De-rated, flat-rated, reduced power T/O

[tornadoflyer]

Is there a difference between de-rated and reduced power T/O? In what context does a flat-rated engine fit in?

Thanks a lot in advance for any comments.

[notdavegorman]

When one derates an engine, the 'datum' from which the thrust calculated from is shifted. For example, B737 engines can be rated at 20,000 lb of thrust, but can be derated down to 18,500 lb. One of the advantages of this is reduced maintenance costs. From this one can reduce the takeoff power setting as far possible given the conditions on the day, in the case of the 737 this is done by fooling it into thinking the ambient air temperature is higher than it in fact is, Boeing calls this the "assumed temperature" technique. If we tell it that the temperature is 56 deg C, the aircraft thinks the air is less dense than it really is, and will reduce the amount of fuel injected into the engine, therefore the thrust output will be consequently lower, reducing noise and strain on the engine.

With a flat rated engine, it will produce it's rated thrust up to it's rated temperature. It won't produce more than it's rated thrust below that temperature - the thrust output against ambient air temperature curve is flat up to the engine's rated temperature, thereafter output is reduced due to falling air density.

[H Peacock]

Surely if you tell your aeroplane that the temperature is hotter than the prevailing conditions it will give you more power. I'm sure everything that I have flown to date has had a worse performance getting airborne on a warm day than a colder day. A warm day is when I would want more -not less - power.
Now if you told the 'aeroplane' that it was colder than for real, then I would expect it to use/need less power.

[flyboyike]

The PT6 in "my" Meridian is thremodynamically capable of 1050hp, but is de-rated to 500hp. That means that an electronic governor will prevent the engine from putting out more than 500hp. There is an override lever on the throttle quadrant that would allow me to bypass the governor and, theoretically, get the whole 1050hp, but that would probably exceed temp and torque limits.

[notdavegorman]

Sorry, not a very good explanation. The assumed temperature will be higher than the engines 'flat' rated temperature, there FMC [flight management computer] will use a lower N1 [fan speed] to give the rated thrust for that particular temperature, i.e. less than the full (or derated) thrust.

To put it another way, although the rated thrust of an engine may be 20k of thrust up to 30 deg C, at 55 deg C the engine might only produce 15k. By entering an assumed temperature, one is instructing the FMC to calculate an N1 speed for that temperature.

[Mad (Flt) Scientist]

Surely if you tell your aeroplane that the temperature is hotter than the prevailing conditions it will give you more power. I'm sure everything that I have flown to date has had a worse performance getting airborne on a warm day than a colder day. A warm day is when I would want more -not less - power.
Now if you told the 'aeroplane' that it was colder than for real, then I would expect it to use/need less power.

Ah, but you're not telling the aircraft; you're telling the ENGINE.

The fact that the aircraft would like more power on a hot day is due to the fact that an engine can only generate a lower amount of power on that hot day; you can't ask it to give the same power as on a 'cold' day because it'll suffer some form of thermal or mechanical damage, through overrunning a limit. therefore by giving the engine an inaccurately high temperature indication you cause it to, in effect, throttle back to protect itself against what it thinks is the risk of high temp operations. That causes a reduction in max thrust.

Other than the engine effects, the temperature effects on the aircraft are somewhat secondary by comparison.

Flat rating, De Rating, reduced Power

The way I like to simplistically think of these are as follows:

If an engine were allowed to generate the maximum thrust its design offers, thrust would be maximum at some absurdly low minimum operating temperature, and lowest at the maximum cleared temperature. Let's say ISA-40 and ISA+40

If you left the design like that, you'd have various design cases driven by the very high thrust at ISA-40, which is of no real practical use, since most operations are in the ISA +/-15 range. So, to avoid having penalties due to an essentially useless thrust level, the engine is run at less than max capability up to some temperature, at which point we decide we NEED the max thrust the engine gives, and then we run as hard as possible above that. The engine is then said to be "flat rated" up to the temperature where we start to run 'flat out'.
A second benefit of this is that the engine is now often run at less than 'flat-out', which saves wear on the engine.

