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shon7
10th Aug 2002, 18:40
This question might be somewhat ambigious from a technical point of view - but here goes:

I have been flying a lot lately and have noticed that sometimes the aircraft starts its roll and the engines start to spool up just as it enters the runway from the taxiway, while at other times it comes to a full stop on the runway and then commences take off.

Assuming that the aircraft has not been given a "taxi into position and hold" clearance - is it preferable to come to a full stop before take off or not? Also are there specific airline policies that state either?

Intruder
10th Aug 2002, 19:01
Different airlines have different policies. Some, for example, require a full stop if the F/O is the pilot flying.

Boeing says it makes no practical difference from a performance standpoint.

None
10th Aug 2002, 19:13
It might be preferred to do a rolling takeoff if there is a concern about FOD. It may also be preferred if there is a concern for engine acceleration (727 center engine stall/stag prevention for example).

On the other hand it might be required to do a static engine run-up prior to takeoff for certain icing conditions.

fireflybob
10th Aug 2002, 23:08
Intruder - I am not so sure you are right about what Boeing says.
On the B737-200 they used to say that a rolling take off was preferred (other than anti ice etc) but if not possible 1.4 EPR must be set prior to brake release for Perf A etc.

Given current traffic densities at many airports the "stop before your roll" technique is a luxury which is not often available.

john_tullamarine
11th Aug 2002, 00:27
The problem of a rolling start (apart from FOD etc considerations) is one of acceleration variations.

On a longer runway at higher commercial RTOW, especially for hot and high conditions, the acceleration is comparatively modest with the result that the aircraft hasn't gone a great distance before the spin up puts the engine at takeoff thrust .... in such cases there is little to worry about.

However, on a short runway at limiting (comparatively light) RTOW, especially for low and cold conditions, there may be a measurable problem in that the acceleration at part thrust settings may be significant. This may then result in the aircraft's having proceeded some distance down the runway prior to the spin up reaching takeoff thrust. In such circumstances, there is the possibility that the rolling takeoff may compromise the sums.

Most operators would get around this problem by doing a few sums and prescribing a below RTOW tolerance within which more rigorous attention to technique is required .......

Capt Claret
11th Aug 2002, 04:19
Shon

On the BAe146 with the operator I work for, we have guidance in the type SOPs (Standard Operating Procedures).

Basically if the aircrafts actual BRW (Brakes Release Weight) is within 600 kg of the RTOW (Regulated Take Off Weight) we come to a halt, advance the thrust levers and don't release the brakes until achieving 60% N1.

If the BRW is within 400 kg of RTOW we must set takeoff N1 prior to releasing the brake as well as ensuring that we do not compromise the line up allowance used to calculate the RTOW.

Why?

Basically to ensure that the RTOW is not compromised so that in the event of an engine failure there is adequate performance for obstacle clearance.

If say we did a rolling start at BRW = RTOW, it would be quite possible to have take off thrust set later than the performance calculations assume. Thus aircraft acceleration would be slower than predicted and the aircraft would be further down the runway when it reached V1. A rejected take off could see the aircraft over run the runway end.

If by spelling out the abbreviations I'm telling you how to suck eggs, please accept my apology

SLT
11th Aug 2002, 10:08
Airbus SOP's suggest a rolling takeoff when possible. I guess the other thing about this is that at airports where runway occupancy time is a factor, it would be always preferable to do a rolling takeoff to keep occupancy time at a minimum to help ATC, even when you are not given "cleared immediate takeoff". We have all been given a standard "cleared for takeoff" when there is traffic at 4 miles. Perfectly OK as regards separation, but why reduce the spacing by stopping??

Our company SOP's across all fleets, and I would imagine in most other airlines as well is not to accept a line-up or take off clearance unless you can taxy on and do just that without stopping.

Hope this helps ;)

Intruder
11th Aug 2002, 19:21
fireflybob:

The Boeing info I have is for the 747-400. My intent was to convey that the standing vs rolling start is not dictated by runway/performance limitations (i.e., there is no practical difference or advantage either way for takeoff roll, V1 calculation, etc). There may well be operational considerations in the decision, including all those that other people mentioned (FOD, anti-ice, ATC, etc).

nitro rig driver
11th Aug 2002, 21:45
i asked this question to a instructor once and the answer was that boeing says
stopping on the end enables the fuel in the surge tanks and the top hat section to flow back into the tanks,otherwise it remains trapped due to the angle of the wing during flight
however i doubt this as they just don/t stay on the end long enough,but i beleive not all a/c have surge tanks now adays

Flight Detent
12th Aug 2002, 19:46
Hi 'Nitro rig driver' -- say what!!!

As far as B747 is concerned, it's purely an engine run-up rolling, verses the run-up static, with less runway used up, when it's critical, doing the run-up to 1.1 EPR stopped, takes only about 5 - 8 seconds, and is worth the v/short delay.

Cheers

bsevenfour
25th Aug 2002, 10:42
Situations when it is better to do a static run up

1> In icing conditions it is usually specified.

2> If you have on an aft C of G and low gross weight, conditions which can lead to very low nose wheel sterring effectiveness.

3> If take off performance is critical. However for aircraft such as the 747-400 the performance difference between a rolling and a static take off is minimal.

4> If FOD is a concern.

5> Slippery runway conditions.

On certain engines / aircraft you are also strongly advised NOT to do a static run up if the crosswind component is greater than 20 kts due to the increased possibility of engine surge.

Flight Detent
26th Aug 2002, 08:47
Forgive me if I missed something here,
weren't we dicussing rolling take-offs vs stopping prior to actually commencing the take-off,
a static run-up is a different matter all together, me thinks!

Cheers

crj-jockey
31st Aug 2002, 00:19
Hello,
it allways depends on your calculations and the prevailing circumstances.
All our T/O calculations are based on a 60m rolling T/O-allowance (60m rolling until T/O thrust). However, there are certain circumstances requirering a static T/O(see statemant above).
Nowadays, we do our calculations with a pilots workpad (Laptop), which allways provide your specific stop margin in event of a RTO. So the final decision rests with the PIC considering all prevaivailing circumstances.

Flight Detent
2nd Sep 2002, 20:15
As it always does !!!
:confused:

Steamhead
4th Sep 2002, 14:08
Try stopping on the runway at LGW on a nice day , with the "A"
team in the tower, and in the rush hour.
It would be enlightning to learn some new swear words.

Regards

GlueBall
6th Sep 2002, 22:03
Our SOP: Rolling takeoffs, unless otherwise dictated by ATC.

Dan Winterland
7th Sep 2002, 11:42
On the 744 (with GE's, i.e. N1 power ref, not EPR) there is no difference.

On my previous type (VC10), we used to get another tonne's worth of RTOW by stopping and running up to 80% N2 before releasing the brakes.