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Are Flex / De Rated take offs safe?

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Are Flex / De Rated take offs safe?

Old 19th May 2008, 03:58
  #21 (permalink)  
 
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The benefits of flex/assumed temp/ reduced power take offs are undisputed.

I doubt there is an Airline in the world that is not doing them as routine.

As already well stated, not only are there benefits in less long term wear and tear, but it vastly reduces the stresses on any particular take off, directly reducing your chances of engine failure.

More is always there if you need it, in the sim, however with an engine failure at or after v1, normally we do not push the power up on the remaining engine (s) unless we absolutely need it.All performance is calculated on that reduced power.

Like to be very careful with what we have left !
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Old 19th May 2008, 04:54
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Not one thing from Garret, RR, or Pratt has come through that has told us that it would save money in real terms.
You obviously dont talk to engine manufacturers very much.

For the B787 engine guarantees, the manufacturer wants us to operate it with an average of 28% reduced thrust to comply with "THEIR" requirements! For the RB211, a 1% EPR reduction results in a 12% increase in blade life, plus in our case brought the engines up to the guarantee levels.

I just wont do it on take off.
Then i suggest that you dont fly for an airline, we have constant ACARS monitoring of the engine conditions, any pilot who constantly refuses to use reduced would get to have "tea and biscuits".

Even our corporate fleet that includes 747-400's use reduced

You seem to have a concept that reduced will always put you right over the fence, this is totally wrong, look at any B777 taking off for a relatively short sector, ie, 6 hours. Look how much runway he used, and i will almost guarantee that he was using reduced!


Mutt
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Old 19th May 2008, 05:53
  #23 (permalink)  
ssg
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Ok Mutt..what is the overhaul hour and cycle limitation on these engines...what do fan blades cost on this engine...what was your last estimate for a overhaul when you put it out to bid...how long are your engines going before Hots...

----------

I don't have a problem with flex...empty plane, long runway, repostion flights, ...big deal.

With 200 people in the back, crappy weather, obsticles ahead, ice in the forcast...and some operators use flex to the nth, as far as they can take it..

You agree with that?

Last edited by ssg; 19th May 2008 at 06:12.
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Old 19th May 2008, 09:16
  #24 (permalink)  
 
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Still getting my head around the flex concept, particularly the bit where u do one set of calculations, then recalculate for the (higher?) temp.

Do bizjets such as CJ3 have a flex power schedule?

On such aircraft that will do maybe 200hrs per year, and is often sold before a hot section or engine overhaul time is reach, is there a benefit? I can understand the benefits of an airline aircraft doing 10 sectors a day year on year.

I am lead to belive that flex power take-offs use a little more fuel because due to the lower initial climb thrust the time to cruise alt is longer and more time is spent on in the lower high fuel consumption altitudes during the climb.

Do I also read it right that for max range achievable on the aircraft, full power and max climb power then long range cruise? Would be interested in seeing some comparisons.

Assuming a new CJ3 is delivered, and flown by the same crew. How does one look after turbine engines. What abuses them. In short, how would you get your engines to a hot section/overhaul in the best condition.
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Old 19th May 2008, 12:28
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There's a great read on the subject here. It's written around the 737, but the principles apply to other ships.

Actually, since I'm a graphical kind of guy, I like a graphical presentation but I have not yet found one online. It should include a chart of typical airfield-limited performance weights vs OAT, and a chart of N1 (or EPR) vs OAT. Using these it's fairly easy to understand how ATM or flex is computed.

And don't forget, even though you're running the engine at a thrust level equivalent to a 40C day, the wing is flying in (e.g.) 20C air. Thus your V1, Vr etc. TAS is quite a bit lower. All this means extra margin.
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Old 19th May 2008, 12:51
  #26 (permalink)  
 
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SSG

Would you, or someone else who flies biz jets, tell me what performance rules you chaps use please.
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Old 19th May 2008, 16:29
  #27 (permalink)  
 
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Would you, or someone else who flies biz jets, tell me what performance rules you chaps use please.
I'll answer that. Light transport category airplanes use the same performance rules that large transport category aircraft use. The training standard is the same, too. Most have reduced thrust takeoff calculation data available. The same SOP's apply in business class turbojet aircraft that apply when flying a B373 or 747. The posters arguing to the contrary are doing so based on their own ideas, and certainly not what's been taught them at well recognized and industry approved training centers.

Still getting my head around the flex concept, particularly the bit where u do one set of calculations, then recalculate for the (higher?) temp.
There exist several programs for operating on reduced thrust. Assumed temperature is just one of them. It's used in business class turbojets as much as large Boeing products. As you know, an aircraft will experienced reduced takeoff performance with an increase in density altitude. With an increase in temperature, we experience an increase in density altitude. We experience an increase in takeoff distance. If we then compare that distance to our present field and find that we can meet all the safety requirements to take off (including stopping and going, appropriate climb gradients, obstacle clearance, etc), then we can operate with a thrust setting that equals that same performance.

