Braking Distances

[RNP]

Hello all,

Question on braking distances with and without use of reverse thrust with autobrake.

Assuming a dry runway (where there is no environmental advantage for reverse thrust), with the use of no reverse thrust, idle, 2nd detent or max reverse....in theory should the distance covered on the runway be the same?

In this scenario will the autobrake system not work on a rate of descent and work harder with less reverse and not so hard with more, therefore covering the same distance?

[Clandestino]

Answers might be type specific.

A320 autobrake always uses just enough braking to achieve the desired decceleration rate (on landing, at least). So more reverse means same stopping distance but less brake wear.

[Intruder]

Depends on the aircraft.

On the 747 all autobrake modes except MAX/FULL/RTO will give a programmed deceleration rate. On a dry runway using normal modes, the brakes will work harder without reverse thrust, but landing roll will be the same. In MAX/FULL/RTO mode, braking distance will be reduced a bit with reverse thrust.

I cannot think of a case where at least idle reverse would not be used...

[Sean Dell]

Unless brake temps hit around 200 degrees - where brake wear is increased! Keep them less than 200 or greater than 300!

[safetypee]

RNP, assuming that I have correctly understood your question, then ‘in theory’, your theory would be correct. :)

However, I suspect that there are several embedded assumptions which would invalidate it in practice.
For nil, or low levels of reverse where the component of deceleration from reverse thrust is less than the commanded auto-brake level, then the auto-brake will control the value of deceleration and provide the same distance.
This assumes that the level of auto-brake deceleration is indeed constant. It might not be depending on brake system characteristics and/or capability, e.g. the level of braking deceleration might reduce with speed (but then the low level of reverse might make up this shortfall).
Higher reverse thrusts might create a component of deceleration greater than that required by auto-brake and thus the runway landing distance will be proportionally shorter depending on the level of reverse used and/or auto-brake level selected, but this depends on how the effectiveness of reverse reduces with speed.

The above does not considered the any aerodynamic effects, or the runway surface; even dry runways have a range of surface characteristics and frictional qualities, any one of which would change the runway landing distance.

Thus I suspect for the braking question, like many things in aviation, there are few ‘absolute’ theories; the important aspects are to understand the assumptions within the ‘theory’, and ask the ‘right’ questions in operation.

See here for info: FSF ALAR Tool Kit Briefing Notes Chap 8.4. Braking Devices. (www.flightsafety.org/alar/alar_bn8-4-braking.pdf) Figs 3 & 4 indicates some of the variables.
Also see: Flt Ops / Flying Technique … “Landing on Slippery Runways”. (www.smartcockpit.com)

[Intruder]

Unless brake temps hit around 200 degrees - where brake wear is increased! Keep them less than 200 or greater than 300!
I don't know a single pilot who attempts to target brake temps -- or, for that matter, one who KNOWS how to target brake temps on landing roll!

The closest I can come to your theory is the FACT printed in our 744 FHB that with Autobrakes 1 selected, full reverse will cause brake modulation to full release, causing excessive brake wear.

Of course, that doesn't apply to ANY airplane with steel (vs carbon) brakes...