Evaluating Turbine Engine Operating Characteristics

AC 25.939-1 · FAA

Public domain · FAAAdvisory Circulars

Overview

The Evaluating Turbine Engine Operating Characteristics (AC 25.939-1) is a public-domain FAA advisory circular, republished here as a free chaptered HTML edition with a linked table of contents and the official PDF.

Publisher: FAA
Document: AC 25.939-1
Pages: 11
Chapters: 3

Key points

This advisory circular provides guidelines for evaluating turbine engine operating characteristics for subsonic transport category airplanes.
Compliance with § 25.939(a) can be assessed while determining compliance with other Part 25 requirements such as performance and controllability.
Adverse engine operating characteristics are classified into three levels of severity: mild, moderate, and severe.
Factors affecting engine operating characteristics include engine installation effects, atmospheric conditions, and pilot technique.
Flight test evaluations should consider all airplane configurations and assess transient and stable operation of engine accessories.

Frequently asked questions

What is the purpose of this advisory circular?

The purpose of this advisory circular is to provide guidelines for the evaluation of turbine engine operating characteristics for subsonic transport category airplanes.

Are the guidelines in this advisory circular mandatory?

No, the guidelines are not mandatory and do not constitute a regulation; they are derived from FAA experience.

What are the three levels of adverse engine operating characteristics?

The three levels of adverse engine operating characteristics are mild, moderate, and severe, each defined by specific operational impacts.

What factors can affect engine operating characteristics?

Factors include engine installation effects, location on the airplane, atmospheric conditions, and pilot technique.

What should flight test evaluations consider?

Flight test evaluations should consider all airplane configurations and the effects of engine accessories and equipment on operating characteristics.

Appendix 1

AC 25.939-1 3/19/86 Appendix 1 APPENDIX 1. SUMMARY OF DEFINITICWS Leve 1 of severity is determined by the occurrence of one or more of the most severe characteristics.

M LO M E A E SEV RE AD YES YES YES AUDIBLE SURGES OPERATING INSTABILITY MINOR/ YES YES BRIEF ENGINE ROTOR ACCELERATION NORMAL SLOW NONE ENGINE LIMITS EXCEEDANCE NONE BRIEF SUBSTANTIAL NONE TEMPORARY SUBSTANTIAL POWER LdSS/Roro~ SPEED DECRtASE NONE MINOR REQUIRES ENGINE VIBRATION SHUTDOWN CREW ACTION REQUIRED FOR RECOVERY NO MAYBE YES NO YES ENGINE FLAMEOUT NO NO NO YES ENGINE DAMAGE PRESSURE LOSS NO NO YES HAZARDOUS CABIN Page l

Next: Appendix 1 →

Appendix 1

AC 25.939-1 3/19/86 Appendix 1 APPENDIX 1. SUMMARY OF DEFINITICWS Leve 1 of severity is determined by the occurrence of one or more of the most severe characteristics.

Next: Appendix 2 →

Appendix 2

3/19/86 AC 25.939-1 Appendix 2 APPENDIX 2. RECOMMENDED TEST CRITERIA ADVERSE tN GIN E OPERATING CHARACTERISTICS ALLOWED ENGINE OPERATING CHARACTERISTICS TESTS MUUtKAlt NU'4E MILD ::>tVERE TAKEOFF & LANDING OPERATIONS Taxiing (Calm Air) X

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X X In Crosswinds or Tailwinds Takeoff (Calm Air} X

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X In Crosswinds or 1"ai l winds X Approach & Landing (Calm Air) X

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X In Crosswinds or Tailwinds X At Speeds Below Reverser X X © "Cutoff" Speed Reverse Thrust (Power) X X Backin9 ~

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TRANSIENT POWER C()!DITIOOS Jam Accels & Decels X

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Bodies (Interrupted Deceleration) X

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Airplane Stall Recovery X X

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("Idle" to "Max. Power") For the Above Transient Tests: At Abnormal Airplane & Eng. Conf. X X Cold Soak Descent & Recovery X

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ENGINE AIRFLOW DISTORTION TESTS Sideslips, Windup turns or

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Symmetrical pull-ups, Approaches to Power-On Stalls: 1. VMo to stall warning X @Alt.well above T.O. & Land. X X 2. Speeds below stall warning X X @Alt.well above T.O. & Land.

X X ©