Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Are Flex / De Rated take offs safe?

Tech Log The very best in practical technical discussion on the web

Are Flex / De Rated take offs safe?

Old 18th May 2008, 08:58
  #1 (permalink)  
Thread Starter
Join Date: Dec 2003
Location: USA
Posts: 194
Are Flex / De Rated take offs safe?

Curious if airline pilots are happy always burning up as much runway as possible to save on supposed engine wear?
ssg is offline  
Old 18th May 2008, 09:50
  #2 (permalink)  
Join Date: Jan 2006
Location: on the Blue Planet
Posts: 255
Safe? Yes, they are. Have done 'em / seen 'em done for years, and until proven the other way, no problem.
Supposed engine wear? Well, heard that on the 320 Bus series the savings are in the order of $150 per degree of flex... good and easy way of saving dough
SOP? Yes, in most airlines flexing is part of the SOPs. TOGA might of course be used in windshear and eventually LOVIS ops.
Happy about it? Any decrease in safety due to flexing as opposed to using full thrust for TO should be small. Continuing this line of thinking one could argue about fuel planning, auto brake used on lo instead of hi, etc etc

Well, increasing safety to a max one could finally stay on the ground

live 2 fly 2 live
F4F is offline  
Old 18th May 2008, 10:08
  #3 (permalink)  
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,584
F4F - troll feeding not permitted here.
BOAC is offline  
Old 18th May 2008, 10:16
  #4 (permalink)  
Join Date: Apr 2003
Location: Europe
Posts: 2,973
Do you ever take off at MTOW?
Same idea, however a flex TO is inherently safer!
Now, if you accept MTOW take offs, if you accept departing at shorter rwy's, if you accept tail wind take offs, if you accept to take off when the temperature is higher than standard, if you accept derated engines, if you accept intersection take offs...then why would flex be a problem?

Good question though.
PENKO is offline  
Old 18th May 2008, 10:28
  #5 (permalink)  
Join Date: Sep 1999
Location: ME
Posts: 5,433
As an airline, we have done approximately 1,800,000 takeoffs during the last 15 years using reduced thrust, to date we havent had 1 incident where the result would have been different if that takeoff was at Maximum Installed Power.

So to answer your question, the answer is YES.....


Last edited by mutt; 18th May 2008 at 10:51. Reason: Inaccurate statistics
mutt is offline  
Old 18th May 2008, 10:36
  #6 (permalink)  
Join Date: Aug 2007
Location: Godzone
Posts: 391
120 million?

21,917 a day?

man, you guys are busy!
toolowtoofast is offline  
Old 18th May 2008, 11:48
  #7 (permalink)  
Join Date: Feb 2008
Location: Wingham NSW Australia
Age: 79
Posts: 1,349
toolowtoofast too quick to get the maths correct

Sorry toolowtoofast, 1,800,000 de-rated takeoffs in 15 years is what Mutt posted. That is about 330 per day, not unreasonable for a large airline. Don't know for whom Mutt works, but if it is a major carrier he is quite likely 'spot on'.
Old Fella is offline  
Old 18th May 2008, 11:57
  #8 (permalink)  
Join Date: Nov 2000
Location: Dre's mum's house
Posts: 1,432
When Flex / Derate / assumed meets all the runway and obstacle clearance requirements it has a further, mostly, hidden benefit: if an engine fails the thrust on the live is lower hence more rudder deflection is available to stabilise the flightpath.

Edited to change the word "control" to deflection"
The Real Slim Shady is offline  
Old 18th May 2008, 12:20
  #9 (permalink)  
Join Date: Sep 2002
Location: La Belle Province
Posts: 2,114
Is it "safe" - yes. Even if Mutt's single incident were in fact a Hull Loss, a rate of 1 per 2 million FH is about half the overall acceptable accident rate, and therefore would likely not be of major concern. But it also clearly is not "as safe", since that one incident did, in fact, occur.

