What factors contribute to achieving proper surge margin, and which inspection indicators relate to surge risk during an overhaul?

• A. Visual inspection of color.
• B. Check fuel viscosity only.
• C. Ensure compressor stability and adequate inlet pressure recovery; inspect for leaks, incorrect clearances, and incorrect spray or ignition; use test cell results.
• D. Random guess.

Answer: (C)

Surge margin is about keeping the compressor operation away from the surge line by ensuring stable airflow and proper fuel timing across operating conditions. The best way to support that during an overhaul is to verify compressor stability and ensure adequate inlet pressure recovery, while checking for issues that can upset the flow or timing. Leaks in the inlet or bleed paths reduce pressure recovery and disturb the mass flow, making surge more likely. Incorrect clearances between blades and casing alter the rotor-stator interactions and can destabilize the flow, raising surge risk. Incorrect spray or ignition timing or delivery can cause unstable combustion pressures, which can drive the compressor toward surge. Finally, validating these factors with test cell results provides concrete data on the engine’s surge margins under real operating conditions.

Visual color checks don’t reliably indicate surge margin, and checking fuel viscosity alone doesn’t address the flow or stability aspects that govern surge. A random guess isn’t a method for assessing surge risk either.