Civilian jet take off power

Hi guys

After watching a mayday episode, I found out that pilot don't push the throttle at max or military max power for take off, but rather calculate a certain EPR value.

I was wondering why it was the case for such critical phase for an airplane. Sure the first obvious reason would be fuel consumption, but why not reduce the throttle once a certain secure airspeed and altitude is reached?

Can somebody explain this?

Reduced engine wear = $$$. There's enough thrust at lower power setting so pushing it all the way is unnecessary engine stress.

Google:

Assumed Temperature Thrust Reduction

FLEX takeoff

^^

Like he said, engine wear. It won't change a lot for the fuel

Typically you have either Take-off Rated Thrust (TRT) which is limited to 5 minutes. Or MCT which stands for Maximum Continuous Thrust. So you take off in TRT and then set MCT. Usually pilot sets certain engine parameters, releases brakes and co-pilot adjusts to TRT. Once aircraft gets of the ground and cleans up, pilot will set MCT.

Those values are calculated by density pressure, temperature and other atmospheric conditions.

Why? Engine wear and tear. Most engines can go up to 105%+...but they are limited by ITT or other limitation. So if you run at 700C for X, that engine will have to be replaced or refurbished. The only time you want max thrust out of your engine is if you don't want to hit terrain...given that it has a 1.0 k/d ratio.

FYI difference between MCT and TRT is small....680C and 700C for engine I'm playing around with right now.

To clarify the previous post, the Captain/Pilot Flying is holding the thrust levers prior to takeoff. On takeoff he'll "stand up" the thrust levers or place them in an approximate position near takeoff thrust. He'll then command the First Officer/Pilot Not Flying to set thrust to that aircraft's rated takeoff thrust. (MCT is the usual term.) This keeps his head up/on the runway rather than fiddling with thrust levers and engine gauges.

that is why I love this forum, thanks a lot for the clarification guys :)

Don't know why MCT and 'Full Rated Thrust' is entering this discussion because it doesn't belong... Also, procedures vary among companies. Boeing recommends the captain assumes control of thrustlevers during take-off at all times. Setting of thrust is not necessary by either pilot as auto-throttle does it better/quicker after pressing the TOGA button. :-)

Assumed Temp (or FLEX which is the same thing) for reasons stated previously.

Definitely for reduced engine wear. We use Derated combined with assumed thrust take off power settings. This can get as low as 25% N1 lower than a full rated thrust take off. The largest savings in wear and tear exist in the first 5%. That wear and tear is huge. GE owns our engines and we lease from them. We get credits for each reduced power take off we do.

thanks

By the way, what is the marging between the needed thrust power (calculated with small security marging I assume) for take off and the under limit just on the edge of stall during take off?

Not a pilot but as far as i can tell, that question doesn't make a lot of sense. When talking about TO power in GA, you think in terms of "will i clear that obstacle standing close to the runway", not "is this enough to not make me stall in the climb". If you would take off with just enough power to make it to VR, you'd definately stall out because of the induced drag after you rotate.