Page 20 - Volume 15 Number 4

The bleed valve on a King Air C90’s PT6A-21 engine in the 6 o’clock position. You can see part of the inlet screen to the right.
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. KA
Dean Benedict is a certified A&P, AI with over 45 years of maintaining King Airs. He’s the founder and former owner of Honest Air Inc., a maintenance shop that specialized
in Beech aircraft with an emphasis on King Airs. In his new venture, BeechMedic LLC, Dean consults with King Air owners and operators on maintenance management and supervision, troubleshooting, prebuys, etc. He can be reached at dr.dean@beechmedic. com or (702) 773-1800.
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
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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.
APRIL 2021