In case of total failure of the hydraulic system, most large aircraft brake systems can be operated using a ______ emergency system.

• A. Electrical
• B. Mechanical
• C. Hydraulic
• D. Pneumatic

Answer: (D)

When braking is normally powered by hydraulics, you need a backup that doesn’t rely on those hydraulics. A pneumatic emergency system uses compressed air to actuate the brake pistons, providing braking capability even if the hydraulic system is completely failed. Air pressure can be stored or supplied from the aircraft’s systems, so the brakes can still be applied without hydraulic pumps or power generation. Electrical backups would still depend on power availability and could fail in a total system loss; a mechanical backup would require a direct, heavy linkage for every wheel and isn’t practical for large aircraft; hydraulic backups wouldn’t help at all if the whole hydraulic circuit is down. Therefore, compressed air is the sensible independent source for emergency braking on large jets.