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I have mostly seen pilots slowly pushing the throttle to toga during takeoff. But why? Why don't they just directly put the throttle to toga? Wouldn't that save runway length?

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    $\begingroup$ “Slow is smooth, smooth is fast.” $\endgroup$
    – StephenS
    Nov 11, 2021 at 17:42
  • $\begingroup$ Until the 1970's fuel crisis changed engineering considerations, most motor vehicles would break traction under heavy throttle or steering. And airplanes have even less traction with the air than do cars with the road. $\endgroup$
    – dotancohen
    Nov 14, 2021 at 9:20
  • $\begingroup$ @dotancohen there are 2 parts to your comment, and I don't understand either of them: (a) ABS, ESP etc. are safety, not fuel efficiency improvements, and AFAIK came around in the late 80s and 90s; (b) "breaking traction" with the air would be something like cavitation in ship propellers? But even if cavitation would be possible in air, I think increasing thrust too fast wouldn't cause it because of the inertia of the turbine? $\endgroup$
    – rob74
    Nov 14, 2021 at 16:55
  • $\begingroup$ @rob74: I mean that cars when engineered as simple devices are fitted with tires that have enough traction for most conditions, but can sometimes be overpowered by aggressive throttle and maneuvering. The engineering considerations combine tires with an engine powerful enough to overcome their traction under some conditions. Likewise, an aircraft engine can overcome the limits of the the vehicle's rudder and ailerons. $\endgroup$
    – dotancohen
    Nov 15, 2021 at 8:29

7 Answers 7

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The engines may accelerate slightly differently, resulting in an asymmetric thrust. Spooling up slowly at first (normally to about 60% N1) prevents this. After that, you can accelerate up to full TO/GA (or press the button) without any significant asymmetry.

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A pure engine-side viewpoint:

Engines don't really like their power level being changed quickly. Doing so, one increases the thermal and mechanical stress over the engine parts, as well as the probability of engine failure, unstable operation or flame-out. One doesn't want some of these things happening during take-off.

Aircraft engines are pretty much reliable these days, but one should not expend their luck quota for no good reason.

(These considerations are not limited to aviation, any seasoned truck driver does the same with the throttle pedal unless there is a compelling reason to act quickly.)

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    $\begingroup$ Even more broadly, being gentle with machines unless necessary is good engineering pratice $\endgroup$
    – Chris H
    Nov 12, 2021 at 9:53
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    $\begingroup$ Aircraft turbine engines are reliable these days mainly because there is an electronic control unit that knows how fast it can increase the fuel flow without the engine stalling and slows the advance of thrust if the pilot is too fast. $\endgroup$
    – Jan Hudec
    Nov 12, 2021 at 12:01
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High-powered single prop planes may react violently if the throttle is opened abruptly. Torque reaction and P-factor should be smoothly dealt with...

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    $\begingroup$ This is true, an inexperienced P-51 pilot (he had the money to buy one but didn't get enough training) jammed full throttle on go-around and it flipped the airplane over for a fatal accident. $\endgroup$
    – GdD
    Nov 11, 2021 at 15:29
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    $\begingroup$ And smaller planes, like your single engine Pipers & Cessnas, may stall if you just shove the throttle to the firewall. $\endgroup$
    – jamesqf
    Nov 11, 2021 at 16:24
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    $\begingroup$ ...so rather than high powered, I'd say high power to weight ratio... $\endgroup$
    – Jpe61
    Nov 11, 2021 at 18:45
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    $\begingroup$ I've tried it. First time in a rotax powered ultralight (Ikarus), I was tad bit too agressive with the throttle, we almost ended up off the runway. The plane will not flip as in the example mentioned by GdD, but it will pull to the side extremely hard. Granted I was not accustomed to the control response either, but the power to weight ratio is quite extreme. After liftoff we would have been able to climb very steeply, SEP's I've been in don't come even close. Instructor adviced to retard throttle and climb out on partial power to keep forward visibility better. $\endgroup$
    – Jpe61
    Nov 12, 2021 at 7:57
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    $\begingroup$ @Jpe61, you were on the ground where the gear provides stability against rolling, while the GdD's example was during go-around, so presumably still in the air where it could roll freely. It's a good argument to do some training on go-arounds so you get the feel how much rudder and aileron you need to add when advancing throttle quickly. $\endgroup$
    – Jan Hudec
    Nov 12, 2021 at 12:07
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On smaller planes, the way it was explained to me was a mix of letting the engine come up to speed more easily as well as smoothly applying the left turn tendency from the inertial changes from the prop spin.