That's a FLAT RATED engine.

If now, in practice, I find I don't actually need or want even the amount of thrust that engine gives for a specific aircraft design, I could choose a different flat rated thrust level. I redefine my thrust vs temp curves, and now have a different set of engine performance from the same physical engine. In this case I have a DE-RATED engine. (If I go the other way, I could squeeze more power from the engine and in effect RE-RATE or UP-RATE the engine). In effect, derating/rerating is a way to put a different engine on the aircraft (in terms of its impact on the design) while keeping the same actual parts.

Finally, assuming I have a given engine on my aircraft, with a given flat rating, because the OEM decided they wanted 20,000lbf per engine for the design mission, but I only need 18,000lbf per engine to meet my mission-of-the-day (long runway, light aircraft, whatever). Ideally I'd like the OEM to give me exactly the right engine/airframe for each flight, but that's not practical. What IS practical is to change the engine behaviour 'on the fly' by reducing the thrust artificially for today's flight only; that's where a 'reduced thrust' takeoff comes in. It's a way to get the right match of thrust and mission for every flight, without having a hangar full of engines all at different RATINGS that I have to swap on and off of the plane every day.

[divinehover]

An engine is flat rated up to a certain temp. This means it's performance is unchanged by a increase in temp up to it's flat rated temp. Eg. Engine A is a 600HP enigine flat rated to 30deg. This means it will provide 600HP up to and including 30deg. Thereafter it will loose power with a Temp rise.

Assumed Tempreture take off (Boeing) and Flex take off (Airbus) are the same thing. Let's say an engine is designed for 25000 pounds of thrust under ISA conditions. An a/c needs to take off from Heathrow (sea level) for a flight to Paris (short hop, little fuel, light a/c). This aircraft has a Max design take off weight of 70 000kg. On this day it only weighs 55 000kg though. The pilot calculates that at this weight the a/c could get airborne (legally) should the temp rise to say 65deg. We then limit the take off thrust N1 or EPR to provide the thrust that would only be available at 65deg (eg. 23500 pounds thrust). Max design thrust is available if the Thrust levers are advanced by the pilot.

I speak under correction regarding Derated take off thrust. The B747-400 can do a 10% or 25% derate take off. Here the engine is detuned via the Fuel control unit by the percentage requested.

In both derate and assumed temp the a/c must still comply with all take and climb performance requierments for that paticular runway and subsequent climb segments.

[barit1]

The whole concept is based on the typical airline situation where the fuel+payload are well below what the aircraft will carry out of a given runway, or (viewed another way) there's excess runway available for the given TOGW.

ANY engine has less power/thrust available as OAT increases, because of temperature and/or rpm limits. This is regulated by the engine control system, or by power management charts on older engines.

If an engine is flat rated to 30º C, it means that while power/thrust decreases above this OAT, but is not permitted to increase (ie held flat) below that temp.

So if the airplane has excess performance at a temp. of (say) 15º C, we look up the max OAT for which the airplane would be legal for this runway, and that might be 38 or 43º C. So we assume that temp for the TO planning.
Advantages: 1) the engine is running no harder than it needs to; and 2) the airplane wing is flying in air cooler than the assumed temp (meaning a lower groundspeed for each IAS).

But there are operational restrictions on Assumed Temp. method (no contaminated runway, etc.) so Derated performance sections in the AFM are provided.

[Flight Detent]

Hi all,

All varyingly interesting explanations, I see some don't understand this subject as well as others.

I thought I would add a couple of points:

The difference between 'assumed temp' and 'derated' thrust is that derated is a fixed amount of power decrease, usually 10% and 20% reductions, and assumed temp is a varying amount, dependent on the maximum temp the aircraft is capable of legally using the runway.
Of course, the two may be used together if the conditions are right.

