---
title: "CS 25J939 APU operating characteristics — CS-25 · Avioverse"
description: "APU operating characteristics must be investigated in all aeroplane operating conditions from APU start until shutdown to determine that no adverse…"
url: "https://avioverse.aero/regulations/easa/cs-25/subpart-j/cs-25j939-apu-operating-characteristics"
---

# CS 25J939 APU operating characteristics

Large Aeroplanes (CS-25) · CS-25 · EAR revision 26 Jan 2023

IR Implementing rule

## CS 25J939 APU operating characteristics

(a) APU operating characteristics must be investigated in all aeroplane operating conditions from APU start until shutdown to determine that no adverse characteristics (such as stall, surge, or flame-out) are present, to a hazardous degree, during normal and emergency operation within the range of operation limitations of the aeroplane and of the APU.

(b) Reserved

(c) The APU air inlet system may not, as a result of air-flow distortion during normal operation, cause vibration harmful to the APU.

(d) It must be established over the range of operating conditions for which certification is required, that the APU installation vibratory conditions do not exceed the critical frequencies and amplitudes established under CS-APU 120.

\[Amdt 25/1]

IR — CS-25 · ED Decision 2005/006/R · CS-25 Easy Access Rules · EAR revision 26 Jan 2023

[All rules in SUBPART J – AUXILIARY POWER UNIT INSTALLATIONS](https://avioverse.aero/regulations/easa/cs-25/subpart-j)

Consolidated from the EASA Easy Access Rules (revision 26 Jan 2023, extracted 13 Oct 2026) for convenience. Not the official publication — verify against the Official Journal of the European Union and the EASA publications before operational use.