744 Autothrottle Disengage During Reverse Thrust

[NSEU]

Hi, folks.
I've read that the A/T is disengaged during Rev Thrust ops. However, I am unable to find a link between the reversers and the A/T (FMC). I've spent a few hours looking through schematics, wiring diagrams and training notes.

I'm wondering if:
1) there is a physical link in the thrust lever switchpacks between the reverser levers and the A/T disc switches
2) there is an obscure link in the WIU's between the Engine Accy Box and the FMC
3) the EEC's tell the A/T the engines are in reverse (perhaps via the EIU's)

I'm running out of ideas.

Thanks.

Cheers.
NSEU

[NSEU]

Some more possiblities...

The EIU's send the FMC
1) flap position data ,and
2) thrust lever resolver angle (thrust lever RVDT angle). This signal is used in the logic for engine trimming, but I guess it could have other functions, too.

I'm not sure what flap position data is given to the FMC, but during reverse ops, Group B (I think) LE flaps are retracted.

Cheers.
NSEU

[Short_Circuit]

Flap position is supplied to the FMC for
MAX manoeuvre SPD,
MIN Buffet SPD and
Flap Speed Schedules, and probably more.

Thrust REV is supplied to the FMC via EIU,s (can only assume) for T/R ops without disengaging the A/T (GA ARM but in a dormant Mode being at IDLE stops + time) the BMM does not go into enough detail.

Thrust lever resolvers send all thrust commands to the engine EECs via electrical data as there are no pulleys & springs anymore, just smoke & mirrors.

[NSEU]

Flap position is supplied to the FMC for
MAX manoeuvre SPD,
MIN Buffet SPD and
Flap Speed Schedules, and probably more.

True, but the question is whether this LE/TE flap data from the EIU's is detailed enough for the FMC to realise that half the LE flaps are not in the fully extended position.

In the absence of other data exchange, I'm starting to think more and more that the A/T uses TLA/TRA info for recognising rev thrust.

Thanks for the feedback.
Cheers.
NSEU

[Jetdoc]

You might want to look in ATA 76 Engine Controls and see if you can find the microswitches S181 thru S184. Check to see how they interface with the autothrotlle system. You might also have some luck looking in ATA 34 under FMS Thrust Management. The drawings and schematics in my incredibly tempermental Boeing PMA will not open. Anyone got a B747-400 AMM in pdf?

[NSEU]

You might want to look in ...

This was the first place I looked There are no obvious links in the schematics or the wiring diagrams.

T'is indeed smoke and mirrors

[h3dxb]

Like I remember the function, when Rev Lever is pulled >6 deg and the Thrust Reverser switch is activated.
> Switch controls the thrust reverser isolation VLV solenoid and
> inhibits the control input from EEC/TCM discrete cards to the ECU.
The ECU takes over now and controls the thrust according to TAT/T12 and P0

Cannot remember if the A/T disconnect switches are activated when the lever is pulled to REV. or they only pulled by flightcrew

rgds

[NSEU]

"> inhibits the control input from EEC/TCM discrete cards to the ECU."

Sorry, I'm not sure I understand this. Are you referring to the trim function?

"Cannot remember if the A/T disconnect switches are activated when the lever is pulled to REV. or they only pulled by flightcrew"

Same here. I'm not sure if the crew are required by procedure to manually disconnect the A/T with the switches, but since my training notes say the A/T is disconnected by the reversers and since the reversers are pulled immediately after landing, it seems pointless manually disconnecting the A/T (during a full autoland).

Rgds.
NSEU

[Short_Circuit]

I think we all have overlooked the obvious.
During land the A/T is in SPD mode. SPD mode is not avail on GND. Therefore it will disengage on TD as a function of Air/Gnd inputs

[SMOC]

I think we all have overlooked the obvious.
During land the A/T is in SPD mode. SPD mode is not avail on GND. Therefore it will disengage on TD as a function of Air/Gnd inputs.

But, during an autoland, SPD becomes idle at 25' RA and then disconnects with selection of reverse thrust, it also re arms 10sec after the reversers are stowed again.

