Re: A320 braking methods

From: (Robert Dorsett)
Date:         22 Oct 93 01:05:26 PDT
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>> Wheel brakes are how the airplane is stopped.  Many people think the
>> thrust reversers play a large role: it's marginal, not affecting roll-out 
>					^^^^^^^^^^
>> distance by more than 10-20%.
>	Robert, is that a quote or your opinion?  The reason I ask is
>	that this is the second time, I think, that I read this, and
>	the earlier time already I had wondered to myself:

Well, that specific figure was from Webb's _Fly the Wing_.  

But let's just consider empirical evidence.  I'm an avid airplane-watcher.  
737's at Mueller typically complete their turn-off within 25 seconds
after touch-down.  Figure a touch-down speed of 135 knots.  That's 228
feet per second.  Figure four seconds (typical) for selection and actuation of
thrust reversal--pilot's first responsibility upon landing is to keep the
nose pointed in the right direction; this takes time.  So does smoothly
spooling down, selecting reverse thrust, and advancing the throttles again.

By the point of selection, our airplane's traveled at most 912 feet (at 
most, because wheel brakes are much more responsive, and would be applied
within two seconds after touchdown).  The engines must be cut down to idle, 
and then accelerated.  That's another three to four seconds, and we're 
probably down to around 90 knots, up to ~1500 feet of pavement, 7 to 10 

Probably takes another 7-10 to slow down to 40 knots.  But thrust reversers 
are stowed at 65 knots, due to the danger of compressor stall and particle 
ingestion.  So just with this back-of-the-envelope calculation, we're looking 
at an interval of 30-40 knots that the thrust reversers can "help out" on.  
How effective are they? Well, figure 30,000 lbs of thrust on a twin, landing 
weight of 120,000 lbs.  Assume 80% power (I think Webb says 65%), at a 60% 
efficiency.  That's a net yield of 16,800 lbs of thrust, or a deceleration 
rate of 4.48 ft/sec^2.  Or 1.36 m/s^2, which is 80% of the brakes-only 
antiskid threshold of 1.7 m/s^2 on the A320.  And you won't be getting
30,000 static lbs of power as a reference, since there's a power loss 
as the airplane moves through the air.  Maybe 5% less, but we'll ignore

But again, this is only in a 40-knot interval.  Distance traveled, assuming
uniform 3.4 m/s^2 deceleration during this interval, starting at 100 knots,
ending at 60 knots, is only around 807', or 6 seconds of time.  So out of a
landing roll of 3000' or so (regulatory runway landing distance of (1500' + 
3000)/0.6 = 7500', so this is probably a bit high), the thrust reversers 
are only responsible for around 50% of that 800', or 400', or 13%.  On a
wet runway, in this interval, the "contribution" figure MAY be a bit higher, 
with inop antiskid.

Clearly, thrust reversers aren't HUGELY relevant.  Some reverse thrust is
going to be generated during the acceleration phase after door deployment,
so let's double it's contribution: 26%.  But this is all gravy: IF the
airplane lands where it's supposed to, we have a large amount of runway left
over for BRAKES-ONLY deceleration.  The thrust reversers will merely serve
to increase the margins, raising the "excess" pavement length by 26%.  

No thrust reversers?  You still have 166% normal touchdown distance to 
complete your brakes-only roll-out.  The penalty of NO thrust reversers
is, therefore, nonexistent, as far as the standards go.  

This is all back-of-the-envelope, your mileage will vary.  But this is all
best-case, at sea level.  Throw in a hot day, high altitude situation, and
the influence of the thrust reversers will go way down.  The effectiveness
of braking won't change as much.

>	how many accidents that had planes overrun the runway, could
>	have been avoided with an additional "10 to 20" of ...
>	(you know what I mean: breaking power, runway, whatever..)

How many accidents  could have been avoided if 400% reverse power had been 
available after the pilot touched down 70% down the runway?  Such questions 
aren't very relevant.  Landing distances have generous amounts of error 
built into them.   One might as well ask why we allow airplanes to brake 
under their own power, and don't just use arresting gear and tailhooks to 
stop the airplane within a few feet, no matter WHERE one lands on the 
runway.  :-)  

Thrust reversers aren't magic cure-alls.  They're meant to be used in
specified ways, and are subject to both mechanical limitations and the 
laws of nature.

I think a good chunk of your reasoning (and similar thoughts have been
expressed on the net) is that thrust reversers make a HUGE difference in
roll-out.  They don't.  They LOOK awfully impressive from seat 35A, along
with the ground spoilers.  But take along your stop-watch on your next 
flight.   :-)

>	my point is that I see no good reason to describe it as "marginal"
>	and wonder if it is a good idea to do so (no criticism intende
>	but rather plain puzzled)

I hope the above clarified things somewhat.


Robert Dorsett