Question about OEI
 

My english is a little poor.
so please understand some weird.
I have some question about OEI.
1. In the condition of OEI, If the pilot use 60% of 2.
5minute power(i.e 1.5minute) and back to OEI cotinuous power, do the pliot have 40% or 100%(reset) ?

2. If the pilot use 100% of 2.5minitue power and back to 10minute power, can the pilot use all of 10minute power? um..I think there is some effect to engine, so some change will occur.

I think I understand your question.
(Your English is slightly worse than mine, but English, well “Australian” is my only language.)

1. 40% - measured on some modern machines. But the concept is that you should have got into a situation where you need a little less power - eg closer to VY.

2. Most people understand that reducing momentarily below a timed power limit does not “reset” that limit.
EG In a SE using the 5 min “TO” limit for 5 min, then reducing the power for 10 seconds does not “give” you another 5 min TO power.
The “limits” are based on different things for different machines. Often the TO power might be for example heating up the transmission, so after some time, like some “cruise”, you could use it for the next TO.
Some engines require inspections (or more) after utilising the OEI limits (I believe).

Engines are designed for a "normal" sortie - take-off power for up to 5 minutes, climb power for up to 20 mins, max continuous for however long.

If you use take-off power on take-off and again on landing, all that is happening is that you are using up the engine life faster than expected - you will see metal in the oil and some chip detectors well before the normal overhaul time. It is expensive, but if you NEED to use the power, then use it. Don't go splat in the trees to avoid going over a limit. But tell your engineers.

The OP is referring to OEI - One Engine Inoperative - Not normal take off power.

There is not an easy answer.

Originally the time limits (30seconds, 2 minutes, 5minutes) were at least partly based on preventing engine parts from overheating. I think (but please don't quote me on this) that static overheating is not the primary issue anymore, but slowly progressing deterioration through high temperature creep on the one hand and material fatigue on the other hand. I will refer to both as "fatigue" from now on.

The fatigue life of an engine (and the attached gear boxes) changes drastically with the power draw. The following numbers are made up, but they should be roughly the right order of magnitude:

- Engine life at MCP: 100000 Flight hours (FH)
- Engine life at TOP (5min): 10000 FH
- Engine life at OEI low (2min): 10 FH
- Engine life at OEI high (30sec): 0.5 FH

Then one assumes certain conservative flight spectra. E.G. 1 take off and landing every 10 flight minutes (5minutes TOP and 5minutes MCP six times per FH), 1 OEI low every 100FH and 1 OEI high every 1000FH.

Then one combines the fatigue damages per flight hour from each of these phases:
D=5*6/60/100000 + 5*6/60/10000 + 2/100/60/10 + 0.5/1000/60/0.5= 0.000105

The combined lifetime for this example is therefore:
L=1/D = 9524 FH

This number is rounded down and 9000FH lifetime is put in the manual.

There is an additional level of complexity, because the temperature is lower in new engines than in old engines at the same power. Therefore, the heat creep also depends on the age of the engine. Another level of complexity is the fatigue sequencing (I used the simple Miner-Rule in the example above), which is tested on real engines with very demanding power cycles.

I actually brought this up with other engineers, and we could change the rules to a engine lifetime damage accumulation, or usage dependent flight hour consumption in stead of the time limits. However the current system seems to work fine and we are not sure that there might not be an overheating or similar issue that we don't know about, because it is proprietary information of the engine manufacturers. This would allow the pilots to fly for longer than 5 minutes at TOP, however the lifetime and maintenance intervals would have to be shortened, if this happens more frequently than in the design assumption (e.g. more than 30minutes per flight hour, or more than 18 minutes per flight hour, depending on helicopter manufacturer).

High engine power is damaging, therefore you should only use the OEI ratings when you really need them. You may need the 30 second rating to get to Vtoss (which shouldn’t normally take anywhere near 30 seconds, unless you are eg winching) and you may need the 2 minute rating to get to Vy and maybe climb a bit. After that you can back off to continuous. And after that, you only need to use the 2 min or 30 second ratings if you slow down below Vy/Vtoss. Which you shouldn’t do except for landing.

So the question should really be “how little can I use the OEI ratings”, not “how much can I use the OEI ratings”. And also you should bear in mind that these ratings are created as part of the certification process and to comply with the certification process, the manufacturer probably only needs to demonstrate that they can be used once in an engine failure on takeoff, once for a baulked landing go-around, and once for the final landing. After that, the engine can be toast according to the certification requirements and you are into uncharted territory.

Q1.
The DECU has inbuilt timers that count cumulative time limits to some algorithm programmed by an engineer that a pilot need not know about.
Q2.
The real answer to your question is you use as much OEI power as you need for as long as you need it. If you need 30 second power for more than 30 seconds, use it. The DECU does not stop you using 30 second power for more than 30 seconds. Same for the 2.5 minute limit.

Nick Lappos had a jar of beans analogy for engine fatigue - your engine has a finite number of beans in its full jar when new (jar empty = TBO) - the more you use that engine up to (and possibly beyond) its published limits, the quicker you use up the beans and the sooner the removal/replacement/overhaul time arrives.

Most important is to keep your engines in good condition so that they do provide the required 5 min and 2.5 min power if you need it - compressor washes, Power Assurance Checks and close trend monitoring.