The pilot of a L39 with a digital altimeter carefully preflighted the aircraft following painting. O...

Narrative:

Plane was leaving the paint shop. Very thorough preflight was conducted paying close attention to the pitot-static system and other vents that could have been taped or painted over. Before departure I verified that the digital altimeter matched the standby altimeter at the field elevation. Departed VFR. Once airborne I picked up my IFR flight plan with center and verified the altimeter setting with them. Was cleared IFR routing and eventually up to FL270. While climbing through the teens both altimeter were in agreement. Checked on with new center controller and called FL250 for FL270; controller informed me that the mode C indicated I was FL235; I believe. I told ATC that I was starting to look into it. Looked over at my stand by altimeter and initially thought they were in agreement but were almost actually almost 2;000 ft off. Verified that both gauges read 29.92 (they did) and cycled between main and standby static system. Verified that all switches and circuit breakers were in correct position. ATC informed me that there was an airbus opposite direction lower then me and I told them I had the traffic in sight and it was in fact lower than me. Once I had tried all that I thought could be done I informed ATC that I was unable to verify my altitude and asked to be vectored to nearest airport. They descended and vectored me there and informed me there was a possible separation problem.

Original NASA ASRS Text

Title: The pilot of a L39 with a digital altimeter carefully preflighted the aircraft following painting. Once airborne and passing FL250 for FL270 ATC informed him that his Mode C returned FL235. The flight diverted to a nearby airport.

Narrative: Plane was leaving the paint shop. Very thorough preflight was conducted paying close attention to the pitot-static system and other vents that could have been taped or painted over. Before departure I verified that the digital altimeter matched the standby altimeter at the field elevation. Departed VFR. Once airborne I picked up my IFR flight plan with Center and verified the altimeter setting with them. Was cleared IFR routing and eventually up to FL270. While climbing through the teens both altimeter were in agreement. Checked on with new Center Controller and called FL250 for FL270; Controller informed me that the Mode C indicated I was FL235; I believe. I told ATC that I was starting to look into it. Looked over at my stand by altimeter and initially thought they were in agreement but were almost actually almost 2;000 FT off. Verified that both gauges read 29.92 (they did) and cycled between Main and standby static system. Verified that all switches and circuit breakers were in correct position. ATC informed me that there was an AIRBUS opposite direction lower then me and I told them I had the traffic in sight and it was in fact lower than me. Once I had tried all that I thought could be done I informed ATC that I was unable to verify my altitude and asked to be vectored to nearest airport. They descended and vectored me there and informed me there was a possible separation problem.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	908159
Time
Date	201009
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Albatros (L39)
Operating Under FAR Part	Part 91
Flight Phase	Climb
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Altimeter
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor Flight Crew Instrument
Experience	Flight Crew Last 90 Days 75 Flight Crew Total 3100 Flight Crew Type 20
Events
Anomaly	Aircraft Equipment Problem Critical Conflict Airborne Conflict

The pilot of a L39 with a digital altimeter carefully preflighted the aircraft following painting. Once airborne and passing FL250 for FL270 ATC informed him that his mode C returned FL235. The flight diverted to a nearby airport.