Thread: What Happens When You Slam On the Brakes in a 747?

Originally Posted by stallie

It's not always a case of jumping on the brakes like in a car and hoping the ABS does its bit...

I'd have to check the manual in my bookcase for the Boeing 747, but for the Airbus, the pilots have a choice of three modes for auto braking, LO MED and MAX.

They are the settings on the 'classic' 747. If I remember correctly (it looks as if I've deleted the manual I had), the settings on the 747-4 were 1 through 4, then max. None of them actually give max braking, there's only one setting that does that...RTO...rejected take off.

MAX is armed for all takeoffs and used for aborted takeoffs and will activate above 72 knots - it provides the maximum stopping rate in the minimum distance and is extremely violent for the uninitiated.

RTO is armed for takeoffs, not MAX. The 72 knots is an Airbus thing. Below that speed the spoilers don't automatically extend in an abort, and it's the spoiler activation that trips the autobrake to RTO. Manual selection of the spoilers will cause RTO to activate down to 40 knots (and when active it stays engaged until the aircraft stops, or it's turned off).

LO and MED are pilot choices for landing and these command a deceleration rate of 2m/s^2 in Lo and 3m/s^2 in MED and modulate the amount of braking accordingly. I regularly land on a solid ice runway and the ABS can be heard clicking in and out and some days the light indicating LO or MED flicks off if the commanded deceleration is not being met.

There is no indication on any aircraft that I have flown (747-2/3,767-2/3, 747-4, or A380) of ABS activating. You can't feel it, nor is there any clicking.

On long runways, however it's common to land with no autobrake and coast to the end, sometimes with no reverse at all - especially if there's noone behind you. Saves brake wear and engine wear, but more importantly on tight turn around it keeps the brake temperatures down for a quick tunraround. They can take a long time to cool - and 300°C is the maximum brake temp for takeoff...

Company policy will control this. I would consider it foolish to land with no autobrake selected, though it may well be disconnected shortly after touchdown. Idle reverse should be used in all cases, simply to eliminate the residual forward thrust of the engines. A large engine at idle forward thrust is still producing thousands of pounds of push. For what it's worth though, reverse thrust does very little actual braking. Only about 4000 lbs of reverse thrust will be available from an engine that may well produce 60,000 lbs forward. Biggest benefit of reverse is that it blasts any water off the runway...and so allow the brakes to do their job.

Thanks for the clarifying the Boeing side of things, my comments related to aircraft from the other side of the Atlantic, however the principle still achieves the same goal, no matter what TLA the competing manufacturers use.

Re the speeds, doesn't the B744 have a similar speed (85kts) below which the RTO autobraking will not normally activate?

There is no indication on any aircraft that I have flown (747-2/3,767-2/3, 747-4, or A380) of ABS activating. You can't feel it, nor is there any clicking.

I've never heard it on a paved runway wet or dry, only on solid ice or snowcapped ice runways, and not every time. It depends on the friction on the day (and sometimes that varies by the hour).

As for landing no autobrake selected, naturally it depends on the runway aircraft and payload. I do it frequently given the routes, payloads (or lack thereof) and usual parking positions we use. Horses for courses.

Actually, my comments were a mix of Airbus and Boeing. I've mostly flown B, but currently fly ze French.

Originally Posted by stallie

Re the speeds, doesn't the B744 have a similar speed (85kts) below which the RTO autobraking will not normally activate?

Probably. I can't remember, and my manuals haven't survived the move to new computers. I'll have to track some new discs down next time I go to work..if only for sentiment's sake.

I've never heard it on a paved runway wet or dry, only on solid ice or snowcapped ice runways, and not every time. It depends on the friction on the day (and sometimes that varies by the hour).

Where are you flying that you land on that stuff frequently? Anyway, you won't hear anything in any of the big jets I mentioned, even on icy runways.

As for landing no autobrake selected, naturally it depends on the runway aircraft and payload. I do it frequently given the routes, payloads (or lack thereof) and usual parking positions we use. Horses for courses.

And probably affected by aircraft size. In any event, banned in my company.

Originally Posted by jb747

Where are you flying that you land on that stuff frequently?

YWKS and NZPG in your current steed's second smallest sibling.

Originally Posted by stallie

YWKS and NZPG in your current steed's second smallest sibling.

Jesus...that's way too far from anywhere with only two engines. I'd don't like it down there with four....but then I've been known to need a rat hole in a hurry.

How is the runway marked?

That's way too far from anywhere with only two engines

You don't want to hear about the 12 hour ferry flight of our skiplanes there then For the 'bus though, it's 180 minute EDTO operation.

How is the runway marked?

With black flags. There is an non compliance raised against part 139 for not having numbers and white lines down the runway

Originally Posted by stallie

You don't want to hear about the 12 hour ferry flight of our skiplanes there then For the 'bus though, it's 180 minute EDTO operation.

So I guess you're waiting with baited breath to hear whatever happened to AF447?

With black flags. There is an non compliance raised against part 139 for not having numbers and white lines down the runway

Oh, I'm sure they're there. Just used white paint....

As a further development airbus now allow the pilots to select the exit point off the runway so that the brakes regulate the braking for the exit point.