[pilotindia]

auto thrust

guys my understanding is that the moment the trhust levers are brought to idle the autu thrust is disconnected....and since you can't go to reverse without going thriugh idle///VOILA....here's some more info that i learnt on the sim...on an autoland..if the thrust levers are not brought to idle at 10 feet( reminder only) and are kept up at the climb gate the auto thrust remains engaged...and it is the auto thrust which gets thtust down to idle....I tried this down to 50 kts with the thrust lever at the climb gate....the thrust remained at idle...but the moment i disconnected the A/P the thrust went up to climb ,,,the auto thrust remained active.

[h3dxb]

According AMM 34-61-00/001 9. Thrust Management it says under B(3)
that A/T is only engaged when engage logic is true, so on GRD at REV is no logic engaged, like GOAROUND or SPEED or VNAV.

But couln't find a simple logic or input table for it, looks like software (they had already software ? )

Was thinking before when the Reverser switches in throttle pack are activated, comes a signal for disengaging ,but isn't.

rgds

[NSEU]

"if the thrust levers are not brought to idle at 10 feet( reminder only) and are kept up at the climb gate the auto thrust remains engaged..."

Climb gate? Sounds very Airbus-ish?

[spannersatKL]

Also on the RR engine variety the A/T is an EPR mode and with the engine in T/R mode its in N1 mode so no Auto thrust....Also who would want or need Auto Thrust in Reverse!!!!

[Jetdoc]

You've really chosen a tough question to answer NSEU. I am confident in saying that on the Classic 747s, the switches in the A/T switchpack disengaged the A/T, howevr, I am stumped on this one. I can only suspect that the Trust Lever Angle which is transmitted to the FMC probably provides the disengage logic.
I am fairly confident that this problem will be solved before the climb gate is found though.............

[Short_Circuit]

After reading all BMM, SSM, WDM Ch 22, 31, 34 & 76 until my eyes bled, there is no input to the A/T system that will command any change of logic when T/R is selected. (Especially when selecting T/R the EPR reversion mode is entered N1 control, as stated earlier on the RR).
The EIU's will receive and I quote from BMM:

G. Engine Interface (Fig. 68)
(1) Digital information such as EPR, EGT, rotor speeds, thrust reverser
position, engine rating and oil data is received by the EIUs on
ARINC 429 data buses from the Electronic Engine Controllers (EECs).
This data is processed for display on the EICAS primary, secondary
and compacted formats, and the EICAS performance, EPCS, engine

exceedance and configuration maintenance formats.

During LAND the A/T is in SPD mode until 30ft when it enters flare IDLE mode and becomes dormant. G/A ARM mode is enabled awaiting a TOGA input to go into G/A ENG mode. If TOGA is initiated and flaps are still deployed T/L will advance, there is a good chance it will increase thrust even during R/O & taxi off the RWY (something to try in the sim thanks guys).
After this I will give up

[NSEU]

"After reading all BMM, SSM, WDM Ch 22, 31, 34 & 76 until my eyes bled, there is no input to the A/T system that will command anything when T/R is selected. "

Not sure what you mean by "command anything"

Did you see my earlier comment about the FMC (which is the A/T on the 744) receiving thrust resolver angles? TRA's below 34.88 are reverser angles. This value could be used as a reverse thrust signal.

[Short_Circuit]

I'm wondering if:

1) there is a physical link in the thrust lever switchpacks between the reverser levers and the A/T disc switches

no (ref BMM & WDM)

2) there is an obscure link in the WIU's between the Engine Accy Box and the FMC

no (ref BMM & WDM) See 3) below

3) the EEC's tell the A/T the engines are in reverse (perhaps via the EIU's)

Maybe (ref BMM)
TRA data to EIU’s, from EIU's to FMC for "The base EPR value corresponding to the engine with the second highest thrust resolver angle position".(EEC trim) & T/L position for A/T servo loop.

Electronic Interface Unit (EIU) (Fig. 13)
(a) All engine related data to and from the FMCs are routed through
the EIUs. The high speed data consists of ….. & Thrust resolver angle (TRA) .....

[NSEU]

http://www.pprune.org/rumours-news/3...n-icn-33r.html

Interesting...

[Short_Circuit]

Interesting indeed!
It puts this thread to bed.