What If..... 747-400 Throttles

[QAVION]

Just wondering what would happen to the throttles on a 747-400 if they were (substantially) staggered when the TOGA switch was pushed on takeoff. Would the A/T servo only drive the throttles forward until the first engine had reached it's target?

Someone once told me that the servo stops driving when the second one reaches its target thrust, but if the stagger was substantial, wouldn't the first engine be overdriven? (Perhaps to the point where the EEC would have to intervene to stop the engine damaging itself).

Perhaps someone with some sim time coming up could help here? ;)

Thanks.
Rgds.
Q.

P.S... and would the same thing thing happen in the air if, say the A/T was engaged with the throttles staggered?

[ 11 December 2001: Message edited by: QAVION ]

[vheijens]

Let's see what the manual (AOM) has to say...

AUTOMATIC FLIGHT - Autothrottle system

When the A/T is armed.
The EEC (FADEC) provides thrust equalization, controlled through the FMS.
So when the TO/GA button is pushed on T/O the T/L will go forward till one reaches the target position. But is is the thing you see. Now internally the ECC will make sure there will be no assymetric thrust, even if the T/L positions vary.

If you trigger the GO levers (only when the A/T system is armed) the ECC will direct the engines to there assigned target and the A/T servo will move in the right position, giving the pilots a clue of what is happening (at an A340 the T/L don't even move enymore).

Now for the fans:
For take off:
- push the TO/GA buttons at an IAS less then 50kts --> this will engage (when armed) the A/T in THR REF and advances the thrust levers to the limit as indicated. (flaps pos. must be out of up)
- at 65kts IAS, A/T servo disengages and HOLD mode is displayed
- if not engaged by 50kts IAS, it cannot be engaged until above 400ft AGL.
-Second push of the TO/GA button in flight cancels any T/O thrust derate. THR REF remains displayed and EICAS displays full rated thrust limit:TO
During the HOLD (de-cludge) phase, thrust can be manually adjusted.

OK so far for the take-off. Maybe will some other guy post the inflight situation.

Hope to be of some help.

http://users.pandora.be/linda.basstanie/TopFly.gif

[QAVION]

"Now internally the ECC will make sure there will be no assymetric thrust, even if the T/L positions vary."

Thanks, TopFly, but I'm not sure that trimming will work in these circumstances (I was told that FMC trimming will not operate if the throttle levers are not within a T/L knob's width of each other(?). I don't know what the range of allowable trim is for when the A/T is engaged, but for when the A/T is only armed, the range is miniscule (for RB211's something like 0 to 0.05EPR maximum trim command, depending on airplane altitude and throttle stagger, according to my training notes).

I'm not even sure that trimming operates on the ground. The lowest I can see it happening is at 400' above the runway (in association with a thrust lever movement of 3 degrees). Doing this unlocks the EEC's from thrust hold mode) and allows the FMC to trim the engines. Or does the FMC trim the engines between 50 and 65Kts on takeoff?

Rgds.
Q.

[ 12 December 2001: Message edited by: QAVION ]

[VnV2178B]

QA,

as far as I can tell from the requirements for EPR trimming it will happen on the ground as the lock-up occurs at 65 kts.

The unlock has four conditions,
airspeed over 300 kts.,
altitude over 4500' above runway,
altitude over 400' and throttle moved by more than 3 deg. or
altitude over 400' and FMC commanded unlock.

That's looking at it from the EEC point of view, what happens to the throttles in the cockpit I don't know.

VnV...

[Cornish Jack]

Qavion
A couple of points re' the 744 throttles.
1. They are ALL driven by a SINGLE servo motor so if they are physically staggered they will remain so unless they are signalled to fully closed - they would then pick up equal positions again.
2. The throttle trim works to trim EPRs or N1s against the value set on the second highest engine. It is effective for a specific engine ONLY if its throttle knob is within half a knob's diameter of the remainder. For your hypothetical situation, the backed off engine would not trim.

As far as the potential overboost is concerned, there should be no problem if the EECs are in NORM as they will automatically prevent overboost and spool speed exceedance - throttle physical position doesn't come into it. The power set for GA will be whatever shows on the Primary EICAS as the GA reference thrust.

[QAVION]

"as far as I can tell from the requirements for EPR trimming it will happen on the ground as the lock-up occurs at 65 kts."

VnV2178B... By "as", I understand that you don't mean "at the time" lock-up begins. Your statement might be read in a couple of ways ;)

"2. The throttle trim works to trim EPRs or N1s against the value set on the second highest engine."

My training notes do mention this, but I wasn't sure if this also meant that the throttles would advance (in response to pushing the TOGA switch) until the second throttle lever reached target thrust.

