You use full power on every takeoff, right? Full power should be used in nearly every piston airplane. Most light airplanes need all the power they can muster. Even if you’re in a lightly loaded twin using a 10,000-foot runway, full power should be used—if only for the fuel-enrichment feature built into most fuel systems that helps keep cylinders cool at high power and low airspeeds typical of climb. Because of this added cooling fuel, a partial-power takeoff could actually cause damage when you think you’re babying the engine. Finally, every takeoff performance parameter in light-airplane flight manuals assumes you’re using all available power. Anything else and, well, your results will vary—for the worse.
But in turbine airplanes there’s an abundance of power on tap—so much so that if the weight, altitude, and temperature (WAT) limits are favorable, it saves engine wear and fuel to perform reduced-power takeoffs, also known as a flex or reduced-thrust takeoff. Modern jets and some turboprops have flight management computers (FMC) that calculate how much thrust is needed given the WAT limits entered into the box by the pilots. Older turbine airplanes may require pilots to delve into the performance charts to determine the same information.
Methods used to actually calculate a reduced-thrust takeoff setting vary, but in general, pilots enter a higher temperature into the FMC to trick the box into reducing power accordingly, to mimic degradations in performance that such a temperature would create. This is called an assumed temp.
So even though it’s 15 degrees Celsius at the airport, if WAT limits allow, a flex temperature of, say, 57 degrees could be used. In turn, the FMC commands the engine controls to supply the thrust that would be used with temperature at 57 degrees C.
While not nearly as fun for us pilots as unleashing all the ponies behind the thrust levers, flex takeoffs are the economical choice for jet operators—for several reasons. Perhaps the biggest reason is that going easy on the engines can greatly extend their lives. And when turbines can cost millions of dollars each, it makes economic sense to get as much life out of them as possible. Operating temperatures and pressures are lower, which reduces strain on nearly every component in the engine.
From a maintenance standpoint, modern engines are replaced on condition, unlike the calendar time/time-between-overhaul method used for piston engines. While a typical Continental or Lycoming piston engine is good for 12 years/2,000 hours, a jet engine could go tens of thousands of hours before needing replacement. Sure, they get inspected at prescribed intervals, but if everything is good they can be buttoned up and sent on their way again.
In addition to engine longevity, flex takeoffs reduce fuel burn and overflight noise for the neighbors around the airport. In fact, if the airplane’s really light, takeoff power may actually be less than the prescribed climb power. If you’re riding in the back of a jet and you hear the power increase after climbing beyond about 1,000 feet above ground level, you can be sure the WAT limits were quite low. This will never occur on a hot day in Denver, though.
Airport and atmospheric conditions can dash attempts for a flex takeoff. Wind shear, runway contamination (standing water, snow, slush), and inoperative equipment are common reasons to abandon a flex takeoff in favor of a full-power takeoff. Crews themselves may elect to do a full-power takeoff if, for example, they are departing behind a heavy jet and want to clear its wake turbulence. As always, if the crew believes that a flex takeoff may jeopardize safety, they can abandon the plan and use max thrust.
So while flex takeoffs may be the norm in jet operations, don’t try to emulate the big boys by doing partial-power takeoffs in your light single or twin.
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