In recent months I’ve fielded quite a few calls on the autofeather system in King Airs. In one case I was engaged to assist a shop with unraveling an autofeather mystery. The system was inoperative on one side and they had taken every conceivable step to fix it, with no luck.
As the tech outlined the actions they had taken, it sounded much like what I would have done. They swapped things from side to side. They replaced pressure switches and relays. Maybe they went a little overboard in changing every switch and relay in the system, but nothing fixed it and they were desperate to solve the problem.
It happens. When the source of the problem proves elusive, the tendency is to change everything and hope for a fix. As the story unfolded, there was one thing I kept wondering about – engine torque – is it really getting down to 200 foot-pounds?
Remember: Prop blades are kept in flat pitch by oil pressure. Engine torque drives oil pressure to a pressure switch in the autofeather system. When the oil pressure drops, the prop feathers. But that’s not happening here, even with a new pressure switch. My thoughts were: Either the engine is way out of rig, the torque meter is off … or both.
Sure enough, the torque meter was out of calibration. This prevented the 200 lb-ft switch from activating, so the prop would not feather. This was a brain teaser, I admit. The autofeather system in King Airs is fairly simple and it works great; but to troubleshoot it (or to help your shop troubleshoot it), you really need to know how it works. Accordingly, I’m repeating part of my article, “Autofeather Review” from 2018.
Autofeather Test
On takeoff, the autofeather system arms when the power level reaches 92% N1 or higher. But to test this on the ground in your preflight run up, you’d have to stand on your brakes and hope your fillings don’t pop out while pulling each power lever back one at a time. Fortunately, the “Test” position of your autofeather switch eliminates this problem. In Autofeather Test, the power lever switches in the pedestal (the ones set at approximately 92% N1) are bypassed, which enables you to test autofeather function at a much lower power value. Note: The Test position of the autofeather switch is spring-loaded so that you can’t mistakenly leave it in that mode.
Auto-ignition and Autofeather
Each engine has a high pressure switch on the torque manifold that actuates at approximately 400 to 500 foot-pounds of torque. These pressure switches have a dual function: they turn auto-ignition off and they arm the autofeather system (they don’t activate it, they just arm it). It is common practice to test auto-ignition and autofeather in the same power run-up. At this point in your ground run-up, you would have the auto-ignition switch in the “On” position and you’re holding the autofeather switch down in the “Test” position.
Both the autofeather and the auto-ignition annunciator lights are green. Below 400 lb-ft your auto-ignition annunciators will be on. As the power levers are advanced toward 500 lb-ft, the auto-ignition greens go out and the autofeather greens light up. Most King Air training programs use the phrase “two greens off and two greens on” as a memory technique to teach this relationship. Just don’t expect the autofeather greens to come on simultaneously. The pressure switches on the torque manifold can trigger anywhere between 400 and 500 lb-ft, and engine N1 acceleration is another variable. You want two greens off and two greens on … eventually.
The main thing to remember about autofeather arming is that each engine arms the opposite side. This is where new King Air pilots can get easily confused. The left engine arms the right side autofeather and vice versa. Imagine advancing your power levers and the left autofeather light comes on but the right light does not. You have to fight the instinct to continue advancing your right engine power lever, because at this point, the right engine just armed the left-side autofeather and gave you a green light. In this scenario, the right-side autofeather is not yet armed and the left power lever must be moved further forward. So, if your left light comes on first, continue advancing your left lever until you get the right-side autofeather light. Conversely, if you get the right light first, keep advancing the right lever until the left-side autofeather light comes on. It’s counterintuitive in the beginning, but you’ll get the hang of it.
Autofeather Test Continued
Now you have two greens on, so autofeather is armed on both sides, and it’s time to test the system. Starting with the left side, pull the left power lever back while holding the autofeather switch in the Test position. As you pull back through 400 lb-ft with your left power lever, the right light should extinguish (the right side is now unarmed and cannot go into feather).
