From Chas:
I just noticed on the last two flights that, when above say 3,000 feet, ATC and my altimeter are disagreeing by 100 feet. That is using the current reported altimeter setting. However, on the ground the altitude is dead on. I just had my IFR cert done last month, and there was negligible error through 20,000 feet. Can an A/P make some adjustment to the altimeter, or must it go in for an overhaul? What are the chances that there is some problem with the hose, or moisture in the hose?
Bob Steward, A&P IA, with expansions by the Editor:
ATC radar reads your encoder, not your altimeter; the encoder sends its altitude reading to the transponder. The encoder, which is “blind” and does not adjust for the local altimeter barometric setting, is factory set to 29.92. While your transponder reading at ATC should match your altimeter if the altimeter is also set to 29.92, you will seldom have that altimeter setting in “real life”. It’s not uncommon to see a 400′ difference between the two, due to altimeter barometric settings that are far from the 29.92 standard.
This is why ATC calls you and ask you to “Say altitude!”, so they can see the effective altitude offset on their end. If you mistakenly give them the altitude you are assigned, instead of the one you are really seeing on your altimeter at the time (assuming you have the correct local baro dialed in), then they put in the wrong offset. For the rest of the flight in that sector, you will show up wrong on their end. That’s one reason why you should not try to scam ATC when they ask you, just because you are off by 100 feet at the time they ask.
There is a Mode C correspondence check (and adjustment) that the Avionics tech SHOULD do at transponder certification intervals. Its not part of a Pitot-Static check, though it’s convenient to do it then.
Make sure you have no trapped water in the Pitot-Static system, as that will cause erroneous readings on the affected instruments. If you are parked outside on the ramp, you should periodically check for this.