JGoodish
Cleared for Takeoff
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- Jun 10, 2006
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JGoodish
I recently had the biennial static/altimeter/transponder inspections completed on my Cherokee, with no problems noted.
Subsequently, I was inside Class B airspace when I discovered that my Mode C was reporting an altitude 300 feet higher than what was actually being flown. I had the encoder re-calibrated in the aircraft, and flight testing afterward resulted in a dead-on Mode C altitude within 15 minutes, but off by 300 feet after 1 hour (I might expect this to be reversed). Encoder was pulled from aircraft, calibrated on the bench, and permitted to run on the bench for a few hours with no drift noted. Reinstalled in the aircraft, encoder now reports anywhere from 150 to 300 feet high when compared with aircraft altimeter set to verified altimeter setting.
The encoder is a Narco AR850, mated to a King KT78 transponder. I had the Narco encoder installed a few years ago to replace an aging and inaccurate unit from another manufacturer.
Does anyone have thoughts on what may be the issue, or is it likely a faulty encoder? If the latter, why would it run fine on the bench but be off when in the aircraft?
Thanks,
JKG
Subsequently, I was inside Class B airspace when I discovered that my Mode C was reporting an altitude 300 feet higher than what was actually being flown. I had the encoder re-calibrated in the aircraft, and flight testing afterward resulted in a dead-on Mode C altitude within 15 minutes, but off by 300 feet after 1 hour (I might expect this to be reversed). Encoder was pulled from aircraft, calibrated on the bench, and permitted to run on the bench for a few hours with no drift noted. Reinstalled in the aircraft, encoder now reports anywhere from 150 to 300 feet high when compared with aircraft altimeter set to verified altimeter setting.
The encoder is a Narco AR850, mated to a King KT78 transponder. I had the Narco encoder installed a few years ago to replace an aging and inaccurate unit from another manufacturer.
Does anyone have thoughts on what may be the issue, or is it likely a faulty encoder? If the latter, why would it run fine on the bench but be off when in the aircraft?
Thanks,
JKG