At approximately 200 lb-ft of torque, the left prop should feather. A different pressure switch on the torque manifold triggers the feathering. It activates a solenoid on the overspeed governor when the torque drops to 200 lb-ft. It’s oil pressure from engine torque that keeps prop blades in flat pitch. When the oil pressure dumps, the prop feathers. Autofeather function is wired on the same side … in other words, the left engine controls the feathering of the left prop and vice versa. Only the arming of each autofeather system is wired to the opposite side. There’s a good reason for this and I’ll explain that shortly.
Flickering Lights and Oscillating Blades
I am often asked about autofeather lights blinking during test. When a prop goes into feather, particularly while on the ground, it increases the engine torque. In the example above where you just feathered the left prop, if the torque pushes far enough above 200 lb-ft, the left autofeather light will come back on and the left prop will flatten out. Flat pitch reduces torque. If it falls back down to 200 lb-ft, the left annunciator light will go out again and the left prop will feather again.
This flicker of the annunciator lights and oscillating of the prop blade is not unusual during autofeather testing on the ground. But it’s equally normal for that annunciator light to go out and stay out. There are lots of variables (engine rigging, N1 settings, pressure switch adjustments) that influence whether or not you’ll have a blinking annunciator during autofeather test. Either way you are good to go. Now let’s bring the left engine back to speed and do the same with the right engine.
Autofeather Test – Last Step
Once you have brought each engine, one at a time, down to 200 lb-ft and feathered each prop, there’s one more check to do. Assuming that you feathered the left prop first and now you have your right prop in feather, pull back the left power lever and make sure the left prop does not go into feather. Also make sure that the righthand prop comes out of feather. This is a crucial test. It ensures all switches and wiring are operating properly. If, in this last test step, the left prop feathered along with the right, you have a problem. Or, if the right prop doesn’t come out of feather you have another problem. Either way, your autofeather system needs attention.
Test Versus Arm
You are now ready for takeoff, put the autofeather switch in the “Arm” position and take the runway. You go through 500, 600, 700 lb-ft of torque but the autofeather lights don’t come on. Why? Because you have not yet gone through 92% N1. When you pass that N1 threshold, your autofeather annunciators should come on. Both sides are armed and ready to go.
The Test position of the autofeather switch is spring-loaded for a good reason. It makes it impossible to accidentally leave the system in Test mode and risk unintentional arming of the autofeather system at an insufficient power level.
The King Air automatic feathering system allows only one prop to feather at a time; they will never go together. You will recall the left engine arms the right side auto feather (between 400-500 lb-ft in Test and above 92% N1 in Arm) and vice versa. Imagine this: Your left engine fails on takeoff and the left prop goes into feather; it is now physically impossible for your right prop to feather, because the left engine, being well below 400 lb-ft, has unarmed the right side autofeather.
Leaving the Switch in Arm
In my opinion, autofeather is most crucial during takeoff. Some leave the switch armed in cruise, but at FL 250, if you had an engine failure and the switch was off, you’d still have plenty of time to cage the problem engine. On approach, even if the switch is in the Arm position, the system is unarmed as long as you are below the 92% N1. However, if you need to make a go-around and your switch is in Arm, the system will arm as soon as you push the power up high enough.
Maintenance
Where’s the maintenance tip in all of this? How you squawk an autofeather problem can make your mechanic’s job simple or complex. Many a pilot has dropped the aircraft off for maintenance on a Sunday night and realized they forgot to write up their squawks, so they leave a hastily scribbled list in the cockpit that includes “Autofeather inop.” Of course I can troubleshoot from square one and work my way to the root of the problem. But if I get more precise information from the start, the job goes faster and that saves you money.
How about this squawk? “Autofeather tests good on ground but L/H annunciator fails to illuminate on takeoff.” Aha! That sends me straight to the righthand power lever switch in the pedestal. Or this: “Autofeather will not test.” Great! I can verify that the greens are not coming on at 400-500 lb-ft torque, and then I know to zero in on those torque switches. The more specific the information you give to your mechanic, the faster the problem will be diagnosed and fixed. I hope this helps.