During ground school, several students have told me about their engines having to go into maintenance for first-stage compressor F.O.D. In each instance, a single blade has been bent with the damage being caused by a soft or dull object. I’m sure it was ice. The King Air intake system is the result of a millions of flying hours coupled with a thorough research program. Closed circuit television cameras located in the intake has shown the effectiveness of deploying the ice vanes in a timely manner. Yet, every year Pratt & Whitney issues a bulletin informing pilots on the need to extend the ice vanes. I’m going to throw in my two cents and hopefully save a few PT-6 engines. The King Air flight manual is explicit when it comes to icing. “Temperature requiring Engine Anti-Ice” +5ºC or lower in visible moisture. The pilot’s interpretation of what is icing conditions is sometimes used as justification to delay deployment of the ice vanes. Many King Air pilot’s wait until the see ice appear on the bottom of the windshield wiper before considering extending the ice vanes. Flying at night adds to difficulty in ascertaining if icing is present. To correctly deploy the ice vanes, you have to understand how the FOD is getting into the engine. It doesn’t build-up on the engine intake and then suddenly break off and go through the engine screen into the engine. The mass of the ice will stop it from turning the corner and hitting the screen. But if the ice vane is not extended moisture will collect under the screen and freeze. Then as you descend into warmer air, a piece breaks off and FOD’s the engine. Snow will do the same thing. So the key to eliminating this problem is to extend the ice vanes at +5º C when in visible moisture. If its night and you can’t tell if visible moisture is present, extend them anyway!
Of course, FOD isn’t only ice induced.
The following actions should be taken by pilots to reduce the potential of F.O.D. during ground operations:
1, Following maintenance, ensure that the mechanic hasn’t left anything in the intake. Even a small safety wire clipping can F.O.D. an engine.
2. Avoid run-ups in areas containing loose gravel or sand.
3. Avoid use of reverse at low ground speeds.
4. Do not use reverse to position the aircraft on the ground.
5. In newer King Airs, consider extending the ice vanes just prior to landing, especially if you are planning on using heavy reverse.