The headlines never stop giving...(apparently I used this title before).

[Gabriel]

Again, the braking in this situation is fully automatic.

Well, it requires the pilot to retard the throttles. That not only reduces the thrust to idle but also activates the brakes in RTO mode and extends the spoilers to full.
About overriding, I can be wrong but I thought that you had to arm the autobrakes to RTO before take-off for them to kick in when you retard the throttles and that, once they activated, you can override it by turning the autobrakes selector to "off". I am not implying that any of this has anything to do with this incident.

[3WE]

Not sure what all the nit picky debate is...

What NEAR-TOTAL-DISASTER Movie doesn't include a sweat-laden brake application where the stop is- at best- at the very very end of the runway.
The media citing that a passenger noted strong braking...umm, yeah, as Brian said...isn't that SORT OF what you WANT to happen.

As to whether the brake PEDALS were used...nice details to know, but that doesn't really change the bottom line...I would guess that at some point when the plane had gotten nice and slow, and turnoff was available, the autobrakes might have been shut off.

Interestingly- truck drivers have a procedure of intermittent braking- supposedly allowing air in-between the brake pads and the drum allows somewhat better cooling...Looks like airplane brakes have so much crap sandwiched together that once you get it hot, there isn't that much air / air flow to cool things down very quickly...Nevertheless, if you see that you can make a fat, dumb and happy stop, are we out of line to think that the pilots might want to switch off the brakes and coast a bit?

[Evan]

Now that I reread the avherald article I would say although I do not have brake cooling charts for that aircraft, that at least 1 hour of the 3 hour delay was for brake cooling.

And one hour for tires from what I hear.

[Evan]

About overriding, I can be wrong but I thought that you had to arm the autobrakes to RTO before take-off for them to kick in when you retard the throttles and that, once they activated, you can override it by turning the autobrakes selector to "off". I am not implying that any of this has anything to do with this incident.

You can override it by depressing the brake pedals manually.

[Gabriel]

You can override it by depressing the brake pedals manually.

I heard something like that but I didn't want to mention because I don't know how it works.

I know that, in a landing, if you set up the autobrakes to 1, 2 or 3, you can brake harder than what the autbrakes is giving you (as long as the antiskid isn't already antiskidding) by pressing the rudder pedals beyond the point of braking that the autobrakes is already giving you. In that way you are very literally overriding the autobrake.

But what happens if it is set to MAX? You are not going to get any more braking by pressing the brake pedals to the bottom.

Will pressing the brake pedals deactivate the autobrakes? (so if you release the pedals there is no more braking)

And will all this still be true for the RTO setting?

[Evan]

Will pressing the brake pedals deactivate the autobrakes? (so if you release the pedals there is no more braking)

And will all this still be true for the RTO setting?

AFAIK, yes, manual braking informs the system that you want to brake manually. It also makes perfect sense since RTO autobrake is only there in case you don't get around to manual braking fast enough.

Maybe BoeingBobby has some first-foot experience with this...?

[BoeingBobby]

AFAIK, yes, manual braking informs the system that you want to brake manually. It also makes perfect sense since RTO autobrake is only there in case you don't get around to manual braking fast enough.

Maybe BoeingBobby has some first-foot experience with this...?

Read my post to Gabe, you cannot override RTO brakes. At least in the 74 you can't. I am sure that Boeing uses the same system in all of it's aircraft. But I have been wrong before. And I have no idea at all in the Bus. ATL can maybe chime in.

[BoeingBobby]

Well, it requires the pilot to retard the throttles. That not only reduces the thrust to idle but also activates the brakes in RTO mode and extends the spoilers to full.
About overriding, I can be wrong but I thought that you had to arm the autobrakes to RTO before take-off for them to kick in when you retard the throttles and that, once they activated, you can override it by turning the autobrakes selector to "off". I am not implying that any of this has anything to do with this incident.

