Aircraft Certification Staff Instruction (ACSI)

ACSI No.: 5
Issue No.:  1
Issue Date:  1998-11-10
AARD File:  5009-3-5

Aircraft Certification Staff Instruction (ACSI)



1.1 The Mode S (S for Select) Transponder has been designed as an evolutionary addition to the Air Traffic Control Radar Beacon System (ATCRBS) to provide the enhanced surveillance and communication capability required for Air Traffic Control (ATC) automation. While providing the usual 4096 Identification Code and pressure altitude information to ATC interrogation, Mode S also provides for selective interrogation to enhance surveillance functions and data link communication functions.

1.2 Mode S performs all the functions of Mode A and C transponders, and has data link capability. Mode S transponders are an integral component of all Traffic Alert and Collision Avoidance System (TCAS) II installations and replace the Mode A and C transponder for TCAS II equipped aircraft. A Mode S transponder may be installed to replace a Mode A or C transponder without necessitating the installation of TCAS.

1.3 One new feature of the Mode S Transponder is that each aircraft is assigned a unique address code, which is broadcast in unsolicited "SQUITTER" transmissions occurring approximately every second. ATC or another Mode S equipped aircraft will use this address for interrogation or communication purposes.

1.4 Mode S Transponder specific advantages are as follows:

  1. Surveillance of a large number of aircraft with better accuracy and increased surveillance reliability;
  2. High degree of data integrity in ground-to-air, air-to-ground and air-to-air data link; and
  3. In TCAS equipped aircraft, the TCAS transmits coordination/interrogations to the other aircraft via the Mode S link in order to ensure the selection of complementary Resolution Advisories.


The purpose of this ACSI is to provide HQ/Regional Aircraft Certification personnel and delegates with general information and procedural guidelines concerning unique aspects of Mode S transponder approvals.


Each Canadian registered aircraft with a Mode S transponder must receive a unique code assignment through their Transport Canada General Aviation Division.


4.1 Airworthiness approval is required and must be obtained by Canadian aircraft operators who install Mode S transponders. Operators should contact their regional Transport Canada Aircraft Certification office for approval details.

4.2 Since there has been no experience with respect to the use of specified data associated with Mode S transponder approvals, such data should be reviewed by Regional Aircraft Certification and accepted if deemed adequate.

4.3 It is expected that additional Mode S capability, such as Datalink and Automatic Dependent Surveillance will be incorporated with future software updates. Approval for this added capability will be based on separate requirements.


Mode S testing procedures and guidelines are contained in AC 20-131A dated 29/03/93 and FAR 43 Appendix F. AWM Chapter 571 appendix F provides guidelines with respect to ATC Transponder Performance Tests. Additionally, the following verification checks must be made since they are not addressed in the above-mentioned documents:

5.1 Mode S Address

The Mode S address is unique to each aircraft and is hardwired into the airframe side of the transponder installation. If the Mode S address is missing or invalid, the aircraft will be "invisible" to TCAS. The preferred method of verifying the address is by using a commercially available Mode S Test Unit, which can decode the address from the transponder transmission. Alternatively, the address can be decoded from the data bus used to output Mode S information using appropriate test equipment. A check of hardware pin coding from the airframe side for address verification has not been found to be acceptable as it does not confirm that the transponder connector is picking up the coding.

5.2 Air/Ground Discrete

The Mode S Transponder transmits this discrete. TCAS equipped aircraft receive this information and TCAS drops the tracking of an aircraft if it is "On Ground". A hardwire check of this discrete is not considered to be adequate verification . The discrete should be functioned and a Mode S Test Unit used to decode the discrete from the transmitted signal. Alternatively, the discrete may be decoded from the data bus which outputs the Mode S information.

5.3 Max Airspeed Discrete

This discrete is used by Mode S to transmit the maximum true airspeed (TAS) capability of the host aircraft. TCAS on other aircraft uses this information when tracking the aircraft to determine a time parameter for its next interrogation. If this discrete is incorrect, TCAS may not interrogate a threat aircraft until it is unacceptably close. The preferred method of verifying this discrete is by using a Mode S Test Unit to decode the Mode S transmitted signal. Alternatively, the discrete can be verified by decoding it from the data bus used to output Mode S information.


The Mode S Transponder can function independently without TCAS II. However, most Mode S control panels have switch selector positions for TCAS II functions. If an aircraft has such a Mode S control panel, without TCAS installed, placarding of the unavailable functions is required.

Original signed by:

K.J. Mansfield
Director, Aircraft Certification

Date modified: