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.
(Although a warped or disconfigured nozzle can affect flow area and cause performance problems, many nozzle-related performance issues will involve heat damage visible to the naked eye.) Next, check the blade paths of the first and second turbine wheel for loss of plasma or other thermally bonded coatings. As these applied coatings tend to "chip off" in pieces, they are usually easy to spot. Don't forget to look down the exhaust stacks at the power turbine for visible damage. If you've checked these items and used the manual's troubleshooting section and still find no faults, it is likely your engine has damage too far internally to be seen in the field. Check in with technical support — tech reps are there to serve you. Consider sending your engine to a Rolls-Royce-Authorized Maintenance Center (AMC) for a run on the test cell. Some repair and overhaul vendors have portable test cells that can assist you with troubleshooting performance problems in the field.
Be careful not to mask the true source of a performance problem by guessing. An inefficient turbine can make power when mated to a highly efficient compressor and vise versa. Solutions of this nature often deprive the engine of its true potential and, in the long run, can cost you time, money, and aggravation.
Troubleshooting the Rolls-Royce Model 250
By David Marone
The second pump level scavenges the gearbox by pumping oil past the lower chip detector, through an internal scavenge tube, into the pump, past the upper chip detector and then on to the cooler, tank and oil filter. As gearbox scavenge oil passes both the lower and upper chip detectors before filtration, it is common to see metal on both plugs. This occurrence should be interpreted as a lower plug indication. For clarification, remember that chip detectors are merely oil sampling devices and do not catch all metal scavenged past them. This is how metal scavenged from the gearbox can show up on both upper and lower plugs while metal solely on the upper plug is an indication of an externally scavenged bearing. (Note: Of the four externally scavenged bearings, No. 8 and No. 1 fail more often and with relatively less warning after the first signs of metal. Please be aware of this when troubleshooting an "upper plug only" metal generation event.)
Although most lower plug or "metal on both plugs" indications turn out to be a gearbox problem, remember that the No. 2 bearing on the aft end of the compressor and the No. 5 bearing on the forward end of the turbine are both scavenged through the gearbox. Inspect these bearings closely before condemning your gearbox. The No. 5 bearing can be easily removed in the field for inspection. The No. 2 will require assistance from an AMC.
The most common cause of turbine smoking is the No. 5 carbon seal. Unfortunately, it is not the only source so we must troubleshoot. To get started, ask this initial question: "Does my engine smoke on start-up, shutdown, or continuously?" If your engine smokes on shutdown, time how long it takes from fuel cut-off until smoking appears. Smoke beginning four to six minutes after shutdown can be a malfunctioning or improperly assembled external oil check valve. (Make sure the poppet is not installed backwards.)
Smoking that occurs immediately after shutdown is usually from these three sources, in order of likelihood: No. 5 carbon seal, turbine internal oil control seal failure, or malfunctioning oil scavenge circuit.
Troubleshooting the No. 5 carbon seal is straightforward. Using a mirror and a flashlight, inspect the extreme aft edge of the exhaust collector internal cone at the six o'clock location for signs of fresh oil or oil residue. If your seal leak is moderate to severe, you may see oil puddling at the bottom of the exhaust collector. (Note: On slant-mount engines, MD 500's, Soloy Hillers, etc. Oil leaking past the No. 5 carbon seal will run down the forward face of the No. 4 turbine wheel due to gravity. Instead of puddling, these engines will leak oil, aft, into the turbine and may also smoke on start-up. Look for oil residue on the forward face of the No. 4 turbine wheel.) At this point, if these tell-tale signs of oil are not present, the No. 5 carbon seal is most likely NOT your problem. Restart the engine, shut it down and quickly remove an exhaust stack. (Please remember that they are HOT!) Now, using a mirror, look into the turbine and observe whether smoke appears to be "rolling forward" from a location aft of the No. 4 turbine wheel. If this is the case, your culprit is likely a turbine internal seal problem OR the oil scavenge circuit. Troubleshoot the scavenge circuit BY THE BOOK. As a minimum, perform the "baby bottle" check and visual inspection on the No. 6/7 bearing strut. If this oil scavenge passage is restricted by coke, your turbine will smoke. Next, ensure you have sufficient scavenge vacuum from the pump. The Rolls-Royce maintenance manual has a step-by-step procedure. Low vacuum will usually require a gearbox repair and is not always caused by the pump alone. If you still do not have a solution, check in with technical support. Chances are this turbine is headed for the shop.
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.
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."