Decathlon 8KCAB
Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Total 550; Flight Crew Type 13
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway; Inflight Event / Encounter Fuel Issue
Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Aircraft
Flying from ZZZ to ZZZ1. Calculated fuel reserve on the ground and confirmed as we watched the tanks at 60 minutes of fuel left at the time of the incident. We started our descent into ZZZ1 talking to TRACON when the engine momentarily lost full power; but quickly regained it. My student and I both noticed it; and verbalized the anomaly. We had mentioned a few minutes previous ZZZ2 passing off our left wing; but noted that it was uncontrolled and closed; with unconfirmed runway condition information. When we felt the engine shudder a second time; my student and I [advised ATC] and began heading for ZZZ2. They advised us it was closed; and we were tempted to go an additional 5 miles to ZZZ3 with better services and a guaranteed cleared runway; but the engine slipped again; and we confirmed with TRACON that we were going to ZZZ2; closed or not. We copied down the TRACON phone number to reach them on the ground. The engine then maintained full power all the way down to the runway and to the fuel farm. As it is a Tailwheel; the tanks tip back when on the ground and both tanks read empty; while in the air during our power issues they read just under 1/4 on each side; which should have been just under an hour of fuel to go for the remaining 10 mins ETA to ZZZ1. Once clear of the runway; I tried TRACON again; and [another] airplane relayed our safe message to TRACON as we were too low to reach them.Once parked and shut down; I called TRACON and confirmed we were safe on the ground. We elected to divert to the uncontrolled; closed airport out of an abundance of caution to avoid flying over populated areas with a questionable engine. Once on the ground; we decided to drain the rest of the tanks' contents fully and have a mechanic inspect the aircraft before we attempted to fly again. We are on the ground waiting for the mechanic as I write this report.
Flight Instructor flying BL8 aircraft reported the engine malfunctioned during descent.