A/T Wakeup / Speed Protection

[emerydc8]

Hi Hardy,

I noticed while trying some stalls in the clean configuration (A/T disconnected at the TQ and AFDS off) that the A/T would wake up into SPD mode and apply power when the speed got close to the stall speed. I don't recall seeing this issue discussed here, but maybe I just can't find it. Is this wakeup feature a safety option that Boeing added later ? I happened to find the post below on pprune and, at least in one sim, it appears there is no wakeup feature. Any information you could offer to help me understand this would be appreciated, as usual. Thanks. Jon D.

[ADDENDUM]: I found this video that appears to show a sim climbing in VNAV SPD and the A/T is disengaged and thrust pulled to idle. It definitely does not have any autothrottle wakeup here. https://www.youtube.com/watch?v=IJKU19Rn0GA

I tried the same scenario in PSX and it acts pretty much exactly like the video -- no wakeup. But when I try disconnecting the PSX A/T in ALT or V/S or when level in VNAV PTH, when the airspeed gets about ten knots below the top of the yellow band, the A/T activates in SPD mode and power is added. This even happens without the AFDS on. For instance, with no A/P or F/D and the A/T disengaged, while doing an approach to stall, it will go to SPD mode around ten knots below the top of the yellow band.

I know the TOGA switches will manually reactivate the A/T, but is there any alpha protection (nothing is mentioned in the engineering manuals)?

If the F/D is still on in VNAV, FLCH, G/S, etc, will anything re-engage the A/T automatically? i.e. including F/D mode automatic mode changes.

Managed to do this in the sim FL370 with normal modes such as SPD . LNAV . VNAV PTH auto throttle disconnected (SPD mode blanks) thrust reduced to idle this is what happened.

A/C slows towards MERS (amber band) once below MERS maybe 10kts the airspeed box is highlighted in amber and "airspeed low" EICAS (if fitted).

VNAV PTH changes to VNAV and the A/C goes into pitch hold, shortly thereafter VNAV has and amber line struck through it and the pitch F/D command disappears.

The A/C would have descended in pitch hold around 10knots above the red bricks. (Because the amber hockey stick varies in size so the mode changes vary its not exact).

Now if you're nasty and hit ALT HOLD (pitch F/D command reappears) the speed reduces to the buffet and stick shaker below the red bricks.

Some figures, A/C was above optimum below max alt forgotten what weight.

Maintaining FL370 pitch was 7 degrees up airspeed 222 / M.696 with major buffet no stick shaker just below the red bricks.

Recovery was pitch down to the horizon or a little more the A/C descended at over 5000 fpm then Max thrust set which took at least 15-20sec to achieve. Once above MERS level off and accelerate towards bug speed. Recovered at just over FL340.

So to answer you question no the A/T doesn't wake up at anytime but the A/C will attempt to do the right thing and give you plenty of indications that its time to do the right thing.

[Hardy Heinlin]

Hi Jon,

it has been implemented a year ago:

http://aerowinx.com/board/index.php?topic=2637.0

See: "0.3-0016. Autothrottle wake-up system installed on all models (no option)."

Here's the original discussion: http://aerowinx.com/board/index.php?topic=2597.msg26186#msg26186


Cheers,

|-|ardy

[emerydc8]

Thanks, Hardy. I thought I remember reading something about this somewhere but couldn't find it. Figures you have crossed that bridge already. If Speedbird and Skybird both say it has been implemented on the B744, then I'll believe it. I'm presuming that PSX uses these conditions:

The low speed protection acts at [Min MAN Speed - 8kts]

It engages the SPD mode of the A/THR in the following conditions:
-A/THR in ARM and fully serviceable (Active or not)
-Above 100ft Gnd for Approach / Above 400ft Gnd after Takeoff
-The AFDS vertical mode FLCH SPD, VNAV SPD, TOGA are not the active modes

Does anyone have any official bulletin or written doc on this from Boeing? Was this implemented on all B744s?

[EDIT]: I ask this question presuming that the wake-up feature was an add-on after the airplane was delivered.
Maybe it was always there. I just didn't think the 744 had the wake-up feature like the 777.

[Britjet]

Just did a quick test of this in big sim.
Autopilot engaged, Alt HOLD, AUTOTHROTTLE armed but disconnected (mode blank). Thrust levers closed.
Full stall ensued with no wake-up.

Peter.

[emerydc8]

Thanks, Peter. I think we can say with a fair degree of certainty that there is no "wakeup" feature on the 744 like there is on the 777. 

[Avi]

Hi,

Allow me to quote Hardy (since he is not here) from here

Thanks. From Google I'm now learning that this is feature is installed on the 747-8 and some 747-400, and that they have been considering installing it on all models as recommended by the FAA in 2011 or 2013 (Asiana accident).

