Transponder Phraseology?

Report Text: I called ATC asking for a Basic Service. I was given the QNH which I entered into my altimeter; Sky Demon and GPS confirmed the same altitude – 2400ft. After approximately 4 minutes I was called by  ATC warning me that I was about to enter their controlled airspace at 2600ft!

(controlled airspace minimum 2500ft.) I informed them that my instruments confirmed that my altitude was 2400ft. ATC asked to change to Mode A. Due to my relative low hours I had no Idea how to change from Mode C to Mode A. I therefore switched off my transponder and immediately descended to 2000ft.

On reaching my destination, I spoke to the avionics engineer about my recent experience. “Ha!”, he said, “your encoder is transmitting the wrong reading. ”

Now I know more detail about my transponder, which at the time was reading 200ft. out. On my next local flight I called  ATC for an altitude check which now confirmed a 100ft. error; understand the maximum acceptable error is 200ft. I wonder how many other GA pilots have infringed or not infringed due to their transponder error?

Lessons Learned: Having completed my flight training and gaining my PPL, I had no idea how to change to Mode A. Never fly close to Controlled Airspace and periodically ask ATC to confirm you your transponder altitude reading.

CHIRP Comment: This report was previously published in GA FEEDBACK Edition 72, the report is reproduced here to remind controllers that GA pilots may be unaware of the terminology ‘Modes A & C’. In many GA aircraft the transponder switches are labelled ‘Altitude on/off’ and there is no requirement for pilots to be aware of Modes A & C.

Therefore it is vital to stick to standard terminology from CAP 413:

“Squawk Altitude” means select the altitude reporting feature.
“Stop squawk Altitude” means deselect altitude reporting.
“Stop squawk Altitude, wrong indication” means stop altitude report, incorrect level readout.

From an Air Transport CHIRP report

Author: FTN Editor

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