B737 Elevator Feel Shift Module

[Derfred]

Does anyone have any detail of the inputs to the B737 Elevator Feel Shift Module?

This module doubles control column feel force "during a stall". (What kind of force are we talking here for full aft? Beyond average human strength?)

The graphic in the FCOM is very limited, and only indicates it gets data from the SMYD, which in turn gets data from ADIRU L and ADIRU R.

In particular, what kind of false air data could trigger a nuisance activation? Single AOA, or a combination of data?

I don't think it's as simple as stick-shaker => EFS activation. I think you have to be deeper into a stall condition to get EFS activation, but the FCOM is very vague. Even speed trim is supposed to get a look in at some point in a stall condition, but no detail on what would trigger that either.

It is also interesting to note that the EFS module only uses hydraulic system A pressure. So, in theory, if you were willing to sacrifice half your hydraulics, you could turn it off (if the need arose) - but I don't know what that would cost you in elevator actuator authority. But on the other hand, you really have no way of knowing if it's actually operating or not (unless you "feel" it switch on in control column force as you pull into a stall).

[Mansfield]

Does anyone have any detail of the inputs to the B737 Elevator Feel Shift Module?

This module doubles control column feel force "during a stall". (What kind of force are we talking here for full aft? Beyond average human strength?)

The graphic in the FCOM is very limited, and only indicates it gets data from the SMYD, which in turn gets data from ADIRU L and ADIRU R.

In particular, what kind of false air data could trigger a nuisance activation? Single AOA, or a combination of data?

I don't think it's as simple as stick-shaker => EFS activation. I think you have to be deeper into a stall condition to get EFS activation, but the FCOM is very vague. Even speed trim is supposed to get a look in at some point in a stall condition, but no detail on what would trigger that either.

It is also interesting to note that the EFS module only uses hydraulic system A pressure. So, in theory, if you were willing to sacrifice half your hydraulics, you could turn it off (if the need arose) - but I don't know what that would cost you in elevator actuator authority. But on the other hand, you really have no way of knowing if it's actually operating or not (unless you "feel" it switch on in control column force as you pull into a stall).

Pat Boone's 737 Management Regerence Guide claims that an erroenous activation of the EFSM can cause a FEEL DIFF PRESS light, because the application of only system A pressure results in a differential between system A and B pressures in the elevator feel module. This would explain Boeing's inclusion of the FEEL DIFF PRESS light in their list of things that might appear in the event of a faulty AoA input, and indeed, the crew of LNI043 recorded this in the maintenance history.

Pete Smejkal's The Ultimate 737 Technical Handbook claims that the EFSM will increase the control forces up to 4 times the normal (as opposed to the FCOM statement of doubling the normal). He claims also that this was done to make it more difficult for the pilot to overpower the STS nose down trim applied when approaching the stall.

Tthe FCOM says that a FEEL DIFF PRESS light may illuminate because of blocked elevator feel pitot (on the tail) or a hydraulic system failure. There is a small note in the indications section that says it also may illuminate because of an erroneous EFSM operation. This seems to be the note that Boeing has amplified in the emergency AD.

[737 Driver]

Our tech manual says it takes one SMYD and increases the forces up to four times. So one malfunctioning AOA can apparently also make it that much harder to apply nose up column pressure.

[737 Driver]

Pertinent excerpt:

Normal Operation

During normal operation, there is no EFSM hydraulic output pressure to the dual feel actuator. The feel pressure is not changed by the EFSM. The EFSM receives 3000 psi nominal system A pressure. The pressure reducer provides 850 psi to the mode valve. The mode valve remains closed until SMYD 1 or SMYD 2 commands the dual coil solenoid valve to open during a stall.

EFSM Operation

At stall onset, either SMYD sends a signal to energize the dual coil solenoid valve on the EFSM. The EFSM operation occurs when all of these conditions occur:

• Stick shaker is active

• AOA is 8 to 11 degrees more than thermal anti-ice (TAI) biased stick shaker AOA

• EFSM is not inhibited due to low altitude or the airplane is on the ground.

