Q2) Why is the risk per flight decreased with a reduced-thrust takeoff?
The Book says:
1) The assumed/flexible temperature method of reducing thrust to match the takeoff weight does so at a constant thrust-weight ratio, making the actual takeoff distance and takeoff run distance from the reduced-thrust setting less than that at full thrust and full weight by approximately 1 percent for every 3ºC that the actual temperature is below assumed temperature.
2) The accelerate stop distance is further improved by the increased effectiveness of full reverse thrust at lower temperature.
3) The continued take-off after engine failure is protected by the ability to restore full power on the operative engine.
@ Mohit, since you stated that you found only the first reason confusing, I will presume that you don't understand what is and why do we need reduced thrust take off. I may stand wrong, but you can correct me on this.
Reduced thrust at take-off, why on God's earth should I reduce thrust at takeoff? Thats the big question. Why don't we teach such things at flight schools? Why don't we do it with C-152s?
Take-off is a time at which the engine is at maximum stress. As safe pilots, we should try to reduce stress on engine whenever possible. Engine is your friend. By allowing an engine to operate at lower thrust during takeoff, we are reducing max thrust operation times, such times degrade engine life quiet fast.
When do we do it?
1. Long runway: If you are not operating from a short field, you can reduce your takeoff thrust and save on some engine maintenance time for your airline.
2. Less weight at takeoff: Lower weight at takeoff, lower thrust.
3. Great headwind: headwind reduces runway length required.
4. Noise abatement: Some noise abatement procedure may require reduced thrust, its better to listen to less louder noise for longer duration than a loud noise for shorter duration. (Noise is a complex territory)
A reduced thrust takeoff program, at any airline if managed with proper safety insights in mind, can save a lot of engine repair time and money. It also increases safety. An engine which is operated in reduced-thrust take off program, would have had to spend lesser time at stressful high rpms, and hence should be left with more life, before the overhaul is done. On the other hand engines used at max thrust, may have had too many revolutions at high stress and may run out of safe life before next overhaul.
Cars which race everyday have much shorter life than the one who driven gently with care. Same thing goes with aircraft engines.
Shockers in your car will have considerably shorter life if you drive fast on speed breakers. Short term but high stress operation eats fatigue life much faster than average stress operations.
This is the reason why reduced thrust take-off reduces risk per flight. You know you are flying an engine which nobody stressed too much. Its gotta be safer.
Lets come to the books explanations here.
1) The assumed/flexible temperature method of reducing thrust to match the takeoff weight does so at a constant thrust-weight ratio, making the actual takeoff distance and takeoff run distance from the reduced-thrust setting less than that at full thrust and full weight by approximately 1 percent for every 3ºC that the actual temperature is below assumed temperature.
Assumed/flexible temperature method
Jet engines have limitations on turbine temperature, you can't let it go high. If temperature goes too high the engine may get internal damage. When the air outside is hot, they have an inbuilt mechanism to reduce max RPM to make sure the temperatures at the turbine don't exceed the limits.
So if you fool the engine by telling that outside temperature is 40c while actual temperature is 20c, the engine computer will reduce max rpm when TO/GA switch is pressed. You can feed this assumed temperature in the FMC. Some manufacturers call it flexible temperature, the FMC is allowing to let you operate the engine at the chosen flexible temperature you want. Hence the assumed/flexible method designator.
You can still get the max thrust by moving throttle to full position, this will override the assumed/flexible temp setting.
The answer in book is cofusing, seems like it has been verbatim copied from an interviewee's reply. The response shows rote learning rather than understanding.
De-Rating
This is not an actual reduced thrust takeoff. I listed here to remove confusion. De rating is done to by maintenance staff to limit maximum thrust, for example from 25kn Flat Rating to 23 kn De-rated thrust, this can be done for several reason, such as engine life and making two slightly different engines have same thrust. Pilot cannot control this during flight or take-off.
2) The accelerate stop distance is further improved by the increased effectiveness of full reverse thrust at lower temperature.
Which aircraft will have better accelerate stop distance:
A) the one with 50kn thrust
B) the one with 45 kn thrust
The answer to above question, reveals the in-correctness of the reason 2. Or may be I don't understand what author is trying to say. Can any one on the firum help here?
3) The continued take-off after engine failure is protected by the ability to restore full power on the operative engine.
This seems to me a sentence which is out of context. Apparently copied from some text and inserted in the book without thinking about the relevance. I don't know how this ability reduces risk per flight as compared to having the full thrust already.
I hope this helped.