As an example, using example numbers which are for illustration only, we have a 10,000' runway. We calculate a full thrust takeoff and find we can takeoff in 5,500'. We then run an assumed temperature thrust reduction analysis, and determine that at 40 degrees C, we would use 7,000' of runway. Our peformance calcualtions show that we can reduce power enough to use that 7,000' of runway and still climb out, or stop from V1, safely using the reduced thrust. The assumed temperature is a reference number only, and at no time are we prevented from pushing up the power to maximum thrust if required. However, the performance data assumes the reduced performance, and shows that we are able to lose an engine at that lower thrust and continue the takeoff with the reduced thrust to give all the necessary performance (climb gradients, etc).

Normally reductions based on assumed temperature are done up to 25% of maximum thrust (meaning 75% thrust avail). Other systems are available to calculate reduced thrust. We use a computer based program that takes everything into account including specific conditions, NOTAM'd runway conditions, etc. We don't use more than a 25% reduction. A whole litany of conditions exist and are clearly spelled out in our Aircraft Operations Manual detailing when we can and can't use reduced thrust. We have the option at any time at pilot discretion of using full thrust, and we're required to do so at least every seven days for no other reason than that's one of our requirements; a full thrust takeoff logged every seven days.

Assumed temp is just one method of determining a reduced thrust setting. Rather than an arbitrary number, it's a reference that compares takeoff performance under a known given set of conditions (increased density altitude), in order to start with meaningful data to compare to the runway and departure conditions in use. If the performance experienced at the assumed higher temperature could still work right here, right now in these conditions, then we can reduce thrust to replicate the same distances and performance we'd experience at that higher temp. In so doing, we reduce engine operating temperatures, save wear, tear, money, fuel, and still meet every margin of acceptable safety.
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Old 19th May 2008, 16:53
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Thanks Guppy
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Old 19th May 2008, 17:02
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Here's a powerpoint presentation on the subject:

http://www.captainpilot.com/performa...r%20derate.ppt


From Boeing:

http://www.captainpilot.com/performance/04_Takeoff.pdf


http://www.smartcockpit.com/pdf/plan...structor/0024/

Cheers
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Old 19th May 2008, 17:28
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ssg -

Methinks you don't comprehend where V1 comes along the length of the runway. If you're doing the numbers right, you'll always have sufficient stop distance after V1, regardless of where Vr abides.

In any case, you'll always be safer when the assumed temp is 40C, than when the actual temp is 40C.

So far you haven't shown us any sign that you understand these basics.
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Old 19th May 2008, 17:31
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Guppy, you might be right in principle, however the regulations governing GA and airlines are different, at least if mentioned GA aviation is not working under an AOC and of course everything just applicable in europe (JAR), dunno enough about the rules in other parts to comment about that.

We had the problem with a small business jet we operated some years ago (yes, we are an airline, but still can operate small stuff), it couldn't operate under JAR OPS rules into a certain airport, but it could quite happily outside of JAR OPS. To be able to fly into the home airport of the ownder of that jet we had to get a special permission that we could fly without having to factor the landing distance according to JAR OPS, as a result it was only allowed to fly there for private flights of the owner but not for normal charter operation.
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Old 19th May 2008, 17:54
  #32 (permalink)  
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OK You may have commitment issues. I think you don't (won't) understand V1. You still want a choice after V1, which is commitment time. If you won't adhere to Runway calcs., (your own, if you actually take the time to consider them), then don't. I think you have trouble with authority (even if it's your own!). V1 is, after all, a command. You don't get that V1 is a commitment to fly, not a notice of enhanced awareness.

"Fly The Force, Luke". No. No. No.
 
Old 19th May 2008, 18:00
  #33 (permalink)  
 
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Barit, the link that you posted about FLEX Temp clears up a lot of questions, great !!

But surprised me with this:

"It Increases fuel burn.
Strange, but true. This is because:
  1. Assuming an uninterrupted climb, it will take longer to reach the more economical cruise altitude than a full thrust climb.
  2. Engines are less efficient when not at full thrust. "
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Old 19th May 2008, 18:26
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Guppy, you might be right in principle, however the regulations governing GA and airlines are different, at least if mentioned GA aviation is not working under an AOC and of course everything just applicable in europe (JAR), dunno enough about the rules in other parts to comment about that.
This is because you're considering the wrong regulations. Operational regulations are irrelevant. What you need to consider are certification regulations. A two engine transport category airplane, be it a Lear 60 or a B737, must still meet the same performance standards and design criteria with respect to minimum gauranteed performance, and they're still flown the same...especially with respect to takeoff and landing. Transport category aircraft are transport category aircraft, big or small.

