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ricfly744
27th Jun 2009, 17:45
Dear all,

I have been reading about performance for years, and still, there are a few gaps I need to fill, and even people who know much more than I do don't seam to know some specific points.
Recently a Boeing Performane Engineer came to our company for a presentation, It was for me a great opportunity to ask what I always wanted to know, and his answer was not so clarifying.
WHY MANY PUBLICATIONS, AND EVEN BOEING TAKEOFF AND LANDING PERFORMANCE PRESENTATIONS STATES THAT REVERSE THRUST HAVE NO EFECT ON STOPING DISTANCE ON DRY RWY WHEN AUTO BRAKES ARE USED (IT MAY HAVE EFFECT ON SLIPPERY RWY), AND WHEN YOU LOOK AT THE QRH UNFACTORED LANDING REFERENCE TABLES YOU CAN SEE THAT IT DOES, AS THE LAST COLLUM AT THE RIGHT FOR DRY AND WITH SOME AUTO BRAKE SETTINGS (DEPENDING ON ACFT TYPE) IS NOT ZERO.

So far I could not get a definitive explanation.
Is it the same for Airbus performance figures?

By the way, JAA requires the CRZ CG for performance calculation (CDU/FMC input that will generate MAX ALT information) to be in the most fwd figure (8.5 for the B744). We have a default of 20 in my company. What are you using? do you have a procedure to change the CG value in flight? Are you using 8.5 ?

Thanks a lot in advance for all good information and interest.

Ric

Mad (Flt) Scientist
27th Jun 2009, 21:04
There's a couple of links in this post (http://www.pprune.org/tech-log/267975-autobrake-usage-737-classic.html#post3226621) that talk to the topic of autobrake and TR interactions. Slides 23 onwards in the second item are illustrative.

Basically, autobrakes target a deceleration rate from all sources. So adding TR to the mix just means the brakes work less hard in some circumstances, making no overall change to the stopping performance.

BOAC
27th Jun 2009, 22:07
MFS - the poster was querying the logic for an addition to LDR with 1 or 2 revs locked out, dry.

ricfly - airlines differ on this as do Captains! Some airlines strictly forbid fiddling with it. It makes little difference in practice.

Mad (Flt) Scientist
27th Jun 2009, 22:37
Apologies - I thought he was questioning the first statement in caps, not the second.

The statement about the autobrakes targeting a decel remains valid, so the only way that they should be a factor in the final distance is if there is some portion of the landing roll where the AB cannot reach the target decel without TR "help".

Looking at the 737 info it does look somewhat odd, in that there's no TR penalty for AB3, the same penalty for both 1 or 2 TR inop for AB2, a different (much larger) penalty for AB1 and a penalty only for 2 TR inop for AB max. It has to be down in the details of how the system engages would be my expectation - I'll also note that the numbers are very small in all but the AB1 case (20ft and 110ft) which is getting close to the limits of the methodology for calculating stopping distances - if you compare the ref distances for the same AB selection but different braking actions, there are cases where even though the target decel is apparently available in both cases, the actual distance quoted differs - and by more than the 110ft we're seeing in the TR cases.

411A
28th Jun 2009, 01:45
It also makes a difference whether you are looking at UKCAA, FAA or others certification/performance data.

SuperRanger
28th Jun 2009, 02:07
i believe the 'additional' distance is due to the ramp-up delay for AB1 & AB2.

BOAC
28th Jun 2009, 08:08
the ramp-up delay for AB1 & AB2. - ok - why would that change with/without REV? I could accept that it may be down to the delay in the A/B sensing the reverse ie it might stay at a higher setting a little longer thus reducing the LDR with REV?

MFS - I have always assumed that it was, as you say, a number-crunching event from computer derived data and that B (and AB) had to publish it for liability reasons. I can see no reason for any addition below max.

Cough
28th Jun 2009, 08:20
Maybe at high speeds the deceleration with reverse thrust exceeds that generated by AB1 (and therefore the brakes are actually off initially - only come on later when the reverse thrust is less effective). The landing distance is less as you have been rolling out with the brakes partly off thus the autobrake has not controlled the deceleration.

Same for AB2 - less effect though.

A37575
28th Jun 2009, 11:32
Thrust reverser efficiency at high speeds is quite dramatically demonstrated in the simulator (B737-300). With a runway length of 3657 metres sea level standard temp and 61 tonnes a rejected take off using both engines at V1 150 knots using manual speed brake, full reverse and no brakes results in an overun with the aircraft going off the end around 45 knots ground speed.

Ditto same weight etc but using speed brake, RTO autobrake and no reverse results in stopping around 400 metres from the end of the runway.

Ditto with slippery runway -RTO autobrake -manual speed brake and no reverse results in the aircrft gently sliding off the end of the runway around 40 knots. In other words proving what we all know about longer stopping distance if the runway is wet.

The final reject with slippery runway, full reverse and manual speed brake and RTO autobrake results in the aircraft stopping with about 300 metres to spare. The point being the stopping effect with full reverse really comes into its own on wet or slippery runways. The faster and earlier you apply reverse on a wet runway the better for stopping distance. Not a bad demonstration for those new to jet airliners although obvious to the more experienced.

ricfly744
30th Jun 2009, 10:21
Yes, It is really a small detail that we all considerad very clear in our performance understanding and in fact there seams to be more to it. As we have observed, the numbers are "small" but they are there, and that goes against what we all had in mind.
How about the CRZ CG? anybody?

Thanks for your posts. I really lean a lot reading this forum.

Ric