During starting the MAX engine, if N2 stagnates at 18%, what is the most likely cause?

During starting the MAX engine, if N2 stagnates at 18%, what is the most likely cause?

• A. the Bowed Rotor Motoring (BRM) function is active
• B. Low fuel pressure
• C. Hydraulic leak
• D. Ignition fault

Answer: (A)

When starting, the high-pressure spool (N2) should accelerate as fuel and ignition bring the compressor up to speed. If N2 stalls and holds at about 18%, it’s a sign a protective motoring function is actively preventing further acceleration. Bowed Rotor Motoring is designed to apply a controlled motor load to the rotor when a rotor bow condition is detected or during certain diagnostic checks, which stops the spool from freely accelerating. That intentional load keeps N2 around a low, steady value—often in the mid-teens to around twenty percent—rather than allowing it to rise toward idle speed. That’s why this is the most likely cause. Other faults like low fuel pressure, a hydraulic leak, or an ignition fault would typically show different behavior (no ignition or no fuel to drive acceleration, or inconsistent motion), not a steady plateau at 18%.

When starting, the high-pressure spool (N2) should accelerate as fuel and ignition bring the compressor up to speed. If N2 stalls and holds at about 18%, it’s a sign a protective motoring function is actively preventing further acceleration. Bowed Rotor Motoring is designed to apply a controlled motor load to the rotor when a rotor bow condition is detected or during certain diagnostic checks, which stops the spool from freely accelerating. That intentional load keeps N2 around a low, steady value—often in the mid-teens to around twenty percent—rather than allowing it to rise toward idle speed. That’s why this is the most likely cause. Other faults like low fuel pressure, a hydraulic leak, or an ignition fault would typically show different behavior (no ignition or no fuel to drive acceleration, or inconsistent motion), not a steady plateau at 18%.