Helicopter Maintenance magazine(HMM) is four years old! In that short span of time, we have become the No. 1 helicopter maintenance magazine in the world. Thank you to our readers and the fantastic companies that take the time and effort to do an article with us. We hope that you will continue to support us with your readership and articles as we continue to grow. For this birthday moment, and for many requests we have received, we are going to take a trip down memory lane.
The Rolls-Royce M250 Turboshaft engine is one of the more populous engines in the helicopter industry. One of our earliest articles was on this engine and we are bringing it back for those who have requested it, and for those who never saw it as they did not know about us at that time. The following interview is with the team at Premier Turbines (PT).
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 M250 engines and a member of the M250 FIRST network.
HMM– Before we get into the details of this article, how about starting us off with some general background information?
PT – The Rolls-Royce 250/T63 Turboshaft Engine was originally designed in June 1958 to meet military requirements for a 250 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 M250 engine grew to where it is today. The first test flight in a helicopter was in February 1961. Since its inception, the M250 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.
HMM – What is the typical time between overhaul (TBO) on the engine?
PT – 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.
HMM – With so many flight hours amassed and so many engines still in service, what are some of the lessons learned in the field by your product support specialists?
PT – 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.
HMM – What procedure would you recommend to eliminate this problem?
PT – 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.
HMM – What other inspection items have proven to be a concern?
PT– 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 vibrate loose and wear the mounting pads of the turbine. This necessitates the gearbox cover and exhaust collector to be re-worked back to specification, and is an additional maintenance cost. Just looking at them and saying they look tight isn’t good enough.
Another lesson learned is on the 300-hour inspection of 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 done 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 nozzle to make sure that no streaks are visible.
HMM – With the Rolls-Royce M250 engine in use in many helicopters operating in salt water environments, what can an operator do to minimize the problem of corrosion?
PT – 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.
HMM– How about any maintenance tips concerning hot and high operations?
PT– Since engine start procedures are affected by altitude and temperature, engine fuel controls should be adjusted according to seasonal changes in temperature and when 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 still push 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.
We hope that you have enjoyed our trip down memory lane and this article. Another special thanks to Premier Turbines and its Rolls-Royce engine specialists. Let us know if you’d like to see an article on a specific topic.
Rolls-Royce M250 Turbine Engine
Evolution of the M250 Series
• T63-A-5 (civil designation M250-C10), rated @ 317 shp, certified in 1962.
• T63-A700 (civil designation M250-C18), rated @ 317 shp, certified in 1965.
• Offered full thermodynamic rating structure and improved TBO (750 hrs).
• Model 250-B15 turboprop rated @ 317 shp, certified in 1969.
• A total of 6,410 Series I turboshafts and 95 Series I turboprops were produced.
• M250-C20 (Military T63-A-701), rated @ 400 shp, certified in 1970.
• M250-B17 turboprop rated @ 400 shp, certified in 1971.
• M250-C20B (Military T63-A-720), rated @ 420 shp, certified in 1974.
• M250-B17Bturboprop rated @ 400 shp, certified in 1974. (limited by PRGB)
• M250-B17B turboprop rated @ 420 shp, certified in 1979. (improved PRGB)
• M250-C20R rated @ 450 shp, certified in 1986, featured improved compressor.
• M250-B17F rated @ 450 shp, certified in 1988, featured improved compressor.
• M250-C28 rated @ 500 shp, certified in 1976, featured single stage centrifugal compressor with beefed up gearbox and turbine airflow improvements.
• M250-C28B rated @ 500 shp, certified in 1978, featuring improvements in compressor, combustor and turbine airflow.
• A total of 936 Series III engines were produced.
• M250-C30 series rated @650 shp, certified in 1978.
• M250-C30R (Military T703-A-700), rated @ 650 shp, certified in 1983, featured a electronic supervisory fuel control system.
• M250-C40 and C47 (Military M250-C30R/3), rated @ 650 shp, certified in 1996, features improved compressor airflow, air cooled turbine nozzle, low smoke combustor and a full authority digital engine control system (FADEC).