Narrative:

[A few days prior to the event] I had operated the aircraft and during the run-up I noted a 350 RPM drop on one magneto. I terminated the flight and returned to the ramp; I informed maintenance of the discrepancy. I was informed that one of the sparkplug electrodes had welded itself closed and this was the cause of the RPM drop. The aircraft maintenance log noted that the defective sparkplug was replaced and all sparkplugs were cleaned. I had flown the aircraft with a different student [earlier in] the day. The flight consisted of performing touch and go's; no issues or discrepancies were noted.the mission to the flight was to perform a VFR night cross country to meet requirements for the student's private pilot certificate. My student and I performed all the required preparation for our night cross country. Both my student and I conducted a preflight inspection; no discrepancies were noted; and I deemed the aircraft airworthy. The engine started quite easily since it was warm. The starting and before taxi checklists were completed; and the student taxied to the departure end of runway xy. The student performed the engine run up and no discrepancies were noted. Both magnetos showed approximately 75 RPM drops each; engine oil pressure and temperature gauges read in the middle of their green arcs; and the fuel pressure was in the green arc. Overall a normal healthy seeming engine run up. The student then prepared the rest of the aircraft for the flight; and completed all required checklists. The student performed the takeoff. On the takeoff roll the student verbally called out that tachometer showed power indications; the engine instruments were in the green; and the airspeed was alive. I looked and the tachometer showed approximately 2300 RPM when were on the ground; and 2500 once we were in the air and accelerating. I also checked the engine instruments; all read in the green arcs. The only abnormal point of the takeoff was that my student rotated at 50 KIAS rather than the normal 60 KIAS; and he set climb speed at vx rather than vy. In the post flight debriefing he said he was slightly confused regarding our earlier discussion regarding aircraft performance charts and short field takeoffs.the student climbed to 800 ft AGL and we were cleared to turn south; the student continued to climb at vy on a southerly heading. At approximately 2500 ft MSL tower gave us a frequency change and the student changed radios to the flight service station frequency. The student opened our flight plan; and then switched frequencies to get VFR flight following. At approximately 3000 ft MSL and 3 NM south of ZZZ the engine made a loud bang noise; and continuous vibrations were felt. The aircraft had noticeably slowed down. I took control of the aircraft and started a right turn towards ZZZ. I ensured the throttle was at full and looked at the tachometer; which read 2000-2100 RPM. I performed the engine troubleshoot checklist; and leveled the aircraft to prevent our airspeed form decaying further. I asked the student to change fuel tanks per the checklist; no noticeable improvement was apparent from any of the corrective actions. I called ZZZ tower and said we had severe engine roughness; and intended to make a right base entry for runway xx. Tower cleared us to land on runway xx; requested souls on board and offered emergency services. I reported cleared to land runway xx; two souls on board; and requested emergency services. I maintained my altitude until I determined I could glide to the runway; at which point I retarded the throttle; extended the flaps to 40 degrees; and initiated a forward slip to lose altitude. The engine did not run rougher at a lower power setting. I performed the landing on runway xx and exited the runway. I requested to taxi across runway xy to the ramp; I was then cleared and taxied to the ramp. On the ramp; I performed a full power static run up. At full throttle; the tachometer indicated approximately 1900 prm; and all the engine instruments were in their green arcs. I then checked both magnetos; neither showed a drop of more than 100 RPM. Next; I performed the securing engine checklist and shut down the engine. I tied down the aircraft.I watched the maintenance personnel investigate the engine. They performed an idle engine run up and noted that the left rear cylinder was cold. They then checked for compression and found none. Next; they removed the cylinder cover to check the movement of the valves; pushrods; and rocker arms; all moved and showed no signs of deformation. I then got close to the engine and looked down on the top of the cylinder and noticed a large crack on the cylinder head; the maintenance personnel removed the left rear cylinder. Upon removal; a crack was observed originating from one sparkplug hole which propagated around the heads circumference past the opposite sparkplug hole. The chief mechanic commented that the engine only had about 800 hours since it was overhauled.

Related reports:

Google
 

Original NASA ASRS Text

Title: GA flight instructor reported severe engine roughness after takeoff that resulted in a return to the departure airport.

