By J.D. Marshall
Foreign object damage is a mutual problem for turbine engines, more so with a “straight” turbine or turbo fan because they do not incorporate an air intake/ inlet screen for protection. The PT-6, however, does utilize an intake screen which is made of a metal mesh spaced in ¼ inch squares. This is by design, because the Pratt & Whitney PT-6 engine is constructed of parts all greater than a ¼ inch in size, which means the engine, theoretically, cannot FOD itself. However, P&W apparently does not consider cotter pins, safety wire or rivet heads dropped by a fatigued technician to be a true part of the engine and these components can breach P&W’s FOD Fort Knox. These items are commonplace during engine inspections; they can be smaller than ¼ inch and can produce significant damage to the “cold” section and beyond if ingested. In addition to the inlet screen, Beechcraft incorporated an inertial separation system into the King Air PT-6 engine installation design. Though they are commonly referred to as Ice Vanes, some models label it as Engine Anti-Ice or Engine Ice Protection and there are several variations on how these vanes are actually actuated. Older models used two mechanical “T” handles with no redundancies while later designs have an electrical equivalent and also incorporate standby actuators. Some King Airs have an electrical primary system with mechanical backup “T” handles that are red and labeled for emergency extension. It’s important to note that once the manual system has been actuated, the override linkage must be reset and use of the electrical system should not be attempted. Though the primary function of the ice vanes are to prevent the solid state of H2O from driving up your maintenance costs, it has been suggested that their use on the ground can also prevent other objects from wreaking havoc on the expensive turbine constituents.
Let’s first categorize the foreign object and the severity of damage which can be incurred. The foreign object itself can be classified as one of the following:
- Build Material
Ice can form under the inlet screen, on intake lip or any piece of solid material subjected to the airflow when precipitation and subfreezing temperatures exist. As I mentioned above, the engine itself does not integrate an air induction anti-icing mechanism, but the King Air airframe does. As we all know, the engine anti-ice vanes should be deployed in temperatures of +5 degrees to +10 degrees Celsius and below when operating in visible moisture. However, the use of this anti-ice mechanism cannot protect the engine from damage if the buildup is downstream of the ice vanes. This build up can break away and be ingested by the engine while descending from freezing altitudes into warmer temperatures. In 1982, P&W circulated a Field Note to PT-6 operators describing how ice buildup under the inlet screen can break off and be ingested causing first stage compressor blade damage, usually in the form of a single bent blade. They also emphasized the importance of deploying the ice vanes before entering icing conditions.
Earth is a generalization that groups all naturally occurring elements that could produce damage when ingested, e.g. rocks, pebbles, stones, sand, dirt, etc. For King Air pilots, the laws of physics prevent the heavier of these elements from making the turn upward into the inlet, but sand and dirt can be ingested causing premature wear and possibly minor damage especially when using reverse thrust at lower airspeeds. Therefore, it is considered good form to have the ice vanes deployed during ground operations, temperatures permitting. Another important note, on some King Airs, like the B200, extending the inertial separators also simultaneously opens the Ice Vane Bypass door, which is commonly mistaken for an oiler cooler door. This reroutes the air flow away from the oil cooler, and therefore could potentially increase oil temps to the upper limitation. In fact, Beechcraft recommends the ice vanes be retracted during operations in temperatures as cool as +15 C. Though Beechcraft does not specify, it can be assumed that this is an airborne, climb/cruise power restriction. I personally use +30 C as a limitation for low idle, ground operations with ice vanes extended, however oil temperatures must be carefully monitored regardless of OAT.
Build material represents metal components used in the construction of the engine which includes the previously mentioned elements. This could also include miscellaneous spare parts from other vehicles, machinery, or equipment that operate on or near airport runways and taxiways, for example fuel trucks, tugs, and grounds keeping equipment. Just like earthen elements, only the lightest of these items pose a threat for the PT-6. A more likely scenario of build material FOD’ing an engine is when it is left behind by a weary mechanic, for example, a ¾ inch stainless steel cotter pin that was dropped during a maintenance event and not properly recovered, dislodged by turbulence and swallowed up by the compressor.
As to the categorization of severity, they are as follows:
- Minor – meaning, the spoiled components can be easily blended without replacement.
- Moderate – meaning replacement of the first stage of ingestion is required.
- Severe – meaning multiple turbine stages are required to be repaired or replaced.
- Very Severe – meaning all stages, cold and hot, require replacement.
Luckily, for us pilots, most aircraft insurance policies include coverage for engine ingestion. It is worthy of your time to find out if your policy does or does not, in fact, cover this type of event. A minor FOD incident can result in a $100,000.00 repair bill, and a major event could cost up to $300,000.00 or more.
The following pictures reflect damage from what was assumed to be a metallic element or elements ingested by a PT6A-61 engine at the first stage compressor section, which by the way, retails for around $58,000.00. It is my opinion that when a foreign object with any significant mass first contacts the 30,000 + RPM turbine, it does not simply impact and pass through, but instead “bounces” from one blade to another much like a roulette ball before being chewed up and consumed by the rest of the engine. This means a single trifling cotter pin could inflict a significant amount of damage to your engine if given the right amount of access.
Fortunately, the P&W PT-6 engine is one of history’s greatest propulsion developments, though not originally intended for aircraft; it has proven to be a truly reliable adaptation. Coincidentally, it was paired with one of history’s greatest flying machines, venerably christened as the King Air.
J.D. Marshall is an ATP, CFI, and A&P and has been flying and maintaining aircraft for twenty years. For the last ten years he has flown in the charter and corporate aviation environment, acquiring over 6000 flight hours of experience in more than 110 different aircraft consisting of 45 different types, taking off and landing at over 400 different airports throughout the United States,Canada and the Caribbean. He has experience in the King Air C90 A/B, E90, 200, B200, 300, and 350.