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.
By Mike Venables
Principal, TriLink Technologies Group
A Pratt & Whitney Canada technician puts the finishing touches on a freshly overhauled engine being returned to service.
I am a Pratt & Whitney Canada PT6A-135A and I'm mounted on the wing of a hard-working corporate King Air C90.
All of my controls are hydro mechanical, and my siblings and I are the most common variant of the PT6 extant.
My extended family numbers more than 50,000, and together we've accumulated over 380 million hours of flight time. Throughout my career I've performed many missions and I've worked relentlessly, which means now I'm in need of some serious R&R. Removal and replacement, that is, and maybe even a shower.
The shower I just referred to was actually an engine wash that can restore some lost performance. The wash eliminates stubborn deposits that can degrade the aerodynamic efficiency of my compressor blades. Depending on where I am flown, compressor and compressor turbine desalination washes may also be vital to making sure that I reach my TBO.
Salty environments, coupled with small amounts of sulfur in jet fuel and the high temperatures (around 1500°F) in the combustion chamber, cause "sulfidation," which is particularly hard on the special coatings on my turbine blade—and, if these coatings get worn away, corrosion is a real threat.
The most important thing that anyone can do to ensure that I reach my TBO is to enroll me in an engine trend monitoring (ETM) program. ETM can spot performance degradation before it becomes apparent to the pilot.
The most effective ETM is an automated program with onboard data recorders that both record performance data and forward it wirelessly for remote analysis. Reports are then available online and any performance degradation is immediately obvious. Problems such as blade fouling can be corrected before they cause permanent damage and shorten my life.
Borescope inspections are relatively easy to do on-wing and can also spot minor problems before they become major, possibly resulting in early removal or even an inflight shutdown.
My normal TBO is 3600 hrs, with a hot section inspection (HSI) due at 1800 hrs. One of the unique advantages of the PT6 is that the HSI can be done on-wing. The airflow is the reverse of what one would expect, with the air intake at the rear of the engine and the exhaust at the front.
Because the power turbine is now toward the front of the engine, the power turbine and propeller reduction gearbox can be removed from the engine while leaving the main part of the engine (the gas generator) on the aircraft. With the power section removed, it's easy to inspect both the compressor turbine and the power turbine without taking the entire engine apart.
Inertial particle separator (IPS) system. When activated by the pilot, a small door forward of the engine intake closes and another further aft opens (indicated by red circles). This forces the intake air around a sharp bend and debris carries on out the back of the intake duct.
This unconventional arrangement provides several other key advantages over the more conventional front-to-back airflow. First of all, the air entering the compressor can easily be forced to go through a 90° turn when the pilot activates the inertial particle separator (IPS). (See diagram on p 88.)
The forward door closes, diverting the air and forcing it to make an abrupt 90° turn as it enters the engine. Most, if not all, of the supercooled water droplets cannot make the turn and continue straight on and out the back of the engine through the aft door, which is now open. The same is true for dust and debris, greatly reducing the chance of foreign object damage (FOD).
For normal operations, the forward door is open and the aft door is closed. The "reverse flow" arrangement also improves aerodynamic efficiency through the turbine section, and the power turbine shaft is much shorter, saving weight.
It's possible to get an extension to the normal TBO of 3600 hrs. Such an extension normally requires a very well defined and controlled operating environment, such as a scheduled airline with each aircraft operating on a regular route. Since I'm employed by a corporate operator with no fixed route structure, I'm due for my overhaul now as I've just reached 3600 hrs.
Just to give you some idea of the internal stresses that I must endure, as I mentioned above, my combustion chamber temperatures can easily exceed 1500°F. My power turbine spins at 33,000 rpm, while my gas generator (compressor and compressor turbine) spins at speeds in excess of 37,000 rpm.
While my power turbine produces some 750 hp (measured at the prop), my compressor turbine has to produce about 1500 hp to drive my compressor.
Once I'm removed from the wing, the first step is to remove my starter/ generator and tacho-generator, as these items are considered airframe accessories and must be overhauled separately. Then it's off to the overhaul shop.
My chief pilot has decided to send me to one of P&WC's 30 owned and designated repair and service facilities for my overhaul.
An overhaul can take 30–45 days, and the operator has the choice of either grounding the aircraft or renting an engine while I'm away. "We do offer mobile repair team (MRT) service to customers if that's their preference," says Jeffrey Quick, Pratt & Whitney Canada's PT6A customer service manager.
"The MRT will come to the customer's location, wherever it might be, remove the engine and lower it into the shipping crate. If arrangements are made in advance, the MRT can also have a rental engine shipped to the customer's location and installed immediately so downtime is minimized." P&WC has some 800 rental engines located across its global network.