C150 instructor pilot and student reported an engine failure resulting in an off airport landing.
Synopsis
C150 instructor pilot and student reported an engine failure resulting in an off airport landing.
Narrative
The engine of the Cessna 150 I was flying with my CFI cut off during cruise flight. The plane is kept outside under cover. I arrived at the field around XG:30 AM. I deiced the frost from the wings; and did my pre-flight. There was a small amount of water in the fuel but I was able to sump until no more water showed up. The plane started up easily but was running slightly rough. It has been behaving like this for the past month or two. The owner has replaced both magnetos within the past month with an electronic magneto system.We took off around XH:30. After completing slow flight and several chandelles; we leveled out at 5000 ft. Suddenly; the engine began to fluctuate RPMs from 2400 down to 1000 and back up several times. Then the engine completely cut out. We were unable to restart and out of gliding distance to the nearest airports. We chose to land the plane in a cleared field. After several hours; the FAA released the plane and the owner was able to start it up in the field. He reported that it ran a little rough. To my knowledge; no determination on the cause of the engine failure has been determined. The owner is replacing the carburetor. Water in the fuel may have been a contributing factor especially because Cessna 150's are known for this issue. The new magnetos may also be suspect. My CFI and I worked well together to plan out our descent and choose a safe landing spot.
Second reporter narrative
Lost engine power while in cruise flight to a practice airport with less traffic. The day's plan of action for my commercial student in her flying club airplane was to practice procedures; specifically steep spirals to landing. As an instructor; I've flown this model of aircraft for a few hundred hours. I practiced or taught more than 30 engine out landings in the past few months in this aircraft. Recently the weather had changed from mild temperatures to cold temps being in the twenties at night; so the aircraft would seem to start much differently than in the warmer temperatures. In the warmer temperatures the airplane would start and quickly be able to maintain 1000 RPM; but when it was cold outside the airplane would cut off if you tried to bring it back to 1000 RPM for idle after starting. This was one possible warning flag that we missed and which was attributed to the cold temps.Another warning flag was on a previous flight happened a few weeks earlier. This happened with another student. The aircraft had a much different type of engine surge; which I'll describe as mild; with only a 100 RPM drop. This lasted a few minutes while over an airport and then returned to normal. Fuel samples from the before the flight had a few drops of water. I witnessed and double checked to make sure no more water was coming from either tank. The fuel is also a mixture of auto fuel and 100LL. This aircraft has a STC for auto fuel and is used to keep the cost down for the club. That first time the aircraft surged I attributed it to possibly water contamination. I reported this to the AP and the club replaced the spark plugs. I ran the airplane again the it didn't sound or feel right; so I reported it again to the AP and he replaced one magneto after it failed the mag check. After replacing the magneto the airplane still didn't sound like a perfect; but passed all checks on the ground. The airplane just felt different than in the summer. While flying at roughly 4200AGL I noticed the engine was surging from 2400 RPM to 1000 RPM and each time the RPMs would surge the range was getting larger. For example the first time it was from 2400 to 1000; then 2400 to 900; 2200 to 700 and it lasted about 40 seconds until the engine died. The oil pressure and engine temps were in the green. The airplane was full of fuel and in the on position. After the engine quit; I told my commercial student to fly the airplane at best glide and point it to the airport while I ran the checklist. After running the checklist; the airplane still wouldn't start. At the completion of the checklist; I [requested priority on frequency] and we squawked XXXX. After [requesting priority] twice within roughly 30 seconds; a controller got on the frequency and gave a us a discrete frequency. His voice was hard to hear because someone else was stepping on him while trying to relay information. The controller assured us that he knew where we were and that he'd send services after touchdown. My commercial student identified a field that look suitable to land. My student maneuvered the aircraft in a position where we could better identify any possible hazards in our intended landing area. Once we were close to 2000 AGL; I asked if my student was comfortable with me taking the landing and agreed to allow me to take the off airport landing. At 1000 AGL; I took the controls. As I got the airplane on a downwind I noticed power lines running across the field we were going to land on; so than I quickly maneuvered the airplane to an adjacent field. Once in position similar to a high final approach; I slipped the airplane and put full 40 degrees of flaps to slow down our ground speed and use as much of the available 2300 foot long field. The rows aligned with our make shift landing strip which help with not damaging the aircraft. The surface was a little more bumpy than landing in a traditional grass strip. The airplane didn't sustain any damage and we walked away from the aircraft and called ZZZ Approach with a number they provided us on a discrete frequency after landing. Approach gave us another number to call and we talked to someone who told us to call; so they could better find us. We dropped a pin using an iPhone and provided our address. Operator didn't know what to think when we called; they asked how can they help us. We didn't really need any services since we were both okay and the airplane was not damaged. We were told that service were on the way and we said we are just fine. CRM really played a key role in allowing us to run the checklist; get services on stand-by and fly. Assigning duties and task made it easier to accomplish the goal at hand and that was to aviate; navigate and communicate. Understanding what my student was capable of performing allowed me to delegate.Still haven't heard what cause the airplane to stop running; but the FAA rep didn't find any water in the fuel or metal in the oil. The tanks were still three-quarters full. The FAA rep and the owner started the airplane; but it didn't sound great to either one of them. The AP/Owner stated that it was likely a carburetor with a stuck float. I know the aircraft has an auto fuel STC and it's possible the fuel was possible tainted with ethanol fuel that maybe caused the float to stick. This is only my theory; because the owner still hasn't take the carburetor off the airplane.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.