Date: 202108
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Operator: Other Flight School
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Engine
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Pilot Not Flying; Instructor
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Glider; Flight Crew Commercial; Flight Crew Flight Instructor
Human factors: Confusion; Distraction; Situational Awareness; Time Pressure; Workload; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I departed the ZZZ in the afternoon on a training flight with a student who I had been working with in Aircraft X. I had been flying this aircraft most of the day and had observed that we had seven quarts of oil and 25 gallons of fuel; plenty for the local training flight; before we departed. Leaving the airport; we proceeded southbound to practice stalls and steep turns at altitude. At this point in the flight; the aircraft's engine was running smoothly; and the engine instruments were indicating normally. Having spent several hundred hours teaching in this particular aircraft; nothing seemed amiss. After completing these maneuvers; I tactfully asked my student to realign the directional gyro and smoothly pulled the throttle to idle as soon as his right hand left it; simulating an engine failure. As my student completed his required tasks and circled above his selected field; I periodically 'cleared' the engine - smoothly increasing the power to 1500 RPM to ensure that it was running normally during the exercise.When [we] reached an altitude of roughly 800 ft. AGL; I instructed my student to go around. He applied full power and the engine performed normally as we began to climb out. After gaining several hundred ft.; the engine began to shake violently with almost no warning; and a significant loss in power was noted. I reduced the power as the engine felt as if it were going to shake itself off of its mounts. I took control; pitched for best glide; and ran the memory items from the engine failure inflight checklist without success. Gliding towards a field; I began to adjust the power setting and after increasing power from idle several times; the shaking began to dampen. Nearing the field; the engine began to run smoothly again and I elected to climb away from my landing point and proceed to ZZZ1; a X;XXX ft.-long grass strip 3 or 4 miles away. I figured that the much safer landing at ZZZ1 was worth the risk of another 'engine failure' on the two-minute trip over.On the way; my student tuned in ATC and [explained our situation]. After an uneventful landing under power at ZZZ1; I was met by a breathtaking amount of firefighters; EMTs; and [other public officials]. After hearing that I had landed at ZZZ1; the aircraft's owner; who happens to be an A&P Mechanic; drove out to the field. After inspecting the airplane and performing a run-up; the owner determined that there was nothing visibly wrong with the aircraft or the engine and elected to fly it back to its home base of ZZZ. The next morning; it was flown to a repair station in ZZZ2 where we are currently waiting for a diagnosis of the problem.In my opinion; engine roughness and power loss of the duration that I experienced 30 seconds to a minute could either be carburetor icing or fuel contamination. The air temperature was nearly 80°F with a very high temperature/dewpoint spread; making carb icing unlikely. Furthermore; carburetor heat had no effect on the engine shaking and power loss. Fuel contamination could be another possibility. However; I did observe my previous student sump the tanks when the aircraft was last refueled and we were an hour into the flight when the power loss occurred; which makes contamination seem unlikely. Regardless; I was happy with my decision to land on a nearby grass strip instead of attempting to get a suspect airplane back to the airport and feel that I set the right example for my student.




Synopsis: Flight Instructor reported engine roughness and a loss of power during the student pilot's climb-out after completing a maneuver.