From my - admittedly - limited knowledge of physics, the energy generated from the engines needs to go somewhere; will it go into the brakes or does it just dissipate into the air?
If a jetliner is giving TOGA thrust but never releases the (parking) brake, will the brakes heat up?
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$\begingroup$ Aren't the real questions: Why would one do so, and how long will it take to realize the mistake (if not indented)? $\endgroup$– U. WindlMay 4 at 8:10
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$\begingroup$ @U.Windl it was more a hypothetical scenario anyways. I know there are plenty of safety features that this shouldn't happen (for too long) $\endgroup$– NicoMay 4 at 16:18
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$\begingroup$ Also a quick "thank you" to everyone who commented here, I appreciate it :) $\endgroup$– NicoMay 4 at 16:18
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$\begingroup$ This question deserves a lot more upvotes, simply because of the amount of debate it sparked. Well done Nico!!! $\endgroup$– Jpe61May 5 at 8:57
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$\begingroup$ Absolutely, and now I ask: name one form of propulsion that does anything other than to impart force on what it is trying to move? Simple thought experiment. Take a block of wood and set it on the table. Apply force with your hand. At will accelerate until force = friction (and aerodynamic drag!). The Power of the block is Force x Velocity. Now nail the block to the side of a merry go round. Relative to where you place it there is no V. But the block + the merry go round recieve the force, same as the jet and earth. Easier to see if the merry go round moves. $\endgroup$– Robert DiGiovanniMay 5 at 16:09
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2 Answers
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Brakes absorb energy through dynamic friction. If the plane doesn't move, the brakes do not absorb any energy, and do not heat up.
The energy is going into the air either way, whether the brakes are applied or not.
The only difference a parking brake makes is whether the reaction force, arising from Newton's third law, acts on the aircraft alone or on the aircraft+Earth system. Momentum will be imparted to that system.
The additional ballast of M⊕ is about 1019 times the weight of the heaviest airplane, so it reduces its acceleration by a factor of 1019. Planets are a heavy load to move.
More technical:
The amount of momentum imparted to the air and to the aircraft is equal, but the amount of energy is not.
Energy absorbed by the brakes is equal to E=F*S, where F is braking force and S is distance traveled by the plane. If the brakes hold the plane, F=T, T being thrust, but S=0, and E=T*0=0. i.e. the plane won't move.
This assumes the brakes work perfectly and hold. Normally the brake will be released at half power. Past that, if the plane is lightly loaded, or has a lot of thrust, or if the runway is wet, the plane can start skidding, so you'll get movement and heating at the tires... And then this is what can happen.
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4$\begingroup$ @Nico, think of it like this, the brakes aren't absorbing any energy from the engines, they absorb only the kinetic energy of the airplane. If ground speed is zero there is no kinetic energy to absorb. $\endgroup$ May 3 at 22:55
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2$\begingroup$ @Nico I've done power assurance runs on CRJs where you run the engines up to TO thrust with the parking brake on. The brake temp indications are unaffected. It's as if the tires were just rigidly attached rubber feet like on the legs of a chair. There has to be movement between pads and discs. $\endgroup$– John KMay 4 at 3:27
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2$\begingroup$ Is the tire-to-ground friction sufficient to keep the airplane stopped with the engines at full power? If the tires are skidding across the ground, the brakes will heat up a little via conduction from the heated tires. $\endgroup$– MarkMay 4 at 3:33
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12$\begingroup$ Never ask ChatGPT anything technical if you expect a correct answer. $\endgroup$ May 4 at 4:31
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To be precise the engines do not generate energy, they convert chemical energy into kinetic energy, which can be measured as heat and thrust.
As long as the wheels of the aircraft are not rotating, no energy is transferred by/via the brakes, as there is no movement between brake pads and brake rotors that would cause friction create heat.
In any case, over time all of the energy released by this process dissipates into the atmosphere, and eventually into space.
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$\begingroup$ "Heat dissipates into the atmosphere". Good. Where does the thrust go? $\endgroup$ May 5 at 17:06
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$\begingroup$ @RobertDiGiovanni Here is an entire discussion we had on that. Summary: with the brakes engaged and engines running, the earth and the air particles are gaining kinetic energy (both in opposite directions) - that's where the mechanical energy "produced" by the engine goes. Due to friction between earth and air, this kinetic energy will eventually be converted into heat. $\endgroup$ May 6 at 0:17
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1$\begingroup$ @AdityaSharma thanks. I won't worry about the friction of air slowing the planet's rotation. Indeed, lucky for those at the equator, earth has a boundary layer! But correctly, we realize fuel moves air, reaction (of varying efficiencies) provides propulsion force. Locking brakes means plane + earth become a much more massive object. To summarize, no friction heat will created unless the wheels rotate or skid. Otherwise, it goes to earth. Boring bookkeeping, but that is the science. Our TOGA plane makes the world a bit warmer, and adds (or subtracts) from its rotation. $\endgroup$ May 6 at 4:20
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