Engines ratings and takeoff performance
 

Hello there,

I am currently investigating the (vast) topic of takeoff and climb performance in airliners, and I am looking for some information regarding thrust settings during the initial part of the climb. This is an ATCO speaking, so if you spot any inconsistencies in what I think I know, please bear with me and don't hesitate to correct me ! :)

I have read up about derated and flex/assumed temperature takeoffs. I understand that the selected climb thrust, while usually lower than the TO thrust, might be higher sometimes (derated takeoff but imposed climb on the SID for example). I know that jet engines produce less thrust as the air gets thinner (higher or hotter). Now I need some glue to make all these pieces of information fit together :-)

• If an engine is rated for 27K lbs @ ISA+15, does that mean that, barring all reduced-thrust considerations, a TOGA takeoff will use those 27K at up to ISA+15 & sea level, or are there other restrictions? [Status : Not answered, but I assume this is correct]
• What is the most common limiting factor for a derated/flex takeoff? I have a feeling it has to be runway length, with higher V-speeds implying longer TODR and ASDR, but that's mere intuition. [Status: not anwered yet]
• Where can I find information about the maximum climb thrust for a given engine? So far I have only found maximum takeoff thrust and maximum cruise thrust at 35'000ft for example (this from the CFM56 website). [Status: ANSWERED. What I need is MCT as in Maximum Continuous Thrust. Thanks !]
  
• So far I have found quite a lot of information on turbofans, no so much on turboprops. I have read somewhere that props also perform derated takeoffs (any word on flexible takeoffs?). What happens when they switch to climb thrust, if there is such a thing (or is it just a prop pitch change) ? [Status: not answered yet]
  

Thanks for your valuable input :)

LeCoyote

"Flex" or derated takeoff thrust can be anywhere from 5 to 25% below max takeoff thrust, depending on the airplane, engine, and autothrottle computer implementation. It is calculated so that the minimum thrust is used to satisfy the runway and climb limitations for the airport (but NOT necessarily SID climb restrictions). Full rated takeoff thrust is seldom used, but exercised once or twice a month.

A common implementation is to use 2 fixed derates (e.g., -10% and -20%), with the option to use a calculated "flex" or "assumed temperature reduced thrust" instead. The "flex" or reduced thrust can be anything down to the max allowable reduction (e.g., -25%).

Climb thrust is usually a bit lower than the derated takeoff thrust. However, when using a large T/O reduction or derate, the climb thrust reduction is often based on full T/O thrust, which then may make climb thrust a bit MORE than T/O thrust.

FLEX is NOT Derated.

FLEX is reduced thrust.

Derated is a fixed reduction for which there is a complete set of separate data.

A derated thrust can also be reduced.

Quite right. And while flex, assumed temperature or reduced take off thrust (all meaning the same thing) can be up to 25% reduction a combination of derate (fixed lower thrust rating) and reduced thrust can be more than 25% reduction.

For example on the 737-800 you have full rated thrust of 26k lbs and two fixed derates of 24k and 22k. You can reduce each of the derates further by up to 25% using the fixed temperature method. The fixed climb derates (climb 1 and climb 2) are based on full rated thrust which means you will most certainly get higher thrust when reducing from take off thrust to climb after using derate 2 and a higher assumed temperature.

When using a fixed derate like this you have to be aware that they also impose different base performance data like different Vmc speeds.

Max climb thrust is limited by time (manufacturer reccomendations, allthough usually not a factor being a reson why we have climb thrust) speed (n1) and temp (EGT). Having said that there is no such thing as max climb thrust as climb thrust is just a reduction in thrust after a safe altitude is achieved to save money.

Not one of your questions but info you might be interested in;
People (pilots/journo's/ pax) often say that reducing thrust for take-offs is to conserve fuel.They are wrong. A reduced thrust take-off uses more fuel than a full thrust take-off to reach cruise altitude. It often saves money...but not fuel. I find that interesting, please disregard if you find it boring :}

Thank you for that Framer, this is a common misconception. Many Pilots believe a reduced power take off saves fuel.


On our most extended range limiting routes I always used full power for take off and Max continous thrust in the climb until it equalled climb power.


Never exceeded any limits and it made a significant, favourable difference in fuel burn.