Narrative: [A few days prior to the event] I had operated the aircraft and during the run-up I noted a 350 RPM drop on one magneto. I terminated the flight and returned to the ramp; I informed Maintenance of the discrepancy. I was informed that one of the sparkplug electrodes had welded itself closed and this was the cause of the RPM drop. The aircraft maintenance log noted that the defective sparkplug was replaced and all sparkplugs were cleaned. I had flown the aircraft with a different student [earlier in] the day. The flight consisted of performing touch and go's; no issues or discrepancies were noted.The mission to the flight was to perform a VFR night cross country to meet requirements for the student's private pilot certificate. My student and I performed all the required preparation for our night cross country. Both my student and I conducted a preflight inspection; no discrepancies were noted; and I deemed the aircraft airworthy. The engine started quite easily since it was warm. The starting and before taxi checklists were completed; and the student taxied to the departure end of runway XY. The student performed the engine run up and no discrepancies were noted. Both magnetos showed approximately 75 RPM drops each; engine oil pressure and temperature gauges read in the middle of their green arcs; and the fuel pressure was in the green arc. Overall a normal healthy seeming engine run up. The student then prepared the rest of the aircraft for the flight; and completed all required checklists. The student performed the takeoff. On the takeoff roll the student verbally called out that tachometer showed power indications; the engine instruments were in the green; and the airspeed was alive. I looked and the tachometer showed approximately 2300 RPM when were on the ground; and 2500 once we were in the air and accelerating. I also checked the engine instruments; all read in the green arcs. The only abnormal point of the takeoff was that my student rotated at 50 KIAS rather than the normal 60 KIAS; and he set climb speed at Vx rather than Vy. In the post flight debriefing he said he was slightly confused regarding our earlier discussion regarding aircraft performance charts and short field takeoffs.The student climbed to 800 ft AGL and we were cleared to turn south; the student continued to climb at Vy on a southerly heading. At approximately 2500 ft MSL tower gave us a frequency change and the student changed radios to the flight service station frequency. The student opened our flight plan; and then switched frequencies to get VFR flight following. At approximately 3000 ft MSL and 3 NM south of ZZZ the engine made a loud bang noise; and continuous vibrations were felt. The aircraft had noticeably slowed down. I took control of the aircraft and started a right turn towards ZZZ. I ensured the throttle was at full and looked at the tachometer; which read 2000-2100 RPM. I performed the engine troubleshoot checklist; and leveled the aircraft to prevent our airspeed form decaying further. I asked the student to change fuel tanks per the checklist; no noticeable improvement was apparent from any of the corrective actions. I called ZZZ tower and said we had severe engine roughness; and intended to make a right base entry for runway XX. Tower cleared us to land on runway XX; requested souls on board and offered emergency services. I reported cleared to land runway XX; two souls on board; and requested emergency services. I maintained my altitude until I determined I could glide to the runway; at which point I retarded the throttle; extended the flaps to 40 degrees; and initiated a forward slip to lose altitude. The engine did not run rougher at a lower power setting. I performed the landing on runway XX and exited the runway. I requested to taxi across runway XY to the ramp; I was then cleared and taxied to the ramp. On the ramp; I performed a full power static run up. At full throttle; the tachometer indicated approximately 1900 PRM; and all the engine instruments were in their green arcs. I then checked both magnetos; neither showed a drop of more than 100 RPM. Next; I performed the securing engine checklist and shut down the engine. I tied down the aircraft.I watched the Maintenance personnel investigate the engine. They performed an idle engine run up and noted that the left rear cylinder was cold. They then checked for compression and found none. Next; they removed the cylinder cover to check the movement of the valves; pushrods; and rocker arms; all moved and showed no signs of deformation. I then got close to the engine and looked down on the top of the cylinder and noticed a large crack on the cylinder head; the maintenance personnel removed the left rear cylinder. Upon removal; a crack was observed originating from one sparkplug hole which propagated around the heads circumference past the opposite sparkplug hole. The chief mechanic commented that the engine only had about 800 hours since it was overhauled.

Data retrieved from NASA's ASRS site 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.