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Author Topic: why not stabalise engines with brakes on?
ZS-BOK

 

 

 

 

 

 
Question posted 24th May 2001 13:53

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

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

747FOCAL

 

 
Post posted 24th May 2001 17:44 

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

ZS-BOK

 

 
Post posted 24th May 2001 18:25

thanks for that, sounds good enough for me.

Does this mean the freight boys do it?
Agaricus bisporus

 

 

 

Lightbulb posted 24th May 2001 18:37

Less chance of FOD ingestion.

Less chance of crosswind induced surges

Boeing say no appreciable difference in runway used.

everybody

 

 

 

Unhappy posted 24th May 2001 18:38

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.

ZS-BOK

 

 
Post posted 24th May 2001 18:54

thanks for that, sorry about the spelling

cheers
Iz

 

Post posted 24th May 2001 18:59

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

john_tullamarine

 

 

 

 

 

 

 

Post posted 24th May 2001 19:23

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

ZS-BOK

 

 
Post posted 24th May 2001 19:23

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.
foghorn

 

 
Post posted 24th May 2001 19:30

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.
ZS-BOK

 

 
Post posted 24th May 2001 19:43

Probably was Jo'burg on a hot day.

cant make too many mistakes on this forum, you blokes are sharp!
ZS-BOK

 

Post posted 24th May 2001 19:50

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

everybody

 

 

 

Thumbs up posted 24th May 2001 20:02

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.

drop bags bar

 

Post posted 24th May 2001 20:22

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

747FOCAL

 

Post posted 24th May 2001 23:57

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.

john_tullamarine

 

 

 

 

 

 

 

 
Post posted 25th May 2001 07:45

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 ....
cpdude

 

 

 

 
Post posted 25th May 2001 08:05

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.
john_tullamarine

 

Post posted 25th May 2001 08:34

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

jonno

 

 

 

Post posted 25th May 2001 15:23

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!!

Clarrie

 

 
Question posted 25th May 2001 19:06

Several posts on this thread refer to "especially if the aircraft's light" and or/ "short runways".

For me the consideration is not clear. Are you considering that if light and performing a rolling takeoff that a greater percentage of the TODA will be used prior to setting takeoff power than if heavy?
mutt

 

 

 

 
Post posted 26th May 2001 07:48
747FOCAL,

I would be extremely interested in Boeing documentation to back up your claim of 600-700 feet.

We are presently the only operator in the world who still operates ALL models of the B747, including the B747-SP. We presently proclaim that the difference between standing and rolling takeoffs, when expressed in distance is negligible. This data is directly from the manufacturer.

Now you tell us that we are wrong?????

BEagle

 

Wink posted 27th May 2001 14:02

It is, of course, very satisfying to ex-fighter pilots' souls to stand on the brakes and set max. thrust in a 4-jet with min. permitted fuel, rotate briskly at Vr and then climb at V2 giving the local area the 'sound of freedom'! We can do it with 80000 lb of thrust in an aircraft weighing about 200 000 lb - and boy, does it go!! But not something that the travelling public would like to experience, I imagine, nor something that the fun-detectors would condone either!

Centaurus

 

 

 

Wink posted 28th May 2001 04:08

In 737 simulator with crew from SE Asia. Their company procedure was the PNF called " Stabilized" after PF had set the throttles, then once the call was made the PF would shove open the power for take off. I suggested to them that a specific call was not needed - just use your eyes to see the N1 were stabilized. That did not work, and during the exercise the call was always made.

So I quietly instructed PNF to " forget" to call stabilized. The PF then opened up to "stabilized" power and motoring past the 1500 ft marker around 30 knots still at low power looked accusingly at the PNF and said " Hey! You have forgotten to call" Stabilized!"....

So much for commonsense.

john_tullamarine

 

 

 

 

 

 

 

 

 

 
Post posted 28th May 2001 14:38

You note that -

Several posts on this thread refer to "especially if the aircraft's light" and or/ "short runways" (and wonder at this ...)

The concern is one of the dynamics of the acceleration in a limiting case (ie minimum runway available compared to runway required) - clearly, if the situation is one with significant excess runway, then the worry is rather lessened.

If an aircraft is heavy then there is

(a) a relatively high breakaway thrust required to commence rolling

(b) a reduced acceleration with whatever the particular aircraft can put out at rated thrust.

In the event that the pilot spins up and advances the throttles (thrust levers if you must) without undue delay, iaw the AFM assumptions, then, by the time that the aircraft has moved any significant distance in a rolling start, rated thrust is set and the overall result is similar to what might be obtained with a standing start on the brakes.

If the aircraft is light, then the situation is quite different and a pilot must exercise great care in setting thrust. Centaurus' tale above presents a useful example of the problem, albeit in an endorsement setting.

