Scott Cochran – Contributing Author
The following is an interview with Scott Cochran, Premier Turbines’ West Coast Regional Manager for the Rolls-Royce 250/T63 Turboshaft Engine. For more than 40 years, Premier Turbines has been a leading provider of repair and overhaul services for business, general aviation, government and military turbine engines, components and accessories. They are also a world-leading Authorized Maintenance Center (AMROC) for Rolls-Royce Model 250 engines and a member of the Model 250 FIRST network.
HeliMx: Good morning, Scott. Some of our readers may not be familiar with the Rolls-Royce 250 engine. How about we start with some general background information?
Cochran: The Rolls-Royce 250/T63 Turboshaft Engine was originally designed in June 1958 to meet military requirements for a 250 shaft horsepower (shp) engine for light observation aircraft. Out of that program, several helicopter manufacturers were asked to look into the feasibility of adapting this engine for helicopter use. From this beginning, the Model 250 engine grew to where it is today. The first test flight in a helicopter was in February 1961. Since its inception, the Model 250 engine has logged more than 200 million flight hours. Over 30,000 engines have been delivered to date, and there are about 14,000 still in service. Today, output shaft horsepower varies with different models between 420 to 715 shp.
HeliMx: What is the typical Time Between Overhaul (TBO) on the engine?
Cochran: It depends on the model and the series. On the Series 2 engines, the gearbox is on condition, and the turbine and compressor are both rated at 3,500 hours. Series 4 engines have a compressor that is rated on condition. The impeller on a Series 4 engine has different life limits depending on the model. The gearbox is on condition and the turbine is rated at 2,000 hours TBO on all models except the C40 engine.
HeliMx: With so many flight hours amassed and so many engines still in service, what are some of the lessons learned that you and other product support specialists have found when you are out in the field supporting customers?
Cochran: One of the problem areas we still find that has a large cost impact to the operator is maintenance of the N1 and N2 tach drives. The male and female parts of the tach drives are square and fit together. If the tach drives are not lubricated properly, the drives lose their squareness from lack of lubrication and become rounded and slip.
HeliMx: What procedure would you recommend to eliminate this problem?
Cochran: Following the instructions in the Rolls-Royce 250 Operations and Maintenance Manual (OMM), lubricate the tach drives as instructed and at the proper intervals. If the N1 and N2 tach drives are lubricated as called out in the OMM, the reduced wear and tear will prolong engine life and reduce maintenance costs, as the tach drives cost approximately $6,500 each to replace.
HeliMx: How often should the N1 and N2 Tach drives be lubricated?
Cochran: Every 100 hours.
HeliMx: What other inspection items have proven to be a concern?
Cochran: Another lesson learned on the 100-hour inspection is a low torque value on the compressor and turbine mounting stud nuts. The OMM calls for 80 inch pounds and the mechanics tend to forget to add the drag torque value on the nuts. When this is added in, the actual torque value is closer to 110 inch pounds. This ensures the nuts stay tight. The area of concern is where the turbine and gearbox come together. If the nuts holding the turbine in place are not torqued to the proper value, they will vibrate loose and wear the mounting pads of the turbine. This necessitates the gearbox cover and exhaust collector to be reworked back to specification, and is an additional maintenance cost. Just looking at them and saying they look tight isn’t good enough.
A 300-hour inspection lesson learned is a maintenance tip relating to the engine fuel nozzle. The nozzle should be removed, cleaned and re-installed during the 300-hour inspection. However, if your aircraft has no airframe fuel filter, it is required to be done at the 100-hour inspection. Depending on the quality of the fuel that is being used, this may have to be done more frequently.
Preferred cleaning of the fuel engine nozzle is with an ultrasonic cleaner. If that is not available, use a toothbrush to remove the carbon buildup. When carbon is allowed to build up on the fuel nozzle, it can cause streaks on the combustion chamber walls and liner, which leads to holes in the wall of the liner and burn holes through the vanes of the first and second stage nozzles. After cleaning, run clean fuel through the fuel nozzle to make sure that no streaks are visible.
HeliMx: With the Rolls-Royce 250 engine in use in many helicopters operating in salt water environments, what can an operator do to minimize the problem of corrosion?
Cochran: The gearbox housing, which is made of magnesium, poses a prime risk for helicopters operating in a salt water environment, and appropriate steps should be employed to keep the helicopter as free from salt water corrosion as possible. A daily fresh water wash of the compressor is a must to help minimize salt water corrosion. A down side is that if the fresh water is being supplied through galvanized pipe, impurities from the pipe can make their way into the engines. If at all possible, a de-mineralized/de-ionized fresh water source should be used.
Another way to extend engine life is the addition of an airframe air filter. The old Engine Air Particle Separator (EAPS) filters that came with most aircraft did a good job on large particles, but not on small particles. Most of our customers are using the barrier filter from Aerospace Filtration Systems, commonly known as AFS, with amazing results. This filters out the small particles and reduces erosion on the compressor blades and vanes, which increases the life of the compressor and maximizes engine output power.
HeliMx: How about any maintenance tips concerning hot and high operations?
Cochran: Engine start procedures are affected by altitude and temperature. Engine fuel controls should be adjusted according to seasonal changes in temperature and if operating at higher altitudes. The automatic adjustment bellows in the fuel controls can only adjust so much, which is why a manual adjustment should be performed seasonally.
Another lesson learned is on engine start procedures. Some pilots are still pushing the starter button until they reach 18 percent N1 speed and then open the throttle. Commercial Service Letter (CSL) 1176 advises to open the throttle at 12 to 15 percent. This puts less stress on the first stage nozzle and wheel and helps stop them from burning up.
Last but not least is to reduce gearbox seal leakage with the advent of the new generation synthetic oils. Before replacing the seal, if you look closely at the shaft where the seal is to be replaced, you will find a shiny spot where the seal lip has been riding on the shaft. We recommend using Scotch Brite to remove the shiny spot. This better allows the seal to properly seat itself and minimize leakage and not burn the lip of the seal.
In closing, your local Rolls-Royce Tech Rep is always available 24/7 to help answer your questions and concerns, and help you better maintain and extend the life of your Rolls-Royce Model 250 engine. Spending time with customers, so they can get the most out of their engines, is what we do.
Cochran first started working on helicopters in 1977. Most of those 33 years has been spent repairing and overhauling helicopter engines. He is currently the West Coast Rolls-Royce Model 250 Regional Engine Manager for Premier Turbines. Cochran is an FAA-certified Airframe & Powerplant mechanic. Besides teaching RR 250 maintenance classes, he is also a Rolls-Royce approved Train a Trainer. Cochran lives and works in the Seattle area.