Ask the Captain: Do jets use full power on takeoff?

Most takeoffs use 'derated' thrust to save engine wear. For each takeoff, performance is calculated, the necessary power setting is determined and the thrust setting is made.(Photo: Joe Raedle, Getty Images)

Question: Please settle a bet: Do airliners generally use 100% power on takeoffs? My friend's dad used to fly for TWA and says that they do.

— submitted by reader Roger, Memphis

Answer: Most takeoffs use "derated" thrust to save engine wear. For each takeoff, performance is calculated, the necessary power setting is determined and the thrust setting is made. Usually this is below the maximum available level and is known as a derated thrust takeoff.

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Q: I have noticed the 737s with winglets are using much longer takeoff rolls. I suspect this is due to derating the engines' takeoff power. My concern is that there is only so much runway available and it would seem prudent to get in the air as quickly as possible.

— Michael, Atlanta

A: Reduced power or derated takeoffs are normal and frequently used. The performance is carefully calculated prior to each takeoff, ensuring that the proper speeds, power settings and flaps are used for the weight of the takeoff on the runway intended and at the proper temperature. When using this method during takeoff, it is always possible to increase to full power if the situation requires. The captain always makes the final decision whether to use full thrust or derate.

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Derates improve engine life and reliability. In addition to lowering operating costs, they decrease the likelihood of an engine failure.

All jets use some form of derated or reduced-thrust takeoffs. The 737 is not unique in this procedure.

Q: I have noticed on takeoff that the pilot seems to rev the engine to 1/2 or 3/4 thrust (with no movement), reduces power, then goes to full throttle for takeoff. This is not specific to a certain plane and happens on 737's, 757's, A320s, etc. What is the reason for this "throttling?"

— Jeff, Boston

A: What you are hearing is the engine being brought to takeoff thrust in steps. Initially the pilot brings the thrust levers partially up, allowing the engines to increase their RPMs. If they accelerate at slightly different rates this short pause reduces the yawing caused by the asymmetric acceleration. It also allows the engine to stabilize, reducing the chances of an over-temperature condition from developing.

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Once the engines have attained the proper RPM, then autothrottle is engaged or manually takeoff power is set. You may hear what sounds like a slight decrease for a moment, but that is only the engines settling at the partial thrust setting until they accelerate to takeoff power.

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John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems

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Source: http://www.news.theusalinks.com/2013/03/25/ask-the-captain-do-jets-use-full-power-on-takeoff/