Troubleshooting the Rolls-Royce Model 250

Troubleshooting the Rolls-Royce Model 250 By David Marone May 2000 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...

Continuously smoking turbines often exhibit an increase in oil consumption due to the advanced nature of their deficiencies. Common causes are moderate or severe failure of the No. 1 or No. 5 carbon seals, compromised oil scavenge circuit, turbine internal seal problems, or, on rare occasion, a cracked No. 1 bearing oil pressure tube inside the compressor front support. (If you suspect either the No. 1 carbon seal or the pressure tube in the front support as your source, look for oil leakage around the fifth-stage bleed air valve to confirm.) Use the same troubleshooting suggestions as for the smoking on shutdown engine and don't forget the Rolls-Royce maintenance manual and CSL's. (CSL 3050)

Smoking on start-up when unaccompanied by smoking on shutdown is often caused by a static transfer of oil from the airframe reservoir to the engine. If your smoking seems to occur only on the first start of the day or after long shutdowns, consider checking the gearbox anti-siphon valve or airframe oil supply check valves. On slant-engines, you may notice oil draining from the burner drain valve.

Be careful not to confuse start-up smoke associated with a delayed light-off as an oil-related problem. This smoke will be lighter in color and is likely due to ignition or combustion components.

Vendor accountability
Most operators expect strong, reliable engines for a fair price. And rightfully so! A weak powerplant, in many airframes, can significantly limit your mission profile. The cost of turbine R&O (Repair and Overhaul) represents a major percentage of total aircraft DOC (Direct Operating Cost). A few questions: Is it reasonable to expect my engine to last to full TBO every time? If not to full TBO, how long should it last? How much power is realistic to expect?

Before answering, lets begin with a bit of a reality check. No repair and overhaul vendor can produce an engine that makes TBO every time. Many hard-working utility shops will experience a lower MTBUR (mean time between unscheduled removal) than a Bell 206L-III flying light duty in southeast Alaska. Different engine/airframe combinations and different operating environments impact engines with great variation making "apple to apple" comparisons a challenge. However, by using statistics, certain empirical deductions can be made and initial fleet baselines can be established for trending of critical operational and economic Measures of Performance (MOPs).

You cannot manage what you cannot measure!

Of the many insightful MOPs you can collect on your fleet, perhaps the three most valuable are Average Fleet Power (AFP), MTBUR, and holistic engine DOC. Intuitively, the first two are directly related and together affect major influence on the third. Let's consider each separately. Depending on airframe, environment, and mission the Model 250 powerplant sometimes "coasts" and sometimes needs every bit of rated power and then some. Consider the positive impact of an engine that can achieve 110 percent of rated power. This means approximately 35 to 40 degrees C of extra TOT margin. How do you make a pilot smile? Take out a "spec" engine and install one with a 40-degree C temperature margin. In addition to the pilot's smile, you get an engine that can degrade normally with reduced risk of "low power" unscheduled removal (higher MTBUR). You also now have an engine that often costs less to overhaul as its turbine has seen less temperature for a given amount of work over the life of its TBO.

Finally, hold your engine vendor accountable for positive trends. Watch for an increasing Average Fleet Power and increasing MTBURs. Once established, these trends should drive your holistic DOC down towards an optimum for your fleet.

Quality doesn't come cheap and in the turbine business you do get what you pay for. Although it can be tempting to shop for the lowest transaction price, many operators have found that the optimum DOC for their fleet was realized only after an investment in quality and a long-term commitment to being "trendy."

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