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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, 20:29
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That's progress for you...
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Old 19th May 2008, 20:47
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"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. "
To address each point:

1. On a snapshot basis, I have to agree. However, if you routinely employ reduced thrust, your engine fuel burn remains closer to a new spec, and thus the long-term effect is a trip (and fleet) fuel savings. (It's not only EGT we're preserving, but fuel burn as well)

2. Not all engines are really "less efficient when not at full thrust". The best modern engines actually have an SFC "bucket" or J-curve i.e. most efficient at cruise.
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Old 19th May 2008, 20:51
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"Full thrust" is a misleading term, as it may cause one to suppose it means whatever power a full thrust lever setting can deliver. Instead it is better read to mean the maximum power setting under a given set of conditions...which will nearly always be substantially less than the maximum temperatures, EPR's, EGT's, TiT's, ITT's, N1's or RPM's that the engine can actually deliver.

For most turbine engines, 90% N1 or RPM is closer to the max limit under a set of conditions, and closer to the most efficient operating range for that engine. That, of course, is very "ballpark," and not at all specific to a given type.
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Old 19th May 2008, 20:55
  #44 (permalink)  
ssg
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Assumed temp...this is conforting...

Increases TODR.
Therefore you may come to rest on the stopway after a stop from V1.
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Old 19th May 2008, 21:09
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And yet you want to stop AFTER V1???
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Old 19th May 2008, 22:07
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SSG

I guess it's "pedal to the metal" every time the lights go green at an intersection ?..Your Ferrari wouldn't last too long

(And carefully used engines can stay on the wing for much longer than 3,500 hours, in excess of 14,000 springs to mind.....ask RR !)
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Old 19th May 2008, 22:45
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ssg, you have quite a gap in your education. Engine hours mean virtually nothing in the airline world, and haven't for three decades.

Airline engines are now removed either for cycle limits (rotating parts and pressure vessels) or for condition monitoring alerts. They may even be removed for excess fuel burn.

Any engine removed at 3500 hours would trigger a technical investigation by the airline and the OEM. It's not unusual for an engine to run 20,000 hours or more.

(When a KC-135R had a FOD during Desert Storm - 1991 - there was no USAF maintenance crew in-theatre that had any experience in engine change on the bird, and a contractor crew had to be flown in...)
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Old 19th May 2008, 22:58
  #48 (permalink)  
ssg
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I am all ears...

Since airline engines don't follow TBO hourly limits (loaded question- hint hint)

Just how do they determine when to overhaul the engines...?

And since I know what cycles are...which pressure vessels on the engines are you talking about...
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Old 20th May 2008, 00:22
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Boresope inspections and trend monitoring are among methods used to plan for engine repair/replacement. A Pratt JT-9Q engine runs upwards of $2,000,000 for parts alone to refurbish the hot section.
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Old 20th May 2008, 02:09
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SSG---you should really look up reduced thrust rules for yourself http://rgl.faa.gov/Regulatory_and_Gu...7?OpenDocument then argue with performance experts and the like ---

Last edited by Pugilistic Animus; 20th May 2008 at 04:49.
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Old 20th May 2008, 04:22
  #51 (permalink)  
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Page doesn't come up...

Nothing sucks worse then barely having enough runway, barely getting second segment, barely having the numbers...and flex forces pilots to simply reduce power to fit thier runway and climb gradient...how fun...

If Flex just gets you balanced field and second segment numbers, then the pilots better have it together for every emergency...time has proven they don't...extra runway saves lives...shorter fields adds risk...

You can't dispute, reduced thrust burns up more runway, less time to stop if need be.. The sooner you get off the ground, and up in the air, the further you are away from impacting terra firma...the more runway you have infront of you during an abort, pre, during, or post V1, the better the chance of keeping people alive..

Just because flex turns every takeoff into an excuse to burn up more runway to save corporate some money on the overhauls doesn't mean it's safer. If the argument is to keep your 10000 hour trend monitored engine up in the air another 10000 hours..then take a walk in my shoes...our engines come off at 3500 hours whether we baby them or not. So we use them...

Now which is safer my engines with 2000 hours or yours with 10000 hours?
Even if the blades look nice every inspection, take it from a guy that had a number one bearing failure at 35000 ft...you can't boroscope for that...oil analyis means didly unless it's a slow, slow problem...that gets caught at the next 150 hour insp...

And if I sit in the back of your airline, burning up 8000 ft of runway, at 70% power,...it's little comfort to me when I see the plane rotate and then grass starts going by...

Besides if flex ever did get popular in corporate, which it's not...I just can't see some GV burning up 8000 ft of runway at 50% power, billionare in the back, white knuckles watching the plane lift off at the fence...

He will walk up to the capt and ask why the plane lifted off so slow.......'because I was trying to save you money' .....that's where Flex ends in corporate

You guys want to run the planes to the end of the runway to squeeze some more time out of your engines..fine...but the rest of us don't...and look at our safety record....not to bad huh?

We can afford to go max rated, we can afford to burn up less runway, we can afford to be as safe as possible..