Cheers,

[emerydc8]

Thanks for the reminder. If you can find anything on Google (or anywhere else for that matter) that the wakeup was actually installed and not just recommended by the FAA (the FAA is regularly ignored because they are a captured agency), I would be interested in seeing it. I have spent many hours over the past ten days scouring the Internet and I could find nothing that says this mod was made on any 744.

[Avi]

Hi,

Unfortunately I can't give a real information about what is installed and where but I wrote my post because it sounded like you came to a concrete conclusion from a single test in the sim.

I must admit I began to be so confused here (and I'm sure I'm not the only one) between what there is on the B777 and what there is on the B744, and, what there is on some B744s and on others not.

Cheers,

[emerydc8]

I agree it is confusing, probably because the use of the word "wakeup" could describe two different but related issues.

The wakeup feature on the 777 occurs when the throttles are disconnected but still armed (FMA A/T window blank) and the speed gets low. So, if you look at Peter's sim scenario above, the 777 wakeup feature would have actually activated the A/T in SPD mode. I am pretty sure that there is no such feature on any 744, but like I said I am open to any new information on this. It should be noted that this wakeup feature did not keep the Asiana 777 from crashing because even it would not work when the throttles are in HOLD and the pitch FMA is FLCH or VNAV SPD.

The other feature (actually a trap that is common to the 747/757/767/777/787) occurs where the A/T is in HOLD while in FLCH or VNAV SPD mode and the speed gets low due to disconnecting the A/P and taking over manually. This FLCH trap was the sine qua non of the Asiana accident.

In 2010, the FAA test pilots accidentally discovered the FLCH trap while testing the 787 and brought it to Boeing's attention. Boeing stood its ground and the FAA backed down. As a result of the Asiana accident, in July 2014, the NTSB (not the FAA) made two recommendations to Boeing:

Revise the Boeing 777 Flight Crew Operating Manual to include a specific statement that when the autopilot is off and both flight director switches are turned off, the autothrottle mode goes to speed (SPD) mode and maintains the mode control panel-selected speed. . . .

[D]evelop and evaluate a modification to Boeing wide-body automatic flight control systems to help ensure
that the aircraft energy state remains at or above the minimum desired energy condition during any portion of the flight.

To my knowledge, Boeing has not redesigned or modified how the 777 logic works while in FLCH or VNAV SPD with the A/T in HOLD mode. This same logic applies to the 744 as well. After the Asian accident, we all read the articles about the NTSB recommendations and presumed that if the NTSB recommended it, Boeing must have complied. I don't think this is the case; but even if it is, I don't think any such modification has trickled down to the ageing 744 fleet.

I would invite anyone who can come up with more information on this to post it here.

[Avi]

[D]evelop and evaluate a modification to Boeing wide-body automatic flight control systems to help ensure
that the aircraft energy state remains at or above the minimum desired energy condition during any portion of the flight.

This should also change the definition of V/S pitch mode where today the FCC commands the A/P (or F/D) to maintain pitch to achieve the selected V/S. In this case the aircraft can get too slow even with full thrust or too fast even at idle power. If Boeing implements this recommendation, when getting close to the speed limit the FCC should stop doing this.

It looks like the NTSB expects from Boeing to change the way they think from there are pilots on board whom know what they are doing to there are completely idiots on board whom have no idea what they are doing and we need to protect them at all time. A little bit sad.

Cheers,

[emerydc8]

My complaint about Boeing is not the design and logic of the AFDS, but the apparent lack of desire to make detailed information available to those who really want to understand it. I don't think Boeing is going to make any changes to the 777 or 744 AFDS other than maybe add another warning light or horn. The EASA recommended similar changes and Boeing ignored them too.

This should also change the definition of V/S pitch mode where today the FCC commands the A/P (or F/D) to maintain pitch to achieve the selected V/S. In this case the aircraft can get too slow even with full thrust or too fast even at idle power. If Boeing implements this recommendation, when getting close to the speed limit the FCC should stop doing this.

I flew the DC-8 and DC-10 for 14 years using V/S as the primary altitude change mode. There was no FLCH or VNAV. The DC-10 was advanced (it actually had an IAS hold feature!). I never once came close to stalling or overspeeding the aircraft. There are some good arguments that V/S mode is actually better than FLCH or VNAV in a lot of situations. I think Peter would agree. The wings will not come off the airplane when you select V/S.

You are right though -- at some point you are either a pilot who will fly the airplane, or a passenger who will let it stall. I like Boeing's philosophy much more than that other company.

[Hardy Heinlin]

I just posted a question in that other thread in the hope that the user Skybird will receive a notification of the new thread comment:
http://aerowinx.com/board/index.php?topic=2597.msg36027#msg36027

[Hardy Heinlin]

A/T wake-up is now optional in PSX 10.0.9-beta2:

http://aerowinx.com/board/index.php?topic=3685.0