[ATTACH=CONFIG]16765[/ATTACH]

Yes they have to be armed to work. I am not sure that you can reach down and turn them off manually while they are engaged or not. But I will make a call tomorrow and find out for sure.

[Gabriel]

Yes they have to be armed to work. I am not sure that you can reach down and turn them off manually while they are engaged or not. But I will make a call tomorrow and find out for sure.

So how would a medium speed rejected take-off be performed from a procedural point of view. Let's say that you are in a 12000 ft runway and at say 80 knots you retard the thrust levers. Are you "condemned" to a max braking run from that point all the way until you are stopped on the runway?

Honest question.

[Evan]

So how would a medium speed rejected take-off be performed from a procedural point of view. Let's say that you are in a 12000 ft runway and at say 80 knots you retard the thrust levers. Are you "condemned" to a max braking run from that point all the way until you are stopped on the runway?

Honest question.

No. RTO autobraking is inhibited below 90kts (wheel speed). But above 90kts it can be disarmed by manual braking, advancing the thrust levers or lowering the ground spoilers (I wouldn't recommend the last two, but...). This is according to my 2002 copy of the Boeing 737NG FCOM. Specifically, it indicates that the autobraking DISARM light (above the rotary switch in your picture) illuminates if you do any of these things. I take that to mean the autobrakes are no longer activated.

[Evan]

Read my post to Gabe, you cannot override RTO brakes. At least in the 74 you can't. I am sure that Boeing uses the same system in all of it's aircraft. But I have been wrong before. And I have no idea at all in the Bus. ATL can maybe chime in.

See my post above. Could be that different builds behave differently. Especially with the 737...

[BoeingBobby]

Just got off of the phone with a check airman from Atlas. I stand corrected. They can be overridden with the toe brakes or by lowering the spoilers (the preferred method, much more gentle). But he also reaffirmed what I said about the RTO itself. RTO auto-brakes in the 74 is 3000 psi right now. Max manual is no different and is probably a little bit lower.

[3WE]

...RTO auto-brakes in the 74 is 3000 psi right now...

This seems wrong...I would think that the brake pressure would vary some with the traction/speed/lift/weight/surface conditions?

ABS is generally a little bit more than full-on-full off, isn't it? (Especially on a big, shiny aeroplanie?)

Ain't no aerobrakeengineer, but have slammed on the brakes in a car or two.

[BoeingBobby]

This seems wrong...I would think that the brake pressure would vary some with the traction/speed/lift/weight/surface conditions?

ABS is generally a little bit more than full-on-full off, isn't it? (Especially on a big shiny aeroplanie?)

Ain't no aerobrakeengineer, but have slammed on the brakes in a car or two.

No matter what I seem to tell you it's wrong! Not only did this come from a check airman, he showed me in the books as well, so you believe what ever you care to. RTO is full hydraulic pressure (3000 psi in the 747) right now!

[3WE]

No matter what I seem to tell you it's wrong! Not only did this come from a check airman, he showed me in the books as well, so you believe what ever you care to. RTO is full hydraulic pressure (3000 psi in the 747) right now!

Understood.

Unfortunately there's two little problems:

1) I am a curious SOB. Curious as to how and why things work the way they do. (Apologies for that).

2) Traction and friction and braking action vary a lot:

-New, dry, grooved concrete and a heavy airplane

vs.

-Old, third-world, wet, slick asphalt (maybe with a little oil on it too) and a light load.

Something makes me think that it takes a whole lot less brake pressure to lock things up on the wet, slick, asphalt and a lighter load.

...and this thing called "Anti-Lock Braking Systems"...that prevents wheels from locking up where you lose directional control and braking efficacy- maybe it adjusts pressures too?

I just don't think there's 3000 PSI of pressure at the brake-actuating pistons for any and all RTO's...

...Maybe upstream from some brake-by-wire system there's 3000 PSI- and maybe that even tells the computer to give her all she's got available...but that is not a measurement of the actual braking effort.

If you cannot figure this out, I am sure Gabriel can give a much more verbose explanation.

That is all.