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A pneumatic braking system has indisputable advantages over a hydraulic braking system for large and heavy aircraft in terms of braking force and secondary perhaps in terms of weight (pipes and hydraulic fluid). However it requires engine input to compress the air and generate the pressure and force required for braking, all of which use fuel.

What is the cost of a pneumatic braking system from a fuel point of view as opposed to hydraulic in relation to weight for a large Jumbo like the Airbus A380 or Boeing 747 if one had to consider using hydraulic brakes for any of these aircraft?

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    $\begingroup$ >>indisputable advantages<< I question this - if there were advantages then aircraft would be using pneumatic brakes, but they aren't. A pneumatic system off the engines usually runs at around 50psi, whereas a hydraulic system runs at either 3000psi or 5000psi - MUCH more effective for braking. $\endgroup$ – RAC Sep 6 '18 at 9:27
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    $\begingroup$ Negligible at most. An A380 flies at 100MW for dozens of hours per trip yet you care about a few KW for brake actuators that operates a few minutes or less per trip? $\endgroup$ – user3528438 Sep 6 '18 at 17:12
  • $\begingroup$ The fundamental issue is air is compressible which makes it more difficult to control. See: aviation.stackexchange.com/questions/21426/… $\endgroup$ – user71659 Sep 12 '18 at 0:34
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There is an existing triple redundant hydraulic system in those planes required used for the control surfaces. Those are not something you want controlled by pneumatics.

This means the weight of the compressor, buffer tank etc. is already there and using it for brakes means you only need to pull a line from a supply/return line (which would also be near the gear to actuate it anyway) and adding a servo valve.

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Interestingly there is one example of this entering service and provides a decent reference point. The Fokker/Fairchild F-27 had an all pneumatic undercarriage (breaks, retract, and steering). When compared with similar aircraft (role/size/era) like the Hawker Siddeley HS 748 or the Handley Page Dart Herald the F-27 actually preformed a bit worse although that by no means indicates the brakes are the cause of this.

All told the system had a flaw in the early implementation:

I flew the FH227 on the DEW line in the early 70's. We operated from gravel strips to resupply the radar sites. A definite step up from the DC3's and the C46's.In warm weather (+20c or higher) we would from time to time use the water meth injection for t/o. The gear was pneumatically operated.If there was any moisture in the lines in the winter (-50c or lower),the gear could freeze in the up position.The company modified the system with a standby 3000 psi nitrogen bottle with a separate set of lines which could be used to lower the gear if this situation happened (only once that I know of).Operating in the arctic in shirt sleve comfort was a very nice experience.

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