Reverse Thrust
 

Hello, hopefully 'security' wont move me on as I am not a pilot, but I have a question I need answered if anyone can help me?

On say a B737 or an A319 what % of the braking action upon landing is provided by reverse thrust under normal (dry) conditions?

I realise there are probably many variables to be taken in to account with this question, but if anyone could point me in the right direction that would be appreciated.

Long runway and not limited of short turnaround we use thrust reverses idle and let the wheelbrakes take the most of the load.
(737 with carbon brakes and cooling fans)
Short turnarounds and limited runway length we use thrust reverses for what it's worth (not much on 737 with cfm engines)
The jurrasic vertion with clamshell reverses were much more effective.

Yes, a lot of variables but the simplest answer (for an A320) is a decrease in the landing distance required of between 2-8% on a dry runway. Main variables are:

a. Landing Config 3 or Full
b. With or without autobrake
c. Autobrake setting (Low/Medium) if used.

Hope that this helps,

YS

A couple of points.

Firstly it all depends on how much reverse you use. That sounds obvious but it's not clear in your question.

Secondly, autobrake settings are pre-programmed deceleration rates. The landing distance is not affected by the use of reverse.

The only variable is how much reverse is used which causes the braking effort to alter accordingly.

Thanks Phil H, Yellow Sun and FlapsOne,

Appreciate your help. This question came around whilst having of a couple of beers with a friend. He was fairly insistent that reverse thrust accounts for more than 50% of the braking action on a typical landing roll. I was sure it was much less than this, say 20% or so. I was of the impression that whilst the noise is impressive, the brakes were by far the major factor in slowing the aircraft to taxi speed (under normal, non time constrained turnarounds). From what I have read (graphs and the like) it isn't too easy to work out a rough percentage of brakes vs reverse thrust under any preset (autobrake, etc) braking conditions.

I can well understand that this question is odd in that I am asking for percentages when there isn't one answer, but that's us non-pilots for you!!

I also realise that I've probably not managed to clarify my question at all and for that I apologise.

I would say for most landings that 20% is much nearer the mark although if you are going to the end of a long runway you may use no wheel braking at all and let reverse idle and drag slow you down. In terms of the total available braking capacity however reverse thrust accounts for a very small proportion.

At the risk of sounding pedantic...
 

...the term "braking action" refers specifically to how much traction the tires can manage on a given surface.

It would be a true statement to say there is "aerodynamic" braking and "mechanical" braking, but even aerodynamic braking refers to the drag produced by the "flaring" of the aircraft and not so much the reverse thrust of the engine.

I think the term you're looking for might be along the lines of "effect of deceleration".

The truth of the matter is this: Reverse thrust is more effective at high speeds and wheel brakes are more effective at slow speeds. So in reality, on a normal landing, most of the deceleration just after touchdown is thanks to the reverse thrust (no brakes). But later in the landing roll, most of the slowing is thanks to the wheel brakes (stow the reversers).

I hope that makes sense.

Thanks Max Angle and zerozero,

Possibly a dumb question, but to help me reinforce all this to my mate am I correct in thinking that you guys are all pilots?

And yes zerozero it was understood.

Norks
 

I've been called worse, but yeah, I'll admit it. I'm a pilot.

:8

Ok... My take on this....

When we are talking about a max energy stop, then reverse thrust will not contribute significantly to the landing distance on a dry runway. On a wet then contaminated runway, then the contribution becomes proportionately much higher...

Short turnaround... Tend to use autobrakes off and 'Normal reverse' till we are around 60kt and then follow through with the brakes to a taxi speed. So question is in that case, what percentage of braking is done with reverse? I would suggest quite a bit. I don't think you can ever put a generic figure on it, because it so depends on the piloting technique used, the slope of the runway, wind component, weight etc.

FlapsOne just brought me into the 90's:Does this mean that autobrake raw data comes from an accelerometer? Or from wheel rpm's?

Or does it depend on the aircraft details?

Autobrake settings are pre-programmed decelerations rates indeed.So,when using long rwys,and braking 1,with reverse idle or more,almost NO braking is applied on wheels,the deceleration rate is reached with reversers only.
You can check this also,by monitoring brake temp in such a case.Of course,as speeds decrease the efficiency of reversers decrease also,so much braking force applies on wheels.
You may try this by selecting no autobrake,deploying reversers and monitoring deceleration rate.It will be enaugh on a long ,dry rwy.From around 80kts you should apply brakes also,due to reversers efficiency.

My point is that deceleration (= negative acceleration) is measured somehow and used as a control parameter in a closed-loop system - eh? Seems like an excellent idea for "performance-driven" landings.

And if it's such an excellent idea for landings, isn't there some thought to the same concept for takeoffs? I.E. some kind of acceleration measurement used to insure that mis-set thrust, TOGW error, dragging brakes etc. don't create a sub-par takeoff?