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Overriding the A/T servo in FLCH or VNAV

Started by Hardy Heinlin, Sat, 9 Apr 2016 06:42

Hardy Heinlin

Hi,

I'm starting a new thread which shouldn't be mixed with facts about the 777 and the Asiana accident.

It's just about the 744 now ...

And not about flare modes (this will get an extra thread too).

Otherwise I can't track all the information anymore ... :-)

...

OK, here's a quote from a 744 book and I'm trying to decode the grammar of that quote:

"The pilot override logic occurs when the throttles are moved to a position that differs from the servo command by 8 degrees in the FLCH or V NAV IDLE descent modes. This causes the A/T mode to change from THR/IDLE to HOLD on the PFD."


Possible interpretations regarding FLCH:

(A) ... in the FLCH climb and descent modes or VNAV ...

(B) ... in the FLCH descent modes or VNAV ...


Possible interpretations regarding VNAV:

(X) ... or VNAV_PTH idle descent mode.

(Y) ... or VNAV_SPD descent mode.

(Z) ... or VNAV_PTH idle descent mode or VNAV_SPD descent mode.


In my opinion, it's (B) and (Z).

Why (B)?

Because the text fragment "in the FLCH" is obviously to be completed with the last words "descent modes". If these last words wouldn't refer to FLCH, FLCH would appear as a name without "the": "in FLCH". That is, THR in FLCH climb cannot be overridden.

Why (Z)?

Because the most famous case of all manual override functions is that -1250 fpm non-idle feature of VNAV SPD which goes from THR to HOLD, similar to FLCH SPD. So if the override function is available in a non-idle FLCH mode, why shouldn't it be available in VNAV SPD descent as well?


So the mode logic provides the following functions:

FLCH SPD descent:
• retarding to idle stop: THR; reaching idle stop: HOLD.
• retarding to target above idle: THR; reaching target: THR (or maybe HOLD?)
• any retarding: THR; overridden by 8°: HOLD.

VNAV SPD descent:
• retarding to idle stop: IDLE; reaching idle stop: HOLD.
• retarding to target above idle: THR; reaching target: HOLD.
• any retarding: THR; overridden by 8°: HOLD.

VNAV PTH idle descent:
• retarding to idle stop: IDLE; reaching idle stop: HOLD.
• retarding to idle stop: IDLE; overridden by 8°: HOLD.


Cheers,

|-|ardy

emerydc8

Hi Hardy,

I agree with everything you said except that I think you will see HOLD if you are doing a FLCH climb and you manually override the throttles. I understand your argument about the semantics of the statement in the manual, and I know you don't want to get into a 777 discussion, but I think that if the logic on the 777 applies to both FLCH climbs and descents, then it probably applies to the 744 the same way. So my vote is for choice A on the first question and Z on the second.

Jon D.

Britjet

I'm with the Hardy camp on this one - but will try it on Monday.
Peter

Will

My friend whom I referenced on the other thread, the former 744 pilot and current 777 pilot, isn't totally sure but seems to agree with B. He writes: "That sounds right at first, you would just be fighting the servos in a climb and making a lot of noise without actually disconnecting A/T. Wouldn't expect HOLD there, I'm pretty sure the servos would keep up the good fight." He is going to talk to a 744 engineer at this company and get back to me. I'll update this thread when he does.

As for parsing the sentence, I agree with Hardy that the word "the" before FLCH makes it sound as if the modifier "IDLE descent modes" applies to both FLCH and VNAV. However, the broader context could be about things that happen in IDLE descent, meaning that the absence of any comment about climb modes doesn't necessarily mean that this behavior doesn't occur there; perhaps HLOD mode in climbs is discussed elsewhere. I'm not advocating for that position, just saying that there may still be room for ambiguity.
Will /Chicago /USA

emerydc8

Hi Will,

Your friend said, "That sounds right at first, you would just be fighting the servos in a climb and making a lot of noise without actually disconnecting A/T. Wouldn't expect HOLD there, I'm pretty sure the servos would keep up the good fight."

I'm curious if he's aware that his 777 would go into HOLD under this scenario.

Jon D.

Will

(Not to go on talking about the 777...) Yes, I believe he knows that. He sent me some company training materials that were distributed after Asiana, which is where I got the part posted on the other thread about the FLCH going from HOLD back into THR. He also said they were encouraged to do this in the simulator to see it happen. But he did say he can't recall ever pulling the throttles back while in a FLCH climb in flight in either aircraft. In any event, he's not sure about the 747. I'll post back what he hears from the engineer he knows.
Will /Chicago /USA

emerydc8

Thanks. It will be interesting to get the final answer on this. I was talking to a friend today who is a classic captain getting ready to transition over to the 744. He couldn't believe that this information wasn't made available by Boeing, especially in light of Asiana. Maybe that's what happens when lawyers and engineers write the manuals.

