---
title: "ETSO-C139a A1 — CS-ETSO · Avioverse"
description: "Audio Systems and Equipment 1 Applicability This ETSO provides the requirements that Audio Systems and Equipment that are designed and manufactured on or…"
url: "https://avioverse.aero/regulations/easa/cs-etso/subpart-b/etso-c139a"
---

# ETSO-C139a A1

European Technical Standard Orders (CS-ETSO) · CS-ETSO · EAR revision 1 Oct 2026

IR Implementing rule

## ETSO-C139a A1

Audio Systems and Equipment

1 Applicability

This ETSO provides the requirements that Audio Systems and Equipment that are designed and manufactured on or after the date of this ETSO must meet in order to be identified with the applicable ETSO marking.

2 Procedures

2.1 General

The applicable procedures are detailed in CS-ETSO, Subpart A.

2.2 Specific

None.

3 Technical Conditions

3.1 Basic

3.1.1 Minimum Performance Standard

The applicable standard is that provided in RTCA DO-214A, ‘Audio Systems Characteristics and Minimum Performance Standards for Aircraft Audio Systems and Equipment’, dated 18 December 2013 as modified by Appendix 1 to this ETSO.

3.1.2 Environmental Standard

See CS-ETSO, Subpart A, paragraph 2.1.

3.1.3 Software

See CS-ETSO, Subpart A, paragraph 2.2.

3.1.4 Airborne Electronic Hardware

See CS-ETSO, Subpart A, paragraph 2.3.

3.2 Specific

3.2.1 Failure Condition Classification

See CS-ETSO, Subpart A, paragraph 2.4.

4 Marking

4.1 General

See CS-ETSO, Subpart A, paragraph 1.2.

4.2 Specific

None.

5 Availability of Referenced Documents

See CS-ETSO, Subpart A, paragraph 3.

\[Amdt ETSO/11]

\[Amdt ETSO/17]

IR · ETSO-C139a — CS-ETSO · ED Decision 2022/018/R · CS-ETSO Easy Access Rules · EAR revision 1 Oct 2026

Appendix Appendix

## Appendix 1 to ETSO-C139a A1 — Correction to RTCA DO-214A, ‘Audio Systems Characteristics and Minimum Performance Standards for Aircraft Audio Systems and Equipment’

A1.1 Introduction

This Appendix corrects RTCA DO-214A, ‘Audio Systems Characteristics and Minimum Performance Standards for Aircraft Audio Systems and Equipment’, regarding an optical character recognition error in the value of a test capacitor in a test procedure not detected during the peer review process. The value was ‘1E-14 F (0.01 pF)’ instead of the required ‘1E-8 F (0.01 µF)’, so it was six orders of magnitude too low for interphone channels.

In addition, this Appendix clarifies RTCA DO-214A, ‘Audio Systems Characteristics and Minimum Performance Standards for Aircraft Audio Systems and Equipment’, regarding an ambiguous requirement for the HOT microphone feature in Section 1.4.5.

A1.2 Correction

In Section 2.8.2.9 of RTCA DO-214A, ‘Audio Systems Characteristics and Minimum Performance Standards for Aircraft Audio Systems and Equipment’, ‘0.01 pF’ is replaced by ‘10 nF’.

\[Amdt ETSO/17]

APPENDIX · Appendix 1 to ETSO-C139a — CS-ETSO · ED Decision 2022/018/R · CS-ETSO Easy Access Rules · EAR revision 1 Oct 2026

[All rules in SUBPART B – LIST OF ETSOs](https://avioverse.aero/regulations/easa/cs-etso/subpart-b)

Consolidated from the EASA Easy Access Rules (revision 1 Oct 2026, 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.