I work on the ramp servicing small turboprop aircraft (ATR 72 and Q300s) and recently I’ve noticed that they don’t really require us to indicate when we’ve chocked the aircraft, however the larger jets do, as there is something they have to do involving the parking brake. So my question is what is so different about larger aircraft that they need to rely on chocks being in place? Aren’t all hydraulic brakes the same?
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2$\begingroup$ I assume it has more to do with turnaround timing then type. $\endgroup$– Giacomo CatenazziCommented Jul 8 at 6:43
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7$\begingroup$ Not sure how the parking brakes on a airplane works but I believe they can not be engaged immediately after landing, when still hot. And extra safety doesn't hurt. Cars are also choked before a mechanic would work on a car, even if the both the P-gear and parking brake are engaged. $\endgroup$– user3528438Commented Jul 8 at 7:56
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1$\begingroup$ @user3528438 While the parking brake shouldn't be engaged when the brakes are extremely hot, after a normal landing the parking brake is used when arriving at the gate. It's kept on at least until the engines have shut down and the ground crew has chocked the aircraft. $\endgroup$– BianfableCommented Jul 8 at 14:49
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3$\begingroup$ Reading the answers, the right questions seems to be: why to small turboprop aircraft not require the chocks? $\endgroup$– jcaronCommented Jul 8 at 16:12
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10$\begingroup$ ...in aviation everything is about "if that fails then this" is the backup... $\endgroup$– GabeCommented Jul 8 at 19:08
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6 Answers
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First, chocks are an added level of safety. At many airports it is a local restriction that the passenger bridge cannot be connected before the chocks are in place.
During landing and long in-taxis the brakes sometimes get quite hot. Having chocks in place lets pilots to release parking brakes while on stands. This helps brakes to cool down as air is able to flow between hot brake discs. The ability of brakes to absorb energy is dependant of their initial temperature so it is often required to cool the brakes before the next departure in anticipation for a potential rejected takeoff at high speed.
Lastly, all(?) transport category aircraft have hydraulic brakes. During longer turn-arounds like overnight stops brakes tend to slowly lose their pressure, leaving the chocks the sole medium keeping the aircraft in place.
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9$\begingroup$ 787 and A220 have electromechanical brakes, which do not have the slowly-lose-pressure failure mode of hydraulic parking brakes. $\endgroup$ Commented Jul 8 at 16:15
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Another advantage for chocks is that an observer outside the aircraft can visually confirm that the aircraft is chocked.
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I was a crew chief on C5 and C141 aircraft in Altus, Oklahoma. Everything that was on the flightline had chocks set. In Oklahoma sometimes the flightline would be solid ice with the wind blowing at a million miles an hour. The chocks would help prevent the plane (truck, generator, whatever) from sliding around on the ice. You wouldn't think something like a C141 could be pushed around by wind but they sure were.
Chocks are also like belts and suspenders - where if somehow the parking brake is released, you still have the chocks preventing your plane full of fuel, liquid oxygen, and atomic weapons from rolling into another plane full of fuel, liquid oxygen, and soldiers.
The air force tended to do everything the exact same way every time no matter what the circumstances - that way you get in the habit of "the one right way", and no matter where you happen to be in the world, or how stressed you are, you know the way you've been trained will work.
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$\begingroup$ Were tiedowns available for smaller equipment? $\endgroup$ Commented Jul 8 at 23:40
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Hydraulic bleed-down can be a problem with keeping a parking brake set.
Also, if parking brakes are set, a FBO can’t reposition a plane without a pilot disengaging the brakes. That’s why jets will have a card in the windshield that says “BRAKES OFF”.
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Because of the hydraulic pressure drops as the parking brake applied due to the normal internal leak of brake system so in some aircraft like (737 ) the hydraulic pressure can be stand only for 8 hrs if parking applied so we need chocks to keep aircraft in safe parking position for long times .
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My tangentially related training in aviation maintenance leads me to believe smaller aircraft probably have a separate system involved in setting a parking brake. Something like a cable that has very little variance over time and so could hold the aircraft still for very long periods of time as set.
I say this because most aircraft operate on a, 'primary', 'secondary', 'tertiary' system mentality. Which is to say, there's always a primary system for any core functionality, and then a secondary system you can fall back on, and then a tertiary system to use if both primary and secondary have failed. Tertiary isn't always available, depending on implementation, but there will always be primary and secondary.
For larger aircraft, there is no cable for brakes. It's hydraulics only, as a cable system simply wouldn't be enough to stop the aircraft, making such a cable pointless extra weight. What you'd have instead is a hydraulic primary system driven by the jet engine hydraulic pumps, a secondary electric hydraulic pump provided with power via the APU, and then chocks as the tertiary(obviously only when stationary on the ground, but also the only long term solution for parking).
Smaller aircraft probably use a hydraulic primary, with a cable secondary(which can be used as a reliable long term parking brake on the ground), and chocks become the largely unnecessary tertiary option.
So it becomes important to note whenever chocks are installed on a large aircraft, but not really consequential for small aircraft.
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1$\begingroup$ I'm curious if there are any common GA planes that have a cable brake backup to a hydraulic system. I can't think of any. $\endgroup$ Commented Jul 9 at 15:05