When all of these conditions occur, the SMYDs energize the dual coil solenoid valve. The solenoid valve sends 3000 psi system A pressure to the pressure-operated mode valve.

The mode valve opens and sends pressure between 820 psi to 880 psi from the pressure reducer to the system A side of the dual feel actuator. This increases the control column feel forces up to four times nominal feel. The increased feel force makes sure the pilots cannot easily override automatic stabilizer movement to nose down pitch of the airplane.

Training Information Point

There is no flight deck annunciation that the EFSM is armed or operating during a stall.

[Machinbird]

Here is a schematic of the Elevator Feel Shift Module:

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/640x400/elevfeelshiftmodule_9235dd64a0d796536669c870142469407a952ca7 .png
ELEVATOR FEEL SHIFT MODULE DIAGRAM
The source of this diagram is a detailed posting by Peter Lemme on his Satcom.guru blog on the elevator feel system.
737 Stabilizer Trim (https://www.satcom.guru/2018/11/stabilizer-trim.html)
Take time to read through carefully. The best diagrams I have seen on the components of the elevator trim system.

[Derfred]

Mansfield, Machinbird... thanks very much, both.

I’ve some reading to do.

At first glance the EFSM, STS and indeed SMYD are far more complex than Boeing’s FCOM would have one believe.

Interesting the contradiction of EFSM 4X feel noted above vs 2X feel in the FCOM. I wonder if the 4X feel is an estimated summation of EFSM and STS, should they both activate.

Deducing how these systems might behave in abnormal phases of flight or with various input failure modes appears non-trivial.

But initial browsing indicates that “FEEL DIFF PRESS” accompanied by stick-shaker is a fair sign that EFSM has activated.

Cheers

[Australopithecus]

Derfred I asked for and got a blocked static port on take-off (in the sim) a couple of days ago. Once stall ID was triggered we got a stick shaker on my side (contrary to my systems knowledge about what signal the SMYD uses to trigger the shaker-alpha was normal . The F/O reported that the control forces got very heavy. The windshear warning was also active. I wonder if the data set in that part of our world is entirely accurate or representative.

[Derfred]

Ok, so blocked static on takeoff will cause IAS to start to under-read, the more you climb the more it will under-read.

So climbing at constant real-IAS, (which you would if you immediately identified IAS DISAGREE and pegged T/O attitude/thrust pending the memory items), you would be seeing a constantly decreasing IAS.

In the documents provided above, I recall noting that stick-shaker can be triggered (by the SMYD) by either AOA or IAS.

So that correlates with your experience: IAS trigger in this case.

What is interesting to me from this is that it appears that EFSM has also activated, indicating that it also has an IAS trigger. I haven’t found a reference for that yet, so thank you.

I also saw a note that the SMYD provides feedback to the FCC for windshear detection.

So it seems that the SMYD, in this case at least, is calling the shots, based on it’s inputs. It’s passing on a message to EFSM of “in stall”, based purely on a single IAS input. It may also have used the rate of change of that single IAS input (must be together with other ADIRU inputs such as GS) to trigger an “in windshear” message to the FCC.

I can add one sim experience of stick-shaker with blocked pitot on descent - similar in reverse. Blocked pitot on descent gives decreasing IAS until stick shaker. But never noticed EFSM operation. But it may have been active but went unnoticed because I didn’t try to pitch up at any point in the exercise.

EFSM gives no notification of it’s operation.

Question for you Austral: Did you get “FEEL DIFF PRESS”? According to documents above, erroneous operation of EFSM should give “FEEL DIFF PRESS”, which makes sense when you look at the FCOM schematic - it adds hydraulic pressure to one side of the Elevator Feel only (SYS A).

Simplistic analysis of EFSM schematic in the FCOM would lead one to believe that you should get “FEEL DIFF PRESS” under any activation of the EFSM, not just erroneous.

