An AC690B pilot experiences encoding altimeter problem on maintenance test flight.

Attributes
ACN	464260
Time
Date	200002
Day	Mon
Local Time Of Day	0601 To 1200
Place
State Reference	TX
Altitude	msl bound lower : 7600 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : i90.tracon
Operator	general aviation : corporate
Make Model Name	Rockwell Aero Commander Twin Turboprop Undifferentiated or Other
Operating Under FAR Part	Part 91
Flight Phase	cruise : enroute altitude change
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : single pilot
Qualification	pilot : multi engine pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 85 flight time total : 15075 flight time type : 2000
ASRS Report	464260
Person 2
Affiliation	company : air carrier
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera other controllerb
Resolutory Action	flight crew : overcame equipment problem
Consequence	other
Factors
Maintenance	performance deficiency : testing
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

On maintenance test flight from iah to cll and back to iah. At 8000 ft departure stated they showed 7600 ft. I reset transponder and they showed 8000 ft. Went over to ZHU. Started back to iah. Center gave a heading and descent to 6000 ft. I turned but asked to verify descent to 6000 ft. They said not cleared to 6000 ft climb to 8000 ft. I had never started descent but they showed 7600 ft. I switched to #2 transponder and it showed 8000 ft. After maintenance test flight, I had maintenance facility pull both xponders and do a 91.14 check. I called houston approach and center by phone and was told by both 'don't worry about it because I was on a maintenance test flight.'

Original NASA ASRS Text

Title: AN AC690B PLT EXPERIENCES ENCODING ALTIMETER PROB ON MAINT TEST FLT.

Narrative: ON MAINT TEST FLT FROM IAH TO CLL AND BACK TO IAH. AT 8000 FT DEP STATED THEY SHOWED 7600 FT. I RESET XPONDER AND THEY SHOWED 8000 FT. WENT OVER TO ZHU. STARTED BACK TO IAH. CTR GAVE A HEADING AND DSCNT TO 6000 FT. I TURNED BUT ASKED TO VERIFY DSCNT TO 6000 FT. THEY SAID NOT CLRED TO 6000 FT CLB TO 8000 FT. I HAD NEVER STARTED DSCNT BUT THEY SHOWED 7600 FT. I SWITCHED TO #2 XPONDER AND IT SHOWED 8000 FT. AFTER MAINT TEST FLT, I HAD MAINT FACILITY PULL BOTH XPONDERS AND DO A 91.14 CHK. I CALLED HOUSTON APCH AND CTR BY PHONE AND WAS TOLD BY BOTH 'DON'T WORRY ABOUT IT BECAUSE I WAS ON A MAINT TEST FLT.'

Data retrieved from NASA's ASRS site as of July 2007 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.