A very important point overlooked here so far is that:
The power set during an assumed temperature takeoff can be exceeded at any time the Captain so decides, up to the maximum t/o power available.
The power set during a derated power takeoff cannot be exceeded, it is considered the maximum power available.

The reason for the difference is Vmcg.
For an assumed temp thrust takeoff, the Vmcg is calculated at the maximum thrust level.
For a derated thrust takeoff, the Vmcg is calculated at the derated thrust setting.
This difference can allow, in some cases, a greater takeoff weight for a derated takeoff the a maximum thrust takeoff, look at the figures for light weight takeoffs and shorter runways!

Cheers, FD :uhoh:

[mutt]

The power set during an assumed temperature takeoff can be exceeded at any time the Captain so decides, up to the maximum t/o power available.

This is where it gets fun...... so what is "maximum power"? The installed engine power or the derate power?

Mutt

[aidey_f]

Just to muddy the waters slightly, it depends on how the derate is done.

I'll illustrate this with the A320, although the same applies for the 737. Mechanically the same engine can be fitted to any airframe from the 319 to the 321, and the FADEC can be "pin-programmed" via the data entry plug to set the installed thrust level. Pin program for a lower thrust rating, and there is no way the crew can override this.

However, if you enter a derate (or Flex Temp) via the MCDU Perf page, the maximum thrust available is still TOGA thrust.

[Flight Detent]

Well,

aidey_f - I refer here, as I do with anything the flight crew can operate, to only those things they have control over. The installed rating of the engine cannot be changed by the flight crew, so only that is considered the maximum power rating, regardless of what the engine is actually capable of.
I thought that comment was unnecessarily confusing to this subject!

mutt - It's quite simple really, the maximum rating of an installed engine is what you get when the TOGA is pushed for a full rated takeoff.
"Up to the maximum t/o power available" seems pretty clear!

The derate is the limiting factor when used, it's considered the maximum for that takeoff, otherwise lowering the Vmcg for the derate, then using full rated power when one fails, will have you red-faced sitting in a ditch somewhere adjacent to the runway perimeter!

Cheers, FD

[Mad (Flt) Scientist]

That's rather assuming you have a nice, smart, FADEC running things.

Start using a hydro-mechanically controlled engine and things become much more interesting in terms of what is and is not available to the crew....

[411A]

...and then attach a Flight Engineer firmly to the thrust levers that are connected to that hydro-mechanically controlled engine, and the pilots have practically nothing else to do except point the aeroplane in the right direction...and pull on the pole a tad as the ground gets bigger.

[Flight Detent]

.....and that, "411A" is something we agree on 100%!
FADEC is something I never really wanted (or trusted)
Cheers, FD :hmm:

[barit1]

Most early-early engines had a "manual reversion" mode in which the governor, accel/decel schedules etc. could all be overridden. It took a very steady hand though, or else you would either cook a turbine or flame out. :eek:

[flyboyike]

Most early-early engines had a "manual reversion" mode in which the governor, accel/decel schedules etc. could all be overridden. It took a very steady hand though, or else you would either cook a turbine or flame out. :eek:


That's what the Meridian has, but I was told not only not to touch that lever, but not to even look at it.

[CMN]

That's what the Meridian has, but I was told not only not to touch that lever, but not to even look at it.

Isn't it the 'Manual Override' (That's what it's called in a PC-12) in case the FCU fails ? Comes in very handy when the FCU suddenly fails ang Ng starts to decellerate....lift up handle slowly, and start throwing fuel on the fire....works great!

[flyboyike]

Isn't it the 'Manual Override' (That's what it's called in a PC-12) in case the FCU fails ? Comes in very handy when the FCU suddenly fails ang Ng starts to decellerate....lift up handle slowly, and start throwing fuel on the fire....works great!


That's exactly what it's for. Not to be used at ANY OTHER TIME.

[CMN]

can't see why you shouldn't touch or look at it....
In normal ops there's no need, but it's fairly un-complicated and should be used extensively during training...to familiarize yourself with it's deadband among other things.

Try touching it - it doesn't hurt ;)