I was wandering how realistic the autobrakes were in the NGX?

 

Although it is not documented the auto brakes, depending on level set, acts in different ways, which is not just how much they kick in ;).

 

On autobrakes 2, the braking works at the 100knots and below mark, not above. So a smooth touch down with spoilers and idle reverse is common and then the brakes kick in at 100knots.

 

On autobrakes 3, the braking kicks in immediately. Ryanair pilots commonly use 3 the most, and go to manual braking at 100knots.

 

 

I read a post by Robert last year and from what I can remember [don't quote me on this] I think he said that reverse thrust has no effect on the stopping distance. If full reverse is used, then the brakes ease up a little? I have been told by a 737 captain just now that it doesn't work like that, full reverse will lower the stopping distance.

 

 

Cheers guys, all the best

 

Alex

As far as I understand it the autobrake setting commands a certain rate of decelleration, with a higher setting being equivalent to greater decelleration. If the reverse thrust is already providing most of the decelleration then the computer will ease up on the braking. Thus using a combination of reverse thrust and auto-brake won't change the stopping distance. Reverse thrust plus manual braking will decerease the stopping distance because in that case the rate of decelleration will be greater (unless you manually let up on the brakes once reverse thrust is applied of course).

Yes, as John-Alan said the autobrakes are a rate of descent and not a braking force. If you have autobrakes 1 and full reverse thrust the brakes will barely apply. To your question: yes PMDG has modeled them the same way they are in RW.

You should be able to try it yourself if you have some time over.Try to land with autobrake 1 and try with or without reverse thrust. You should be able to see a difference in the brake temperature indicators.

 

HTH

Manfred

Yes, as John-Alan said the autobrakes are a rate of descent and not a braking force. If you have autobrakes 1 and full reverse thrust the brakes will barely apply. To your question: yes PMDG has modeled them the same way they are in RW.

You should be able to try it yourself if you have some time over.Try to land with autobrake 1 and try with or without reverse thrust. You should be able to see a difference in the brake temperature indicators.

 

HTH

Manfred

 

This is what I thought also, but I just have been told otherwise by a pilot.. :/ confused

Maybe on other types... but the 737NG autobrakes work exactly so. There are decel rate numbers somewhere in the FCOMs...

Hey guys. I will try and remember these numbers from the top of my head. Here goes.

Auto brake 1 has a decel rate of 3 feet/sec, auto brake 2 is 4 feet per second and autobrake 3 is 7 (I think it's 7 but it might be 8). Reverse has an INITIAL decel rate of 6 feet/sec.

I explained this in a post many moons ago so I will do it again.

So if you have AB3 set, when you touch down the AB system is looking for a 7 feet/sec decel rate. So if you do NOT use reverse this is simple. You decelerate at 7 feet/sec. Now if you use full reverse, the reverse give you 6 feet/sec deceleration so the autobrake applies another 1 foot/sec to equal the required 7 feet/sec. Now as you slow down the reverse becomes less effective. So let's say at 100 knots the reverse is only giving 4 feet/sec. Well the AB system needs to apply 3 feet/sec to get the required 7 feet/sec. And of course as you slow further the reverse may only give 2 feet/sec so the AB applies another 5 feet/sec. It gets to the point where AB would be applying almost the whole 7 feet per second if using full reverse until you come to a full stop. Now in the real thing say at 60 knots, disengage the reverse thrust all together you will feel the AB really start to bite to make up for the sudden loss of reverse thrust.

Now if you were using AB2 which looks for a 4 feet/sec deceleration, when you touch down and use full reverse, you get 6 feet/sec so the AB system says "hey I don't have to do anything until I see less than 4 feet/sec. So like I said, as you slow down the reverse becomes less effective and when it gets to the point where it can only achieve 4 feet/sec, that is where to AB system will start to kick in to maintain the 4 feet/sec. The same thing happens in AB1 as it does in AB2.

Hope that helps!

JackColwill

 

Great answer, Jack. Thanks!