APU RPM Red limit is what percent?

• A. 105%
• B. 107%
• C. 109%
• D. 111%

Answer: (B)

The essential idea is knowing the APU red-line speed—the maximum safe spool speed for the APU, expressed as a percent of its rated speed. This limit prevents overspeed damage to the turbine and related components, so there’s a protective threshold that the system won’t allow to be sustained.

For the CRJ-550 APU, that red-limit is 107 percent. Short excursions above 100% can occur during certain operations (like starts or bleed-air procedures), but sustained operation at or beyond the red line is not allowed and the protections will intervene to prevent damage. This is why 107% is the correct limit.

Other values would either be below the defined protective threshold or exceed it; they don’t match the specified red-line limit for this aircraft’s APU.