Narrative:

While climbing to my planned altitude of 5;500 ft I leaned the fuel mixture to compensate for altitude. The airplane I fly has sparse engine instrumentation so I lean it while listening for an RPM drop then enriched slightly for best power. The radio was tuned to the [departure airport] CTAF and the frequency was busy with chatter from surrounding non-towered fields. So in order to better hear the engine I turned the volume on the radio down completely. While continuing to climb I was observing the scattered cloud layer below becoming denser and the visibility deteriorating. I decided land at [an alternate airport] which was less than 10 miles away instead of continuing to [the planned destination airport]. [The alternate airport] is my home base and I had flown from there earlier in the day. At 7 miles out I tuned the tower frequency and attempted to make radio contact. I broadcasted my location and landing intentions. There was no response. I attempted one additional time to raise the tower on the radio. Again no response. I was unsure whether the communication failure was due to a radio malfunction or perhaps that the tower had closed early. It was [a holiday] so even though my preflight briefing did not mention early tower closure I saw it as a possibility. I thought maybe the briefer had overlooked it. Since the radio had been functioning all day I concluded that the most likely scenario was that the tower had closed early due to the holiday. I proceeded to approach the airport as I would a non-towered field. I made frequent position reports and entered the traffic pattern on a left downwind for runway xx. While in the pattern I watched the tower for light gun signals and did not see any; this seemed to confirm my suspicion that the tower was closed. I made position reports on every leg of the pattern and proceeded to land on runway xx. While taxiing back to the parking area an FBO employee intercepted me and directed me to park at the FBO. During shutdown I was turning off the avionics and finally the problem became apparent; I had never turned the radio volume back up. Once I stopped the engine he advised me that I was to call the tower. After mentally reviewing the incident I realized that it was immediately after setting the mixture that I decided to divert because of the weather. My mind was switching gears from one task to the next and I simply neglected to turn the radio back up. I did not thoroughly troubleshoot the issue because the holiday provided what seemed like a reasonable explanation for not hearing a response on the radio. I am not sure what can be done to prevent someone else from making this error. I do know for sure that I will never make it again.

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Original NASA ASRS Text

Title: The pilot of a C150 lowered the volume on the communication panel to better hear the engine during mixture adjustment; but then neglected to recognize it resulting in the inability to hear Tower communications. He concluded that the Tower was closed and landed without a clearance.

Narrative: While climbing to my planned altitude of 5;500 FT I leaned the fuel mixture to compensate for altitude. The airplane I fly has sparse engine instrumentation so I lean it while listening for an RPM drop then enriched slightly for best power. The radio was tuned to the [departure airport] CTAF and the frequency was busy with chatter from surrounding non-towered fields. So in order to better hear the engine I turned the volume on the radio down completely. While continuing to climb I was observing the scattered cloud layer below becoming denser and the visibility deteriorating. I decided land at [an alternate airport] which was less than 10 miles away instead of continuing to [the planned destination airport]. [The alternate airport] is my home base and I had flown from there earlier in the day. At 7 miles out I tuned the Tower frequency and attempted to make radio contact. I broadcasted my location and landing intentions. There was no response. I attempted one additional time to raise the Tower on the radio. Again no response. I was unsure whether the communication failure was due to a radio malfunction or perhaps that the Tower had closed early. It was [a holiday] so even though my preflight briefing did not mention early Tower closure I saw it as a possibility. I thought maybe the briefer had overlooked it. Since the radio had been functioning all day I concluded that the most likely scenario was that the Tower had closed early due to the holiday. I proceeded to approach the airport as I would a non-towered field. I made frequent position reports and entered the traffic pattern on a left downwind for Runway XX. While in the pattern I watched the Tower for light gun signals and did not see any; this seemed to confirm my suspicion that the Tower was closed. I made position reports on every leg of the pattern and proceeded to land on Runway XX. While taxiing back to the parking area an FBO employee intercepted me and directed me to park at the FBO. During shutdown I was turning off the avionics and finally the problem became apparent; I had never turned the radio volume back up. Once I stopped the engine he advised me that I was to call the Tower. After mentally reviewing the incident I realized that it was immediately after setting the mixture that I decided to divert because of the weather. My mind was switching gears from one task to the next and I simply neglected to turn the radio back up. I did not thoroughly troubleshoot the issue because the holiday provided what seemed like a reasonable explanation for not hearing a response on the radio. I am not sure what can be done to prevent someone else from making this error. I do know for sure that I will never make it again.

Data retrieved from NASA's ASRS site as of July 2013 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.