Like all the "lines in the sand" which form part of the underlying assumptions behind the regulations to which aircraft are built and then operated, as we get smarter about operating nearer to the lines on a routine basis there must be concern over degraded average margin of safety. That's one reason for there being an element of "regulatory creep", to stay ahead of (or not too far behind ...) the "design creep" that occurs.
Mad (Flt) Scientist is offline  
Old 18th May 2008, 15:35
  #10 (permalink)  
Join Date: Oct 2005
Location: USA
Posts: 3,218
From my perspective as a pilot, we've used reduced thrust where able in most types of aircraft I've flown, certainly most all turbine equipment, including agricultural, corporate/charter airplanes, and airline equipment. It's been a practice in most types of operations I've flown, from firefighting to ambulance to crop dusting/ag, to cargo, charter, corporate, government, and yes, airline. Where it's safe to do, allows ample takeoff and stopping margin, allows adequate obstacle clearance etc, it's perfectly acceptable.

A reduced thrust takeoff means that one always has the option of pushing up the power as required, though all the performance calculations take into account climb gradients, going, stopping, and obstacle clearance without having to do so. This includes an engine failure; when we calculate reduced thrust takeoff performance, the performance data assumes losing an engine and continuing the takeoff...still at reduced power, still able to make the required gradients. Stopping is a no-brainer; the power will be retarded, ground spoilers deployed, and the aircraft stopped on RTO brakes where installed, or manual braking. Not rocket science, and it's all factored in...without reverse I might add. Reverse only shortens that distance.

Our operations manual spells out exactly when a reduced thrust takeoff can be used, and when it can't. Every reduced thrust takeoff is planned with the specific runway and runway conditions in mind, including any appliable NOTAMS such as temporary obstacles or reductions in length. Every takeoff is planned with an engine failure in mind, as is the departure path after takeoff. Nothing is left to chance.

We utilize reduced thrust takeoffs, and reduced power climbs as part of the nearly universal standard noise abatement departure procedures. We also have reduced climb thrust above 10,000'.

From a mechanic's perspective, reduced engine temperatures make for substantial increases in engine and component life. I've been an aircraft mechanic and inspector as long as I've been a pilot, going back to my early teenage years. I've been working on large radial engines, small pistons, turboprops, turbojets and turbofans for several decades now. I've had these engines apart, boroscoped them, handled every internal part as a regular function of inspection and repair. The differences in operating techniques or procedures show up in burned blades, cracked cans, metal creep, etc. A ten percent reduction in power equates roughly to a ten percent increase in engine life. If this can be done safely, all the more power to the operator...increased engine life also equates to improved engine safety, longevity of components, increased mean time between failures for stressed and hard use items such as turbine wheels and blades, etc.

A common method of operating reduced thrust is to use an assumed temperature. This isn't a wild idea made up by flight crews, but comes directly from the engine and airframe manufacturers after ample testing and design. One assumes a takeoff at a much higher density altitude based on a plethora of criteria and data, and determines if the aircraft could be safely flown off the current runway under those conditions. If it could still do so, still meeting all go and stop criteria applicable to each segment of the takeoff, then it can also be taken off at a reduced power which replicates a takeoff at the higher assumed temperature. The performance data is recalculated using the reduced power to ensure it matches, and when all data adds up, the reduced power is established for the takeoff. Nothing precludes pushing up the power at any point in the takeoff where required, nor performing a full power takeoff if required. However, it isn't required, and where a reduced power takeoff is performed based on an assumed temperature calculation, an engine may be lost and the takeoff continued at that reduced power, and still meet all the takeoff criteria.
SNS3Guppy is offline  
Old 18th May 2008, 18:21
  #11 (permalink)  
Join Date: Sep 1999
Location: ME
Posts: 5,433
Old Fella, .... toolowtoofast correctly pointed out that i had miscalculated...i therefore edited my original post.

Even if Mutt's single incident were in fact a Hull Loss,
It wasnt a hull loss....plus that was 1.8 million takeoffs, not hours.

mutt is offline  
Old 18th May 2008, 18:49
  #12 (permalink)  
Thread Starter
Join Date: Dec 2003
Location: USA
Posts: 194
Most won't disagree that trying to baby the engines to overhaul might help save some engines...I just wont do it on take off...

Increasing ground roll, decreasing stop distances, decreasing reaction times to a crisis in furthurance of saving a buck on overhaul 5 years later seems counter intuitive...especialy with 200 people in the back....

I wonder how many lives could have been saved had ther pilots hit V1 5000 feet earlier, and had more time to accelerate, they were faster when they tried to horse it off, or more runway to stop on.

But hey, a thousand flights a day using flex can't be wrong..and someone thinks it must save some money...but not all airlines use it, they chose safety first, probably more from a liability standpoint rather then a love of safety...

