Some things rob your engine of performance. Dirty air, dirty fuel, and air leaks can all limit your engine’s potential. Not all the enemies of the engine are performance robbers. Sometimes the target is the engine parts themselves.
Internal and external parts can be adversely affected by corrosion. In the air stream, we see high-temperature corrosion or sulphidation, as it is known. We see corrosion on compressor parts. Right now, I want to focus on two components: the air inlet case we discussed previously and the rear reduction gearbox housing.
As air enters the engine, it flows through the inlet case. This causes distress on the coatings of the inlet case. If any material is in the air, it acts as a blasting media and can wear the housing coatings. Even water at high speed, the air entering the inlet case can be abrasive. The engine maintenance manual contains some very specific inspection and repair criteria. Even if you lose the coating and start to get some corrosion, there are steps you can take on wing to protect your part and nurse it along to the following maintenance/overhaul time.
I started thinking about corrosion because of some issues we have seen with the rear reduction gearbox housing. This is an area of the engine that you cannot visually inspect. It is difficult to determine if you have an issue until it is too late. What is the problem, and what can we do? Glad you asked.
The RGB rear housing is a coated part. The base material is treated, a surface sealant is applied, and then finally, a primer and two-part aluminized epoxy coating is applied. If you have been around PT6s for a long time, you may remember the previous version of the surface sealant. The coating used to be varnish. You may remember flakes of varnish in the oil. It was a real problem. The new coating is a much harder substance, and it is a significant improvement over the varnish. All this protection, and we still see issues.
When we do a power section disassembly, we visually get the opportunity to see the outside of the RGB rear housing. A gap exists between the power section exhaust duct and the RGB rear housing. Most of the gap is taken up by an insulation blanket. This barrier is essential as it protects the oil contained in the power turbine and RGB rear housings from the temperature of the hot gasses being exhausted by the engine. However, there is a small pocket at the end of the insulation blanket that would allow moisture to be held and corrosion to form.
There may be several contributing factors to the root cause of the corrosion problems. There is typically a lot of carbon buildup in this area. Potentially carbon is generated from the rotor and stator air seals in the engine. A small amount of oil works past the air seals and then burns when the engine is hot. This carbon could hold moisture in this area. There are also three different materials; the housing, the exhaust duct, and the insulation blanket. This gives us the potential for dissimilar metal or galvanic corrosion. When two dissimilar metals contact one another, one of the metals undergoes galvanic corrosion. This process also requires the presence of an electrolyte. The electrolyte can be moisture, dirt, or oil, which we already have established are in this area. This is where I believe the issue comes from.
What can you do to give your RGB housing a chance? Pratt and Whitney Canada first tell us to ensure we give every opportunity to protect the engine from moisture. When the engine is inactive, we put desiccant bags into the exhaust to remove moisture. When we perform an engine wash or rinse, we make sure that the exhaust duct drain is free from carbon and water flows freely from it. After the wash, we also run the engine enough to get the temperature up to dry any possible moisture. These are the immediate things you can do to give yourself the best opportunity to protect your part.
The air seals are the other part of this equation. If your power section has not been inspected in many hours or if you continuously operate from less-than-smooth strips, it might be time to consider an inspection. Want just to ignore it? Recently we heard of a pilot a few miles away from landing and noticed steadily decreasing oil pressure. Upon landing, the oil pressure went to zero PSI. Further engine inspection revealed that his housing had a hole, which allowed the engine oil to leak out. This is the worst case. Many housings we find on inspection have corrosion beyond repairable limits, and we just have to replace them. I just wanted to share what could happen.
Be aware of the enemies of your engine. Follow best practices. Consult your maintenance manual. Listen to your engine. I like to remind everyone that the PT6 will tell you when it has a problem.
Robert Craymer has worked on PT6A engines and PT6A-powered aircraft for the past three decades, including the last 25+ years at Covington Aircraft. As a licensed A&P mechanic, Robert has held every job in an engine overhaul shop and has been an instructor of PT6A Maintenance and Familiarization courses for pilots and mechanics. Robert has been elected to the NAAA board as the Allied-Propulsion Board Member. Robert can be reached at robertc@covingtonaircraft.com or 662-910-9899. Visit us at covingtonaircraft.com.
