Mike Guntner of Montreal’s Essential Turbines says the 420-shp RR 250 weighs 158 pounds dry and turns the N1 at an incredible 53,000 rpm at 104%. Additionally, the maximum stabilized turbine outlet temperature is 1,490 degrees F! While that seems like a lot of operational stress for an engine, the major strain on the turbine is cyclical temperature differentials. Each start/stop must be recorded as they limit the life of the power plant due to metal stresses associated with rapid increases or decreases of exhaust gas temperatures – i.e., during startup and shutdown.

Another engine killer is flight in sandy or dusty areas. Airborne particles erode the plastic compressor case halves and the resulting air “blow-by” around the edges of the blades will eventually cause the engine to fail its power check. This results in premature overhauls and extremely high-cost hourly operation. I’ve seen this happen in less than 100 hours of Australian desert operations in a fleet of JetRangers – very expensive! This wear is avoidable by choosing more suitable landing surfaces, wetting down the pad area or making no-hover autorotations for landings on suitable surfaces, to reduce engine and rotor system speeds thereby reducing erosion. (Some cracks in the plastic lining are normal due to the thermal shock from normal operations.)

PILOT PERFORMANCE

Much of a turbine’s reliability depends on pilot handling. You can extend the lives of your engine and yourself by observing pre-flight inspection requirements, starting limits and in-flight parameters. Use flying skill as opposed to heavy applications of power to accomplish your goals. While you may think highly of your skills, you don’t want to have to put them to the test during a real powerless emergency landing caused by inappropriate engine handling. Oh, I know your autorotations are first-rate, but during practice you pick your landing area and don’t flare into tall trees or rocky mountainsides.

Essential Turbines says that prudent pilots should operate the antiice system from time to time to ensure the anti-ice air outlet slots at the front of the compressor aren’t plugged. If the TOT doesn’t rise when checking the anti-ice, suspect plugged slots and turn to your maintenance team for help.

A hot start will result after an abort at any time that the combustor drain valve gets plugged and someone attempts another start. Immediately after pressing the starter button a “boom” will be heard as the pooled fuel ignites and spreads a gusher of flame out the exhaust duct. On hearing the whoosh, the pilot should keep motoring the engine while closing the throttle to keep a cooling airflow circulating through the engine to purge the burning fuel and minimize damage.

Ever have an engine that smokes after shutdown? This is likely a leaky seal and not a problem as long as the oil consumption remains within limits (one quart in five hours). So what’s the deal when you are flying along and the oil pressure goes to zero? Most folks would instantly enter an autorotation and perhaps wrap the bird up in a ball at the termination thereof. Before plunging the pole through the floorboards, Essential Turbines recommends checking the torque meter pressure. If it is functioning normally (with its oil pressure supply), then the engine oil pressure gauge system has failed and you can rest assured that oil is in fact circulating. However, if the torque meter shows no torque – that would be a good time to practice your autorotation skills. (This engine will operate for approximately 30 seconds without oil supply and then the #8 bearing will fail.)

Guess the primary cause of hot starts? If you said rapid or early opening of the throttle, starting with a tail wind or not motoring residual heat out of the engine from a previous shutdown – you were wrong. It’s a weak battery. Although the others can contribute to “warmish” starts, care and feeding of your battery is the best investment in reasonable start temperatures and a long, reliable engine life.

How about oil blowing out of the gearbox vent? While this could have been the result of overfilling the reservoir, it is also possible that excessively high gear box pressure exists. You can lower this pressure by installing a different size diffuser vent orifice.

Have you ever had a 250 compressor stall during start-up on a freezing day? Did you recently wash the engine? If water freezes in the bleed valve venturi it causes the valve to close prematurely and the starter motor will be unable to overcome the back pressure.

Is your transmission oil level increasing on your Bell helicopter? A transmission making oil isn’t magic. It’s almost undoubtedly a failed double lip garlock seal on the freewheeling unit at the power takeoff drive pad. A bad seal allows oil to seep from the engine/gearbox supply into the transmission.

Don’t forget those two-minute idling periods to reduce oil coking. Cooling down the turbine’s interior below the “coking” temperature precludes carbon formation. Also, trend monitoring is very important and has saved my bacon on at least one occasion. Regular in-flight power checks and monitoring engine coast-down times from throttle “chop” while listening for unusual noises can lead to maintenance checks that eliminate

FADEC INSTALLATIONS

For those of you with FADEC installed on your RR turbine, (Bell 407 for instance), do not disregard a FADEC fault indication. Immediately determine the warning’s significance.

Although the ECU box is a handy step, don’t tread thereupon as the contents are delicate – and expensive to replace when crushed. Also, avoid water spray washes with pressure exceeding 30 psi near FADEC components and ensure caps are installed when connectors are removed as any form of contamination can result in faults – grounding your bird.

MORE MAINTENANCE

Jim Taylor of Rolls-Royce analyzed 234 engine events (a euphemism for pricey problems), and advises 48% were maintenance- or overhaulrelated. The other biggie is the basic engine systems themselves. “Operational” incidents caused only 10% of the difficulties – making pilots look pretty good in this data. So let’s look at avoidable maintenance issues.

Operations in a corrosive or erosive environment require daily engine gas path rinse/washing. Previously, many methods of rinsing have been employed to clean compressor sections in the field; however, we are now advised to use a dispersed water stream rather than a strong jet of water to avoid damage to the engine. So much for borrowing the fire department’s high-pressure hose. Taylor also reminds operators to use only approved cleaners, such as isopropyl alcohol.

The RR rep cautions operators to respect the 300-hour PC filler inspection as dirt accumulation results in slow or hung starts and may reduce power output.

Essential Turbines asks if you ever notice coppery residue in the oil change. The #1 bearing in the compressor front support is the only copper cage bearing in the engine. Remember, non-magnetic copper residue will not be found on the chip plug – so you will need to look in the drained oil. If you find copper, ground the helicopter until the bearing is replaced.

While it may not make RR happy, Mike Guntner recommends Extex compressor wheels as replacement items simply because they are better. He also emphasizes regular cleaning of the fuel nozzle to avoid carbon buildup which erodes the N1 rotor gas producer blades and eventually results in loss of power. And he wants everyone to be aware of the new AD 2006-13-06 on the tie bolt that requires replacement by Oct. 31, 2011. Also, the new replacement RR tie bolt in the gas producer wheels now has a cycle life of 9,000 starts.

If your helicopter has a gearbox breather line installed and suffers a smoking habit from the right exhaust, you likely need a larger orifice diffuser vent because high gearbox pressure is overriding the air/oil separator system and blowing oil out the air/oil separator breather gear vent. If changing the orifice doesn’t solve the smoking routine, the garlock seal of the air/oil separator gear has failed and the gearbox case must be split to replace same.