Page 19 - Volume 15 Number 4

(Exception: Very old -20s have the valve inside the inlet screen.) The valve closes at approximately 84- 86% N1 (compressor speed, often referred to as Ng). This variable threshold is dependent on outside barometric pressure and compressor efficiency. All compressor bleed valves are operated by P3 compressor discharge air as well as P2.5 air vented from the compressor.
Compressor Bleed Valve Failures
When King Air pilots see low torque coupled with high engine temperature, some will immediately think there is a problem brewing in the hot section. While this might be the case, I have learned through experience that a bad compressor bleed valve can create the same scenario.
Let’s consider the bleed valve in the PT6A-20, -21, -28 and -135A engines, which should be closed by 86% N1. Imagine you are accelerating your engines on the ground, the N1s pass through 86% and suddenly one engine starts running hotter than the other. Additionally, the torque on the hotter engine is not coming up as it should, so you add more fuel to get the torque up to where it should be, but now the temperature is going up even further. Hmmm ... You take off anyway but find yourself fretting over the hotter engine.
Remember this: If your compressor bleed valve is stuck open, you will see low torque and high temperature. More specifically, you will have low torque, high ITT, raised N1 and raised fuel flow. This points to a bleed valve failing to close properly.
Coke Bottle Check
There is a simple and decidedly low-tech way to check out engine bleed valve function on the ground. Although known for years as a “Coke bottle check,” any bottle will do, plastic or glass. Please note: I am
APRIL 2021
not suggesting you actually do this yourself. This is something for your maintenance technician to orchestrate with his team.
He will take a length of tubing about 3/16- or 1⁄4-inch in diameter that will reach from the bleed valve to a safe distance from the aircraft.
The tube is secured very close to the bleed valve and the other end goes in the bottle, which is filled with water. As soon as the engine is started, air bubbles should come out of the tube, indicating the bleed valve is open. Then, the engine is accelerated; once it passes through
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