Narrative:

After fixing the aircraft at pre departure clearance I determined that, since the problem manifested itself worst at altitude, the aircraft should be test flown to verify that the problem had been fixed. I determined that 17500 ft would be sufficient for the test since that is the altitude I use often at my home base for flight tests. The controllers at my home base are familiar with our flight test profiles and are not alarmed when we climb to 17500 ft or even 17900 ft to altitude test an aircraft. The first factor arose when I assumed it not a problem to do the same in relatively uncrowded airspace above pre departure clearance. The second factor arose when I did my preflight runup and mistook the transponder for the ADF because of the discrete codes' similarity to am radio stations' frequencys. I therefore did not change the previous transponder setting to 1200 before proceeding with the flight. The flight proceeded to 17500 ft where I leveled off for a short cruise segment to cool the engines. I scanned the altimeter and noticed I had risen to 17900 ft. I immediately descended to 17500 ft where I remained until descent back to pre departure clearance. Upon landing, the county sheriff stopped by and gave me the ZAU phone number to call. The controller stated that the transponder in my aircraft had reported FL180 requiring him to rerte many other aircraft. He seemed equally distressed that I did not request VFR flight following for the flight and exclaimed that the procedure for their airspace was apparently not the same as for denver. He did not, however, seem distressed about the incorrect squawk code on the transponder. Factors affecting the situation were: 1) the atypical flight profile of the aircraft. This left the controller with no clues as to the aircraft's intentions, ie. Was the climb going to stop at 17500 ft? Which direction is he going next? 2) the assumption by me that the controller would not be alarmed or concerned by an aircraft at 17500 to 17800 ft. Also, the assumption that because there were no transition rtes marked on the IFR or VFR charts above the airport, that the airspace would be relatively empty of traffic below FL180. Judging by the controllers comments, that was not the case. 3) the inaccurate transponder and/or altimeter error (I set the altimeter to field elevation), or the incorrect barometric pressure correction factor entered into the computer at ZAU -- whichever it was that caused the altitude readout to go to FL180 or higher. Future corrective action should include: 1) a transponder check with the controller after takeoff, 2) an attempt to obtain VFR flight following for test flts and leave it up to the controller as to whether his workload can handle the additional flight. Callback conversation with reporter revealed the following information: the reporter was flying a beechcraft duke, testing it to see if a crankshaft seal would now stay in the engine. At altitude, the seal had popped out as a vent had become clogged causing crankcase pressure. Pre departure clearance is an uncontrolled and unmanned airport, so ZAU had to call the sheriff to get in touch with the reporter. 'What fun to have the sheriff meet the aircraft!' the reporter found that his encoder was off by a very few hundred ft. He has had several occurrences with light aircraft encoders not working properly after sitting on the ground for an extended period of time. The reporter has heard nothing further from the FAA on this matter.

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Original NASA ASRS Text

Title: POSSIBLE INCURSION INTO CLASS 'A' AIRSPACE.