I hear Flex is used primarily to keep the 3000 pilots flying the fleet from overtemping, over torquing, ect the engines..really about keeping pilots from breaking the engines and less about trying to save on overhaul costs...

Operators could have thier pilots fly around at 5% less on cruise to probably get the same intangible results on overhaul costs...

None the less...

When you have one pilot flying the plane, vs 1000, it's probably a good idea to put in place some margin on operating limitations...always a throttle jockey in there that will add 5% over max rated on take off by accident, then it might be a tear down inspection...flex could eliminate type of error...now if guys are using FADEC ect...this is moot...

Even so..John T brought up a good point where the argument at Quantas was how far should flex go..the crews bitching for some kind of margin...rather then being forced to make every take off a 'fly it to the fence' type of thing...

Anyone in here that flies planes that other people fly will probably hope that the last ten guys were easy on the plane..In corporate, typicaly, your the only one that flies the plane...so that type of error is very much reduced...

I know corporate pilots that have threatened to quit because the idea of a partnership and having a bunch of newbies fly the creampuff without them not being there, was to them unsafe...to many variables..not to mention increased operating costs, like brakes, ect from having a bunch of pilots run though the plane vs one..

Well, I hope the next time I fly on airlines, I can push a button on my seat that says says 'vote yes or no for flex take off today"
ssg is offline  
Old 18th May 2008, 22:09
  #13 (permalink)  
Join Date: Jun 2003
Location: Glasgow
Posts: 206
Isn't the idea of the assumed temp method that it reduces wear on the engine such that x number of years down the line someone doesn't set rated thrust then have it go bang at an inopportune moment hence providing a safety margin in itself although it appears somewhat immeasurable. As has been mentioned one always has the discretion of setting full thrust at any stage without any penalty. What are ssg's thoughts on taking off at RTOM which is field length limited given the assumptions that go into how heavy the aircraft is declared to be? This would have the same effect on an assumed temperature take off if I understand it correctly but at least you could be happy in the knowledge you have full thrust set.
Port Strobe is offline  
Old 18th May 2008, 22:16
  #14 (permalink)  
Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
assumed temp reduced thrust take offs are safer than taking off at perf limited max weight as assumed temp calculations assume a lower 1/2 rho.

Is it unsafe to take off at max weight?

The statistics for long term engine wear and the chance of engine failure are conclusive. Anyone with any doubt should talk to RR or GE. They will be happy to send you some data.

So yes Flex/De Rated take offs are not only safe but statistically safer than full thrust take offs.
FE Hoppy is offline  
Old 18th May 2008, 22:50
  #15 (permalink)  
Thread Starter
Join Date: Dec 2003
Location: USA
Posts: 194
You've been sold a bill of goods...

If Flex made sense to save money, trust me every cheap skate millionare with a jet would make his pilot do this as well.

Not one thing from Garret, RR, or Pratt has come through that has told us that it would save money in real terms...
ssg is offline  
Old 18th May 2008, 23:33
  #16 (permalink)  
Join Date: Jun 2004
Location: Australia
Posts: 1,844
It seems, ssg, that no amount of argument can sway you from the conviction that the only significant reason for using Flex / Reduced thrust is cost savings.

The truth is that significant cost savings do occur, but, (and it's a big but), even if no cost savings arose, I would (and do) strongly advocate it's continued use for safety reasons.

The probability of engine failure is directly proportional to the stress placed on the engine, and the widespread use of Flex / Reduced thrust very significantly reduces engine stress, and thus very significantly reduces the probability of engine failure. One of the predicaments faced by the statisticians in assessing the reduction in the number of engine failures is that as the said engines did not fail, no absolute figure can be placed on the engine failures prevented by the use of Flex / Reduced thrust. The one good comparison that CAN be made is with operators who previously used full thrust for all takeoffs, and then changed policy to using reduced thrust. The failure rate decreased dramatically.

So, even if no cost savings arose, or even if it was slightly more expensive, the increase in safety due to decreased engine failure rate makes such policy worthwhile.


Old Smokey
Old Smokey is offline  
Old 18th May 2008, 23:41
  #17 (permalink)  
Join Date: Nov 2003
Location: Over the Moon
Posts: 712
Its common sense to look after your jet.

