Wind Speed and Takeoff Length Calcs

[DeepC]

Yesterday, I dropped by at Luton to see how folks were coping in the strong winds. Unfortunately in the 10 minutes that I was there I saw no planes land but two Easyjet planes take off.

What got me thinking was the fact that one of the Easyjet planes did not 'backtrack' but took off from the intersection. This must knock a good couple of hundred metres off runway length.

The wind was strong probably at least 25mph gusting to 35+ straight down the runway.

I was wondering if the wind speed is taken into account in the calcs for takeoff run or was this plane just running light or empty?

Any thoughts?

DeepC (Non-Pilot)

[john_tullamarine]

Wind is accounted by entering the wind value in the relevant charts or computer analysis program.

To provide a bit of fat for wind speed variations from that reported, while the pilot enters the reported wind component speed, the "behind the scene" calculations are based on half the reported windspeed for headwind components and one and a half times the reported windspeed for tailwind components.

This is all based on steady winds and doesn't take any account of turbulence.

[DeepC]

Thanks for the comprehensive reply.

If anyone from Easyjet or Luton airport reads this..

How often does a 737 take off from the intersection on RWY 26? Taking a 'full' 737, what sort of steady wind speed would make it possible to take off from the intersection?

Thanks

DeepC

[hadagutful]

I'm not an RPT pilot but I am absolutely amazed to hear that a 737 did an intersection departure.

Regardless of wind conditions and take off distance required I would have thought that it was good policy to use all the runway available. The runway behind you is fairly useless if something untoward happens during take-off run.
Could they be in such a hurry for departure or would ATC request it for traffic sequencing?

In all my years of flying and observing aircraft I don't think I have ever seen a larger RPT jet aircraft execute an intersection departure.

Would be interested in comment from pilots of such aircraft.

[quid]

hadagutful-

It's not at all unususal (or unsafe) to do intersection takeoffs.

1. With a few exceptions, most carriers use "balanced field" V speeds. Meaning that as long as you have enough runway for your takeoff weight, the V1 will be the same, even if the runway is longer than required.

2. Also, with few exceptions, most carriers' SOPs require that once you've reached V1, the takeoff will be continued, regardless of the runway remaining.

So...if you need 6000' for takeoff and the runway is 12,000' long, if you choose to make an intersection takeoff with 10,000 remaining, no problem.

Of course, every takeoff is different, and the PIC evaluates each one. If there are factors to be considered such as slippery, icy, tailwinds, three engine ferrys, etc., an intersection departure would be ruled out.

[john_tullamarine]

... not quite.

BFL is fine and a convenient, simple way to do runway sums .. but takes no note of obstacles.

Each takeoff, whether full length or from an intersection, must be reviewed against ALL the requirements (WAT, TOR, TOD, ASD - AEO and OEI, obstacle clearance, etc).

Just because you can schedule a departure for a 12000 ft full length runway at a certain weight .. which just happens to require a TODA of 6000 ft ... certainly DOES NOT necessarily mean that you can takeoff at the same weight from a 10000 ft intersection .. unless you redo the sums and nothing is sufficiently critical to compromise the proposed takeoff.

Routinely departing full length is nice for the pilot but, at the end of the day, the operator is there to make a dollar ... and the use of intersections is part of the dollar maximising equation... so long as the operator uses competent ops engineers who do the sums for each scheduled RTOW page ....

[quid]

jt-

...sure, there are other factors. That's why I said "as long as you have enough runway for your takeoff weight,". That weight may well be limited by "climb" or "obstacles", tire speed, brake energy, etc.

[jed_thrust]

So, my question has to do with tailwinds.

When you enter a 5 kt tailwind for the takeoff calcs, the regulations will work out 7.5 kts for you (150%).

This is all fine until I use zero: what happens when I line up and there is a (say) four knot tail wind? Legally speaking, can I take off?:confused:

[Three Mile Final]

All terribly interesting, but is "hadagutful" getting confused between the alpha or bravo intersections with the runway and the charlie intersection ??? (wot is how this thread was started by DeepC).

There is a hell of a lot of difference. From Alpha or Bravo you maybe give away a couple of hundred metres (of the 10,500 feet or so) whereas I would agree nobody (except me in a Warrior) is going to take off from Charlie which would lose you 4,000 - 6,000 of runway.

Having said that, the last time I was on an Easy 737 on 26 from Alpha we rotated at charlie, which (not having my Jeppesson to hand) is about 4,000 ft.

[mutt]

I am absolutely amazed to hear that a 737 did an intersection departure

We routinely do intersection takeoffs in B744/B777 aircraft.
Takeoff calculations are based on departing from that point.

Absolutely nothing wrong with the procedure.

Jed_thrust, considering that in most cases the aircraft is planned to cross the end of the runway at 35 feet following an engine failure. Then look at the weight decrement per KT tailwind and calculate how overweight that aircraft would be with a 4 KT tailwind. Legality aside, it wouldnt be extremely sensible.......

Mutt.

[DeepC]

It took off with out backtracking after entering the runway via Alpha1.

Therefore he lost about 1/5th of the total tunway length of 2106m.

From where I was sitting it looked like he rotated between the charlie intersection and the bravo intersection.

Looked safe, I was just surprised having flown from LTN many times in the last five years and never having seen a 737 take off like that.

DeepC

There was no need for him to expedite as there was no other traffic in the next 10 minutes afterwards.