I tried my PCAS MRX-A for the first time today and it appeared to be working properly accept the "TRA FL" consistently reported 200ft lower than my transponder was reporting (as verified by ATC). Any ideas on cause and/or correction procedure?

I'm not certain, but could this be the issue?


In New Zealand, the ATC radar system for the controllers automatically applies the area QNH for the controller, so on their radar screen, your transponder altitude is displayed in multiples of 100 feet corrected from ISA pressure altitude (QNH 1013).

Thus, ATC should be roughly reporting your altitude within 100 feet of your altimeter.

If you wanted to check, then you'd need to look at what your transponder altitude was displaying on the actual transponder in the aircraft and not ask ATC as they will report your actual altitude. Some transponders display this on a screen such as 015 (1500 feet) and many do not.

Another simple way to check is set QNH 1013 on your altimeter sub scale and see if the altitude displayed on your altimeter then matches your PCAS.

Also, bear in mind the actual tolerances for transponders is +/- 200 feet, so in fact 200 feet out is at the limit, but just inside the allowed tolerances.