(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.
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