Take-off Distance : What are the considerations

[richie-rich]

ok I know temperature, Height of the runway from seal-level, Pressure height, Runway gradient all would contribute at the time of take-off. I am sure I have missed something out as well.

Why would we require more take-off distance if the temperature is relatively warm than it is when cold? I know the theory, but my instructor has failed to come up with a convincing theory on this.

Would appreciate if you guys care to elucidate on these points to a student pilot. :ok:

Richie

[john_tullamarine]

(a) engine thrust is down hence acceleration is down

(b) the TAS required to achieve the necessary IAS/EAS is up

hence ... more concrete required ...

[Tinstaafl]

Things that affect T/O dist required:

1. Air density. The more molecules passing over/around the wing the more lift that can be produced for a given speed. Alternatively the same lift for a lower (true) speed. The lower the (true/nil wind) speed needed then the less distance needed to accelerate to that speed.

Also the engine(s) will be capable of more thrust, increasing the acceleration ability --> less distance needed to get to speed.

Things that affect air density:

* Pressure eg higher altitude or a low pressure system: lower pressure --> less dense
* Temperature: higher temp --> less dense
* Composition eg humidity: increased humidity --> less dense

2. Weight: More weight means a greater distance required to accelerate to a given speed for the same amount of force causing the acceleration.

3. Slope: Uphill means some of the thrust is 'wasted' fighting gravity. Downhill means gravity adds to the total acceleration force. Greater acceleration --> less distance to get to a given speed.

4. Surface: Anything that increases the drag on the wheels eg long grass, rough surface etc will retard the acceleration of the a/c --> greater distance required.

5. Wind: A headwind reduces the t/o distance. As far as the wings are concerned they're already moving at whatever the headwind component is, even if the a/c hasn't yet moved. End result is a lower grounspeed needed. Opposite for a tailwind of course.

6. Configuration: Extending high lift devices eg flap will reduce the take-off roll. The penalty is a reduced climb angle. Take-off distance isn't just the wheels on the ground roll but also an initial climb to a given height (15', 35' or 50' typically, depending on circumstances). Using 0 flap will give a longer roll but a steeper climb once airborne. Using full flap would give the shortest roll but a very shallow climb (ignoring handling/rotation/additional drag during the roll). Somewhere in between is usually an optimum for the conditions.



Note that solving a take-off distance problem isn't limited to just getting airborne. The subsequent climb out over any obstacles must also be considered. The climb angle after take off is also affected by most of those items above.

It's no good being able to get airborne in the runway distance available but not being able to climb steeply enough to clear the trees/buildings/hills...

[chiglet]

During the recent "kerfuffle" at Manch {UEFA cup, or whatever]:D
Several a/c were offered 24L with "reduced R/W length" Some a/c belonging to a "Big Airline" were unable to accept, 'cos they "didn't have the figures":confused: I mean, "You have a TORA of 2400M, and they don't have the figures, puleeese
One a/c when asked why 24R said "It's a shorter taxi!":rolleyes:
we aim to please, it keeps the cleaners happy

[Notso Fantastic]

Well the way the 'Big Airline' does its TO performance calculations, the figures are for individual RWs and specific points down that RW, so if the intersection offered is not in the book, it becomes a generalised calculation that can take several minutes. But of course, you will know better how it should be done than the airline with one of the largest and most varied route networks in Europe/Asia/Oceana/Africa!

[chiglet]

NsF,
The crew "concered" had at least 30 mins to "calculate" said figures:mad:
we aim to please, it keeps the cleaners happy

[overstress]

Chig:

If you were on the receiving end of the conversation, the big airline crew are used to being spoon fed (I work for them as well) and probably couldn't be bothered to either work it out or plug the figures into ACARS to receive the computer version