Which Exit?

[donnlass]

When aircraft come into land at MAN (or any airport) some of a similar size ie A330's or even 747's end their landing roll and exit the runway at different points.

One may roll to the end of the runway and taxi past the Concorde Hangar while some stop short and exit beside Rompa.

How come there is such variation in where aircraft turn off? Is it due to their landing wieght, fuel load etc or do ATC direct them to turn off at these points?

Cheers.:confused:

[The_Steed]

I think there are a lot of different reasons why (just a SLF speaking by the way so please correct me!) such as...

- Size of aircraft (i.e. 747 needs longer to stop than a 737)
- Different styles of exits (i.e. high speed exit angled specifically away from the runway compared to a 'normal' 90 degree turn)
- Following traffic (i.e. need to get off the runway quickly)
- Fuel burn (presumably these days you would be looking for the shortest distance to the stand)

I remember landing at Edinburgh with Nortjet (back in the day) and in my opinion we were coming in quite fast. There was one exit from the runway about 2/3 of the way down and if you missed that you had to go all the way to the end, do a 180 and backtrack. The pilot must have realised that after landing as there was some impressive braking, some interesting noises from the brakes (I was sitting above the MLG) and everyone leaning forward with the deceleration. Dunno how much extra fuel he would have used if he had missed the turn, but I guess that's a reason too - $$$$$$$

[SNS3Guppy]

Generally, we turn off the runway when it's safe to do so, as opposed to being told to exit at a certain point. Not uncommonly, when air traffic control does request that we exit at a certain point, we will advise them which one will be using...it may not be (and often isn't) the one that ATC advised we should use.

A large airplane such as the B747 takes a lot of runway when heavy. Think about it this way: our center wing fuel tank, if filled, holds more weight than a loaded DC-9...and that's one fuel tank. With a 630,000 lb landing weight, a lot of runway is required, when we're heavy.

Brake energy and the heat we develop during landing dictate which runways we can use and the amount of braking we'll need on the runway. The turnaround time from one landing until the next departure is important to consider, also. Hot brakes can delay turn times. Long taxi times, such as one gets at larger airfields like Amsterdam, can heat tires substantially, as well.

The runway that's to be used, as well as it's relationship to the taxi routing, and taxiway or access occupancy, are important considerations.

We generally roll to the end unless some other extenuating circumstance exists, but we also generally operate close too maximum landing weight, too. Other operators may or may not be the same.

The same may be said for departures. Not all departures begin at the beginning of the runway, but may be planned on intersections, for a variety of reasons. It's up to the crew to determine what they can, or cannot accept.

Many airports use separate runways for departures, and for arrivals. Arrival runways are sometimes shorter than departure runways (departing aircraft are heavier and require longer distances for takeoff). The runway access points may not be the same for both runways, and delays crossing either runway, or routing to the terminal or other destination on the airport, vary, and may vary throughout the day.

[Hartington]

Two thoughts/replies.

Many years ago, arrived in O'Hare and experienced very heavy braking. Knowing O'Hare uses things like intersecting runways I just assumed it was normal until the flight deck apologised and explained that the plane in front of us had been a far eastern 747 and he thought we might like to get to the terminal first.

The other incident was a 737 into Belfast when I had the privelege of being in the cockpit jumpt seat. On finals the first officer turned and said "we're now going to show you a short landing in a 737". OK, I thought. We dropped the mains on the runway as soon as we could followed by reverse thrust, lift dumpers, heavy braking. Turned off the runway and taxied to the terminal. After they had turned everything off I asked why. Well, came the response. There are three exits one at each end and one in the middle. If had had to go to the end we would have had a long taxi back to the terminal and we'd have been late.

[Dan Winterland]

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.

[Gulfstreamaviator]

So true, and will make many friends in the cabin.

glf

[chevvron]

Echoing what SN3SGuppy said, too many controllers say 'vacate next left/right' whereas I always say 'vacate next convenient left/right' so that the pilot doesn't need to brake sharply. If a particular turnoff is not available and it's the one normally used, I advise the pilot before he/she lands. Course where I am now we only have turnoffs at the runway ends, so if I see someone land and brake sharply, I advise them to 'roll to the end and vacate left/right',