Actually, a 'Reduced Thrust Take-Off' conserves engine life, or, allows you to keep an Air Conditioning Pack ON for the take-off (dependant upon aircraft type).

In the last 5 years of tech instruction I don't think I've come across anyone who thought Reduced thrust was to save fuel. That would be a sample of around 1000 pilots on 4 continents. Careful with those sweeping statements framer.:ooh:

Ok Hoppy, I'l be careful not to let my sweeping statements get toooo out of control;

...........................bugga......I think I need help, next thing ya know I'l be saying things like "chicks can't parralell park" and " FE's need to chill out more " :eek:

Hello there,

Thank you all for your feedback !

Thought as much. I guess what I am more interested in finding is then Max Continuous Thrust. I found figures that suggest that MCT is approx 93% of the max TO thrust (CFM56-3B-2, GE90-76B), and apparently this kind of data is easier to find on the Internet (EASA has some figures). I will look that up. Thanks !

Something I forgot to mention in the initial post: this is for use in an professional ATC simulator. We are trying to get the A/C performance right, and this obviously cannot be achieved until we reproduce reduced-thrust takeoff behaviour in a not too shabby way :-)
In order to do that, one of the important points is to figure out the limiting factor in thrust reduction calculations. In other words, if for example today's A319 can fly a FLEX 49°C but not 50°C, is it usually because of runway length (ASDR > ASDA, my guess) or something else ?
I understand that a required climb gradient (obstacle clearance) could be a restriction here, but I am after the most common limitation really.

Again, thanks for your input. I will edit the original post to reflect the status of each question.

Cheers,

LeCoyote

Really depends on the airport, runway and environment. In real life most often i do see an obstacle limit on 737 700/800s, however field length is of course limiting on short runways (2000m or less). On runways in excess of 3000 to 3500m it is nearly allways obstacles that limits our performance, not any field limit (ASDR, TORR).

Hi Denti,

Interesting... Let me see if I get this right: you look up your tables, and at some point the N-1 climb gradient does not allow you to clear the obstacles, so you use the next available thrust rating (be it assumed temp or derate), or something like that ?

What would be the climb gradient in that case ? I work at LFPG, where the airport is surrounded by flat fields, so obstacles are not really a problem usually. This is the reason why I am asking about this.

At any rate (no pun intended...), I assume that this would happen when you have an obstacle that reaches higher than the usual, obstacle-free 1.9%, right ? Or am I still missing something here ?

Cheers,

LeCoyote

May I suggest that in this thread, the term MCT is ambiguous, and instead, Max Climb or Max Continuous should be spelled out.

Max Continuous is a certified rating; the certifying agency FAA/JAA/etc has been satisfactorily shown that the engine will run all day at Max Con as long as fuel and oil hold out. It is intended for OEI or similar unusual situations.

Max Climb is a limit imposed by the OEM for warranty purposes; Its intent is to maintain parts life to expected levels, preventing premature engine removals.

Max Climb is less than Max Con at low altitudes, of course. But engines may have performance curves that slowly increase Max Climb as the bird climbs, so at TOC Max Climb may actually converge with Max Con. If so, this is intentionally done to meet climb profile requirements.

We, and i suppose most airlines for that matter, do not use books with tables. Instead we use an EFB-tool that checks performance for given parameters and selects the lowest possible combination of derate and assumed temperature while using or discarding improved speeds. So the performance gets lowered until we just fulfill the legar requirements to clear a given obstacle situation and staying within take off distances given.

It is sometimes interesting what kind of obstacles are there to be considered even at pretty much flat fields and i wouldn't even call the area surrounding LFPG as flat. But i have to say we only use that field as alternate for our next door stop at LFPO.

Hi there,

I am OK with the basic concept of getting the lowest possible thrust setting while still being able to comply with the requirements.

What I am more interested in is which of these requirements is likely to be the most limiting ? Would it be the n-1 climb gradient if the runway is long enough ? Can it be the 1.9%, 1-engine-inop requirement (or is it 1.6%, I get confused...) ?

In LFPG, the AMSR is 2000ft in a rather wide radius around the airport (apart from the city of Paris obviously), so I assumed we were in the middle of a rather flat area; I am ready to accept that there can be flatter areas :)

Cheers !