This publication provides recommendations and examples, further to the applicable regulation, certification specifications, and AMCs. They have no legal value and are only advisory.
Should I choose CS-ACNS, or AMC 20-24 as part of the certification / compliance basis for installation of ADS-B (for a new TC or a change to a TC/STC)?
The intents of CS-ACNS and AMC 20-24 are different. The former provides the means to comply with the rules set forth in the Commission Implementing Regulation (EU) No 1207/2011 (amended by (EU) No 1028/2014) (‘SPI regulation’) and supports applications where the ADS-B could be used to replace Radar (RAD) applications with a 3 Nm separation between aircraft. The latter is intended to support Non-Radar Areas (NRA) applications, with a 5 Nm separation and less stringent integrity and continuity needs.
For certification of ADS-B installations, CS-ACNS applies to aircraft with a maximum certified take-off mass exceeding 5700 kg or having a maximum cruising true airspeed capability greater than 250 knots.
Compliance to CS-ACNS (including book 2) has to be demonstrated for the initial airworthiness and changes to those aircraft. For a new TC, CS-ACNS is expected to be part of the certification basis.
For other aircraft, the applicant may elect to use AMC 20-24 instead of CS-ACNS.
What are the differences between CS-ACNS and AMC 20-24 regarding requirements for ADS-B?
There are several differences in terms of parameters required, continuity of the function, ES version used, as shown in this non-exhaustive list:
The term ‘remote’ corresponds to quantitative probability of 1E-5 / F.H.
The terms ‘major’ and ‘minor’ are Failure Condition classifications corresponding to quantitative probability objectives of respectively 1E-5 and 1E-3 / F.H.
Should a change to type design installing ADS-B out capability be classified as Minor or Major?
Due to the ‘major’ failure effect associated to ADS-B Integrity and Continuity conditions (due to potential disrupting effects at ATC level) and the novelty of the function and potential extension of the operational capabilities, the changes that use CS-ACNS are expected to be classified as Major.
However, changes using only AMC 20-24 may be classified as Minor if certain conditions are met:
- The transponder is ETSO-2C112b approved and complies with the requirements of ED-102/DO-260 or DO-260A, and
- The GNSS receiver is approved under any of ETSO C-129A, TSO C-129, TSO C-129A, ETSO C-145/C-146, or TSO C-145A/C146A, and
- The interface between the transponder and GNSS received is direct (no routing through other equipment).
What does the Continuity requirement imply and how is Continuity calculated?
- The assessment should encompass all equipment contributing to the ADS-B function and not just the transponder.
- The reliability of each component contributing to the function (e.g. transponder, control panels, position and velocity source) has to be considered,
- The probability of failure of the power source failure has to be considered,
- Common modes have to be considered.
Our analysis show that the Continuity meets the Implementing Rule / AMC 20-24 requirement, but not the CS-ACNS requirement.
CS-ACNS requires compliance to a qualitative probability of ‘remote’, which corresponds to a quantitative probability of 1E-5 / F.H. This is more challenging to meet than the Implementing Rule Annex II and AMC 20-24 figure (2E-4 / F.H.).
A deviation to this requirement may be accepted on a case-by-case basis and under conditions by the Agency after review of the compliance data. Such a deviation is published by EASA under reference ‘Deviation Request CS-ACNS#1’.
No deviations to the requirements of the Implementing Rule will be granted.
Is a dedicated ADS-B failure annunciator required in the instrument panel?
CS ACNS.D.ADSB.090 requires that a means is provided to indicate the non-operational status or failure of the ADS-B Out system without undue delay.
This can be an indicator dedicated to an ADS-B failure, however a single transponder failure indicator may be acceptable if the crew are able to distinguish between ADS-B device or function failures and other transponder failures using suitable troubleshooting procedures.
AMC 20-24 does not contain an equivalent requirement.
What are the requirements on the GNSS source?
The requirements are described in AMC 20-24 and CS-ACNS.
- Position and velocity information must come from the same source.
- ETSO-C129A is the minimum equipage qualification in Europe.
- For compliance with CS-ACNS, the applicable ETSOs (C129a/196/145/146) alone does not guarantee that the unit is suitable as an ADS-B position source. Further demonstration is required (see in particular AMC1 ACNS.D.ADSB.070 and .080).
- The System Design Assurance (SDA) parameter can be set to 2 without analysis if the GPS and ADS-B unit are both approved (to either ETSO C129a, C196(), C145(), C146()) and directly connected (i.e. direct wiring from GPS to receiver with no routing through other equipment). Aircraft with other architectures require a system safety analysis to set the SDA.