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?

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 :ok:
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 - troll feeding not permitted here.

Do you ever take off at MTOW? :ok:
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.
:)

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.....

Mutt

120 million?

21,917 a day?

man, you guys are busy!

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'.

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"

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.

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.

Old Fella, .... toolowtoofast correctly pointed out that i had miscalculated...i therefore edited my original post.

It wasnt a hull loss....plus that was 1.8 million takeoffs, not hours.

Mutt

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"

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.

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.

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...

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.

Regards,

Old Smokey

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?

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....

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.

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.

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 !

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.

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

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?

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.

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. :)

SSG

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.

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.

Thanks Guppy

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

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. :confused:

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.

ssg
 

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.

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. "

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.

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.

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.

Very clear sns... Thanks a lot.

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.

Its also pretty clear you can't grasp the concepts behind flying large jets safely.

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!

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.

Mutt

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?