regarding engines

[1515Blue]

question:
what do you mean by or how do you "flat" rate an engine????
is this the same as derating? and what are the advantages???

thanks!

[turboshaft]

Similar but distinct. Derating an engine is done primarily to reduce engine temps and thereby extend life. The disadvantage is obviously a reduction in available performance. Flat rating an engine can also extend life (depending on operational usage), but is principally done to guarantee performance at non-ISA 'standard' altitudes or ambient temps.

Cheers,
t/shaft

[1515Blue]

thanks but how exactly do you flat rate an engine??

[Thunder Child]

Via the FMC I believe.........

[planett]

In the turboprop world, this can be done by limiting torque to a value below what the engine is capable of producing. In some cases far below. The result is max power limitations due to torque on all but the hottest weather. It's satisfying to see max torque set (or just available) through 10000' without reaching temp limits.

A good example is the TPE 331 - 10 installed on Metro II's, using the -3 gearbox. No need to re-write the Flight Manual for performance, since no credit is given for the excess power available, but the benefits are evident on warm days or at altitude.

[Capt Claret]

Perhaps I have the wrong end of the bull, so to speak. I always thought "flat rating" refered to the maximum OAT at which the engine would produce rated power.

For example the ALF-502 produces 6970 lb thrust at sea level at up to 15 degrees C, that is, it's Flat Rated to 15 degrees C. Where as the LF507 produces 7000 lb at sea level to 23.3 degrees C, that is, it's Flat Rated to 23.3 degrees C.

[mutt]

If you look at a net thrust chart, you will see the amount of thrust that the engine is capable of producing versus the temperature, in most cases that i have seen, jet engines are capable of producing their RATED thrust up to ISA+15 or 30C, after that the amount of thrust decreases. The term used is that these engines are FLAT RATED to ISA+15 (Or whatever the temp is).

As a crewmember you have NO control over the flat rated temperature, it is a certified level dictated my the manufacturer.

Totally different to DERATED.


Mutt

[Flight Detent]

Right 'mutt',

it's all pre-determined by the settings on the fuel control, with it's determination of the OAT and P/ALTITUDE combinations!

No input allowable from the crew (or maintenance), it's factory set!

DERATE is set by the crew, as part of company policy, to lower the internal temperatures of the engine, and, as a result, reduce the power output, to extend the service life of the engine.

Cheers

[SuperRanger]

1515,

what acft do you fly? on the boeings that i flew B744 (GE & PW), B734 (CFM) and now the B777 (RR Trent), they are all 'flat-rated' to ISA+15__°C. just as mutt said, they produce max thrust at ISA+15°C. you can try this out. when the OAT < 30°C, try entering a SEL TEMP (derate) of less than 30°C, you will noticed then TO thrust will NOT reduce beyond that of 30°C.

SR

[mutt]

Just to take this a little bit further, FAA regulations restrict thrust reductions to 25% of the rated engine, (I presume that the JARs are the same.) This thrust reduction comes with strict limitations on its use, such as the runway surface condition, mels/cdls, periodic checks etc. Initially the crew performed this form of DERATING by fooling the engine into thinking that the outside temperature was hotter than it actually was, these were called "assumed temperature derates".

Although ATM Derates were sufficient for older engines they weren’t sufficient for modern day engines, which are generally overpowered, the engine manufacturers introduced a second method of further reducing the engine thrust, which were called fixed Derates. Fixed Derates are basically take an engine and pretend that its a smaller engine. The size of this size reduction are usually airline selectable and can be as much as 30-35% less than the installed engines thrust.

Fixed Derates have a number of operational advantages:
1: Less restrictive than the assumed temperature method.
2: Permits a greater thrust reduction when combined with assumed temperature thrust reductions.
3: The lower Thrust/VMCG may allow greater limited takeoff weights on certain contaminated runway lengths.

This decrease in takeoff/climb thrust is reflected in lower engine temperatures that increases engine life and reduces maintenance costs. These maintenance cost savings are considered more beneficial than the extra amount of fuel required for the slower climb associated with the reduced thrust.

Theoretically this all sounds great, but I’m still waiting for certain engine manufacturers to financially justify derating engines above 10%.

But you should be able to see that Derates are a totally different kettle of fish to Flat Rating………

Mutt.