Now there are certain things you can do, and get away with in small, light transpor category aircraft that you can't in big airplanes; I've taken off in a Lear 25 and turned downwind at 18,000'. Not something you'll do in a heavily loaded B747. However, each must still meet the same minimum performance criteria. And the training for each is very similiar. Moreover, approved training programs for either one don't teach to stop after V1, and do provide for reduced thrust operations.

But surprised me with this:

"It Increases fuel burn.
Strange, but true. This is because:

1. Assuming an uninterrupted climb, it will take longer to reach the more economical cruise altitude than a full thrust climb.
2. Engines are less efficient when not at full thrust. "
During a reduced thrust takeoff, very often when climb thrust is set after takeoff, it may be to increase thrust, rather than reduce it. During a standard noise abatement departure, climb trust is a thrust reduction. However, during a reduced thrust takeoff, setting climb thrust may mean increasing it to the climb thrust setting.

Turbine engines are most efficient at a high power setting. You can imagine the loss of efficiency at lower power settings somewhat like driving a car up a hill in the wrong gear; inefficient. Part of the reason that a jet engine is most efficient at high altitude is that the powerplant is required to run at a more efficient RPM. At lower altitudes, excepting climb, the power is pulled back into a less efficient power range.

The problem as you can guess is that at low altitudes, airspeed limitations prevent pushing the power up too far. At altitude, the power can be pushed up into an efficient range without exceeding airspeed limitations. At altitude in most turbojet airplanes, you run out of available power before you exceed airspeed limitations...meaning you can cruise in an efficient range. At low altitudes, reducing power even further decreases efficiency...which is part of the reason (performance being the other) that power is often increased after a reduced power takeoff.

Our typical profile includes a departure at reduced thrust, with climb thrust set at 1,000' above the departure elevation. At 10,000' we'll reset the climb thrust, often to a reduced climb by a given percentage; we use .04 EPR if our weight is more than 600,000 lbs, and .06 epr if weight is below that value. This isn't a great reduction, but it's also another place we reduce power slightly below maximum climb values for engine longevity. Around 24,000' we find that we need to restore the climb power to the maximum value for performance reasons, and continue up to our cruise altitude at that power setting.
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Old 19th May 2008, 18:53
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This is because you're considering the wrong regulations. Operational regulations are irrelevant. What you need to consider are certification regulations. A two engine transport category airplane, be it a Lear 60 or a B737, must still meet the same performance standards and design criteria with respect to minimum gauranteed performance, and they're still flown the same...especially with respect to takeoff and landing. Transport category aircraft are transport category aircraft, big or small.
Right, certification wise there is no difference if both are certified to the same regulation. However there are still different operational regulations and those govern performance calculations as well. In fact the safety margins imposed on airline operation are much higher than on non AOC operation, especially for performance calculations. And therefore they do matter if the question is to what regulation you operate an aircraft, which was exactly the question asked by Ashling.

Of course you are right, V1 is V1, and aborting after V1 is not teached in either case.
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Old 19th May 2008, 18:55
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Very clear sns... Thanks a lot.
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Old 19th May 2008, 19:07
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'If your sitting in the back of an airliner, and the pilot uses 4000ft to get off the runway, and has 6000 ft to stop if he aborts...vs buring up 7000 ft of runway and having only 3000 ft to stop...because they wanted to save buck....which one would you choose.

'Sounds like they are endangering peoples lives to save a buck'
I believe that many are in this business to make a buck and their customers don't expect "pedal to the metal"

On the other hand you could alswys attain a performance increase by off loading some of the passengers.
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Old 19th May 2008, 19:55
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Its also pretty clear you can't grasp the concepts behind flying large jets safely.
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Old 19th May 2008, 20:04
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it's in every flight manual that I ever looked at...reduced power climbs use more fuel.
and in every Airplane Flight Manual that i have ever looked at, they dont show ANY DATA for ALL ENGINES! Therefore I dont see where you are getting your information from!

is just a tactic by your boss's to keep you from over temping, torqueing, over limiting your engines
Sorry but we have FADEC's! As i said to you before, go talk to your local RR/PW/GE rep and tell him that you are going to use their engines at full power, ask them how long will they guarantee the engine.

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Old 19th May 2008, 20:24
  #40 (permalink)  
ssg
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lol

I don't have to ask, my engines come off at 3500 hrs whether I like it or not...

Are you trend monitoring? Basicaly using them untill something is broke?
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