A PT6 overhaul seen from the engine's perspective
Turbine service events should be planned in advance. Knowledge of the details can go a long way toward eliminating the uncertainties.
PT6 internal arrangement. The air flows generally from right to left. The rotating group shown in blue is the gas generator, while the red parts represent the power section. The compressor features 4 axial stages and 1 centrifugal stage, with a single stage compressor downstream of the reverse-flow combustor. A 2-stage power turbine drives the propeller through a 2-stage planetary reduction gear set.
Once I get to P&WC, I'll be assigned an event manager who'll follow me throughout the overhaul process. He'll be the official P&WC liaison with my chief pilot. The event manager's first job is the induction process. That means that my log books are analyzed carefully, and all of my maintenance records and the records pertaining to life-cycle-limited parts are evaluated.
"Ideally, engines are shipped with their log books in the crate," says Quick. "Receiving the log books with the engine allows us to initiate the induction process and minimize the turnaround time of the overhaul work without delays."
Some of my components are life-limited and the log book analysis will determine which of these components need to be replaced and which can safely be left in service.
Pratt & Whitney Canada's Customer First Centre includes a multidisciplinary team representing key resources from across the company's aftermarket network. This group is tasked with providing responsive and proactive customer support. Available services include AOG and critical emergency service, technical/maintenance consultation, and engine and spare parts delivery status.
Nick Kanellias is the GA general manager at P&WC, and he describes the internal process in greater detail. I'm installed in a build stand using the aircraft mount points on my compressor casing.
Using this stand, all the other parts can be removed, leaving just the casing, which is a single part. My remaining accessories, like the fuel control, bleed valves and prop governor, are removed and sent for separate overhaul at the accessories shop.
My power section is removed and disassembled. All the gears and bearings in the reduction gearbox are inspected for wear. Each mounting point in the housing is also inspected. These "inspections" include very exacting measurements of all wear faces to ensure that all dimensions are within tolerances.
Each blade is removed from the turbine disk and inspected individually for damage and wear. In addition, each blade is measured for "creep"—the blade growth which can happen when the engine is exposed to excessive temperatures, speeds or other stresses. Blade growth can also cause damage to the turbine shroud.
Parts that can be repaired quickly are sent to the component repair department and are returned in time for use when the engine is reassembled. Parts which take longer to repair can be replaced immediately with exchange or used serviceable parts from the company's existing inventory.
The removed part is repaired and is then put into inventory to replace the part that was used. New parts are sourced from the company's new parts inventory.
My gas generator section includes, counting from back to front, an accessories gearbox, a multistage axial compressor, a single-stage centrifugal compressor, combustion chamber with 39 fuel nozzles and single-stage compressor turbine. The accessories gearbox will be completely disassembled and inspected in the same manner as the reduction gearbox.
The multistage axial compressor is the first to encounter the air as it enters the engine. If the aircraft has been operated in contaminated environments and the pilot has not used the IPS appropriately, this is where the damage will occur first. Eroded coatings, nicks and bent blade tips are all consequences of extreme operations that will usually cause blades to be scrapped.
Each fuel nozzle is cleaned and tested. Using contaminated or nonstandard fuel can interfere with the spray pattern from the fuel nozzles. A streaky fuel spray pattern can cause hotspots that will damage the combustion chamber liner.
Similarly, uneven combustion can damage the vane ring and the compressor turbine blades. Fuel droplets that are larger than normal (caused by damaged or dirty fuel nozzles) do not burn properly and create carbon soot that can damage turbine blades.
Balancing is a critical issue. Given the speed at which the components rotate, any imbalance will lead to vibration which will tend to shorten the life of the engine and possibly even lead to a failure.
Static imbalance occurs when the center of mass of, say, a turbine disk, does not line up with the center of the shaft running through the middle of the disk. Like a loaded die at a crooked casino, if the shaft is rotated gently, it will always stop with the same blade at the top. Couple unbalance occurs when the center of mass at the front face of a disk is offset differently from the center of mass at the rear face of the disk. Dynamic unbalance is a combination of static and couple unbalance at the same time.
When reinstalling the blades in each of my 7 disks (4 axial compressor disks, 1 compressor turbine disk and 2 power turbine disks—the centrifugal compressor is a single piece), each blade must be weighed and installed in the right slot to minimize the static imbalance of each individual disk.
Then each disk must be spun up in a balancing machine. Small weights may be needed to ensure that the disk is balanced from a static, couple and dynamic point of view.
When the disks are then reassembled onto their respective shafts (the complete compressor shaft with its 4 axial compressor disks, centrifugal compressor and compressor turbine and the power turbine shaft with its 2 turbine disks), the shaft assemblies must be rebalanced as a unit.
Getting it back together
Once fully reassembled, I am transferred from the build stand to a test stand. This is mounted in a test cell and fuel and control lines are connected, as are the various diagnostic indicators.
"The test cell and monitoring equipment are state-of-the-art and resemble the test cells we use to evaluate new engines," says Quick. "We test for power generation levels, fuel consumption and vibration levels. We also ensure that there is no oil or fuel leakage."
The testing of the overhauled engine is to ensure that it will perform "like new" and that there are no surprises for the operator once the engine is reinstalled on the aircraft. Once the testing is complete, inspectors conduct a "post-run" analysis of the engine as a final check.
With all testing complete, I am sent to the packaging department in the overhaul facility where I'm once again carefully crated for return shipment to my home base.
Back at my hangar, I am reunited with my starter/generator and tacho-generator and reinstalled on the wing of the King Air. It will likely be another 10 years before I have to go through this process again.
While the overhaul is a complex process that requires a lot of care and precision, if the same degree of care and precision is applied to operating the engine between overhauls, the actual overhaul will proceed smoothly with a minimum of surprises.
Most manufacturers go out of their way to provide extensive educational resources for owners and operators which, if followed, will ensure that their engines operate reliably for many years.
Mike Venables is an aviation consultant and freelance writer. The principal at TriLink Technologies Group, Venables has been involved in the aerospace industry for more than 40 years, including aero engine, airframe, avionics and simulator manufacturers.
1 | 2|