Narrative:

[I] experienced intermittent alternator half-way through a 2.5 hour flight on an IFR flight plan. With severe VFR conditions I elected to cancel and continue VFR. After trouble-shooting I elected to shut all electronics down to insure adequate electrical power for the arrival into a towered airport below bravo. On the ground I fixed a loose alternator ground wire; added 20 gallons of fuel and returned home against a 30-40 KTS headwind. I ran one tank unexpectedly dry but figured I had not balanced the tanks properly (assumption #1). Landed uneventfully and my engine data monitor (edm) with fuel flow; showed 15 gallons remaining which seemed reasonable (mistake #2). Five days later I returned to the airport to change the gaskets on the fuel caps; as part of routine maintenance; prior to the ten minute flight to my local mechanic to check out my repair. I usually remove the pitot cover as soon as I enter the hangar but since I would be working on the airplane first; and preheating; I did not (mistake #3). I took-off with the pitot cover firmly attached and returned; again uneventfully; to remove it. Trip to the mechanic and back was also uneventful until I topped off the tanks for a long cross country flight coming up in a week. The edm still showed 9 gallons aboard and as expected the left tank took the full 32 gallons but I was horrified as the right tank took 31.5 gallons. I racked my brain for two days wondering how the edm could be ten gallons off and how I could have allowed myself to come so close to becoming a statistic. Did the FBO only put in 10 gallons instead of 20? Did I have a fuel leak? Another part of my routine is to download the edm data every 20 hours or so. Analyzing the data for other reasons; I realized some flight time was missing. It dawned on me that when I had shut down all electrical; a week earlier; the edm stopped measuring fuel burn. That accounted for the missing ten gallons. I have come to trust the edms accuracy over the years by constantly checking the actual fuel used every time I top off but failed to realize the ramifications of shutting it down in-flight. All four flights were made with partial fuel loads so there was no chance to check the actual load and my gauges are typically inaccurate except when empty. Lesson learned. I also added the pitot cover to my run-up checklist.

Google
 

Original NASA ASRS Text

Title: An M20 pilot shutdown his electrical system in flight which prevented the engine data monitor (EDM) from recording fuel flow. He later relied on the EDM for fuel remaining which was inaccurate because it was turned off. Actual fuel on board was 10 gallons less than calculated.

Narrative: [I] experienced intermittent alternator half-way through a 2.5 hour flight on an IFR flight plan. With severe VFR conditions I elected to cancel and continue VFR. After trouble-shooting I elected to shut all electronics down to insure adequate electrical power for the arrival into a towered airport below Bravo. On the ground I fixed a loose alternator ground wire; added 20 gallons of fuel and returned home against a 30-40 KTS headwind. I ran one tank unexpectedly dry but figured I had not balanced the tanks properly (assumption #1). Landed uneventfully and my engine data monitor (EDM) with fuel flow; showed 15 gallons remaining which seemed reasonable (mistake #2). Five days later I returned to the airport to change the gaskets on the fuel caps; as part of routine maintenance; prior to the ten minute flight to my local mechanic to check out my repair. I usually remove the pitot cover as soon as I enter the hangar but since I would be working on the airplane first; and preheating; I did not (mistake #3). I took-off with the pitot cover firmly attached and returned; again uneventfully; to remove it. Trip to the mechanic and back was also uneventful until I topped off the tanks for a long cross country flight coming up in a week. The EDM still showed 9 gallons aboard and as expected the left tank took the full 32 gallons but I was horrified as the right tank took 31.5 gallons. I racked my brain for two days wondering how the EDM could be ten gallons off and how I could have allowed myself to come so close to becoming a statistic. Did the FBO only put in 10 gallons instead of 20? Did I have a fuel leak? Another part of my routine is to download the EDM data every 20 hours or so. Analyzing the data for other reasons; I realized some flight time was missing. It dawned on me that when I had shut down all electrical; a week earlier; the EDM stopped measuring fuel burn. That accounted for the missing ten gallons. I have come to trust the EDMs accuracy over the years by constantly checking the actual fuel used every time I top off but failed to realize the ramifications of shutting it down in-flight. All four flights were made with partial fuel loads so there was no chance to check the actual load and my gauges are typically inaccurate except when empty. Lesson learned. I also added the pitot cover to my run-up checklist.

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.