4 MAINTENANCE BEST PRACTICES FOR PREVENTING SALT AIR CORROSION
Engines powering aircraft that operate near the sea face corrosion by salt—but not if you follow some simple maintenance practices. Here’s what PT6A operators need to know.
1. USE A CORROSION INHIBITOR
“If you left an engine in the sea, some parts would literally disintegrate after the water had eaten away at the hardware for a while,” says Francis Marotte, PT6A Customer Manager, by way of illustrating the potentially harmful effects of salt.
While being submerged is not a concern for PT6A customers, exposure to salt in the air when flying is. In regions with high salt content in the atmosphere, operators must be constantly vigilant against corrosion, whether it’s rusting of the airframe or sulfidation of hot-section components.
Fortunately, the right maintenance practices will keep corrosion at bay, no matter where you’re operating. The first step, says Francis, is to apply a spray-on corrosion inhibitor to the hardware, such as CorrosionX or Procyon.
Corrosion inhibitors are recommended for external components if you’re operating in a harsh environment or if you see signs of corrosion on the engine. They’re particularly important for magnesium components, which are more prone to corrosion than aluminum.
How often you need to apply them varies depending on the environment and type of operation. The maintenance manual has some guidelines, but ultimately, operators should rely on their experience and judgement.
2. DO REGULAR BORESCOPE INSPECTIONS
Francis recommends doing a borescope inspection every time you do a fuel nozzle inspection. Look for blisters forming on the profile or platform of the blades.
After the inspection, check your observations against the engine maintenance manual, which will provide recommendations for different stages of corrosion and indicate whether it’s within the acceptable parameters.
Another best practice is to keep records of previous inspections so you can monitor the progression—or lack thereof—of corrosion. This will also help you validate whether the washing interval is appropriate for your type of operation.
3. WASH THE ENGINE INTERIOR...
The key to preventing corrosion is washing your engine. The more salt there is in the operating environment, the more often washes will be required.
In salt-laden environments, the most frequent wash should be a turbine rinse. A compressor desalination wash should also be performed periodically and should always be followed by a turbine rinse. These washes will flush salt out of the engine and prevent sulfidation (sometimes referred to as hot corrosion).
Unlike a performance recovery wash, which is done with soap and water, these washes should be done with demineralized water—the higher the quality, the better. “If you can drink the water, then it’s good for your engine too,” says Francis.
On medium and large PT6A engines, Francis emphasizes that the exhaust duct drain must always be open when you do a wash.
Because of the configuration of the exhaust duct, water could accumulate inside and attack the rear reduction gearbox housing if the drain’s not open at the bottom. Besides removing the plug, you should make sure the drain is not blocked.
He adds that on some engines, it’s also necessary to remove the P3 filter. Newer engines, however, are equipped with a valve.
4. ...AND EXTERIOR
The engine externals should also be cleaned from time to time. During an internal wash, dirt flushed out of the engine may splash and stick to the outside. The external wash will remove this, plus any other corrosives that were already there.
Francis advises using a regular hose with normal pressure rather than a pressure washer, which could damage external parts.
One customer that certainly appreciates the importance of regular engine washes is Harbour Air Seaplanes, based in Richmond, British Columbia, on Canada’s west coast. Operating scheduled services, tours and charter flights, its fleet includes 20 Viking Air single-turbine Otters with PT6A-34 engines, three PT6A-27-powered Twin Otters and a PT6A-140-powered Cessna Caravan EX aircraft.
Director of Maintenance Shawn Braiden confirms that with its planes landing and taking off from salt water every day, Harbour Air prioritizes regular external and internal washes to stop corrosion from progressing.
During 100-hour inspections, we wash the outside of the engine down. This keeps everything clean and easier to inspect. It also washes away any salt that has accumulated on the engine and the surrounding area under the cowls. Each aircraft also gets rinsed down at the end of the day with lots of fresh water, and we do a desalination wash once a week.
He adds that if corrosion is found on the external casing, the maintenance team will slow it down by removing it and touching up the paint. Nozzles are replaced and borescope inspections performed every 400 hours to keep a close eye on the engine interior, in combination with engine condition trend monitoring.
For some customers, such as those operating Twin Otter seaplanes with engines mounted on the wings, there’s no way to prevent some salt water getting inside the engine. Frequent washing and rinsing is therefore indispensable.
For more practical advice on optimizing PT6A maintenance, read our article on ground performance checks.