smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

No Slope Correction For Certain Autobrake Setting

Sun Jan 27, 2013 5:21 pm

On the inflight landing calculations for the 737-800 for a Dry runway with Auto brake 3, there is no corrections to apply for the slope of the runway.

I believe we only accept 2% up/down on a runway but there are corrections for all the other auto brake settings, just wondering why nothing for AB3?
 
Max Q
Posts: 5628
Joined: Wed May 09, 2001 12:40 pm

RE: No Slope Correction For Certain Autobrake Setting

Mon Jan 28, 2013 1:17 am

Why would you need a slope correction for an autobrake setting ?


The Autobrakes provide a predetermined rate of deceleration for each setting not affected by slope.
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tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: No Slope Correction For Certain Autobrake Setting

Mon Jan 28, 2013 1:56 am

Quoting smartt1982 (Thread starter):
On the inflight landing calculations for the 737-800 for a Dry runway with Auto brake 3, there is no corrections to apply for the slope of the runway

Strictly speaking, there is a correction, it just happens to be zero for that particular combination. For the same slope on other runway surface conditions, it's non-zero.

Quoting Max Q (Reply 1):
Why would you need a slope correction for an autobrake setting ?

I was wondering that too, but I think it falls out of the way landing distances are calculated...they come off actual data and, with slope, if you cross the threshold at the correct height your touchdown point is going to move a little bit. The roll distance from the time the brakes come on shouldn't change with slope for autobrakes, but the distance from the threshold to the start of braking will change.

If you look at some of the data, there are obvious trend reversals, which wouldn't happen if you were doing analytic solutions but can happen if you're generating the data with actual performance landing data.

Tom.
 
vikkyvik
Posts: 11760
Joined: Thu Jul 31, 2003 1:58 pm

RE: No Slope Correction For Certain Autobrake Setting

Mon Jan 28, 2013 8:49 pm

Quoting tdscanuck (Reply 2):
I was wondering that too, but I think it falls out of the way landing distances are calculated...they come off actual data and, with slope, if you cross the threshold at the correct height your touchdown point is going to move a little bit. The roll distance from the time the brakes come on shouldn't change with slope for autobrakes, but the distance from the threshold to the start of braking will change.

That's certainly true. For a standard 3 degree glide path, 50 foot TCH, and discounting flare, your touchdown point will move like so:

% upslope - touchdown point (feet from threshold)

0.0 - 954
0.1 - 936
0.2 - 919
0.3 - 902
0.4 - 886
0.5 - 871
1.0 - 801

(that's of course assuming the main gear is at 50 feet when crossing the threshold)

As far as autobrakes go, with a constant deceleration rate, the brakes simply won't have to work as hard if you're landing on an upslope. They'll have to work harder if you're landing on a downslope.

Is that the basic gist of it?
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