Business jet operators should take action to preserve their engines if the aircraft is going to sit idle for more than two or three months, according to recommendations from engine makers. Such preventive measures can be as easy as ensuring oil is coating components properly or as complicated as removing and bagging the engines. Neglecting an engine that sits inactive for a protracted period can cause serious problems that are not covered by a maintenance plan.
Moisture in the fuel or engine oil can attack the bare metal surfaces inside the engine, beginning corrosion in bearings and gears, Mike Bevans, Honeywell’s director of technical sales, told AIN. A Pratt & Whitney Canada spokesperson said that corrosion can set in within “as little as 24 to 48 hours.”
Most of Honeywell’s engine manuals require that each engine be run up to normal operating temperature every 30 days, at least, to vaporize moisture that has collected in the oil and fuel systems. Another purpose is to cover all the internal parts of the engine with a fresh coat of oil to discourage corrosion from starting. The allowable time limit before you take action varies slightly with the engine, but the general guidelines are the same.
“If for some reason the engines can’t actually be run, they should be motored over,” Bevans said. This should be done at a high enough speed to distribute fresh oil throughout the engine. Simply spinning the fan is not enough. The high-pressure spool needs to be rotated, as it drives the accessories, including the oil pump.
Using the same principles but implementing them in a different way, Pratt & Whitney Canada recommends protective measures for engines that are idle anywhere from seven to 90 days. “The best way to maintain an idle engine is to run it every seven to fourteen days and keep it in a hangar that is conditioned and has some type of humidity control,” the Pratt & Whitney Canada spokesperson said.
However, this method can be used only for a few months. If the engines are expected to sit idle for more than 180 days, they must be removed and placed into “long-term preservation,” according to Honeywell processes. The engine is put into a large bag with a desiccant that absorbs moisture, along with a moisture indicator. Finally, the entire bag is evacuated (rid of as much air as possible) and sealed.
Failure to follow these recommendations can lead to the engine being deemed unserviceable, Honeywell warns. Then, depending on factors such as how long the engine sat without preservation, the environment and the moisture content in the oil, an “inspection and repair work scope” will have to be developed to return the engine to serviceable condition. When this type of work scope is required, it is considered abuse and will not be covered by your maintenance service plan (MSP) contract, Bevans said.
Pratt & Whitney Canada confirmed that improper storage can have serious repercussions. The operator may be required to return the engine to an overhaul shop, which can be expensive depending on the level of disassembly required.
Typically, only aircraft in for major maintenance (including interior refurbishment) or in the hands of brokers sit idle for prolonged periods. The aircraft’s location does make a difference; an aircraft sitting idle by the sea is exposed to more corrosive salt air than one positioned in the relatively dry, salt-free air of the Mojave desert, for example. “We have only one preservation procedure and it works for both environments,” Honeywell’s Bevans said.
Rolls-Royce mentioned a risk of erosion in a sandy environment, though. “Humidity is not an engine’s friend,” the Pratt & Whitney Canada spokesperson emphasized. He recommended engines be stored in an environment that has less than 40 percent humidity.
Rolls-Royce has pickling procedures similar to Honeywell’s and Pratt’s, as the firm said that a typical procedure is to perform a full installation ground run and then “adhere to the preservation procedures.” Alternatively, the engines should be removed and stored properly.
Only Pratt & Whitney Canada would put numbers on preservation costs. In the straightforward case of regularly running the engine, costs could come to “perhaps $300 to $400 per run.” This includes the cost of maintaining a hangar, fuel, crew and so on. If you remove the engine and cocoon it in a humidity-controlled container, this could run as high as $15,000 to $20,000.