So does it inhibit “ FEEL DIFF PRESS” under “real” (non-erroneous) activation?

If so, it’s not yet clear to me how it does that, nor how it determines what is real and what is erroneous.

—
Addendum:

Did you note any STS activity?

In my initial post I also noted that STS may also have a role in anti-stall behaviour. FCOM only vaguely hints at that. But the documents above indicate that it certainly does. In reading, it sounds like the grandfather of MCAS.

The vibe I got was that it might also override the FWD electrical limit switch, so it can trim to 0 units continuously. But... it cannot override the aft column cutout switch. So, the EFSM is partly designed to make it very hard for a pilot to pull the column far enough for column cutout activation, so as to allow STS to keep trimming forward until it either reaches zero or it saves the day.

Disclaimer: I’ve only skimmed the documents so far, I still need time to read carefully - so there might be gross misinformation or misinterpretations in my words.

Fascinating, thanks Astral.

[737 Driver]

According to the tech reference I have, both the EFSM and STS will activate with a false stall warning. The EFSM will increase control column pressures up to four times and cannot be turned of (except by shutting off HYD A pressure which is not an approved procedure). The STS is driven by the FCC, so switching to the good side FCC should stop it. The STS will trim continuously nose down, but can be stopped by the control column trim cutout switches (that is, by pulling back on yoke) or by use of main electric trim. However, after a 5 sec delay, STS will start trimming again. Could be mistaken for runaway trim. STS is only active with flaps down, gear up, below 226 knots CAS.

Wasn’t aware of nuisance wind shear warning. Have to follow up on that.

[Australopithecus]

I didn’t have time to look past the projector to see if the feel diff press light was illuminated, and the F/O was doing good work anyway. The windshear* warning, while spurious owing to the delta V, actually triggers a pretty safe response from the PF...15° and TOGA never killed anyone. In our case I was unloaded enough to verify flaps and cross-check attitude and speed on the standby and provide some assurance to PF that the flight path and energy were both good.

Airbus provides a stall warning on take-off procedure: 15° pitch and TOGA. Wadaya know? The procedure was written for false AoA sensor data on rotate. I note that several hero posters (not you guys) have advocated going directly to the Unreliable Airspeed pitch/power right after rotate, as though the second segment climb gradient was suddenly moot.

I see no problem with getting to at least the target accel altitude prior to reducing to 80%/10°

[Derfred]

Agreed, we had to do a blocked pitot on rotate a while ago at MTOW 79.0T. This causes F/D pitch up towards 20 deg then the F/D disappears (which I was pleased to see).

Exactly as you said, we held 15 deg to 1000 feet, then tried 10/80% (no terrain on this occasion). I was also pleased to note that 10/80% gave us over 1000fpm rate of climb. So if in doubt about 10/80%, following an engine out procedure would keep clear of terrain.

It was certainly very confidence boosting trying it at MTOW, because I must admit that prior to then I would have had trouble believing it.

[Australopithecus]

Agreed, we had to do a blocked pitot on rotate a while ago at MTOW 79.0T. This causes F/D pitch up towards 20 deg then the F/D disappears (which I was pleased to see).

Exactly as you said, we held 15 deg to 1000 feet, then tried 10/80% (no terrain on this occasion). I was also pleased to note that 10/80% gave us over 1000fpm rate of climb. So if in doubt about 10/80%, following an engine out procedure would keep clear of terrain.

It was certainly very confidence boosting trying it at MTOW, because I must admit that prior to then I would have had trouble believing it.


That's good to know, and I guess upon reflection, I should have expected that much all engine climb. I aim to try it using flaps 25 next cyclic at whatever weight we can lift.

What's your thinking on source for the pitch limit symbol on the PFD? That must be solely a product of the SMYD too.

on edit, a quick FCOM search shows that the HGS PLI info is based on alpha = Stall ID-5°, or whenever the windshear warning or stick shaker is activated. In typical cryptic fashion, the manual elsewhere states that the SMYD provides pitch limit information but does not elaborate on when or how.