During run-in tests, what condition indicates thermal stability?

• A. Turbine blade color
• B. Consistent oil pressure and stable temperatures across operating range
• C. Rotor speed
• D. Battery charge level

Answer: (B)

Thermal stability during run-in tests is shown by consistent oil pressure and stable temperatures across the operating range. When the engine is brought up through its speeds and loads, the lubrication system must deliver a steady oil pressure so bearing clearances and film strength remain constant. If oil pressure stays within the designed band throughout the test, it means the pump, passages, and lubrication routes are functioning properly and there are no leaks or blockages affecting oil flow as temperature and RPM rise. At the same time, temperatures that remain stable as you move from idle to high power indicate the cooling system and fuel control are balancing heat production with removal across the entire operating envelope. If temperatures drift upward or show large fluctuations, hotspots or inefficient cooling or fuel metering could be developing, signaling a lack of thermal stability.

Other indicators like blade color, rotor speed alone, or battery charge don’t directly reflect how the engine handles heat across changing conditions. Blade color isn’t a reliable, quantitative measure of in‑service temperatures; rotor speed tells you speed, not how evenly the engine is cooling or lubricating, and battery level isn’t connected to the engine’s thermal behavior during run-in.