more easily able to recover.

The 777 and 787 have that also. It will give a nose down elevator command near stall AOA. It can be overridden though. Pilot input will sum with the FBW protection input.

I think Max Q was asking if there is another model that gives a Stabilizer input - not elevator - for stall protection.

BoeingGuy wrote:

fr8mech wrote:

Max Q wrote:

What I haven’t seen before is a system that uses the horizontal stabilizer to input a nose down force if approaching a stall


In the past stick pushers provided a nose down input to the elevators, why Boeing designed it this way makes me wonder if their flight test results showed that the elevators lacked the necessary authority in that scenario


Problem is you can’t override the stab with elevators, it is much more powerful

Aerodynamically, that may be true, but on every Boeing I've worked on (B727/747/757/767, note the conspicuous absence of the B737), an elevator (column) input in the opposite direction will disable the stabilizer trim system. I can't imagine the B737 doesn't have the same set-up.

Each one of the AMM's I looked at has this statement:

The switches employ a pilot's natural reaction to correct a runaway condition.

Now, tell me that the pilot can't override if the MCAS is trimming, I'm concerned.

The 737 has the same set up for all other trimming other than MCAS, and that is also being changed with the new MCAS software. An opposite column input on the 737 will stop stab trimming. It will stop MCAS trimming also with the forthcoming design.

Which basically defeats the purpose of the system.

pikachu wrote:

stratclub wrote:

Kind of a lame article. Why could the MCAS system have not been a Master caution and an EICAS message instead of the MCAS system mysteriously auto trimming the plane? Since a pilot was already manually flying the aircraft such a warning and message would have got their immediate attention.

It certainly would have solved the mystery of what the Lion Air plane was doing. Or even if the system was auto trimming, an aural warning and EICAS message would have alerted them to what the aircraft was doing.

EICAS? In a 737? Why does anybody waste time here looking for answers from children in high school?

Laughably poor understanding and uneducated opinion.

Most of my 30 plus years experience is on aircraft that do have EICAS. I started out working 707/720/737's that had 3 man crews and steam gauges. and Yes, the original 737's did have a 3 man crew. The 3rd man was usually informally referred to as the GIB (Guy in Back)

The display system on the MAX is very similar to EICAS and is called the MAX Display System (MDS) http://www.b737.org.uk/flightinstsmax-m ... m#overview

Since apparently your IQ and maturate never advanced past the 10th grade, how did you ever Obtain an Airman's Certificate with a commercial pilots rating?

It depends on the stall characteristics of the aircraft. Stick pushers are considered usually when you have deep stall problem. This is true usually on T Tail aircraft, where the wing shadows the horizontal tail at high AoA. Aircraft designs like the Max, with conventional tail, don’t have deep stall problem. The issue is that these highly optimized wing profiles used on modern jets are creating some peculiar stall characteristics where the aircraft instead of naturally lowering its nose at a real stall, it stays at these high AoAs. I see that it is difficult to have these newer wings without a closed loop FBW for stall protection.