If your going to make the case..that airliners don't fly to the fence using flex, then don't point the ire at me...start looking around this forum, guys that tell me it takes 7000 feet to get a loaded 737 off the ground...so what about a 747, or Airbus then? Considering the world isn't full of 12000 ft runways...we are left with an average of say....8-10k ft runways...so does flex help to push balanced field from 7000 ft in a 737 to 10k? ...now the pilot doesn't want to reject, fear of hitting the fence...so he goes...and flies this time his unflyable aircraft to the end...into the Potomic, into a Hotel, into a market in Goma...into....

Last edited by ssg; 20th May 2008 at 05:08.
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Old 20th May 2008, 04:48
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Yes, using TOGA on every takeoff would increase the stopping margin.

If that is you only contention, then by all means, I hope you always use 12K plus foot runways, anything else would be unsafe, right?

As plenty have mentioned, large aircraft engines are normally trend monitored and that is what determines when overhaul is neccessary, not a number of cycles or hour limits.

Is Flex/derate unsafe, the numbers would say no. Plenty would say, as they have here, that the greatest stress is put on an engine when it operates at TOGA power.

You want to use TOGA on every takeoff, by all means, nothing can be said that will change your mind. The airlines will continue to use Flex/Derate and the large engine manufacturers will continue to recommend it!
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Old 20th May 2008, 04:56
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Ssg, I've fixed the link---


NO--the parameter that's limiting at full thrust is not necessarily the one that limits on reduced thrust---one could be ASD limited the other Obstacle limited---the main justification was given by the likes of Mutt, Old Smokey---and the other who routinely use this procedure and have satisfactory operational experience using the procedure---now point out to me the EXACTLY the dangers of this method?
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Old 20th May 2008, 05:28
  #54 (permalink)  
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Got the link thanks...

I read AC25-13...thank for taking the time to post this.

And your point...that Reduced thrust 'may' increase engine life and reliability?

Look I will throw you guys a bone..if I had a 40,000 hr engine, a million cycles on the bearings, a lake of burning lava at the end, and a ten hour trip over the Polar Ice cap to do for the next year, in this plane...no money for engines, airline was broke, we had to do the trip,..yeah I would baby those engines like you wouldn't believe..flex, reduced cruise, watch the temps like a Hawk, stay away from FOD and birds, no runups on the dirty ramps...you name it...anything to save those engines....

But is that a safe operation? If you guys can't afford to overhaul your engines at the same intervals as the rest of us...maybe it's not so safe...and it's forcing you to take chances...

If Flex is so integral to extending engine overhaul times based on boroscopes...maybe we should talk about just how far out you guys are taking these engines..and what you will do to keep from having to pay to fix you engines...and your planes...

Let me bore you with a story...in younger years, as a flight instructor in twins, one of our Cessna 310s had a gear problem, nose gear wouldn't come down...so the pilot landed the plane..didn't feather any engines, let the nose come down, trashed two props and two engines...as per the book.. Someone else got to pay the fix.

If it had been my plane, I would have shut down one engine, feathered a prop, bumped the starter to flatten the two blades...assured my landing on our 8000 ft field, then shut down my last good engine, bumped the blades around...nose hits..no blades or engine tear downs...costs me a little skin and paint....

Now which is safer...using power to the end, or being a cowboy to save a buck? If I worked for you guys, there is a little cowboy in me that likes pushing it sometimes,, but I might see passenger after passenger load up and rethink that...

Trust me, I get Flex..but I don't have to pay the bills, I can afford to be safe, my boss isn't cheap, he can afford the engines, and has...we don't need to take chances to save a buck...it's not worth it..

Last edited by ssg; 20th May 2008 at 05:41.
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Old 20th May 2008, 06:41
  #55 (permalink)  
 
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SSG,
Do you ever accept intersection takeoffs?
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Old 20th May 2008, 06:58
  #56 (permalink)  
 
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ssg

It seems to me that ssg may in fact be a watchmaker, the way he keeps winding people up. Or maybe he is just a self indulgent reader who knows how to fix the world and who enjoys reading his own solutions. It is fairly obvious that one thing ssg cannot do, that is accept the view of others even if they have combined years of experience many times longer than ssg has been on the planet. Time for a change.
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Old 20th May 2008, 07:16
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Besides if flex ever did get popular in corporate, which it's not...I just can't see some GV burning up 8000 ft of runway at 50% power, billionare in the back, white knuckles watching the plane lift off at the fence...
I don't think that's legal. Sure isn't on the aircraft I operate. 8% N1 reduction is the limit. In addition to that limit is my own. I always leave myself at least a 1000kg margin. My aircraft is not new and neither am I.
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Old 20th May 2008, 07:42
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SSG if your talking about 50% thrust reductions you clearly have not much of a clue about flex.
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Old 20th May 2008, 07:55
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ssg
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Good Question...

Right...in a jet, no, unless by myself...I tend to choose the longest runways when possible.

-------------

Ashling...Did I say Flex? Did I say Airbus ...Did I say reduced thrust, or Derated?
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Old 20th May 2008, 08:28
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You said Flex when you spoke of a 50% reduction.
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