When you reach max power during takeoff, you’ve got your eye on the ITT and torque. If ITT is high but torque is lagging, you might worry that a hot section problem has developed in the engine. Before leaping to that conclusion, however, consider the compressor bleed valve(s).
All PT6 engines have at least one compressor bleed valve installed just forward of the engine inlet screen, some models have two. These valves (called Bleed Off Valves, or BOVs by many mechanics) serve to vent excess compressor air not needed in the combustion chamber and to prevent compressor stalls.
I don’t want to insult anyone’s intelligence here, but I want to make sure we are all on the same page. If you live and breathe all things PT6, bear with me for a second. The bleed valves discussed herein have nothing to do with bleed air taken off the engine to run the pressurization and pneumatic air systems.
The PT6A-20, -21, -28 and -135A engines have just one bleed valve located at the 6 o’clock position forward of the engine inlet screen. (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 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 ¼-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 the N1 threshold for that particular valve, the air bubbles should stop, indicating that the bleed valve has closed. If the bubbles slow down but do not stop entirely, that indicates the valve isn’t closing all the way and should be changed. Obviously, if there is no change in the bubbles, the valve is stuck open.
It’s extremely rare for these valves to be stuck closed when they should be open. I’ve only seen it one time in my 45 years of King Air experience, but if you’re getting compressor stalls on the ground, have your engine bleed valve(s) checked.
Two-valve Engines
The PT6A-41 and -42 engines have two bleed valves. The low-pressure valve closes at approximately 65% N1; the high-pressure valve closes at approximately 92% N1. When the rear cowls are open, you can see these valves. If you are standing by the rear cowls and facing forward, the low-pressure valve will be in the nine o‘clock position and the high-pressure valve will be in the 3 o’clock position, on each engine.
The -41s and -42s are slightly more difficult to troubleshoot because of the two-valve setup. For example, if the low-pressure bleed valve fails, you might not see much of a spike in temperature or N1 while the engines are between 70-80% N1, which is well past the 65% N1 threshold for the low-pressure valve. Your temp and N1 might be a little higher than expected on the problem engine, but nothing outrageous; however, when you get the engines above 93% N1, it will become very noticeable on that side. Because of this, you might automatically suspect the high-pressure valve is your culprit, but in actual fact, if either bleed valve is stuck open, you will see low torque, high ITT, high fuel flow and high N1. The higher you go N1-wise, the greater these anomalies will be.
In order to discern which valve is bad, your mechanic can do a coke bottle check on each one. On King Air 200s, the bleed valves vent through the cowls so the tubing can be attached there. If the first valve that is checked is found stuck totally open, you still need to check the other valve because it may be only partially closing. With two tubes and two bottles, both valves can be checked on one run.
One Valve on the -60As
For you 300 and 350 drivers, Pratt & Whitney went back to a single bleed valve on the PT6A-60A engines, so there’s only one valve to be checked. The bleed valve on the -60As closes at approximately 92% N1. It is located in the 3 o’clock position on each engine; and, like the 200s, the cowl is vented to allow the bleed air to escape.
Fortunately, removing bleed valves for repair or exchange is an easy and straightforward process on the 200 and 300 series King Airs. The job is a tad more involved on the 90s and 100s simply because access is a little more complex at that 6 o’clock position.
In summary, if lower torque gets your attention, look also for higher N1, higher ITT and higher fuel flow. If you see these things going hand in hand then inform your shop that you suspect an engine bleed valve problem and let them know what you have observed.
If you truly have a hot section problem, you will see low torque, high ITT, high fuel flow and – here’s the kicker – N1 slower than it should be. So, before jumping to the wrong conclusion with a low torque situation, take note of the other parameters, and then you will have a better idea of what is going on with that engine.