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Thread: Which Exit?
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Old 2nd Aug 2010, 04:46
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Dan Winterland
 
Join Date: Jun 2001
Location: Blighty
Posts: 4,789
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Lots of factors.

Weight is an obvious one, but all aircraft will be down to their max landing weights in normal operations and in something like an A320 or a B737, there won't be much variation.

Also, the approach speed between the different types is relevant. A lightweight A330 will approach at about 125 knots. A heavy weight A321 will approach at 155 knots. As the landing roll is all about absorbing the energy on the landing roll, and the formula is Energy = Mass x Velocity squared, you will have to absorb four times as much if the speed is doubled.

The type of brakes are important. If the aircrat has carbon brakes, it will tend to stop quicker. Carbon brakes like to be worked, don't fade if they get hot and the wear is determined by the munber of applications, not how hard you apply them. A heavy classic 747 frieghter will use all the runway, whereas a much lighter passenger 747-400 with it's carbon brakes will stop by the first rapid exit.

The aircraft may have a tight turnaround, so brake cooling time before it can go again may be a factor. It may or may not have brake cooling fans.

Also, it depends on whether idle or full reverse thrust is used and whether manual or automatic braking is used, and if automatic, what setting. Although reverse thrust doesn't tend to have much effect on autobraking as the system tends to slow the aircraft down at a selected decceleration rate.

Airfield policy is also a factor. Busy airports have minimum runway occupancy times and ATC will want you off ASAP.

Also, there may be someone approaching faster behind. Or, you are parking at the other end of the airfield and there is no one behind and ATC don't mind if you "Roll through".


Lots of factors are involved. It's a bit of a "How long is a piece of string" question.
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