Troubleshooting the Rolls-Royce Model 250
By David Marone
Helicopter operators would agree that the Model 250 is a tried and true powerplant. Still, as is the case with most machinery, it can have its days. Preventative maintenance, quality overhauls, and effective troubleshooting are your best defense against costly downtime and unnecessary expense. The following troubleshooting suggestions are intended for those paid to keep rotors spinning. Please remember that these tips are not a substitute for the appropriate Rolls-Royce approved technical data, which should be used whenever performing services on an aircraft engine.
From my days as a field "tech-rep," I've selected a list of three common challenges that kept me gainfully employed for years. After a discussion on each, we'll share a few ideas aimed at assisting you in getting value for your engine maintenance dollar.
Probably the most common cause for unscheduled removal, the low power engine can be a challenge to properly troubleshoot. To get started, ask this question, "Did my engine lose power gradually over time or did it experience a rapid change in performance?" For the gradual loss of power, go back to your previous engine performance records. For most Bell and Agusta products, which measure percentage of shaft horsepower, a loss of about one tenth of one percent per hundred hours is not uncommon. For MD products, which measure temperature margin in degrees Celsius, using one half degree per hundred hours is a ballpark figure. With these numbers as a benchmark, you can calculate estimated power degradation over time. For example, your JetRanger III was a plus four at annual four hundred hours ago. The pilot is complaining that with full fuel and four on board, he is running out of TOT (Turbine Outlet Temperature). You assist the pilot with an engine performance run and find your engine is now a negative two. By our estimate, this engine should have degraded approximately one-half percentage in four hundred hours, not six percent. Troubleshooting this type of rapid degradation should include the following: First, check the easy stuff. Does your bleed valve function properly? The Rolls-Royce maintenance manual has a chart you can use to test for proper bleed valve modulation. Has the compressor been checked for FOD? Gain access to the compressor inlet and look as deeply as possible into the compressor. Case-half removal is recommended as it allows for a better visual on the compressor rotor and inspection of the case half plastic and stators. (Remember, only one case-half should be removed at a time.) Although uncommon, FOD can miss the first stage wheel and cause damage further downstream in the compressor. (DOD, Domestic Object Damage, can start from anywhere in the engine.) If the compressor exhibits no apparent defects, search the engine for air leaks. Cracked compressor scrolls, cracked outer combustion cases, air discharge tube leaks, and leaking airframe bleed air lines are all common causes for high TOT. (MD 500s: Check the flexible braided bleed-air heater line from the left side of the diffuser scroll to the engine bay firewall. C30s: Check the triangular metal gasket(s) on the front face of the diffuser scroll.)
That's most of the "easy" list. Now the fun starts. Calibrate your gauges! Use a deadweight tester on your torque system, check the calibration of the TOT gauge, and compare the OAT gauge with a calibrated thermometer. If you still haven't found a discrepancy, it's time to look in the turbine. Although a borecope can help, removing the outer combustion case is highly preferred. Check the combustion inner liner for signs of heat duress. Although not often the cause of rapid power loss, take a good look at the fuel nozzle. Inspect the first nozzle for trailing edge vane cracks or eroded leading edges.
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