"It is effective for a specific engine ONLY if its throttle knob is within half a knob's diameter of the remainder."

This is a key statement. I previously understood that all engines would stop trimming if any were outside the range.

By the way, by "half a knob's diameter", do you mean only a half knob stagger... or half a knob clear of the other throttles? The former would be very small indeed and would stress the need to line up the throttles properly before TOGA is pushed.

Anyway, thank you gentlemen... and, by the way, C.J., it's good to see you are still with us :D

Cheers.
Q.

[Cornish Jack]

Qavion
Just a couple more points re' throttle stagger. Yesterday's post was from my mental stockpile - what follows is from the books.
"Trim authority is limited to about 5% equivalent thrust below about 20,000' and 10% above. This corresponds to about +/-.03% EPR or +/-1.5%N1 (<20,000) and .05% EPR and 2.5% (>20,000)" (Boeing T.M.)
"For manual operation, trim is inhibited until the leading engine is within .1% EPR (5% N1) of the target thrust to allow intentional stagger during ground handling operations."
"Trim equalisation adjusts the trailing engine(s) thrust to match the LEADING engine up to the available authority limit and then downtrims the leading engine(s) when the difference exceds the authority"
"If the throttle lever of one engine is intentionally retarded beyond trim range (four times the trim authority level relative to the leading engine) that engine is removed from the trim function and trimming continues with the remaining three." (No trim is available with less than three operating)
Yesterday's point about using the second highest thrust as the trim reference (and the half knob diameter) came from one of the tech manuals but I cannot find it again. :confused:
So, to go back to the original question, how much stagger is "substantial"

Thanks for the welcome back, Q, - I still lurk, but less frequently. Redundancy's greatest reward is not having to get up at 6 o' clock daily. :D

[QAVION]

"So, to go back to the original question, how much stagger is "substantial""

Subconsciously, I may have been trying to say "beyond trimming capabilites", C.J :)

Thanks for the many more pieces of useful jigsaw puzzle.... I think that's well and truly sorted out the trimming issue....

BTW, is there anything in your notes about when the A/T servo will stop driving after pushing the TOGA switch (If so, does this agree with TopFly's description?).

Thanks.

Cheers.
Q.

[Cornish Jack]

Q
Yes, Topfly's description is correct and what is of most interest (for me, at least) is that the throttle HOLD activation is on IAS, not G/S. The point being, for instance, if you do a rolling take-off on a strong wind day, if the throttle servo is a bit on the sluggish side, you could well end up with throttle HOLD at less than reference thrust. Both my ex-company's SOPs had a requirement for a thrust check at the HOLD annunciation to cover the possibility and, were it necessary, thrust could then be manually trimmed up BEFORE reaching 80kts.

Cheers

[QAVION]

Many thanks to all.

Cheers.
Q.

[scanscanscan]

I think the 767 throttle hold at 80 Kts is obtained from the ins ground speed.
Also this ins 80kts speed arms the rto auto brakes.
Also the ins is inputed to the yaw dampers.
Possibly the 747 also uses the ins for throttle hold and other functions.

[QAVION]

(scanscanscan)

From the Boeing Maintenance Manual (Ch 22-32-00p33)

"When the N1/EPR mode on the MCP is engaged, the thrust levers are driven until the reference N1/EPR is reached. The T/O throttle hold (THR HOLD) mode deactivates the autothrottle servo by removing 115V ac excitation and zeroing the command when the airplane reaches 80kts CAS. This prevents the thrust levers retarding after T/O thrust is set"

Also, according to the Maintenance Manual, RTO must see 85 kts or greater. The simplified block diagram for the Antiskid/Autobrake Control Unit shows "average wheelspeed" rather than IRS for RTO. RTO is completely independent of the IRS's, that's why you can select RTO without the IRS's being aligned <img src="wink.gif" border="0">

Having said this, there are a variety of manufacturers producing these computers. There are some differences between models :)

Rgds.
Q.

[QAVION]

P.S.....

Those values quoted in my previous message, by the way, were for a 767.

Rgds.
Q.

[fantom]

throttles are part of carburettor assys.
airbuses are not fitted with such out-of date equipment. <img src="rolleyes.gif" border="0">

[scanscanscan]

QAVION...Regarding the throttle hold I did say I think!
However you could persist with the wheel speed sensors, antiskid links, and yaw damper links and also the Rat while we are trying to clarify just what is looking where and at what.
I do recall reading somewhere the ins are involved, hence the post.
I accept the 5kts correction, what pilots are told is often not exact maintenance manuals figures.
Thank you for your help on this.