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From the question 'Do I Set the Parking Brake On the Ramp?' it is concluded that you do not set the parking brake when parking the aircraft.

The question concerns small aircraft which can be parked for a longer period of time. However, commercial jets (jetliners) are also parked at the gate without the parking brake engaged and rely on blocks/chocks placed around the wheels.

Then why is it called the "parking brake" as it is not used while parking the aircraft? Other wheeled vehicles like cars, trucks and buses are always parked on the parking brake.

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    $\begingroup$ "Other wheeled vehicles like cars, trucks and busses are always parked on the parking brake." Not always. With manual transmissions, some people prefer to leave their car in low gear (first or reverse, depending on whether they're parking uphill or downhill) rather than engaging the parking brake. With automatic transmission, "park" is separate from the parking brake and can be used independently. My understanding is that this is particularly common in cold climates to keep the parking brake from freezing. I'm pretty sure I've seen mention of this on Motor Vehicle Maintenance & Repair, but can't find it now. $\endgroup$
    – user
    Commented Mar 22, 2018 at 12:26
  • $\begingroup$ My guess is, you can not really engage the hot bake while standing still for too long. The friction material would bind to the brake surface. I've heard of advice that when a motorcycle comes to a stop light, the rider should use the rear brake while waiting for the light and release the front brake, or the brake pads would bind to the hot disk and leave some residue that can cause the front brake to "pulse". Aircraft brakes may suffer from the same problem, so the parking brake could be only use when the brake is cold. $\endgroup$
    – user3528438
    Commented Mar 22, 2018 at 14:47
  • $\begingroup$ @MichaelKjörling is correct. Automatic transmission vehicles normally do not set the parking brake when parking. Instead, the transmission holds the vehicle in place when 'park' is selected. $\endgroup$
    – reirab
    Commented Jun 27, 2018 at 17:00
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    $\begingroup$ @reirab: And then you set the parking brake manually, just to make sure. $\endgroup$
    – Vikki
    Commented Jun 6, 2019 at 2:51
  • $\begingroup$ @Sean I almost never do that in an automatic unless I'm parked on a steep grade. There's no point. The transmission is plenty to hold the vehicle in place unless the grade is quite steep. Of course, I do set the parking brake when parking a light airplane, as they have no transmission and the wheels will otherwise roll quite freely. $\endgroup$
    – reirab
    Commented Jun 6, 2019 at 5:33

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Parking brakes are used by commercial aircraft at the gate if the ground crew can not set the chocks. The most common time for this to happen is during a lightning storm when ground crews are not allowed on the ramp. Large airports like DFW have automatic parking lights that will guide the aircraft to the proper alignment and stopping point. The crew will then set the parking brake before deplaning the passengers. Some crews will also use the parking brake for ramp holds, de-icing, etc.

The reason why chocks are preferred over the parking brake, is due to the way parking brakes work on aircraft. On commercial aircraft, the pilots apply the brake manually, then turn on the parking brake, which closes a hydraulic valve trapping the pressurized fluid in the brakes. Due to internal leakage of the valve, the brake pressure will slowly bleed down over a few hours. Therefore, after a few hours the brakes will no longer hold the aircraft. Further, most aircraft ramps are slightly sloped away from the terminal, so the aircraft would end up rolling backwards. Due to the bleed down issues, most airlines require pilots to stay in the cockpit ready to apply brakes until the wheel are chocked.

I have personally seen cases of large aircraft being parked with a parking brake, that rolled after the brake bleed down. In one case, a 777 rolled backwards and the jet-bridge tore off its door. Link to News Article

Car parking brakes work differently. Parking brakes in cars have a direct mechanical linkage (typically a cable) that is held in place. Therefore, they do not have the bleed down issue and will maintain the same amount of braking force at all times.

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    $\begingroup$ Ditto for "direct mechanical leakage". I think you meant "linkage". $\endgroup$
    – Robᵩ
    Commented Mar 22, 2018 at 18:14
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    $\begingroup$ Does DFW allow the jet bridge operators to work during lightning storms? When we had a lightning storm start at MCO just after we had boarded, but before pushing back, we all just had to sit there for a couple of hours because the ramp closure meant that the tugs couldn't push us back and also that the jet bridge operator couldn't re-attach the bridge until after the lightning stopped. $\endgroup$
    – reirab
    Commented Jun 27, 2018 at 18:14
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I am going to answer you from an airline pilot point of view. First of all, parking brakes in aircraft work pretty similar to the ones in a car or a truck. And yes they are used to park the airplane by letting go off the toe brakes, which is again pretty similar to a road vehicle.

Once we are at the gate after a flight, we put on the parking brake and shut the aircraft down. The ground engineer usually lets us know through the intercom once the chocks are in place. This is a sort of 'okay' signal to release the parking brake. The reason why we use chocks and keep the parking brake off is mainly to cool down the brakes. Following a typical landing, and sometimes a long taxi to the gate, the brakes tend to heat up. Because airplanes rarely stay on the ground, we need to cool the brakes down to ensure they remain efficient in a possible rejected take off that might occur in the next flight. Keeping the parking brake off makes sure that the brake stators are released from the rotors, allowing better air flow circulation. This ensures proper heat dissipation. In some airplanes, we have brake fans, which helps to cool the brakes. The fan is more effective with the parking brake off. However, for the walkaround, we set the park brake on, even if the chocks are in place. This is so that we can get a more accurate indication of the brake wear indicators.

When the airplane has to be parked at a place for a long period of time, the parking brake can be kept on. But as the parking brake in most airplanes (when unpowered) is channeled through a pressurized hydraulic accumulator, it tends to slowly lose pressure. For example, the A320 accumulator can hold the brakes in place for about 12 hours. If you have to park it for more than 12 hours, you should place the chocks on the wheels to prevent it from moving and hitting something.

enter image description here

The A320 parking brake system. It is powered by the yellow hydraulic system and by an accumulator when the aircraft is off power.

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It’s so a linesman can tow your plane around if need be without you being there or needing to get into the plane.

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    $\begingroup$ I understand that this applies to smaller aircraft but the larger commercial aircraft stay at the same gate in general. $\endgroup$
    – Brilsmurfffje
    Commented Mar 22, 2018 at 14:28
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    $\begingroup$ @Brilsmurfffje Perhaps that may be true in the majority of cases, but there are significant exceptions. For example, aircraft coming into the terminal for overseas arrivals at JFK are always towed to their respective company terminals after discharging their pax. And the last 747 carrier I flew for had only 3 gates at their headquarters terminal and almost always towed from the gate. Also nose-loading 747 freighters are typically towed away from nose loading ports that lead into a cargo facility. $\endgroup$
    – Terry
    Commented Mar 22, 2018 at 17:03
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    $\begingroup$ I would add that chocks are a more reliable - and visible - way to ensure a plane is actually held in place, whether GA or much larger. $\endgroup$
    – John Polson
    Commented Mar 22, 2018 at 17:33
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    $\begingroup$ @JohnPolson Be aware that if you have posted a question, it's not at all frowned upon to edit your answer to make it more complete rather than putting it in a comment. $\endgroup$
    – Terry
    Commented Mar 22, 2018 at 19:15
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    $\begingroup$ @Brilsmurfffje aircraft are regularly towed from gates to maintenance areas overnight or if their return flight is long enough away that keeping them at the gate would obstruct other flights. $\endgroup$
    – jwenting
    Commented Jun 28, 2018 at 10:39

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