Inaccurate Airborne Status Transmitted by Transponders and its Effect on Runway Monitoring and Conflict Alert Systems - Civil Aviation Safety Alert (CASA) No. 2021-03


All air operators, air navigation services providers, original equipment manufacturers and aircraft manufacturers

File Classification No.: Z 5000-35 U
RDIMS No.: 18081002
Document No.: CASA 2021-03
Issue No.: 02
Effective Date: 2021-12-20


The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness to aircraft owners, operators and Air Navigation Services Providers (ANSP) and original equipment manufacturers (OEMs) on potential safety issues related to operation of aircraft equipped with the integrated avionics system transponders.


The Transportation Safety Board (TSB) advised Transport Canada of an occurrence at Toronto/Lester B. Pearson International Airport (CYYZ) where, during a rejected take-off, an Embraer 190 (E190) equipped with a Honeywell Primus Epic integrated avionics system transponder transmitted that the aircraft was in airborne status, even though it remained on the runway.

A second aircraft, a Boeing 777-300 (B777), readying for take-off, had been issued a take-off clearance after the E190 commenced its take-off roll. The E190 flight crew made a radio call to the tower that they were rejecting the take-off, but, at the same time, the B777 read back its take-off clearance on the same control tower frequency and commenced its take-off roll. The simultaneous radio transmissions went undetected and neither air traffic control nor the B777 flight crew heard the radio call of the E190 advising of its rejected take-off.

Immediately after beginning its take-off roll the B777 flight crew observed that the E190 was still on the runway and initiated a rejected take-off. The B777 came to within 3800 feet from the E190.

In the initial assessment in its investigation (TSB investigation A20O0029) the TSB has found that the integrated avionics system in the E190 uses software logic that determines the aircraft to be airborne when the aircraft’s indicated airspeed exceeds 50 knots. The result is that the aircraft transponder may transmit that the aircraft is airborne when the aircraft may still be on the ground.

Transport Canada has determined that the airborne status indication was appropriate because this status was intended to ensure that aircraft position and trajectory data would be transmitted for use by Airborne Collision Avoidance Systems (ACAS) of other aircraft in the vicinity. If this data was not transmitted, prior to the aircraft physically being airborne, ACAS aboard proximate aircraft would not be able to detect and identify the departing aircraft as a potential threat.

The minimum performance standards for transponders are identified in the Radio Technical Committee for Aeronautics (RTCA) document Minimum Operational Performance Standards (MOPS) for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment (DO-181E MOPS). The standards require an override threshold that switches the transponder to airborne mode after 100 knots is reached, regardless of any other sensor input.

For the occurrence aircraft, and most similar airline operations aircraft, the take-off speed is greater than 100 knots and the airborne transponder indication will take place prior to take-off. The use of an override threshold of 50 knots on the occurrence aircraft may be unusually low compared to similar aircraft and may result in a transponder configuration of airborne for a longer period during the take-off roll prior to take-off or prior to returning to 50 knots after a rejected take-off. Other types of aircraft may have different override values up to 100 knots and can be set by the manufacturers.

The B777 was also identified as airborne after reaching 100 knots; the highest override threshold permitted by DO-181E. Lower override thresholds such as the 50 knots threshold used by the E190 lead to a longer period of airborne indication while on the ground during take-off rolls or rejected take-off prior to decelerating back to the threshold value.

Additionally, the runway incursion monitoring and conflict alert system (RIMCAS) used by the air navigation service provider (ANSP) at CYYZ was configured to use data from the aircraft’s transponder transmission as the primary indication that an aircraft had become airborne. Therefore, when the E190 exceeded 50 knots on its departure roll, the RIMCAS identified the aircraft as airborne even though it was not. As a result of this system logic, the RIMCAS did not detect a conflict when the B777 began its take-off roll and did not issue an alert until well after both aircraft had initiated their respective rejected-take-off procedures and decelerated.

In December 2020, NAV CANADA published an Urgent Air Traffic Control (ATC) Information Bulletin for all Toronto Tower personnel. The bulletin cautioned controllers that RIMCAS Stage 1 alerts (visual warnings that appear on advanced surface movement guidance and control system (A-SMGCS) displays advising ATC that a hazardous situation exists) and Stage 2 alerts (visual warnings appear on A-SMGCS displays and is accompanied by aural warning via a tower-wide alarm advising ATC that a critical hazard is imminent) may not be generated when Embraer E-jets and some aircraft manufactured by Dassault, Gulfstream, Learjet, and Textron Aviation (formerly Cessna) are departing. The bulletin also advised that Stage 1 and Stage 2 alerts may not be produced for aircraft or vehicles approaching the active runway when one of these aircraft types is departing, and controllers were reminded to monitor these situations closely. NAV CANADA is investigating options for RIMCAS software mitigations.

Transport Canada has communicated with the original equipment manufacturer (OEM) to address the current software logic to identify aircraft airborne status. Additionally, Transport Canada has communicated with applicable ANSPs who have similar RIMCAS, or any other system, that makes use of data transmitted from aircraft with similarly configured transponders.

This occurrence highlights the importance of understanding how and why data is output, before using data transmitted by aircraft avionics as an input to other systems, particularly air traffic management (ATM) systems. Given the system used transponder data which indicated that the aircraft was airborne, the logic used to classify the E190 as such was appropriate for the intended use.

Recommended action

  1. ANSPs whose ATM systems make use of data transmitted by aircraft avionics should ensure that the limitations of system logic are well understood and that the data is appropriate for the intended use by the ATM system(s).
  2. Air operators should become aware of the airborne status override threshold applicable to their aircraft and provide the information found in this CASA to staff and/or flight crew as a means of awareness.

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact Commercial Flight Standards in Ottawa by e-mail at

Félix Meunier

The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.