Hello again,

I have had a new assignment set at my college in aircraft and airfield performance and this specific assignment is for a distinction. The question is: 'What are the benefits in de-rating for take off. Please describe and explain to your fullest knowledge and research made. Minimum of 2,000 words (WC)" So i have come up with a few key benefits on why de-rating affects aircraft performance and how it can affect other factors such as pax comfort etc

 

By de-rating aircraft engines are not performing on full power which is increasing more stress on the engines. Therefore by de-rating there is less stress on the engines which can potentially make the life span of aircraft engines longer consequently decreasing maintenance on the engines. This can benefit the airline itself as maintenance costs can suggestively be cut.

 

Passengers may benefit from a de-rated take off. As the engines are not performing to their maximum takeoff thrust there would be a less hard acceleration force whilst taking off. As a result this could be much more comfortable for passengers especially nervous passengers. Also with a de-rated takeoff the clime rate could be significantly slower than a full thrust takeoff. Therefore the cabin can pressurise more efficient which can be more comfortable for passengers unlike a higher climb rate where the cabin pressure could not match the outside pressure as efficiently. Therefore passengers may experience blocked ears, blocked noses, tiredness or even headaches if the climb rate is fast.

 

De-rating can effectively reduce the aircraft fuel burn. Therefore because the engines are not working as hard there is less fuel being injected into the combustion chamber therefore this can result in lower fuel costs for the airlines benefiting the airline itself.

 

Lastly de-rating may also reduce noise levels for local residents. The engines are making less noise to what they would be if they were performing at full power therefore the noise can slightly be reduced when de-rating. However noise constraints are usually in power for reducing noise such as SIDS/STARs or even throttling back the engines over a specific height (climb thrust)

 

Right as you probably know 2,000 words is a lot to ramble on about de-rated takeoff's but that’s all i can think of for now i've just briefly thought of these in my head. If you could help me out a little bit and share your knowledge on this topic i would appreciate it greatly!

 

Regards,

Wowk

Write a thesis on how derating decrease the carbon footprint and the environmental impact . Professors luv that ----. Say it Say it.

Write a thesis on how derating decrease the carbon footprint and the environmental impact . Professors luv that ----. Say it Say it.

 

Ahhhh yes!!! That's a very good one!

If you're still battling to reach the min words, less stress also reduces the chance of a mechanical failure at some awkward point - like V1, which results in fewer bits of turbine FOD needing to be swept up afterwards. Could mutter something about minimised peaks in thermal cycling in the metals, depending on how much more you need to fill out.

You could stretch the text a bit when going into the history of the de-rating methods and needs. Maybe even naming the different authorities playing a role there and when/why the methods came up in the first place.

 

As the engines are not performing to their maximum takeoff thrust there would be a less hard acceleration force whilst taking off. As a result this could be much more comfortable for passengers especially nervous passengers.

I never looked at it that way. But maybe you can add that some noise abatement procedures come with a rather big thrust reduction right after takeoff, which make it useful to explain that cutback to the passengers beforehand as they would otherwise wonder why the engines 'run silent' and then would get nervous.

 

The de-rate envelope can not be summarized as 'saves fuel' per se as there are other factors one has to take into account. In fact, you will end up saving engine wear but also burn more fuel on your way up into more suitable regimes for the plane in use. Time is a vital factor.

Roughly spoken, you benefit from placing your vehicle in the optimum flying regime as fast as possible when it comes to fuel savings. For instance, receiving clearance to go faster than 250 kts below 10.000ft may indeed be a nice thing for the companies.

 

In general, you will always have to look at the savings on engine wear versus the increase in fuel burn. This bias might therefore change if the fuel prizes increase. Same goes for the actual noise in place, which is why there are different noise abatement procedures. The 'close in' (ICAO NADP1) and 'distant' (ICAO NADP2) procedures for example, roughly pointing at where you want to avoid noise. At some place, you have to add thrust (and therefore make noise), to avoid it in another segment of the climb.

 

From an aerodynamic viewpoint, you could add that, depending on the plane's design, flying faster means lower thrust demands and therefore noise and fuel consumption than forcing the same design into a regime where induced drag is huge. Concorde (r.i.p.) was a prominent example due to the ways of her low speed flying, deep in the region of reversed command. So her 'de-rate procedure' always started with a rather noisy and fast acceleration to then being able to cut back on the thrust and pass by sensitive areas at some 240+ kts. That's still slow for her, but not as slow (and bad) as some 190 kts for example.

 

This is just to show that the de-rate usage at least has to cover 'how much?' and 'when?' in the planning stage. Not to forget about the 'how long?'.

 

I think you have a theoretical point with the 'more comfortable cabin pressure', but I doubt that the system redundancy and overall output of the pressurization systems practically make a difference for the passengers. Means that the de-rated takeoff and climb may well be as comfortable as the full thrust one, as long as the crew doesn't turn off the packs and forgets about them like I sometimes do in the sim. :blush:

 

On a long shot, you could look at how noise abatement procedures are designed, how they measure and estimate values and how the aircraft design lead to those different stage categories.

 

And, not even finally, you can explain how you de-rate different plane setups. You know, the assumed temperature method, the fixed derate or a combination of both. Or how did/do steam gauge planes without any performance computers determine which de-rate to use?

 

Depending on the manufacturer and company's procedures, you will also find factors which render any de-rate usage to be not recommended. This could mean that icing conditions, heavy rain and crosswinds cancel out the plans on saving engine wear cycles.

 

(Maybe) finally, one could speak about how de-rating can affect safety. It's nice to save engines, but it's not nice to overrun a runway. Sounds a bit rough, I know. But I recall a case where a crew received wrong dispatch information (incl. too low Vspeeds), was happy when entering them into the FMC and later found the rotation and climbout to be 'feeling wrong', fortunately applying full TO thrust and climbing out of that dangerous regime. Here's the description of that one. http://www.skybrary..../books/1247.pdf

 

In short, they've used TO values for a GW of 120.800kg (on an A332) while the plane actually was at 210.183kg. Yikes! Not to mix that up, the de-rate value (FLEX) was only one being wrong then. The VR being 26 kts too low surely is a for some readers I guess.

In general, you will always have to look at the savings on engine wear versus the increase in fuel burn.

I just saw that the wording may be misleading. You increase the trip fuel when sticking to the climb portion for longer than needed due to a de-rated thrust setting. Hence the note on the time factor. Time (and distance) of you being 'in climb' increases, the cruise portion decreases.

 

Well, you will find them doing the calculations for every trip, weather and payload situation. Asking for the overall de-rate benefits as per the thread title would therefore cause the dreaded 'it depends' answer. ^_^ At least from how I understood the subject.

Probably worth asking the same from the professionals on pprune as well.

Don't forget, Flex thrust makes for less asymetric thrust if an engine failure should occur on the T/O roll.

 

Regards,

Ró.

Taking the point from Rónán, this then means that, when you are limited by V1MCG, the takeoff weight can be higher with a de-rate in place than without.

Limited by V1MCG,

Well now you've opened a can of worms LOL Mentioning or bringing up VMCG or VMCA never ends well.... :wacko: ^_^

Haha, I saw the other threads, Rónán. I hear you. But, for the most part, it was a great read and I've also found some nice docs in the Net regarding the whole V1 calcs and concerns on aircraft performance.

 

The linked doc from above, with the wrong dispatch values, was one outcome of it. Quite a read and a bit of a shock in both ways. First, for the actual lack of proper data (and that it went through three pilots) and second, for the good outcome, large tolerances and the 'power of TOGA'. ^_^

You have received great answers! If you need more material, you could head over to smartcockpit.com. Under flight ops, there are couple of texts about using de-rating (also about the dangers).

I have actually spoken to many real world pilots about this. Esentially, they have all told me that it is safer at lower thrust on T/O (especially in the scenario of an aborted takeoff). With lower thrust, as Ronan said, you will have less asymetrical thrust. The engines also have a lower chance of a compressor failure when operating at cooler temperatures and lower thrust.

 

Also, lower thrust significantly extends the life of the engines. Unforutnately, thrust derations result in higher fuel burn due to longer durations to get to climb altitude. But, the engine maintenance costs far outweigh the price of fuel.

Google is your friend

 

Note the difference between "reduced" and "de-rated" takeoff thrust.

it is safer at lower thrust on T/O (especially in the scenario of an aborted takeoff).

 

You could also consider the human factor. Check list require you to have the flaps set before takeoff. Even so, unintetional flaps up takeoffs have been performed (and not all of them where succesfull).