Originally Posted by
OVERTALK
Graybeard said
Can you elaborate upon this please Graybeard?
TCAS R/T: means the TCAS verbal directives?
Both DME? Where does DME come into it?
Sorry: R/T = Receiver/Transmitter, or Transceiver. It's also called a TCAS Processor. R/T is the part of consequence here.
Next time you're doing a preflight, take note of the DME and transponder antennas. In most cases they are identical. That's because they operate on similar frequencies in the 1.0 GHz range. The suppressor line connects the two transponders, the TCAS unit, and the two DME transceivers all onto a single line. The line sits near 0 volts. When any of the 5 units transmits, it raises the suppressor line to above 18 volts, commanding the other units to shut off their receivers during that brief portion of time, to avoid interference one with another.
If one of the units fails, and keeps the suppressor line high, the other units will be deafened, so to speak, while not causing any failure indication on the flight deck. The DMEs will show loss of signal, however. This is a rare failure, but possible.
If the suppressor line is disconnected at one or more places, there will be interference between units. Only a few DME frequencies will cause interference with the transponder and TCAS, on their 1030 MHz and 1090 MHz frequencies. A suppressor line disconnect between the active transponder and the TCAS will cause the TCAS to see own transponder and go crazy. That failure has been hidden in later TCAS units, but the interference is still there, and performance will be degraded.
The likelihood of an open line is probably significant in brand new airplanes. I've had to troubleshoot MD-80s where the phased TCAS installation was incomplete before bankruptcy, and when the plane was put back in service, the suppressor cable was overlooked.
What does R/T mean to you, Radio Transmissions?
If you're further interested, somewhere there is a list of DME frequencies paired with VOR frequencies. Any decent DME installation or overhaul manual should have it. If there's no other explanation for the apparent transponder failure on the Legacy, this would be worth exploring.
GB