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I noticed that when a jet engine airplane is started or is conducting run-up, the airplane remains stationary even though the engine is already burning gas and is supposedly producing thrust.

What keeps the aircraft stationary while the engine is spinning? Up to how much percentage of the engine's maximum thrust can you put the thrust setting on as to not have the aircraft move about its stationary position?

An alternate question would be: "On what thrust setting on each engine (assuming twin engine) will the airplane start to move and accelerate?"

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    $\begingroup$ Brakes are holding aircraft from moving. Idle thrust is enough to taxi airplane. No idea about exact percentage. $\endgroup$ – vasin1987 Jan 11 '17 at 14:20
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    $\begingroup$ Also note that flight idle will vary from airplane to airplane. $\endgroup$ – John R. Strohm Jan 11 '17 at 14:21
  • $\begingroup$ Burning fuel not necessary produce thrust. Up to 2/3 of engine power at full thrust is consumed by engine's compressor. So on idle this part is even higher. The rest is consumed by auxiliary equipment. So there is very little (if any) thrust available. $\endgroup$ – Eugene Jan 11 '17 at 14:57
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    $\begingroup$ Depends entirely on how good the brakes are, and of course on how good the traction is. On an icy ramp, your brakes might not have a lot of effect. Also note that engines can be bolted down for testing. If that can be done with the Saturn V's F1 engine, holding a commercial jet should be a piece of cake :-) $\endgroup$ – jamesqf Jan 11 '17 at 19:14
  • $\begingroup$ @jamesqf They don't actually bolt anything down during on-wing run-up tests, though, do they? I've never seen that done. Of course, when the engine isn't actually mated to an aircraft, then you need some kind of stationary stand to attach it to for a run-up test. $\endgroup$ – reirab Jan 11 '17 at 22:21
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There are two answers to this question depending on how you look at it.

If the brakes are on: Full Thrust wont even move the plane this is a requirement of the braking system.

Brakes off: Somewhere a bit above idle. John covers it in his answer nicely. It will vary by loading and aircraft.

A propeller plane can get moving even at idle thrust. While jets don't do run ups (unless required for maintenance), propeller planes preform a run-up prior to takeoff where the brakes are utilized to hold the plane in place at a high thrust. A proper short field take off will also involve the application of brakes and full thrust.

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    $\begingroup$ This question seems to be about jets which dont do run ups but I will mention it in the prop context. $\endgroup$ – Dave Jan 11 '17 at 14:35
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    $\begingroup$ Well, jets DO do run-ups when required by maintenance. $\endgroup$ – Federico Jan 11 '17 at 14:36
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    $\begingroup$ airports also have dedicated areas (and rules) for that, e.g.: portseattle.org/Environmental/Noise/Noise-Abatement/Pages/… $\endgroup$ – Federico Jan 11 '17 at 14:41
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    $\begingroup$ While the brakes can hold the aircraft during a run-up, it's important to follow procedures or bad things can happen as did with this A340 that crashed at Toulouse. $\endgroup$ – Gerry Jan 11 '17 at 18:03
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    $\begingroup$ @Joshua Only fighter jets and high-end aerobatic aircraft (well, and rockets) have anything close to thrust >= weight. Others are much less. 14 CFR 25.735(d) requires the parking brake to be able to hold the aircraft static at full power on one engine and max ground idle on all others. Part 25 is the airworthiness standards for transport-category airplanes. $\endgroup$ – reirab Jan 12 '17 at 7:34
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I noticed that when a jet engine airplane is started or is conducting run-up, the airplane remains stationary even though the engine is already burning gas and is supposedly producing thrust.

The fact that the engine is burning gas does not necessarily mean that it is producing thrust. The extreme example of this is the Auxiliary Power Unit (APU), which is a jet turbine that never produces thrust, it only produces electrical power and hydraulic pressure.

What keeps the aircraft stationary while the engine is spinning?

Friction. Brakes. Tie-down ropes. Clamps. Anything.

Up to how much percentage of the engine's maximum thrust can you put the thrust setting on as to not have the aircraft move about its stationary position?

That depends on how firmly the airplane is held to the ground.

Here is an example of a 737 being pushed around purely by wind. In other words, in this case, the thrust needed to get the airplane moving is exactly 0.

For the other extreme, it is a certification requirement, that even at full thrust, the brakes must be able to hold the airplane (e.g. in case of a FADEC malfunction).

This is sort-of the opposite of what you asked for, but it demonstrates the enormous power of the brake system: as part of certification testing, the airplane must demonstrate a worst-case rejected take-off with

  • Maximum Take-Off Weight
  • Maximum Take-Off Speed
  • No Thrust-Reversers
  • Brakes completely worn-down
  • 1 Brake completely disabled
  • 5 minutes emergency response-time by firefighters

Note that in this case, the plane stops so fast that the engines don't even have time to fully spool down. So, not only are the brakes able to hold the airplane in place at full thrust, they can even stop it from high speeds at full thrust.

And really, this is pretty much true for all vehicles that use brakes. E.g. automobiles usually also have brakes that are more powerful than what the motor can put out. That's how you do burnouts, after all. In general, a (heavy) vehicle that is moving has more energy than the motor, so if the brakes can stop the moving vehicle, then they can also stop the standing one.

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    $\begingroup$ RAF Waddington. 1980. Dark night. Early in the morning. Winter, cold and raining. A basically horrible night. Vulcan ground crew tired, cold and hungry getting the last aircraft airborne for night exercise. All cleared to taxi away. 30% thrust. No movement. Maybe the tyres are a little flat spotted or stuck to the tarmac. 50%, no movement. OK, here we go, 70% thrust. Big bang, nose jumps up a couple of feet and the aircraft lurches forward. We check and on the pan (ramp), was the remains of the nose leg tie down hook snapped in half by about 30,000 lbs of thrust. Luckily, no other damage. $\endgroup$ – Simon Jan 11 '17 at 18:26
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    $\begingroup$ If air is being accelerated through a turbine there is an amount of thrust being generated, even if it's very small/negligible. $\endgroup$ – Notts90 Jan 11 '17 at 19:02
  • $\begingroup$ Yes, Notts90 is correct. Also, in the case of an airliner, the idle thrust is not so negligible. My understanding is that quite a few of them need to brake occasionally to maintain a safe taxi speed at (ground) idle. $\endgroup$ – reirab Jan 11 '17 at 22:19
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    $\begingroup$ Gatwick, some time in the 1990s. I was a passenger on a 747 which aborted takeoff fairly late because of an engine problem indication (which turned out to be an instrument fault, not an engine fault!) The pilot applied excessive braking, stopped the plane on the runway but seized the brakes. Max thrust on the other 3 engines wouldn't move it. Result: Gatwick main runway blocked for 5 hours. (Amusingly, after 3 engines wouldn't budge it, some optimist decided to try a tow truck - not surprisingly that didn't work either) $\endgroup$ – alephzero Jan 11 '17 at 22:53
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It will vary with the airplane and the engine(s).

The Boeing 737-800 will sit quite happily in one place with the parking brake released and the engines at flight idle (20% N1). She will also taxi slowly at 20% N1, if you bump the power up enough to overcome initial friction and get her moving, and then pull the power back down to flight idle. (I've done this in a simulator.) Usual procedure is to push the power up to about 30% N1, and then pull it back down to flight idle as soon as the airplane starts moving.

Standard practice, and it is on the checklists: if you are stopped, SET THE PARKING BRAKE.

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