Minimum bleed air pressure for engine start?

• A. 50 PSI
• B. 60 PSI
• C. 45 PSI +/- 3
• D. 25 PSI

Answer: (C)

Pneumatic engine starts rely on bleed air to drive the air turbine starter, so a minimum amount of bleed pressure is required to spin the engine fast enough for ignition. For this aircraft, the specified minimum bleed air pressure to start the engine is 45 psi, with an allowable variation of plus or minus 3 psi. That means a usable range of about 42 to 48 psi.

Why that range fits: the starter and related valves are designed to function reliably only when the supply energy meets a certain threshold; if pressure falls below roughly 42 psi, the starter may not accelerate the core to the light-off speed, leading to a failed or hung start. If pressure is within or above the 42–48 psi window, the starter has enough energy to turn the engine over and reach ignition, without exceeding system tolerances.

In practice, you’d ensure the bleed source (APU or another engine) is delivering within that window before beginning the start. Values like 25 psi are clearly too low to reliably start; higher numbers outside the specified tolerance aren’t relevant to the minimum requirement, which is why the 45 ± 3 psi range is the correct choice.