PT-6 Hot Section Inspection

PT-6 Hot Section Inspection

Tips for keeping this workhorse in top flight condition

By Joe Escobar

July 2001

imageOn 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. image
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.

Specific 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.

Engine performance
The following guidelines should be followed to ensure optimum engine performance.
• Compressor turbine blade tip clearance should be maintained close to normal limits. Compressor turbine blade clearance can be improved by maintaining vane ring lug and groove geometry and minimum side clearance of lugs.
• The same power turbine first-stage vane ring class needs to be maintained as was installed on initial engine build.
• The interstage sealing rings should be serviceable to prevent pressure leakage.
• Inspect the compressor bleed valve for proper operation.
• Ensure no pressure leaks are present in the gas generator assembly area.


Maintaining fits
The following conditions should be maintained by rework on the affected components. Accomplish rework by hand stoning and lapping on locally manufactured lapping plates.
Flatness and sealing must exist on the flat sealing land between the small exit duct and the leading edge of the CT vane ring’s outer ring. This should be lapped to obtain a good sliding fit.
Flatness and sealing must exist on flat sealing land between the CT vane ring and lockplate. Lap as required.
A free sliding fit must exist between the lugs of the CT vane ring and the slots in the Number 2 bearing cover flange and CT shroud housing. Hand stone the lugs if necessary.
A free sliding fit also must exist between the combustion chamber liner and small exit duct. Rework by hand as required.
Note that excessive clearance between lugs and slots will cause tangential play and destroy blade tip clearance. Replace or return to an overhaul facility for rework those components with excessive clearance.

image Bulge on small exit duct.

Zero grind fit
One of the most time consuming parts of performing a hot section inspection is the grinding of compressor turbine shroud segments. If the segments are replaced, grinding may be necessary to remove high spots and/or any slight eccentricity of the segments in order to obtain correct turbine blade tip clearance.
Although many operators are performing their hot section inspections themselves, some of them are choosing to send their hot sections to a certified repair station for inspection. These repair stations, like Heritage Turbines, are able to provide a zero grind hot section for the customer. Basically, the CT vane ring, turbine disk, small exit duct, and number 2 bearing cover are sent in for inspection. Once all inspections and repairs are accomplished, the unit is placed on a lathe for all necessary grinding. Working in a controlled environment on a lathe, they are able to obtain very tight tolerances between the turbine disk and CT shroud segments so that when the operator installs the components on the engine, no grinding is needed. The compressor turbine still needs to be checked for clearance, but usually no additional grinding is necessary.
Stan Bernstein of Heritage Turbines explains that this is a valuable service for the customer. He states, "Grinding the hot section segments in the field can be a very time consuming endeavor. Consecutive steps of grinding and measurement are performed until the desired tolerances are obtained. With a zero grind hot section, the customer installs it, and it is ready to go with no grinding necessary."
When it comes to packing the hot section for shipment, do not skimp on the material. Bernstein states "It is vital to protect the components for shipment. All of these items are very expensive, and skimping on packing can easily add up to thousands of dollars worth of damage."
image Taking measurement on compressor turbine for fit.

Once all items are inspected and all defects are corrected, the units are reassembled. Be sure to maintain proper clearances and free sliding fits as directed in the maintenance manual.
Once the engine is reassembled, the an engine performance run should once again be performed to check that all engine parameters are within limits and to ensure proper operation.
The PT-6 engine is an engine with a proven history of reliability. A hot section inspection — along with other good practices, such as engine washes and fuel nozzle checks — can help ensure your engine gives you years of safe and efficient operation.

The Source
Additional resources....
Heritage Turbines
2 Hinckley Road
Barnstable Municipal Airport
Hyannis, MA 02601
(888) 671-7788

Pratt & Whitney Canada
1000 Marie-Victorin Blvd.
Longueuil, Quebec
Canada J4G 1A1
(450) 677-9411