747-400 Autobrake RTO Message Logic

[NSEU]

The 747-400 Aircraft Maintenance Manual tells me that "AUTOBRAKES" Advisory and Status messages appear if manual braking is applied during an (active) RTO.

The message is said to remain until the A/B switch is returned to OFF (some manuals mention a 45 second delay after selecting OFF). The A/B switch does not return to OFF or DISARM by itself in this situation. The manuals say that the RTO function will automatically re-arm for the next takeoff attempt. However, I assume that the next takeoff will not be attempted with the message in view.

Is it pilot procedure to cycle the Autobrake switch to OFF to get rid of the message prior to the second attempt?

Thanks!
Rgds
NSEU

[zerozero]

Hmm, I agree with most of what you describe except the selector will not automatically "re-arm" RTO before the next takeoff. All it knows is that automatic braking was canceled when the pilot applied manual braking (and therefore tripped to "disarm").

If another takeoff is attempted the pilot must manually position the selector (once again) to RTO. The logic has no idea if the plane is taxiing back to the gate or back to the runway. As the selector cycles from "disarm" to "off" the "autobrake" messages will go away and once RTO is re-selected "Autobrakes RTO" message will display once again for takeoff.

I think I got that all right, but if I didn't I'm sure someone will correct me.

Clear as mud?
:}

[NSEU]

Thanks for the response.

If another takeoff is attempted the pilot must manually position the selector (once again) to RTO. As the selector cycles from "disarm" to "off" the "autobrake" messages will go away...

But the switch does not trip with manual braking during RTO application. Only the autobrake valve solenoid is disabled.

The logic has no idea if the plane is taxiing back to the gate or back to the runway.

Well, a wheelspeed of 85 knots might be a clue. An 85 knot wheelspeed circuit in the BSCU has to be latched prior to RTO application :}

I'll see if I can come up with a quote from a manual regarding re-arming. In the meantime, perhaps someone has a sim session coming up soon? :)

Cheers

[NSEU]

P.S. Note that the Autobrake switch is mechanically unable to trip from RTO to the DISARM detent. There is an obstruction. This is why you have to push the switch when going from OFF to DISARM and vice versa.

[BBK]

Hi nseu

In my co we turn the rto switch from disarm to off during the after landing checks. That removes the rto message from the eicas.

For a further take off you would need reselect rto. Bear in mind the take off speeds and thrust derate etc would all need to be re entered into the fmc as well. In effect a further take off needs a full pre flight prep.

The only exception might be where training is being conducted and touch and go landings are flown. I believe in that case the auto brakes would be left off and thrust reversers are NOT used as they retract the leading edge flaps. Hope that helps.

regards

BBK

[NSEU]

Thanks, BBK. Manual placement of the switch seems to be necessary, but there still seems to be a disagreement between the Boeing Maintenance Manual and flight crews over what happens to the switch after manual brake application during RTO braking.

A few years ago a captain reported after a test "flight" that the Autobrake Switch didn't trip when manual braking was applied during RTO. Engineering simply wrote that this was normal as per the AMM.

As I said, the physical construction of the switch won't allow it to move from RTO to DISARM.

Cheers

RTO Disarm Logic (http://www.iinet.net.au/[email protected]/RTODisarm.GIF)

[zerozero]

Ah, I think I understand your questions now.

To be honest, I have no idea what the selector does on an actual RTO as I've never performed one in real life and I never noticed in the sim.

:uhoh:

But I'm headed back to the sim at the end of this month. If I remember I'll try it out and report back here. You might even wanna drop me a PM to remind me later this month.

And to be perfectly honest, in a real life RTO, once you're clear of the runway you're going to perform an "After Landing" checklist and that involves positioning the Autobrake selector to OFF. If you were to attempt another takeoff, as BBK noted, a number of things would need to be addressed.

[L337]

In the sim, after an RTO, when the instructor says... lets try that again....he has to re-arm the switch by moving it from OFF back into the RTO detent for the next take off.

[NSEU]

But I'm headed back to the sim at the end of this month. If I remember I'll try it out and report back here. You might even wanna drop me a PM to remind me later this month.

Thanks! Will do (If someone reminds me) :}

Either the maintenance manual is completely wrong or there are a few mistaken pilots out there ;)

[zerozero]

Hi again.

Back from the sim. Sorry I didn't respond sooner, lots going on.

Basically, things seem to work as we described. I'm not sure what the pilots are telling you on the line. It could be they're mistaken or perhaps the airplanes are rigged differently than the sim.

But during an RTO (in the sim), the selector stays in RTO until it is canceled with pedal pressure, then the selector moves to OFF (not DISARM). There is an EICAS message: AUTOBRAKES. That can be canceled with the 'cancel' switch. (i.e. not just moving the selector to OFF).

In order to re-arm for the next attempted takeoff, just move the selector back to RTO.

Not sure if that helps you, and sorry you had to wait so long for so little info. Feel free to follow up if you need more info.

Best,

[NSEU]

Very helpful, Zerozero ))

I'll try and find a contact in our sims and see if I can confirm this and then I'll make some formal enquiries (This could take many months with our system however :P). Eventually we should get a response from Boeing.

At the moment, there is a definite disagreement between sim and Maintenance Manual. Unfortunately/fortunately, it is not something super critical (the switch goes where it goes and pilots respond accordingly)

Many thanks for your help/enthusiasm )))

Cheers
NSEU

[zerozero]

Glad to know it was helpful.

Hope you get a quick resolution to the matter.

Best,
zz.

[NSEU]

Curious...

Admittedly, this is 3rd/4th party information, but a friend of a workmate who works in our 744 sims said that the switch did NOT go to OFF with manual intervention during RTO when he tried it.

Another pilot said he would get back to me on this also. If he confirms that the switch does not trip, then I can't pursue this any further (manuals agree with sim).

I would be surprised if this is some kind of airline option.

[spannersatcx]

simulators are simulators and not the real thing!

[NSEU]

Agreed, Spanners, but it's nice to know our sims agree with our manuals ))) Real RTO "test opportunities" don't come along too often.

Just had further confirmation that the switch does not move in our sims.

Cheers
NSEU

[gas path]

Real RTO "test opportunities" don't come along too often.

Oh I dunno! What with the number of -524 failures you've had down there in recent months.:} :E:E