What is the tolerance specified for the minimum bleed air pressure during engine start?

• A. ±1 psi
• B. ±2 psi
• C. ±3 psi
• D. ±4 psi

Answer: (C)

Bleed air pressure during engine start must meet a defined minimum, and the specified tolerance shows how much the actual pressure can vary around that minimum and still be acceptable. A tolerance of plus or minus three psi means the pressure can be a bit lower or a bit higher than the exact minimum and the start is still considered okay. This range is wide enough to accommodate normal fluctuations from sensors, duct losses, valve transients, and small engine-to-engine differences, while still ensuring there’s enough pressure to reliably start the engine. If the tolerance were tighter (like plus or minus one or two psi), normal variations could cause false failures or unnecessary start aborts. If it were looser (like plus or minus four psi), you’d risk starting with insufficient or unstable pressure. So the three-psi tolerance strikes a practical balance, making ±3 psi the best choice.