Narrative: AFTER FIXING THE ACFT AT PDC I DETERMINED THAT, SINCE THE PROB MANIFESTED ITSELF WORST AT ALT, THE ACFT SHOULD BE TEST FLOWN TO VERIFY THAT THE PROB HAD BEEN FIXED. I DETERMINED THAT 17500 FT WOULD BE SUFFICIENT FOR THE TEST SINCE THAT IS THE ALT I USE OFTEN AT MY HOME BASE FOR FLT TESTS. THE CTLRS AT MY HOME BASE ARE FAMILIAR WITH OUR FLT TEST PROFILES AND ARE NOT ALARMED WHEN WE CLB TO 17500 FT OR EVEN 17900 FT TO ALT TEST AN ACFT. THE FIRST FACTOR AROSE WHEN I ASSUMED IT NOT A PROB TO DO THE SAME IN RELATIVELY UNCROWDED AIRSPACE ABOVE PDC. THE SECOND FACTOR AROSE WHEN I DID MY PREFLT RUNUP AND MISTOOK THE XPONDER FOR THE ADF BECAUSE OF THE DISCRETE CODES' SIMILARITY TO AM RADIO STATIONS' FREQS. I THEREFORE DID NOT CHANGE THE PREVIOUS XPONDER SETTING TO 1200 BEFORE PROCEEDING WITH THE FLT. THE FLT PROCEEDED TO 17500 FT WHERE I LEVELED OFF FOR A SHORT CRUISE SEGMENT TO COOL THE ENGS. I SCANNED THE ALTIMETER AND NOTICED I HAD RISEN TO 17900 FT. I IMMEDIATELY DSNDED TO 17500 FT WHERE I REMAINED UNTIL DSCNT BACK TO PDC. UPON LNDG, THE COUNTY SHERIFF STOPPED BY AND GAVE ME THE ZAU PHONE NUMBER TO CALL. THE CTLR STATED THAT THE XPONDER IN MY ACFT HAD RPTED FL180 REQUIRING HIM TO RERTE MANY OTHER ACFT. HE SEEMED EQUALLY DISTRESSED THAT I DID NOT REQUEST VFR FLT FOLLOWING FOR THE FLT AND EXCLAIMED THAT THE PROC FOR THEIR AIRSPACE WAS APPARENTLY NOT THE SAME AS FOR DENVER. HE DID NOT, HOWEVER, SEEM DISTRESSED ABOUT THE INCORRECT SQUAWK CODE ON THE XPONDER. FACTORS AFFECTING THE SIT WERE: 1) THE ATYPICAL FLT PROFILE OF THE ACFT. THIS LEFT THE CTLR WITH NO CLUES AS TO THE ACFT'S INTENTIONS, IE. WAS THE CLB GOING TO STOP AT 17500 FT? WHICH DIRECTION IS HE GOING NEXT? 2) THE ASSUMPTION BY ME THAT THE CTLR WOULD NOT BE ALARMED OR CONCERNED BY AN ACFT AT 17500 TO 17800 FT. ALSO, THE ASSUMPTION THAT BECAUSE THERE WERE NO TRANSITION RTES MARKED ON THE IFR OR VFR CHARTS ABOVE THE ARPT, THAT THE AIRSPACE WOULD BE RELATIVELY EMPTY OF TFC BELOW FL180. JUDGING BY THE CTLRS COMMENTS, THAT WAS NOT THE CASE. 3) THE INACCURATE XPONDER AND/OR ALTIMETER ERROR (I SET THE ALTIMETER TO FIELD ELEVATION), OR THE INCORRECT BAROMETRIC PRESSURE CORRECTION FACTOR ENTERED INTO THE COMPUTER AT ZAU -- WHICHEVER IT WAS THAT CAUSED THE ALT READOUT TO GO TO FL180 OR HIGHER. FUTURE CORRECTIVE ACTION SHOULD INCLUDE: 1) A XPONDER CHK WITH THE CTLR AFTER TKOF, 2) AN ATTEMPT TO OBTAIN VFR FLT FOLLOWING FOR TEST FLTS AND LEAVE IT UP TO THE CTLR AS TO WHETHER HIS WORKLOAD CAN HANDLE THE ADDITIONAL FLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A BEECHCRAFT DUKE, TESTING IT TO SEE IF A CRANKSHAFT SEAL WOULD NOW STAY IN THE ENG. AT ALT, THE SEAL HAD POPPED OUT AS A VENT HAD BECOME CLOGGED CAUSING CRANKCASE PRESSURE. PDC IS AN UNCTLED AND UNMANNED ARPT, SO ZAU HAD TO CALL THE SHERIFF TO GET IN TOUCH WITH THE RPTR. 'WHAT FUN TO HAVE THE SHERIFF MEET THE ACFT!' THE RPTR FOUND THAT HIS ENCODER WAS OFF BY A VERY FEW HUNDRED FT. HE HAS HAD SEVERAL OCCURRENCES WITH LIGHT ACFT ENCODERS NOT WORKING PROPERLY AFTER SITTING ON THE GND FOR AN EXTENDED PERIOD OF TIME. THE RPTR HAS HEARD NOTHING FURTHER FROM THE FAA ON THIS MATTER.

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.