This left turn tendency is less of an issue on a dual prop plane with counter rotating props, but it's still a lot harsher on the engine and isn't really needed. If you need maximum power from the start of your take off roll, you can hold the brakes to keep from moving forward until at full power rather than forcing the engine to handle extreme changes.

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Smoothly adding power allows the pilot to look at engine parameters as engine increases RPM/N2 and possibly avoid a failure at a critical time. With multi engine aircraft it also avoids a bunch of asymmetric thrust and a possible runway excursion. Our Falcon 2000LXS manual actually recommends advancing the power levers from idle to takeoff power over a 3 to 5 second period.

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There is rarely only a single reason for something. In the aircraft I currently fly, there is a dynamic counterweight system in the engine. This system absorbs torsional vibration of the crankshaft in normal running. However, rapid power setting changes can lead to the counterweights "detuning" and causing significant damage to the engine, as well as potentially loss of power.

Of course, if the engine lacks a dynamic counterweight system, this reason does not apply. In general however, most engines will last longer if they are operated with care rather than making rapid power setting changes. For regular public transport, minimising the cost to the company is important for financial viability.

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I think you're a tad confused... they don't. Pilots typically push it to full or almost full throttle the second they begin to accelerate for take off. The slow increase you feel is actually not a slow increase in thrust (the force applied to move the plane forward), but the rate at which the heavy plane full of people is accelerating (increasing in speed).

Please remember Newton's second law, which tells us acceleration is Net Force divided by Mass. Massive objects such as a heavy passenger jet (such as a 747, which weighs 183,500kg without any people on it!) would accelerate veeeerrrryyy slowly even with a large force added to move it forward.

We can also use, for example, a muscle car at a drag strip. Why does the driver in the competition slowly push down on the gas? Why doesn't he (or she) just floor it? Well, they're actually adding gas/'thrust'/force very quickly (they have to, to move their heavy cars the entire 1/4 mile so quickly!) but it takes time to take a heavy object and make it go from not moving to moving VERY fast (cars often exceed 100mph by the end of the 1/4 mile, and passenger jets average around 4 or 5 hundred km/h groundspeed).

Hope this helped!

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    $\begingroup$ What you feel is indeed a slow increase in thrust, not a slow increase in speed. What you feel is force. You cannot feel speed. It takes a relatively long time (several seconds) for commercial jet engines to spool up, so you do indeed feel the change in force (and, thus, the change in acceleration) as the engine power is increased. You feel it even more in larger aircraft like the 747 you mentioned than in smaller ones, since larger engines tend to take more time to spool up (due to more rotating mass in the engine that has to be accelerated.) $\endgroup$
    – reirab
    Nov 14, 2021 at 6:30
  • $\begingroup$ Even on light aircraft, though, we don't just suddenly jam in full throttle. We can add power much faster than huge jet engines can, but we do smoothly increase the throttle to full, not just suddenly jam it full open from idle. Doing that would decrease the life of the engine and aircraft engines are expensive. Also, it would lead to sudden changes in forces (e.g. p-factor) that the pilot would have to counter with other control inputs. $\endgroup$
    – reirab
    Nov 14, 2021 at 6:32
  • $\begingroup$ (Also, as a side note, I think you've mixed up your units in the last paragraph. Passenger jets average 450-550 miles per hour ground speed when cruising, not kilometers per hour. Large passenger planes haven't been in the 450-550 km/h range since the before the beginning of the jet age. The Connie was at the upper end of that range, for example.) $\endgroup$
    – reirab
    Nov 14, 2021 at 6:38

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