B744 Autothrottle Wakeup

[cf6-80c2b5f]

Does anyone have any information on the autothrottle wakeup feature of the B747-400? I wasn't aware that it even existed on the 744, but a respected sim instructor is certain that it does, at least in the simulators he used. I can't seem to find anything in any of my manuals. I'm referring to when the A/T is armed at the MCP but disengaged with the throttle switches and will come alive in SPD mode (FMA) when the speed gets low.

Did Boeing add this feature later, possibly as a result of the Asiana 777 crash (even though this feature wouldn't have prevented it anyway). Maybe the paragraph in my Vol. 2 below is referring to this feature, but it is difficult to tell if this is a wakeup feature or if it is assuming the A/T is already awake.

The autothrottle and AFDS independently provide speed protection for all operations except during V/S pitch mode or engine failure above maximum engine-out altitude. Autothrottle speed protection is limited by the reference thrust limit (CLB, CRZ, CON, etc.) and idle. AFDS speed protection is provided through the elevators in the following pitch modes: VNAV SPD, FLCH SPD, or TO/GA.

Thanks.

[Brick32]

No such feature available, does not exist.
Fly the B744 myself for a few years now which is obviously not a guarantee in itself for a correct answer but....

[JammedStab]

Subject to confirmation as I am not current on the machine,

I think the FCOM quote says it all. As well, while in VNAV path with the thrust levers in hold, once the speed decreases to 15 knots below command speed, the autothrottle wakes up and SPD mode is activated.

With airspeed unreliable, merely disconnecting the autothrottle is not part of the procedure(which would leave it armed). Instead the Autothrottle Arm switch itself is turned off depowering the entire system. I believe this is to prevent the wake up feature if indicated airspeed erroneously displays too low.

[cf6-80c2b5f]

Thanks, JammedStab. That is a good point about disconnecting the A/T to prevent unintended activation with airspeed unreliable.

I didn't think the 744 had the wakeup feature like the 777. I read in another forum that it activates under the conditions below, but I can't confirm it with any manual I have, so I was thinking maybe this feature was added recently:

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

[cf6-80c2b5f]

Thanks for the reply, Silberfuchs! I have been scouring the Internet trying to find anything offical on this. I am beginning to believe anything in writing is marked classified/top secret by Boeing/Honeywell.

You're right -- it is appallingly written.

[JammedStab]

As I mentioned earlier, my experience was, when on a VNAV PTH descent with the autothrottle in its typical HOLD function, if we got 15 knots slow from the speed for the descent(as shown in the VNAV descent page), the autothrottle would change to SPD, add thrust and return the aircraft to its target descent speed on the VNAV PTH. Is this a wake-up function?

[cf6-80c2b5f]

Hi JammedStab,

I don't think this is the same as the wakeup that's on the 777. The wakeup on the 777 will actually activate even if the A/T is disengaged (blank FMA) via the throttle switches. Let's say you are level in |SPD|HDG SEL|ALT| at 6,000' and you disengage the A/T and manually reduce power. On the 777, the A/T will actually wakeup and activate in SPD mode to keep you from stalling. I don't think the -400 will do this.

The activation on a VNAV PTH descent when you get slow while in HOLD is different. That happens on both the 744 and 777.

It sounds like the 747-8 does have the wakeup feature like the 777.

[actus reus]

As for the 777 A/T wake-up function, if you descend with 'speed on elevator', the most common mode being FLCH (God's version of V/S) and then do not quite capture the Altitude, i.e. disregard the last few seconds of the flight director commands during the ALT CAP phase, the aircraft will sit there, in level flight with the F/D commanding 'fly down', the pilot disregarding the commanded manoeuvre and the speed decreasing to the stall with NO response from the A/T.

We established that during the certification programme in the early/mid 90's.

Try it next time you are in the simulator.

[cf6-80c2b5f]

The "FLCH trap" was the sine qua non of the Asiana crash in SFO.

In the Asiana crash, those guys were high on the approach and had 1800' (FAF) set in the MCP while descending in V/S at about 1700 FPM to intercept the visual glide path from above. As they approached 2300', they set 3000' (missed approach altitude) in the MCP and continued to descend. When they got to around 1600', someone erroneously pressed FLCH. Of course, the power levers advanced and the aircraft began to climb to 3000'. The pilot immediately disconnected the A/P and pulled the throttles back to idle. At that point, the system went to HOLD and the throttles never activated to prevent the speed from bleeding off.

