The owner of an aircraft maintenance facility; reports about a Turbocharged Cessna P210 Pilot and Instructor; who experienced smoke in the cockpit and power loss; requiring engine shutdown and landing. Heat damage to the engine compartment and cowling was noted. Self-locking nuts that secured the exhaust flange were found loose or not attached.

Narrative

I am the Owner of a aircraft maintenance facility; who is; in part; responsible for quality control and end product to include supervision of subordinates (employed Technicians). As reported to me: The subject aircraft was on the second flight; following maintenance when the Owner/Pilot and his Instructor noted higher than normal engine cylinder head temperature on cylinder 6 and I believe the loss of cylinder head temperature on cylinder 4. Shortly after these indications the Pilot and Instructor reported some smoke in the cockpit and some power decline. They then proceeded to a nearby airport; and suspecting a possible fire; when within gliding distance; moved the engine mixture control to idle cutoff and the fuel selector control to the off position. Following landing; it was noted that the left hand side of the engine cowling had severe heat damage (paint and deformed aluminum sheet metal skin. The Pilots called me to inform of the event. In accordance with the rules; the pilot called the FAA FSDO and reported the event and forced precautionary landing.As I was out of town; I dispatched my business partner also an A&P; IA; who arrived the next day to examine the aircraft and attempt to determine cause. When the cowling was removed it was noted that the cylinder 4 exhaust riser; which is 'slip jointed' into the left hand exhaust riser assembly had separated from the cylinder exhaust flange causing the escape of hot exhaust gases into adjacent areas resulting damage to cowling; electrical wiring; and adjacent cast aluminum induction tubing. It was also noted that at least one of the four exhaust riser flange retaining nuts was lying in the lower cowl area below the number 4 cylinder. This exhaust riser had been previously removed and reinstalled for access to perform other engine work. It is possible; if not likely; that the riser stud nuts were under torqued at installation and this defect was not discovered during final inspection prior to return to service. As the riser stud nut are of the self locking type it may also be possible that the nuts had lost their locking feature through age and being subject to continuous extreme heat. Future further examination will be conducted to determine the exact cause.Two subordinates; one low experience and one very senior; under my charge had performed the work and I returned the aircraft to service. The chain of events and considerations I believe at this point; it may be assumed the subordinate Technician failed to adequately torque the retaining hardware and to properly re-check his work. Further; inspection personnel failed to perform more than a visual inspection; which would not detect an under-torque condition. A low experience Technician's work should have been even more closely inspected and supervised by Senior Technicians. Company installation and assembly procedures were not followed. Company final inspection policies were followed; but were inadequate in this case and shall be revised to include an Inspector torque check of all critical hardware and fasteners. A company policy requiring a flight test and an additional inspection following any critical maintenance shall be instated. A company policy requiring the replacement of all self locking hardware with new (hardware); on critical components during reassembly shall be instated. The company policy manual shall be revised to reflect these and further changes arising form this event. Management will pursue a more active role in human factors management; supervision; and quality control. Even though the submitter was not the person who performed the actual work; the submitter feels and is aware that; as the person returning the aircraft to service; he is ultimately responsible for the finished product to be free of defects to the best of his knowledge.