Engine Operability Testing

We are providing guidance and advice for engineers involved in planning and conducting engine operability tests. Test requirements as well as applicable regulation are reviewed together with lessons learned from previous test experience.

Background

            Engine operability test covers both engine handling and transient performance characteristics. The basic idea is to expose the engine to different ambient, flight, and installation effects conditions while changing the engine operating point from one steady state condition to another. Engine operability margin, i.e. compressor surge margin, core pressures and temperatures limit, needs to be maintained for the different test conditions.

            The engine is delivered by the engine manufacturer with certified compressor margin stack-ups (Part 33) based on analytical and test bench validation. Once the engine is installed on the aircraft a number of items will (negatively) impact the operability margins; these items are inlet distortion effects, air bleed and power off-takes effects, throttle demand, atmospheric conditions (cold day, hot day, blowing snow) and flight conditions (low and high airspeed/Mach, high altitude). Part 25 certification focus on these installation effects on engine operability.

Certification Requirements

3.1       Civil Requirements

·       CS 25.939, turbine engine operating characteristics, and the equivalent FAR paragraphs:

(a) Turbine engine powerplant operating characteristics must be investigated in flight to determine that no adverse characteristics (such as stall, surge, or flameout) are present, to a hazardous degree, during normal and emergency operation within the range of operating limitations of the airplane and of the engine.

(c) For turbine engines, the air inlet system must not, as a result of airflow distortion during normal operation, cause vibration harmful to the engine.

·       CS 25.1143, engine controls, and the equivalent FAR paragraphs:

(b) Power and thrust controls must be arranged to allow – (1) Separate control of each engine; and (2) Simultaneous control of all engines.

(c) Each power and thrust control must provide a positive and immediately responsive means of controlling its engine.

3.2       Additional Requirement

Often airframers will specify their own thrust response requirements including go-around response from a landing configuration (approach idle thrust) and for accelerations in cruise/climb flight.

Test Content

4.1       General

            Testing will generally involve operating across the full aircraft flight envelope, covering from static on-ground cases to in-flight low/high speeds and low/high altitudes (envelope corner points). Additional test conditions other than the envelope corner points might be added if considered as worst operability cases.

Work close together with the engine manufacturer and design office to early identify the critical compressor stability case/s (or pinch point/s) identified during Part 33 certification. Decide whether these cases need to flow down into the test specification for Part 25 certification.

For more details on maneuvers, ground test and flight test including ‘Good Practice” advice, please reach out to us!! We can help with test preparation, instrumentation definition, test execution and more.

We are here to support your team!!