Date: 202105
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Skyhawk 172/Cutlass 172
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
Aircraft component: Cylinder
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Anomaly: Aircraft Equipment Problem Critical; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Object
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; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Environment - Non Weather Related
Primary problem: Aircraft

Narrative: With one passenger; took off from ZZZ1; was circling the west side of [the island] intending to land back at ZZZ1. At 2;000 feet MSL; abeam ZZZ; lost 400 RPM engine power; engine got very rough; with bad vibration. Requested priority handling for Runway XX at ZZZ. Upon reaching threshold; aircraft was much too high and too fast for landing; was still at maneuvering speed from descending from cruise; made low pass and went around for another attempt at Runway XX; at about feet 200 AGL; with throttle at max power setting; and barely able to maintain altitude. Runway surface was shortened to about 2;600 feet by construction. There were two orange construction barriers; one at the end of the shortened runway length; and a second one about midway within the remaining 3;000 feet of runway closed for construction. Construction barriers were not apparent to me as such from the air. I was not able to touch down at the threshold; and touched down just beyond the first barrier; within construction area.At that point; I still did not see that the barriers were barriers (they looked like flat tarps or orange police tape on the ground). Still at high speed from rollout and heading towards the second barrier; my passenger called for me to go to the left. Steering to the left side of the runway; I passed beside the barrier; and avoided hitting it; and with full braking; came to a stop at the end of the paved surface; and shut down. Nothing was damaged; nobody was hurt; but I had barely avoided the construction barriers; made of concrete. But only on the ground and nearby are they visible as barriers. From the air they look like orange flat tarps or bags. The potential for catastrophe in the event of [a] landing is great. If construction barriers used on runways were made of soft rubber; like traffic cones; or constructed to be collapsible on impact; there would be no hazard; and they would still fulfill their purpose.On my part; my landing was poor. My speed and altitude were too high. Reducing power and using a large amount of flaps; or planning on making a downwind to Runway XY instead would have been necessary to avoid the overrun into the construction. My mindset at the time of the accident was one thing 'get on the ground in as few seconds as possible'; based on training for the total engine failure scenario; where there truly is no time. This scenario was not like that; and needed essentially a normal landing procedure. A partial power loss; like this; is a more common [event] than a total complete engine failure. But the general training syllabus based on the FARs emphasizes the total engine failure scenario; almost to the exclusion of other emergencies. I thus suggest that the training syllabus mandated by the FARs be changed to put more emphasis on a partial power loss; and assessing the difference between that and a more total power loss. I intend to take a flight review in the short term; with emphasis on practice for a partial power loss; like this scenario; and being on proper approach speed for short field landings at the threshold in such scenarios. Next day; mechanic found electrodes had been bent away from contacts in both spark plugs on one cylinder; causing that cylinder not to fire. He repaired those plugs; and we did a successful ground run; and per his determination that it was fixed; I took off again; from Runway XY; at the beginning of the usable runway surface before displaced threshold; to return to home base; ZZZ2. At 1;200 feet to 1;300 feet MSL (about 400 feet to 500 feet AGL) after takeoff; the problem recurred. I did a 180; reduced power; and with flaps extended; made safe landing touching down at threshold of Runway XX; and came to a stop within the usable runway.The following day; mechanic prepared to pull the affected cylinder; whose spark plugs had been affected the same way as before; to investigate the problem. He found a small black cylindrical object; 1/4' diameter and 3/8' long; out of hard plastic or scratched metal; in the cylinder; and concluded that it was knocking against spark plug electrodes and bending them. Neither he; myself; nor FAA could identify object or its origin. Mechanic is certain it did not originate from within the aircraft; and concluded that it had to have come in through the carburetor heat intake; which is unfiltered. This is unlikely to have happened during operation as carburetor heat inlet is far off the ground. I believe a careless operator did a run-up beside where my aircraft was parked at an outside tie-down; and that run-up kicked the FOD into the carburetor heat intake. On an earlier unrelated occasion; similar carelessness caused my entire windshield to get pitted by kicked up FOD (I had to replace the windshield); making the scenario plausible as my aircraft is always tied down outside. Pilots are taught to avoid doing runups near persons or other aircraft or vehicles due to this danger. But as I cannot control the carelessness of others while I am not present; and hangaring the aircraft is unfeasible; I intend to get cowl plugs for when the aircraft is parked; to avoid this. While I don't feel that they should be mandated; perhaps the FAA needs to officially recommend them for those who must park outside. The FOD removed; I departed from Runway XY again; and this time; there were no problems; landed safely at ZZZ2. Hence; the FOD had been the cause of the engine problem.




Synopsis: C172 pilot reported recurring engine power loss resulting in two precautionary landings was traced to unidentifiable FOD found in an engine cylinder causing the spark plug electrodes to not work.