Date: 202011
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Daylight
Aircraft: Cessna 150
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Other Training
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Intake Assembly
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Instructor
Reporter qualification: Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Procedure; Human Factors
Primary problem: Aircraft

Narrative: I was conducting a training flight from ZZZ with a relatively new student; who is part owner of the aircraft (I am not an owner of the airplane). The lesson began normally with a preflight inspection by the student and an abbreviated inspection by myself; which showed no abnormality. There were five quarts of oil in the engine; I tightened the cap myself after double-checking. I personally watched the student add 10 gallons on fuel to the aircraft and sump uncontaminated fuel from both tanks immediately prior to flying. The run up was performed prior to takeoff with normal RPM drops on both magnetos and with application of carburetor heat. On takeoff the oil pressure and temperature read normal; and the engine was generating what appeared to be normal maximum power (~2;500 RPM). Takeoff and climb to the west were uneventful; with the student being the pilot flying. Several minutes into the flight I took the controls to demonstrate a maneuver. After performing a clearing turn at 3;000 ft. to prepare to demonstrate stalls to the student; I pressed the mixture in to prepare for full-power recovery from the maneuvers. Shortly after doing so; I noticed a drastic decrease in engine power (roughly 1;500 RPM maximum). After moving the throttle through most of its range to see if any setting would produce more power; I executed the engine failure checklist from memory (fuel on; mixture rich; carb heat on; magnetos on both; master on; primer in and locked). All items were configured correctly; I cycled each magneto individually to see if using one alone would produce more power; but this did not work. As I was doing this I was turning back toward land to the east and searching for a nearby grass runway. I requested priority handling to Approach; who assisted in locating ZZZ which was only several miles away and within gliding distance. A safe landing was made on the runway; which was in good condition and dry. When shutting down the aircraft; I noticed a sharp increase in RPM while pulling the mixture out to cutoff. I called the TRACON on the phone to let them know of our safe landing. After exiting the aircraft; I checked the oil level again; it was still at five quarts; and the cap had been tight. There was no obvious cause of the engine malfunction visible through the air inlets or the oil door. My student did find some acorns underneath the aircraft on the runway; the plane had recently undergone maintenance to remove debris in the engine cowling placed by squirrels that had gotten into the hangar. Several minutes after landing; deputies from the Sheriff's Department arrived to verify our safety; as they had been contacted by the approach controller. I was able to restart the aircraft normally; though in the full rich position the aircraft was not able to produce more than 1;500 RPM and ran very rough. With the mixture pulled out about two inches; I was able to generate a normal static RPM at full throttle. The aircraft was secured so that it could be inspected and repaired by the on-field A&P when he was available later in the week. I was informed by the student several days later that the mechanic found acorns in the air induction system.




Synopsis: Flight Instructor reported loss of engine power during a training flight which caused a diversion and a precautionary emergency landing. Maintenance later found acorns in the induction system.