Safety Investigation Report 2018:1 Factual Information/1.9/1.9.3 Mode S Transponder
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
1.9.3 Air Traffic Control/Mode S Transponder System
This aircraft was installed with a Bendix/King TRA-67A Mode S transponder. The Air Traffic Control (ATC) ground stations interrogate the airborne ATC/Mode S transponder system as shown in Figure 1.9A (below).
The ATC/Mode S transponder replies to the interrogations in the form of coded information that the ground station uses. The ground station uses a Primary Surveillance Radar (PSR) to get radar returns from aircraft within the radar range. To make a communication link with the aircraft in the radar range, the ground station uses a Secondary Surveillance Radar (SSR) to interrogate the ATC/Mode S transponder. The ground station transmits a side lobe suppression signal to inhibit close ATC replies that come from a SSR side lobe transmission.
Figure 1.9A - Air Traffic Control/Mode S Transponder System
Copyright © Boeing. Reprinted with permission of The Boeing Company
On the ground radar display, the Air Traffic Controller (ATC) sees the radar returns, altitude, and a four digit aircraft identifier. The ATC also sees aircraft derived Enhanced Surveillance downlink data on the ground station radar display, such as Magnetic Heading, Air Speed (Indicated Air Speed and Mach number), Ground Speed, Roll Angle, Selected Altitude, True Track Angle, andVertical Rate.
The ATC/Mode S transponder also replies to mode S interrogations from the Traffic Alert and Collision Avoidance Systems (TCAS) of other aircraft. ATC/Mode S transponders with Extended Squitter function provide broadcast of Global Position System (GPS) position and velocity data.
Two transponders are installed on the aircraft. A Transponder selector switch on the Transponder panel in the cockpit allows selection of either the left or the right transponder. During normal operations the crew procedure is to leave the left transponder selected on the panel. There is no automatic switching between the transponders if one fails. It must be done manually by the pilots. Failure of either of the transponders will be annunciated in the cockpit. The Left ATC/Mode S transponder gets 115V AC power from the AC Standby bus. The Right ATC/Mode S transponder gets 115V AC power from the Right AC Transfer bus. Thedual transponder panel gets 115V AC power from the AC Standby bus. The two transponders are powered by highly reconfigurable AC buses; the left one can be powered by the battery if the left AC bus is unavailable (the AC Standby bus can be powered by the left Transfer bus or the battery), and the AC Transfer busses also have their alternate sources.
This system can be deactivated (turned OFF) by pulling the circuit breakers located at the P11 overhead circuit breaker panel or by selecting the Transponder Mode Selector (Transponder Panel) to “STBY” position. The transponder on the occurrence flight was operating satisfactorily up to the time it was lost on the ATC radar screen at 1721.13 UTC, 07 March 2014 [0121:13 MYT, 08 March 2014]. There was no message received from the aircraft to report a system failure.