Small transport on VFR flight following with center is advised of altimeter reading discrepancy that...

Attributes
ACN	186035
Time
Date	199108
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mtj
State Reference	CO
Altitude	msl bound lower : 17500 msl bound upper : 18200
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 16 flight time total : 3770 flight time type : 150
ASRS Report	186035
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe non adherence : far other anomaly other other spatial deviation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While en route from bullfrog, ut, to denver (centennial), co, and after passing montrose, co, I contacted denver ARTCC for VFR flight following. The controller, after acknowledging radar contact, asked that I stop altitude squawk. The controller asked me to verify 17500 ft and stated that I showed an altitude of 18200. I confirmed my altitude and initiated a slight descent. Center confirmed their altimeter setting of 30.4? Which differed from the altimeter setting I received from flight watch over montrose of 30.31. This altimeter setting discrepancy does not account for the 700 ft difference between both my aircraft altimeters and center's readout of my transponder encoder altitude. The pitot static system in the aircraft was biannualed in february of 1991. Callback conversation with reporter revealed the following information: callback to reporter made account situation fairly unique. Reporter could not give an explanation for differing altimeter readouts and stated that later checks with ATC provided no essentially different altitudes. Reporter felt it was a 1 time happening that he could not explain away. Aircraft had not encountered any moisture or precipitation of any kind and both transponders mode C worked correctly. No problem with agency.

Original NASA ASRS Text

Title: SMT ON VFR FLT FOLLOWING WITH CENTER IS ADVISED OF ALTIMETER READING DISCREPANCY THAT PLACED ACFT IN PROX OF PCA.

Narrative: WHILE ENRTE FROM BULLFROG, UT, TO DENVER (CENTENNIAL), CO, AND AFTER PASSING MONTROSE, CO, I CONTACTED DENVER ARTCC FOR VFR FLT FOLLOWING. THE CTLR, AFTER ACKNOWLEDGING RADAR CONTACT, ASKED THAT I STOP ALT SQUAWK. THE CTLR ASKED ME TO VERIFY 17500 FT AND STATED THAT I SHOWED AN ALT OF 18200. I CONFIRMED MY ALT AND INITIATED A SLIGHT DSCNT. CENTER CONFIRMED THEIR ALTIMETER SETTING OF 30.4? WHICH DIFFERED FROM THE ALTIMETER SETTING I RECEIVED FROM FLT WATCH OVER MONTROSE OF 30.31. THIS ALTIMETER SETTING DISCREPANCY DOES NOT ACCOUNT FOR THE 700 FT DIFFERENCE BTWN BOTH MY ACFT ALTIMETERS AND CENTER'S READOUT OF MY TRANSPONDER ENCODER ALT. THE PITOT STATIC SYS IN THE ACFT WAS BIANNUALED IN FEBRUARY OF 1991. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK TO RPTR MADE ACCOUNT SITUATION FAIRLY UNIQUE. RPTR COULD NOT GIVE AN EXPLANATION FOR DIFFERING ALTIMETER READOUTS AND STATED THAT LATER CHKS WITH ATC PROVIDED NO ESSENTIALLY DIFFERENT ALTS. RPTR FELT IT WAS A 1 TIME HAPPENING THAT HE COULD NOT EXPLAIN AWAY. ACFT HAD NOT ENCOUNTERED ANY MOISTURE OR PRECIPITATION OF ANY KIND AND BOTH TRANSPONDERS MODE C WORKED CORRECTLY. NO PROBLEM WITH AGENCY.

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.

Small transport on VFR flight following with center is advised of altimeter reading discrepancy that placed aircraft in proximity of positive control area.