I see a lot of questions about the King Air’s autofeather system in online discussions. It’s not just the newly minted King Air pilots that are asking. Many a King Air pilot with plenty of hours under their belt have had the occasional “synapse lapse,” so I thought a basic review of autofeather would be in order.
The Haves and Have Nots
Not all King Airs have it. Back when I was corporate shop foreman at Beech West in Van Nuys, California, autofeather was not a standard feature. The sales staff ordered all new King Airs from Wichita without it. When they sold the aircraft, they convinced the buyer to add autofeather – this was a major job. You have to remove the leading edges and rip into the nacelles and the interior to run the necessary wiring. Although it kept the shop plenty busy, I thought this was a foolish way to sell a King Air. I never got used to tearing into a brand-new airplane.
Eventually, I got the sales team in Van Nuys to order King Airs from Wichita with autofeather every time. Later, a group of us in the field convinced the factory to add the autofeather wiring into the master wiring loom; this made subsequent installation of autofeather a much more efficient process.
Over time, autofeather became a standard feature, but there were plenty of King Airs built without it, and many of those still don’t have it. This is something to keep in mind if you’re shopping around in the used King Air market – don’t assume they all have autofeather.
If your King Air does not have autofeather, you can research the serial number to see if the necessary wiring was installed at the factory when your aircraft was built. If the wiring is there, then adding autofeather would involve the installation of some switches and cannon plugs in the pedestal, new torque manifolds and pressure switches, and some annunciator panel revisions.
Autofeather is a nifty thing to have, and if an engine fails during takeoff, it is a godsend. Feathering the propeller on the failed engine is one less thing to worry about in a clutch situation.
Autofeather Test
On takeoff, the autofeather system arms when the power level reaches 92 percent N1 or higher. But in your preflight run-up on the ground, it’s a pain to run each engine up to takeoff power for an autofeather test. 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 that problem.
In Autofeather Test, the power lever switches in the pedestal (the ones set at approximately 92 percent N1) are bypassed. This enables you to test the autofeather function at a much lower power value.
Autoignition and Autofeather
Each engine has a high-pressure switch on the torque manifold that actuates at approximately 400-500 foot-pounds (ft-lbs) of torque. These pressure switches have a dual function: they turn your autoignition off and they arm your autofeather (they don’t activate the system, they just arm the system). It is common practice to test autoignition and autofeather in the same power run-up. At this point in your ground run-up you would have the autoignition switch in the On position and you’re holding the autofeather switch down in the Test position.
Both the autofeather and the autoignition annunciator lights are green. Below 400 ft-lbs your autoignition annunciators will be on. As the power levers are advanced toward 500 ft-lbs, the autoignition greens go out and the autofeather greens come on. 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 ft-lbs, 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 easily get 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. 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-hand autofeather light. Conversely, if you get the right light first, keep advancing the right lever until the left light comes on. It’s counterintuitive in the beginning, but you soon get the hang of it.
Autofeather Test Continued
You have two greens on, so autofeather is armed on both sides; now it’s time to test the system. Pull one power lever back while still holding the autofeather switch in the Test position. Let’s start with the left side. As you pull back through 400 ft-lbs with your left power lever, the right light should extinguish (the right side is now unarmed and cannot go into feather). At approximately 200 ft-lbs 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 ft-lbs. Prop blades are kept in flat pitch by oil pressure. 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 come to it shortly.
Flickering Lights and Oscillating Blades
I am often asked about autofeather lights blinking during test. Well, when a prop goes into feather, particularly while on the ground, it increases the engine torque. In the example where you just feathered the left prop, if the torque pushes far enough above 200 ft-lbs, 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 ft-lbs, 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 ft-lbs to feather 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, there’s one more step – pull back the left power lever and make sure the left prop does not go into feather. Also make sure that the right 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 and either way your autofeather system needs attention.
Test versus Arm
You are ready for takeoff; you put the autofeather switch in the Arm position and take the runway. You go through 500, 600, 700 ft-lbs of torque, but the autofeather lights don’t come on. Why? Because you have not yet gone through 92 percent N1. Once you pass that N1 threshold your autofeather annunciators should come on and 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.
A King Air’s automatic feathering system is designed to allow only one engine to feather at a time; they will never go together. You will recall that the left engine arms the right side autofeather (between 400-500 ft-lbs in Test and above 92 percent N1 in Arm). If the left engine has failed on takeoff and the left prop goes into feather, it is now physically impossible for the right prop to feather. The left engine, being well below 92 percent N1, 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 will not be armed as long as you are below the 92 percent N1 threshold. However, if you need to perform a go-around and your switch is in Arm, the system will be armed as soon as you push the power up high enough.
Maintenance
Where’s the “Maintenance Tip” in all of this? Well, 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 you save 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 R/H power lever switch on the pedestal. Or this: “Autofeather will not test.” Great! I can verify that the greens are not coming on at 400-500 ft-lbs 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.
Meanwhile, enjoy the heck out of your King Air.
Note: This article originally appeared in the November 2013 issue of King Air and was republished (with minor revisions and corrections) at the request of new magazine subscribers regarding this subject.
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