On the Brakes or On the Roll?

[YFlex]

Boeing Perf Question: Is there any advantage (w/Take Off Run) to setting static power or on the roll after stabilized?
Cheers!!

[mutt]

Nope, both of them are perfectly acceptable procedures, performance wise, there is no difference between them.


Mutt.

[john_tullamarine]

.. provided that any RTOW penalty is taken into account .. potential problem is a light aircraft on a short, (near limiting) runway where part thrust acceleration from a standing start could compromise the numbers a tad ... which is why the rolling procedure is predicated on entering the runway at a suitable taxy speed.

Overall, the maintenance cost penalties associated with static full thrust takeoffs are the driver in this consideration.

[Flight Detent]

Hi all,

As I understand the Boeing performance manual, rolling and static takeoffs are both quite acceptable, even with reduced power takeoffs.

BUT, the hesitation to allow the spool-up of the engines should not exceed 2 seconds during a rolling takeoff, otherwise runway performance can be compromised.

Thats my understanding.

Cheers :ok:

[calypso]

From the FCTM:



" A rolling takeoff procedure is recommended for setting takeoff thrust. It expedites
takeoff and reduces risk of foreign object damage or engine surge/stall due to a
tailwind or crosswind. Flight test and analysis prove that the change in takeoff roll
due to the rolling takeoff procedure is negligible when compared to a standing
takeoff.
Rolling takeoffs are accomplished in two ways:
• if cleared for takeoff prior to or while entering the runway, maintain
normal taxi speed. When the airplane is aligned with the runway
centerline, apply takeoff thrust by the method described below. There is
no need to stop the airplane prior to adding thrust.
• if holding in position on the runway, release brakes, then apply takeoff
thrust as described below.
Note: Brakes are not normally held with thrust above idle unless a static run-up
is required in icing conditions.
A standing takeoff procedure may be accomplished by holding the brakes until the
engines are stabilized, then releasing the brakes and applying takeoff thrust as
described below.
Note: Ensure the nose wheel steering wheel is released and the airplane is
tracking straight down the runway prior to thrust application.
Advance the thrust levers to just above idle (40%N1) as the airplane rolls onto the
runway at a slow taxi speed. Once the airplane is aligned with the runway, allow
the engines to stabilize momentarily then promptly advance the thrust levers to
takeoff thrust (autothrottle TO/GA). Allowing the engines to stabilize provides
uniform engine acceleration to takeoff thrust and minimizes directional control
problems. This is particularly important if crosswinds exist or the runway surface
is slippery. The exact initial setting is not as important as setting symmetrical
thrust.
Note: Allowing the engines to stabilize for more than approximately 2 seconds
prior to advancing thrust levers to takeoff thrust may adversely affect
takeoff distance."

I hope it helps

C

[YFlex]

Very informative Calypso,
and Thank You All !!
I've been doing RVR600 T/O's in the sim while setting power on the brakes and no one has told me different (may be it's just the sim)
YFlex

[Old Smokey]

Mutt says -

both of them are perfectly acceptable procedures, performance wise, there is no difference between them
On the first point, I'm in complete agreement, and go further to agree with Boeing that the rolling takeoff is the preferred procedure.

On the second point (there is no difference between them), I beg to differ. If the manufacturer has done the performance certification work using the rolling start technique, suitably described as calypso has done, then there is no difference between them in terms of legality and safety, but the laws of physics dictate that, in this case, the standing start will offer improved takeoff distance performance (even if undesirable from other aspects).

Boeing may do this but all manufacturers do not do so, and a lot of readers here should be cautioned against accepting that both are the same. A number (I would think most) manufacturers conduct testing and performance certification based upon the "Standing Start" technique to maximise RTOW, and provide suitable corrections to this, either in terms of a weight or Takeoff Distance penalty, to be applied to the Takeoff Weight extracted from the "Charts" for a Rolling Start. I do performance engineering work for aircraft rangeing from quite small to VERY large, and find that even for the smallest aircraft that I do work for, the correction will vary from Zero (obviously not Runway limited) to several thousand Kg, and going on through to "Can't takeoff at all unless a Standing Start is used". My solution is to produce RTOWs for the optimum (Standing Start) condition, and in the RTOW table supply the deduction to be applied if the actual aircraft weight makes the preferred Rolling Start possible.

There are essentially 3 types of Takeoff procedures - (1) The Standing Start, (2) the Rolling Start, and (3) the Roll-On start. The Standing Start is USUALLY that used for certification purposes by the manufacturer. The Rolling start (brakes release and thrust application within a specified time frame) is preferred over the standing start, but only if either (a) the manufacturer has certified the aircraft in this way, or (b) suitable corrections are made to the standing start calculated data as alluded to by John Tullamarine. The latter technique, Roll-On start, assumes that taxy speed is maintained onto the runway and initiation of the Takeoff roll. Most schools of thought (including my original mentor, McDonald Douglas), equate this with being equivalent with the Standing Start. I tend to agree with them, but am concerned that some pilots may accept 2 or 3 knots '90 degree turn on' taxy speed as satisfying the Roll-On requirement as opposed to the 10 knots that is assumed to make them identical to the Standing Start.

Be aware that there is a big difference between Rolling and Roll-On start (the latter is the one that, for all intents and purposes, is equivalent to the Standing Start).

I won't be drawn into a discussion regarding which aircraft types are involved here. The basic principals are the same, but I fear that someone may use data inappropriate to their own aircraft, and end up using a generic BAe McBoeingBus technique. Check your own AFM!

Keep safe, know the numbers.

Old Smokey