Cpdude

I find your statement that takeoffs are designed to use the maximum extent of the runway rather innovative but, I suggest, a little wide of your operations engineers' intent.
mutt

 

 
Post posted 28th May 2001 20:22

John_tullamarine,

I agree with CPDUDE, most of our takeoffs use fixed derates combined with assumed temperature thrust reductions. We are therefore planning to use up most of the runway. The B727 isnt as flexible as some newer aircraft, but with the B744 we can reduce the takeoff thrust by upto 45%. Therefore making a light aircraft field length limited on even the longest runways.
PPRuNe Towers

Dep Chief PPRuNe Pilot 

 

 

Post posted 29th May 2001 16:13

If the performance rules and criteria are enlightened, accurate and honestly brokered why did it take 15 years of fighting in the alphabet soup groups and committees for there to be any allowance for the distance taken to enter, line up and straighten an aircraft on the runway?

Why was such a pathetic and measly distance, self-evidently required until Thunderbird 2 is employed to place aircraft at the very beginning of the TODA fought over so viciously?
747FOCAL

 

Post posted 29th May 2001 19:04

My post was not clear I am sorry. I meant the difference between actual and the FAR 25 requirement for 115% of Balance Field length is around 600-700 ft. The difference between standing start and rolling is greater than 50 FT unless you roll around the corner at 40 mph and start spoolup, but try it in a 747 and you might end up in the grass. Its going to depend a lot on altitude and temp and all the other stuff that goes into low speed performance.

mutt

 

 

 

 

 

Post posted 29th May 2001 22:43

Legally we don?t have to account for line-up distances (FARS) and until recently we didn?t have the ability to do so. We did however shorten numerous runways where the crew knew that they couldn?t taxi on and line up at the start of the tarmac. But generally speaking on most runways, it didn?t happen, the distance used in line up was lost. I?m not saying that this is right, but there again neither is pretending that it doesn?t snow! (no contaminated runway regulations in the FAR?s)

You believe that the basis of takeoff calculations is flawed. They have slightly changed over the years with the introduction of VEF 1 second before V1, accounting for the aircraft acceleration during the reaction time and even accounting for worn brakes. If you still don?t agree that this is sufficient, what else would you introduce?
john_tullamarine

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Post posted 30th May 2001 05:37

A few more thoughts and comments on your thoughts ...

(a) "Legally we donít have to account for line-up distances (FARs) and until recently we didnít have the ability to do so."

I suggest that prudent corporate governance includes a requirement for the consideration both of

(i) what the FAA might or might not do in respect of AOCs and penalty actions, as well as

(ii) the potential for strict liability after a mishap, Conventions notwithstanding.

How can you suggest that you didn't have the ability to do so ? Australia, for many years now, has adopted a fairly pragmatic approach by doing some simple taxy/lineup tests with reasonable wheel/soft bits clearance margins. I have copies of the original correspondence between the initial operator and the regulatory authority in my patch and it makes for very sensible reading. Even if one didn't go to that extent, it is a very simple exercise to use a CAD add-on to simulate the same thing. Either way, sensible lineup allowances very easily can be derived. You don't need any legal coercion to do so - so long as your procedure is conservative with regard to Parts 25/121, then the System probably isn't going to be too concerned one way or another. But your pilots will be - and we really are only concerned about the accel stop accountability.

(b) "...(no contaminated runway regulations in the FARís)"

I don't work for any FAA-approved operators but .... are we confusing airworthiness with operational rules ? The main reason for a dearth of prescriptive rules for contaminant operations lies in the variability of the animal and the practical difficulties associated with making sensible and repeatable measurements for calculation purposes. An operator cannot just ignore the problem - operational requirements impose a responsibility to make some, albeit imperfect, attempt to maintain a reasonable level of operational safety. We all know that some operators chose to ignore some things, but that is beside the point.

.... and, always, one has to keep in mind ... what is your story going to be at the inquiry ?

mutt

 

 

 

Post posted 30th May 2001 12:07

JT, thanks for the responses, very enlightening stuff.

(A) Aviation is a wonderful world of trade-offs, we didnít account for line-up distances unless there is a known problem, but to balance this, we donít use stopways to increase the takeoff weight. Getting a realistic line up distance when you operate 11 different jet aircraft types is pretty interesting, what works for some will not work for others.

(B) Contaminated runways, lets not go there. That needs a topic all to itself. I shouldnít have used it as an example.

fly4fud

 

 

 

Cool posted 3rd June 2001 16:49

Hi ZS-BOK! I don't know if u are still following the thread which has become slightly technical, nevertheless here my small grain of salt:

On a powerful and lite aircraft (e.g. A300-600 or A310 or similar), the brakes will hold the aircraft standing still, but not the tires !!! The power is such that the whole bird will start with a hopping motion that becomes stronger and stronger until it starts moving forward, rubbing the smoky tires on the ground Hardly good for the tires, the landing gear, the structure or comfort

On a less powerful aircraft (such as the ARJ I'm presently on ) you will apply and wait for a max stabilized power at very short places like LCY, LUG and such.

ZS-BOK

 

 

 

Wink posted 4th June 2001 15:11

Thanks for the reply fly4fud, makes you appreciate the power of those engines.

The topic has got rather technical, so Im going to be quiet an try an learn something.

cheers


Last Updated on 06/07/2002
By Professor Stephen Emery