PT-6 Hot Section Inspection
Tips for keeping this workhorse in top flight condition
By Joe Escobar
On May 30, 1961, the Pratt & Whitney PT6 engine took flight for the first time. Mounted on a Beech 18, that first flight initiated what would evolve into over 36,000 PT6 engines delivered to date, operating in over 160 countries.
The sheer number of PT6 engines in use means that many mechanics out in the field are either currently working on PT6 engines, or have worked on them in the past.
Hot section inspection
Although the PT6 series engines have proven to be very reliable powerplants, there is a certain amount of scheduled maintenance necessary to ensure continued safe operation. One of the manufacturer’s requirements is that of performing a hot section inspection. This inspection provides for a detailed inspection of all hot section components. Timed at the halfway point to Time Before Overhaul (TBO), this critical inspection helps to ensure that the engine will make it to its TBO without any hitches. Hot section inspections are also performed on an unscheduled basis occasionally, as dictated by the maintenance manual for certain performance degradation problems or overtemp situations.
The following steps are for a typical PT 6 hot section inspection. As with any maintenance procedure, always refer to the manufacturer’s maintenance manual for any maintenance work performed. One of the most important preventive maintenance tips for hot section life is to properly maintain the fuel nozzles. Stan Bernstein of Heritage Turbines in Hyannis, MA explains, "Poor fuel nozzle flows are a primary cause of reduced hot section life. Spitting or drooling nozzles can wreak havoc on the hot section components, especially the vane ring."
This photo of a CT vane ring shows the destruction that can be caused by poor fuel nozzle spray patterns.
Fuel Nozzle Maintenance
Engine performance check
An engine performance check should be performed both before and after a hot section inspection. This will help determine the status of the hot section components and establish performance parameters for the engine.
The specific disassembly is detailed in the maintenance manual. Basically, the disassembly involves separating the power section of the engine from the gas generator section. The compressor turbine (CT) disk and blade assembly are removed. The combustion chamber liner is also removed. Other components may also need to be removed for a more detailed inspection.
The following items should be inspected for defects. The maintenance manual provides damage limits and corrective actions.
Look for cracks, distortion, overheating, and corrosion on the gas generator case. The combustion chamber inner/outer liner should be checked for cracks, distortion, burning, fretting wear, hot spots, and blockage of cooling holes due to weld repair or plasma overspray.
The combustion chamber’s large and small exit ducts should be inspected for cracks, distortion, burning and coating loss. Any amount of coating loss is acceptable provided that burning of the parent metal has not occurred.
Check the CT vane ring and shroud housing for cracks, coating loss, erosion of parent metal, or impact damage. The CT shroud segments should be inspected for cracks, distortion, erosion, and metal build-up.
Measure the compressor turbine disk and blade assembly tip clearance. The blades should be examined for tip rub, erosion, impact damage, coating loss, cracks, and shift. The blade rivets should be inspected for condition.
Check for wear, fretting, and distortion of the interstage sealing rings .
The T5 temperature sensing system should be inspected for installation and the wiring harness and lugs also should be examined. An operational check should be performed.
Look for impact damage, erosion, and cracks on the power turbine first-stage disk blades as well as the vane ring and interstage baffle. The exhaust duct should be inspected for cracks and distortion.
Inspect fuel nozzles for dissimilarity of carbon build-up. An operational flow check should be performed. Nozzle sheaths should be examined for fretting wear, erosion, and carbon build-up.
Perform a compressor inlet inspection by removing the inlet screen and examining the compressor inlet area and struts. The first stage blades and vanes should be checked for dirt deposits, corrosion, erosion, and cracks.
Remove the reduction gearbox oil strainer and inspect for foreign matter.
This article on PT-6 hot section tips was written by Standard Aero's Kelly M. McCallum, and originally ran in the September 2001 issue of AMT. Due to several reader inquiries on the article, we have...
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