Hardy Heinlin

Quote from: Will on Sat,  9 Apr 2016 19:01
... the absence of any comment about climb modes doesn't necessarily mean that this behavior doesn't occur there; perhaps HLOD mode in climbs is discussed elsewhere.

I agree, but I can say that it is not discussed elsewhere. The quote is from a section about "Pilot Override Logic"; this section contains further, override related details, but nothing about climb. If the climb would be involved, this section would be the right place to mention it.


|-|ardy


On another page where the available A/T modes for takeoff, climb, cruise, and descent are explicitly explained, HOLD is mentioned in the takeoff and descent phase only, whereas in climb it can provide just THR REF and THR. If HOLD was available in climb as well, it would be mentioned here as well.

Will

Tangentially related to this conversation is the fact that you can enter autothrottle THR and SPD modes on the ground. That, of course, leads to the throttles moving forward as if the aircraft were in flight. For example, if the AFDS is configured for a normal takeoff but is still near the gate, and someone inadvertently bumps the FLHC button, the autothrottle mode changes to THR and the aircraft suddenly develops takeoff thrust while perhaps still on a taxiway or even close to the gate after engine start.

And pulling the throttles manually back to idle just leads to the throttles moving right back up again to takeoff thrust.

Also, hitting V/S or altitude HOLD afterwards changes the autothrottle mode to SPD, but the point is that in either THR or SPD, the autothrottle is commanding maximum thrust while still on the ground. And, of course, if FLCH is selected on the ground before the autothrottles are armed, then the simple action of flipping the "arm" switch will result in the engines spooling up to takeoff power.

I would have thought that Boeing would have inhibited modes SPD and THR on the ground, enabling only THR REF after pressing the TO/GA switches.

Has any pilot ever gotten themselves in trouble with this?
Will /Chicago /USA

emerydc8

For that reason, the A/T arm switch is not armed until taking the runway (below the line) and on the other end it is part of the after landing flow as you exit the runway. The A/T disconnects on landing upon using reverse thrust; but ten seconds after reverse thrust levers are down, the autothrottle is armed until flaps are up. There are usually only a few seconds, if any, before the after landing flows are completed and the A/T arm switch is off.

Britjet

I have never heard of the autothrottle being physically armed on entering the runway. This is not Boeing SOP.
It is armed at all times. There is no NORMAL procedure where the autothrottle switch us moved to OFF at any time, either before, during or after flight.
Sitting on the ramp with engines running and hitting a vertical mode will not cause the autothrottle to engage. Only TOGA will do that.

Peter

Hardy Heinlin


Avi

If you are not confused enough...

I read my (very old) manual about the A/T to try finding answers to the new questions (which discusses in a new thread) but found something that belongs to this one.



I must say that from this quote I don't understand it applies only for a descent mode (on the contrary) so I vote for FLCH climb and FLCH descent (not VNAV) but again, this is very old manual.

Cheers,
Avi Adin
LLBG

Britjet

Big sim test today.
Just using FLCH in a climb. (THR and FLCH SPD).
Retarding all thrust levers - THR changes to HOLD.

I didn't try any other modes, but prob a safe bet that VNAV is the same..
Peter

emerydc8


Hardy Heinlin

I conclude, the 744 book that I quoted is version specific or outdated or wrong (due to omissions).

I'll modify the next PSX update accordingly for all airline models, and will keep that mod until someone proves it's version specific.


Regards,

|-|ardy

Avi

Quote from: Britjet on Mon, 11 Apr 2016 18:17
I didn't try any other modes, but prob a safe bet that VNAV is the same..

Sorry but I don't know about that.
The quote I brought above specifically talks about FLCH, it doesn't mention VNAV so I don't know about it.

Cheers,
Avi Adin
LLBG

emerydc8

Quote
So the mode logic provides the following functions:

FLCH SPD descent:
• retarding to idle stop: THR; reaching idle stop: HOLD.
• retarding to target above idle: THR; reaching target: THR (or maybe HOLD?)
• any retarding: THR; overridden by 8°: HOLD.

So, have we decided whether it goes to HOLD when it sets some thrust above idle to descend in FCLH or does it just stay in THR?

Hardy Heinlin

I speculate this BA sim would go to HOLD.

emerydc8

I agree; although I am speculating too.