Page 20 - Volume 12 Number 1

All aircraft oil systems contain the vernatherm valve, the pur- pose of which is to control oil temperature by directing oil to either flow through the passages of the oil cooler or to bypass around them. During a typical start, for example, the oil cooler bypasses until oil temperature starts coming up. In the development stages of the 200, it was found that the location of the oil cooler – in conjunction with the then new design of the cowling – did not permit sufficient oil cooling under hotter conditions due to insufficient airflow. So, the oil cooler door was added to permit increased air flow across the cooler’s fins. You are correct, the oil temperature itself works a mechanism that overcomes the spring that is tending to hold the door closed. You have probably observed that the door may be wide open at the start of a lunch break, but then is closed an hour or two later. (When it’s closed, you can pull it open with your fingers.)
I wish I understood and could present the exact working of the mechanism better, but I do not have that detailed knowledge. All I can state is that once the vernatherm is directing all oil through the cooler – the bypass going closed – now the next step is to start opening the door.
We have various models of the C-12, but all are with the PT6A-42 engines. Aircraft equipped with the three-bladed prop system use TGT for the engine temperature gauge and the aircraft equipped with four bladed props use ITT on their gauges. ITT versus TGT – why are these different when they both get their temperature sensing at the same position in the engine?
If I recall correctly, the very first C-12 models did indeed mark the temperature gauge as TGT (Turbine Gas Temperature). It was “funny” in that those models that first appeared in 1976 were supposed to be “off the shelf” standard 200s,
yet they incorporated many, many, changes that the folks at Ft. Rucker desired and this was one of them. As time passed, the later C-12 models became much more standardized with the civilian ones and the old familiar ITT came back into use. Exactly when that happened, I do not know. My guess would be that four-blades versus three blades has nothing directly to do with the TGT/ ITT debate, but rather merely shows that most of the later models were built with standard four-blade props. If a previous model were upgraded to four-blades, I speculate that TGT would remain.
In reference to the autofeather system, why was 90 percent N1 chosen as the setting for when the system becomes armed versus any other setting?
The whole purpose of those left and right power lever switches is to allow the autofeather system to differentiate between a desired and commanded power reduction – as should happen when the power lever is retarded – versus an un- commanded power reduction that happens when the engine rolls back even while the power lever has not been retarded.
N1 and power go hand-in-hand, but is not even close to a linear relationship. At sea level, 90 per- cent N1 typically is closer to 50 to 60 percent power, not 90 percent power. By using this value, it almost guarantees that there will not be “nuisance” activations of feathering. Expressed another way, when less than 60 percent power is being carried, almost always flight conditions would not be as challenging as when very high power was required, i.e., takeoff and go-around.
I should probably add that if a significantly higher N1 switch setting were specified, then it increases the chance that autofeather would not arm during a low-altitude, cold-day takeoff. The values that I quoted above are ones applicable to sea level where 100 percent of rated engine
18 • KING AIR MAGAZINE
JANUARY 2018