Re: Tire burn-out during landings

From:         Geoff.Miller@Corp.Sun.COM (Geoff Miller)
Organization: Sun Microsystems, Menlo Park, Ca.
Date:         05 Jan 93 00:24:08 PST
References:   1 2 3
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In article <airliners.1992.194@ohare.Chicago.COM> 
(Robert Dorsett) writes:

>We need to consider three issues: (1), the means by which the tires get 
>"spinning," (2) the actual control benefits by having the tires spinning on 
>touch-down, and (3) the *additional* wear and tear on the brakes, as they 
>must absorb the spinning energy, in addition to performing their normal 
>task of slowing down the airplane.  We could also add a (4), having the
>wheel assemblies spinning at high speed for extended periods of flight
>(outer marker to completion of roll-out), with the ramifications on the
>wheel structure (for one thing, a balancer to stop in-air "wobbling" would 
>be needed).

Regarding item (3), why would there be any additional wear and tear on the
brakes?  The brakes don't come into play until after the tires have made
contact with the runway, by which time the wheels wouldn't be spinning any 
faster than they would've been *without* the spin-up system.  Spinning up
the wheels during approach would only lessen the disparity at the moment
of contact between rubber and pavement, not afterward.

Consider the sequence of events: the rotational speed of the tires at the 
instant after they touch the pavement is a function of the plane's ground-
speed, not how fast the tires were turning at the instant before touchdown. 

As for item (4), I'd think that keeping the wheel/tire assemblies above
some arbitrary threshold of dynamic balance would be trivial.  This could
be checked at some specified interval, say, once a week during an overnight
maintenance period.  It's not very time- or manpower-consuming to jack up 
one set of landing gear at a time and pull the wheels off, although I'm
sure it would add up.

(As an aside, I see a lot of references in various publications to coded
maintenance periods such as "C" and "D" checks, but even in enthusiast
magazines written for the layman, there's never any elaboration given.
Could someone post a list of such inspections, along with a general rundown 
of their frequency and what they consist of?  Are they standard across the
industry, or peculiar to individual manufacturers and airlines?)

>(3) seems the major disqualifier of the idea.  With an inert tire, you'll 
>have *minor* control problems ("bump", and that's it), but the energy absorbed 
>by the tire in *spinning up*, on landing, in itself helps slow the airplane.  

Hmmm; I hadn't considered that.  Still, I wonder how significant that energy
really is, compared to the total energy that must be absorbed to slow an
airplane down after landing.  It sounds kinda like the amount of energy
that's dissipated when a speeding locomotive runs through a sheet of Kleenex.

>The current system is obviously cost-effective enough to be used.  I don't
>have stats on tires handy, but the airlines do get a lot of wear out of them.

That's a fact.  I don't recall the specifics, but I remember being surprised
as an FE to learn just how much exposed cord the airlines allow before they 
consider a tire ready for replacement, compared to what I'd have downed a 130
for.  I'd have expected the airline standards to be higher than those in the 
military, not more permissible.  On the other hand, the military doesn't
have to worry about making a profit...  :)


Geoff Miller			+ + + + + + + +        Sun Microsystems
geoffm@purplehaze.Corp.Sun.COM	+ + + + + + + +     Menlo Park, California