Narrative:

I departed with a student in a cessna 172 with no issues. After getting a frequency change approval from the departure airport; I was about to call the adjacent air force base (AFB). Before I managed to call the AFB; the engine made 2 short bursts of unusual noises. I was concerned that the engine was going to quit; so I opted to turn around and go back to the departure airport. I declared an emergency and landed downwind (straight in for runway xr). My reasoning for landing downwind is that if the engine really was going to quit; I didn't need to waste time entering the traffic pattern and risk losing an engine in the pattern. Also; there is heavy flight training activity at the departure airport; which could delay getting on the ground. My departure airport is infamous for flying wide patterns and having extended downwinds. I believed this to be an unnecessary risk; and given these circumstances; I opted to err on the side of safety and declare an emergency. We landed without incident. Once on the ground; my student and I made a discovery as to the magnetos on the airplane. When the magnetos are on 'both'; the key is not pointing directly to 'both.' this was not identified during the pre-takeoff power run-up. Furthermore; the parallax from the instructor's seat makes the key look like it is even more so in the 'both' position; when in fact; it is only running on one magneto. The problem was identified as having the key in the incorrect position; and thus the engine was not running on both magnetos. This was confirmed with a run-up and test flight. The declared emergency and straight in downwind landing was an act of good judgment to get the aircraft on the ground as quickly as possible in the event that something much worse occurred.

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Original NASA ASRS Text

Title: After a C172 engine made an unusual sound during climbout; so the instructor declared an emergency and returned to the departure airport where he found a single magneto selected although the key appeared to point to the both position.

Narrative: I departed with a student in a Cessna 172 with no issues. After getting a frequency change approval from the departure airport; I was about to call the adjacent Air Force Base (AFB). Before I managed to call the AFB; the engine made 2 short bursts of unusual noises. I was concerned that the engine was going to quit; so I opted to turn around and go back to the departure airport. I declared an emergency and landed downwind (straight in for Runway XR). My reasoning for landing downwind is that if the engine really was going to quit; I didn't need to waste time entering the traffic pattern and risk losing an engine in the pattern. Also; there is heavy flight training activity at the departure airport; which could delay getting on the ground. My departure airport is infamous for flying wide patterns and having extended downwinds. I believed this to be an unnecessary risk; and given these circumstances; I opted to err on the side of safety and declare an emergency. We landed without incident. Once on the ground; my student and I made a discovery as to the magnetos on the airplane. When the magnetos are on 'both'; the key is not pointing directly to 'both.' This was not identified during the pre-takeoff power run-up. Furthermore; the parallax from the instructor's seat makes the key look like it is even more so in the 'both' position; when in fact; it is only running on one magneto. The problem was identified as having the key in the incorrect position; and thus the engine was not running on both magnetos. This was confirmed with a run-up and test flight. The declared emergency and straight in downwind landing was an act of good judgment to get the aircraft on the ground as quickly as possible in the event that something much worse occurred.

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.