When pushback is connected to the nose gear, can the pilot release the aircraft's parking brakes while the pushback vehicle's "hand-brake" is set? Can it hold the weight of the aircraft?
Any regulations or SOPs that are covering this question?
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The tug operator talks to the pilot through an intercom connected directly into the airplane's audio system, and when the pushback is ready to start, the tug will confirm PB release with the pilot before the movement starts.
If the tug operator doesn't have voice communication with the crew, hand signals are used, parking brakes off being two closed fists held up, then hands opened with palms toward the pilot (brakes on is the opposite).
The tug is quite heavy, with powerful brakes, and has no problem slowing down or holding the mass of the aircraft at the walking speeds being used.
If the tug driver has to stop during the pushback, the driver will carefully slow and stop the tug, and depending on how long they expect to wait, or if some other conditions exist like a slope or a strong wind that wants to push the plane along, may tell the pilot to reset the parking brake as well. Or they may just sit there relying on the tug's brakes, especially if it's expected to be a short 10 or 15 second wait. Depends on the situation and the operator's judgement, and airline SOPs that dictate minimum actions.
SOPs for that will depend on the airline. You also have manufacturer's towing (and taxiing) procedures, covered in Chapter 9 of the Aircraft Maintenance Manual, and airline SOPs will typically be based on that, possibly with their own custom procedures thrown in.
Yes, they can. Planes generally* don't have any kind of automated communication with the tug. The brakes, power, etc. are all coordinated by hardwired (voice-only) intercom, (voice-only) radio, hand signals, signal lights, or something similar. So there's no interlock that prevents the pilot from doing whatever they want with the brakes at any time.
Airplane tugs are big, heavy things with oversize brakes, specifically because they're designed to move bigger, heavier things. Typically, the airplane's brakes aren't used at all during the pushback, the tug does all the braking itself. So, yes, its brakes would be quite capable of keeping the plane still without any help.
As far as I'm aware, there are no specific regulations on the use of plane brakes vs. tug brakes. Rather, the airplane's manufacturer and/or the airline will have a set procedure to use, and there may be (depending on jurisdiction) a regulation that says you must follow procedure.
*I don't actually know of any exceptions to this, but I'm not exactly a guru of all things pushback-related either.