Narrative:

Departed teterboro, nj, on 'teterboro 4 departure.' the SID calls for maintaining an altitude of 1500 ft until approximately 4 mi from teb, then climb to 3000 ft. While we thought we were complying with SID, the controller questioned our altitude. He then advised us to immediately climb to 4000 ft to clear traffic, which we did. We were observing 'vagaries' on the VOR signal (sweeping of HSI) and thought, perhaps, our DME led us astray by indicating approximately 4 DME when we were not actually at that spot. En route we compared DME distances and VOR radials and found no discrepancy. The next day, en route ttn-ugn we were told by ATC that our mode C indicated 300 ft high. We adjusted our altitude accordingly and each time we were given an altitude or frequency change, queried the controller as to what altitude he showed for us. Upon arrival at ugn we 'grounded' the aircraft, thus canceling all trips, until the fault could be located. Our maintenance personnel found a defective air data computer and replaced it. As a precaution, we also had them check and swap VOR/DME receiver equipment until it can be ascertained if this could have contributed. They did discover a 300 ft altitude discrepancy, so I suspect this caused the error in the first place.

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Original NASA ASRS Text

Title: DEPARTING TEB CESSNA 650 FLC AFTER A CLRNC BUST AND MODE C DIFFICULTIES GNDS THE ACFT.

Narrative: DEPARTED TETERBORO, NJ, ON 'TETERBORO 4 DEP.' THE SID CALLS FOR MAINTAINING AN ALT OF 1500 FT UNTIL APPROX 4 MI FROM TEB, THEN CLB TO 3000 FT. WHILE WE THOUGHT WE WERE COMPLYING WITH SID, THE CTLR QUESTIONED OUR ALT. HE THEN ADVISED US TO IMMEDIATELY CLB TO 4000 FT TO CLR TFC, WHICH WE DID. WE WERE OBSERVING 'VAGARIES' ON THE VOR SIGNAL (SWEEPING OF HSI) AND THOUGHT, PERHAPS, OUR DME LED US ASTRAY BY INDICATING APPROX 4 DME WHEN WE WERE NOT ACTUALLY AT THAT SPOT. ENRTE WE COMPARED DME DISTANCES AND VOR RADIALS AND FOUND NO DISCREPANCY. THE NEXT DAY, ENRTE TTN-UGN WE WERE TOLD BY ATC THAT OUR MODE C INDICATED 300 FT HIGH. WE ADJUSTED OUR ALT ACCORDINGLY AND EACH TIME WE WERE GIVEN AN ALT OR FREQ CHANGE, QUERIED THE CTLR AS TO WHAT ALT HE SHOWED FOR US. UPON ARR AT UGN WE 'GNDED' THE ACFT, THUS CANCELING ALL TRIPS, UNTIL THE FAULT COULD BE LOCATED. OUR MAINT PERSONNEL FOUND A DEFECTIVE AIR DATA COMPUTER AND REPLACED IT. AS A PRECAUTION, WE ALSO HAD THEM CHK AND SWAP VOR/DME RECEIVER EQUIP UNTIL IT CAN BE ASCERTAINED IF THIS COULD HAVE CONTRIBUTED. THEY DID DISCOVER A 300 FT ALT DISCREPANCY, SO I SUSPECT THIS CAUSED THE ERROR IN THE FIRST PLACE.

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.