Of course, I'm correct...use of ice vanes (inertial separators) is nonsense, unless the runway surface is contaminated....gravel, dirt, loose snow, etc....especially, when maximum performance is necessary off of shorter runways and/or obstacles.
Positively confirmed via discussions recently with a senior FAA ops inspector.
Actually, no. That's the part you're wrong about. You're correct about adhering to manufacturer recommendations...but then you branch off and try to support antique understanding not with current manufacturers recommendations...but with a "discussion" with an operations inspector.
You're apparently not familiar with the Raisebeck separators, either, with which much of the King Air fleet today is equipped, which produce no loss of torque and do not interfere with oil cooling...and for which there is no restriction on operation.
So far as asking a FAA inspector...who has no authority to interpret regulation (and doesn't represent the aircraft or engine manufacturer), it's a nonsensical reference. You might as well have asked Fred Flintstone. The inspector is not the aircraft flight manual, and the inspector is not the aircraft type certificate data sheet. With this in mind, you've already tried to establish your authoritative stance based on having "flown the first commercial king air on the west coast," and now it's based on having had a "discussion with a senior FAA ops inspector."
Why not simply turn to actual material (AFM limitations, for example) instead of ancient history and a talk with an irrelevant source?
Intertial separation is also useful in flight in conditions other than icing, such as the previously mentioned flight in ash, heavy smoke, and debris.
The original poster asks:
Is there a benefit in having the ice vanes open during a normal takeoff/landing on a gravel strip or on a sealed runway in temperatures above freezing?
Yes, there is a benefit, primarily found on the gravel strip. However, one should remember that ice isn't simply found below freezing. One may be departing in conditions above freezing and still encounter engine induction icing and airframe icing, especially when temperatures are near freezing, standing water or slush is on the runway, or one departs into visible moisture (such as low clouds or fog).
The PT6 uses a large screen over the inlet (which is at the back of the engine), which is only useful for stopping debris big enough to have it's own part number. For this reason, many PT6 installations use inertial separation prior to air reaching the engine inlet screen...far easier to simply separate debris, ice, moisture and contamination, than block it with a screen. Some installations also use filters or additional screens prior to the engine in order to prevent compressor contamination or engine damage. This is more commonly found on aircraft which operate in an environment that is at a high risk for contamination (common on ag and firefighting aircraft, for example), but not on aircraft such as the King Air.
Some PT6 installations don't use the vanes at all.
When operating the ice vanes, one needs to be cognizant of the changes in engine parameters while moving the vanes...both when extending, and retracting them. In some installations, the vane moves a door under the nacelle, which opens much like a cowl flap and causes drag. In other installations this is not the case. In some installations a slight reduction in torque and sometimes an increase in temperature will occur, but in other installations this is not the case. One should know what to look for to verify the position of the deflectors beyond the primary indications. I have experienced separation of the deflector door which caused a partial blockage of the air induction and required completing the flight (a divert to a maintenance base) on partial power. No primary cockpit indication occurred of this event, which happened shortly after takeoff and entry into weather. My indication first by watching changing engine parameters, and then by a low hooting which developed from an incipient compressor stall.