In the NTSB hearing, Boeing admitted that they knew about the FLCH trap but decided that after millions of operating hours between the 747/757/767/777/787, and all sharing the the same system, there was no need to change it. So in typical Boeing style, they had some attorneys put a vague note in the AOM warning the pilots about it (no need to give the flight crews any more than we think they need to know).

As an aside, almost five years prior to the Asiana accident, a 777 forum member here was excouriated and mocked by a number of other pilots when he asked about the very same trap that got Asiana into trouble.

http://www.pprune.org/tech-log/342579-777-flch-climb-t-going-into-hold-no-low-speed-prot.html

[actus reus]

cf6...
With you on Asiana but the point I was putting here also included the fact that failure to follow the F/D component of the system in manual flight will defeat most of these protections.

If my memory serves me correctly, back 'when' there was significant discussion about installing a back up system that compared KCAS to the airplane's configuration and therefore not having to disturb the basic operating parameters of the A/T or the AFCS system algorithms.

That approach did not get a jersey but Airbus considered and acted on that approach, amongst others including 'attitude' protection during A/P Altitude capture, following Nick Warner's (RIP) unfortunate crash of the A330 in 1994(?).

Ironically enough, the VERY well respected Captain Warner had previously been the lead on the B757 programme at BAC and it is fair to assume, knew full well the type of shortcomings the A330/A340 A/T system would need to address.

[cf6-80c2b5f]

Interesting history on this, actus. My approach is that whenever the A/P is off, the A/T is off too, except during takeoff and climb.

I have no problem with the system as it is on the -400. My issue is how Boeing hides information from everyone.

[actus reus]

Yes, the 'one off; both off' approach is known to work well even though there is the occasional wiff from the real 'technology is the way to go' brigade (of which I am NOT one) of that being the dinosaur approach.

Without going into too much boring detail of things long past, the original A/Ts were designed to only be used during Coupled Approaches to Autoland, as you know.

There is a myriad of information, both design criteria and 'discoveries' during flight test that do not make their way into the FCOMs or whatever.

Most manufacturer's are very cognisant of the fact that within their customer portfolio, there will be a 'lowest common denominator' who may go off in a wild way by misinterpreting all the information there is to have. I know, it sounds like hubris but what do people need to know is a complex question.

I agree; more information with appropriate caveats if needs be but there is a finite point in that argument.

AI for instance, did not have a FCTM for the A330/A340 as, I believe, they did not see the need for one until it was written for them by an operating airline.

[actus reus]

Oh, and I sorely wish I had seen that post re the 777 from a few years ago. It would have got my support.

[cf6-80c2b5f]

Without going into too much boring detail of things long past, the original A/Ts were designed to only be used during Coupled Approaches to Autoland, as you know.

Having survived more than 10 years in the left seat of the DC-8, I can attest that I have never once seen the real airplane do an autothrottle (some where equipped) or autopilot coupled approach to minimums without something going pair shaped. I know the design intent was there because the sim actually would do it. It was kind of like the PMS on the 747-200 -- nice when it worked, but don't be surprised if it bites you.

[actus reus]

Yes, the -200; oh dear.

A/T design stagnated (excuse the pun) on the 47 due to the fact that at the other end of the A/T switch(es) is an engine(s).

Throttle bar on the 47-100 with the early Pratts, 'sub idle' requiring a minimum level of thrust during descent (RR eng/ -200) and a couple of other problems both, those well known and those not so well known, contributed to the argument that with engines requiring tlc, there was no need for massive development of the A/T system.

As Joe Sutter, the father of the 47, pointed out, the only reason the A/T eventually was further developed was at the insistence of the TPs who wanted those functions to reduce crew workload during extended cruise etc.

Limited flying of a DC8 (-63) but it seemed to have had better control harmony and more acceptable stick forces compared to the 707. I soon realised with the 07 why it was known at the time as 'the airborne bullworker', named after a then popular isometric exercise device as you most probably also recall.