why not stabalise engines with brakes on?
 

Is there any reason why engines are stabalised at the beginning of the takeoff run, not producing takeoff power but using up runway. Would it not be a better idea to do this with breaks set before the run begins? Im shure theres a valid reason, just curious.
cheers

------------------
Rater down here wishing I was up there, than up there wishing I was down here !

That really shakes up the plane and vibrates a bunch. Generally scaring the crap out of some or all of the passengers.

thanks for that, sounds good enough for me.
Does this mean the freight boys do it?

Less chance of fod ingestion.
Less chance of crosswind induced surges
Boeing say no appreciable difference in runway used.
Plus those given above.

btw, breaks is what happens when the brakes stop working.

Setting the thrust on a rolling take-off allows a gentle acceleration - better for passenger comfort, rather than the sudden kick you get - more from oleo release than acceleration - when you release the brakes having set the thrust. So it's preferable.

However, as you suggest, you may for performance reasons need all the runway you can get. On our 45 tonne aircraft, if we're within 2 tonnes of the performance limited weight, or we're using full thrust for performance reasons (we don't normally to be kinder to the engines) then we do a standing start. The brakes aren't released until full thrust has been set and checked.

Lots of airline stuff is really black and white. If the book says you can go from a particular intersection on the runway and there isn't some other factor involved, then you can just go from there and not worry about it - no need to demand full runway length.

thanks for that, sorry about the spelling
cheers

Well, we stabilize the engines at about 10% thrust (1.15 EPR actually) with brakes on, then release the brakes while setting TO thrust. You may not mean the same thing with 'stabilizing' but that's how we do it :)

A few considerations -

(a) FOD has been mentioned
(b) circulation and reingestion
(c) aircraft shake, rattle, and roll is not really a problem
(d) the acceleration question is not a major consideration as the acceleration peak is reached quickly regardless
(e) distance penalties are not a major consideration if the start commences from a rolling taxi and the thrust is set promptly
(f) distance consideration is relevant for a very light aircraft on a short runway as the breakaway thrust is low and the acceleration at lower thrust levels is not insignificant. In the case of a heavy weight takeoff, by the time the thrust is set, we haven't ventured very far down the runway

I can only talk from a PAX’s point of view, but I always thought the stabilizing stage is after brake release and the engines spool up, above idle but only enough for a gentle roll, and only after about 5 seconds you get that nice kick in the butt as full power is reached. Am I mistaken, is this just the engines naturally spooling up to full power, as I hear it takes about 6 secs, and has nothing to do with stabilizing?
I remember a flight with SAA from Cape Town to Johannesburg, in an A300B4, where he powered up until the overhead bins started shaking before he released the brakes, must have been heavy that day, I did see quite a few worried faces.


------------------
Rather down here wishing I was up there, than up there wishing I was down here!

Would be surprised if that was necessary, Cape Town is at sea level and has 3.6km of runway. Maybe the journey was the other way round - JHB on a hot day?

Full power against the brakes is quite common with bigger turboprops at London City where they only have 1200m of runway to play with, especially on hot/windless days: KLM wind their Fokker 50s up to full power before brake release. They usually warn the pax that this is a standard procedure, though.

[This message has been edited by foghorn (edited 24 May 2001).]

Probably was Jo'burg on a hot day.
cant make too many mistakes on this forum, you blokes are sharp!

Will the brakes still hold at full power on a contaminated runway?

Gosh, you're keen !

That's one reason we don't do it contaminated, the other main one being ingestion of contaminant.

We have a whole set of different performance figures for different types of contaminant which (allegedly !) take the rolling start into account.

The is also an airframe fatigue price to pay for full power against brakes.

john_tullamarine- You obviously have never done it in an aircraft with big or 4 engines. FAR25 runway calculations are done based on 115% of flight tested takeoff distances. Boeing says no difference in runway used is BS in a 747-400 you are talking about 600-700 ft of runway.

747FOCAL

(a) my comments were, I think quite clearly, intended to be read as being general, not specific.

(b) you are correct in that I have not operated the 744 - biggest toy in my background is the 727-200 - a dinky toy by comparison, no doubt, but still a lot of fun.

(c) your comments re Part 25 relate to AEO scheduled data - but I think I miss whatever point you are making in that regard ? Perhaps you might elaborate ?

(d) my admitted limited knowledge of the 744 - based on a few ops eng planning projects - suggests that my general observations remain relevant. I will, however, dig out a current AFM for the model and have a ferret around in the bowels of the books.

I would be interested in knowing the Boeing Doc from which your comment is derived as I would appreciate the opportunity to review it for my own CPD. Alternatively, you may be able to email a copy to me ?

(e) thrust/weight remains a very relevant and pertinent parameter for consideration of takeoff performance. The 744 has a big bunch of grunt and one would tread very delicately in respect of low weight shorter field operations in that model.

I look forward to your further commentary ....

747FOCAL,

I also fly the 747-400 and according to our manuals from boeing, "flight tests and analysis have proven the change in take-off roll due to rolling take-off procedure is negligible (less than 50 FT) when compared to a standing take-off". We are directed to set 1.10 EPR approximately and allow the engines to stabilized momentarily before pressing the TO/GA switch.

Are you possibly waiting too long prior to press TO/GA to burn 600' - 700'?

Boeing states the reason for the rolling take-off's as it expedites take-off and reduces the risk of FOD.

Perhaps I no longer need to dig out the AFM ...

Hi,
Just a vaguely related question,
I was on a very empty (passenger wise) AA 777 sitting at the start of the runway at O'hare, the engines were spooled up, the whole airframe was vibrating heavily, and there was a large roar from the engines, but we held there for about 30 seconds on the brakes. It seemed silly to me, the possibility of chunks of ice being taken into the engine I guessed was large?
Why? This was in the winter, just guess work but..was this to burn away de-icer entered into the engine before PACKS switched on to prevent smoke ingress into the cabin? Was this to ensure Engine nacelle and wing anti-ice given a working before take-off to remove ice??
Interestingly we had an engine failure on the flight, with a visible stream of fuel from the engine, and had to emergency land in JFK, any link between the two??
Many thanks for any info.
Tom.

[This message has been edited by tom775257 (edited 25 May 2001).]

Hi all, I'm a '74 Classic operator, and yes, I still think they are the best, anyway -

the initial spool-up is for mainly the assurance that when the time comes to set take-off power, especially when the bird is light, all the engines have actually spooled up, and the aircraft keeps heading in the right direction!
Also, we have actual real eyes watching all the engine instrumentation, in addition to comparing both sides flight instruments, good eh!!