Sure you can take off at rated power all the time if you want you can also deploy the flaps at limiting speeds all the time. Same end result, they will fail more often perhaps with catastrophic results. So you have to figure whether any added safety margin due to reduced time on the ground outweighs the negative of an increased number of failures. The judgement of clever people who look into all this is that reduced thrust take offs are safer and cheaper than the rated alternate. You can disagree if you like SSG but I first suggest you write to the engine and aircraft manufacturers and ask the same question.

Remember too you can often have a spread of V1s depending on whether your operator is go or stop minded.

Also operators specify when you must use rated thrust eg contamination, strong winds, unserviceabilities etc to cater for situations when the risk benefit balance shifts the other way. Limits are also placed on the maximum reduction allowed. I think its 25% but I could be wrong on that number.

The calculations are normaly very conservative. We don't normaly take any benefit from a headwind or QNH (although we could) and allow for 1.5 times a tailwind and correct for a low QNH. If your taking these comfort factors out its because your payload critical and will be rated power in any case.

As others have said if you lose an engine you can increase power if you wish to give more margin but SSG you must understand this is not required to continue safely. You can stick with the reduced thrust if you so wish.

The Airforce do it too by the way.

SSG Reaction times are not reduced. You stop up to V1, at or after it you keep going. Same decision same reaction time wherever on the runway you happen to be. Less runway left certainly however I very much doubt if it makes anything like 5000' of difference as you have a limit to how much you can reduce by and in reality the derates are often not that large.

Horse it off the runway? You need to get this cowboy mentality out of your head SSG. I've had many engine failures in the sim at V1 and have never horsed the aircraft anywhere. Always a smooth rotation and the aircraft has done as it should so maybe the people who build the sims are in on the bill of goods as well.

The vast majority of airliners now have FADEC or EECs of some type that will look after the engines so overboosting is not an issue now for the vast majority.

Out of interest, as I have no experience of Business Jets, do they operate to Perf A? Some of them? All of them?
Ashling is offline  
Old 19th May 2008, 01:44
  #18 (permalink)  
Thread Starter
Join Date: Dec 2003
Location: USA
Posts: 194
Ok...lets talk engines..

All the airline pilots in here that have removed, replaced, bid. boroscoped, bought, sold, leased, argued for parts, and decided which components to install on thier new jet engines...please raise your hands..

I didn't think so.

All the airline pilots in here that have flown one plane to overhaul then actualy seen how thier actual flying habits have affected those particular engines..please raise thier hands

I didn't think so.

Engines are certified and built based max thrust take offs, max cruise speeds ...to make it to designated overhaul times...say 3500-5000 hours.

I won't argue that taking better care of an engine, or running them right doesn't help...they do..that's why my overhauls and hots are really cheap...but if you trying to tell me that the trade off between flying it to the fence in a Flex take off is the exta safety of un worked, more reliable engine...that's really a stretch..

And it still won't make a diff when that flock of birds, bad fuel, puddle of water goes into your ultra reliable, unworked, babied engine, at the end of the runway....
ssg is offline  
Old 19th May 2008, 03:44
  #19 (permalink)  
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 3,954
In the airlines we do not necessarily have fixed overhaul times but rather overhaul on condition of parts. As i said before we had engines that could remain over 40.000 hours on the wing, flying shorthaul operation with a take off every 70 minutes, because they were looked after in daily operations. And that means largely flying them less stressed by using flexed/assumed temperature/derated thrust methods.

We have to do the calculation (and guess what, pilots do technical jobs as well, many of them started as certified mechanics) and the costs saved as well as the amount of maintenance required because of wear is several magnitudes lower than on engines that have to do a full rated thrust take off every single time.
Denti is online now  
Old 19th May 2008, 03:51
  #20 (permalink)  
Join Date: Feb 2005
Location: flyover country USA
Age: 77
Posts: 4,580
There's the added factor of operating your aircraft at a more nearly constant thrust-to-weight ratio. What does that mean?

It's pucker factor. If you typically operate from long runways, at moderate TOGW, at moderate density altitudes, and use rated thrust all the time, you get spoiled.

But then comes that long leg out of MEX. The book says you are fine, but the plane seems so doggy and you've never used so damn much r/w before and HOLY **** WILL WE EVER MAKE VR????!!!

OTOH If you're used to flex takeoffs, that MEX takeoff (while within AFM limitations) won't seem so terrifying.
barit1 is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.