Regardless of the A/P (autopilot) system installed in your King Air aircraft, there are some failures common to almost any system. For example:
All these issues are often traced to a tach feedback issue in the primary servo for the affected axis. Most autopilot servos have a tachometer installed internally, the purpose of which is to provide servo rate feedback to the autopilot computer. This negative feedback limits the primary servo speed when responding to error signals commanded by the autopilot computer.
Interesting Fact
Regardless of the autopilot system installed, loss of the tachometer signal results in a reasonably quick oscillation of approximately two (2) cycles per second. So, if you see the yoke rocking back and forth, or the yoke pumping in pitch, or the rudder pedals oscillating at that rate, it is most likely the primary servo for that axis that is the failure.
Granted, it certainly can be other causes as well, but here’s a tip to determine the issue while in flight: If your yoke is rocking or pumping, for instance, and you grab it and steady it and it momentarily stops while you hold it steady, it is almost certainly the tack feedback in the primary servo at fault.
If, on the other hand, you can still feel the A/P trying to rock or pump the aircraft while you hold the yoke steady, then look elsewhere for the root cause. Likely suspects are the autopilot computer, the airspeed sensor or air data computer, and main and/or bridal cable tensions—either too tight or too loose. Also, check the vertical gyro or accelerometer however not all systems will have all these devices as part of the system.
Your autopilot is not behaving correctly. An easy way to split the system in half is to monitor the V-bars while the autopilot is misbehaving, or hand-fly in the same mode following the V-bars. For instance, the autopilot rolls left. If the V-bars indicate a right turn is needed to bring the aircraft back to level, the issue is likely the autopilot computer, servo, or wiring.
If, on the other hand, when the A/P rolls the aircraft left, the V-bars also show a left turn, staying snug behind the symbolic aircraft on the ADI, the issue is likely the flight director computer, vertical gyro, air data, or for pitch issues, the accelerometer.
To put it another way: If the V-bars follow the horizon, it’s the autopilot computer, if it does not, it is the flight director computer.
First of all, most systems require the yaw damper to be functioning before the autopilot can be engaged. If neither one will engage, troubleshoot the simpler yaw damper system, as likely the autopilot will engage once the yaw damper failure is corrected.
Collins APS-65.
There is a simplistic diagnostic routine that can be run both on the ground or in flight to determine the most likely cause of the failure.
Press and release the test button on the FCS-65 autopilot control panel or press and hold when in flight. A successful test will result in the GA (Go Around) annunciator illuminating on the MSP-65 mode select panel and no other mode annunciators being Illuminated. If additional annunciators are illuminated, remember which ones.
See your Duncan Aviation Component Tech Reps for a complete list of failures.
One last thing. If the test comes back with just the GA annunciator lit, but the autopilot will not engage, it is likely due to the yoke wiring between either the autopilot disengage switch and the autopilot computer. A stuck switch or broken wire will indicate to the autopilot that the pilot initiated the disengage; therefore, it is not an issue that results in a fail code.
Releasing the test button in flight or pressing the test button again on the ground disengages the diagnostic test.
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Honeywell SPZ-200A/500 FCS.
No internal diagnostics testing exists with this system. However, a Duncan Aviation avionics technician can back-pin several pins at the SP-200 autopilot computer to check for voltages and determine the state of the various valid discretes required for yaw damper or autopilot operation.
Loaner units may be available to aid in troubleshooting. Contact a Duncan Aviation Component Customer Account Rep for more information.
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Collins FCS-80 FCS.
This system was designed with a very complex engage logic, requiring more than 20 input discretes and test signals to be valid at the correct time in order to engage. Consequently, the most common issue with this system is a lack of engagement. Fortunately, the system failure is most often one of the computers or other units in the system.
At Duncan Aviation, we have found that, in many cases, the best way to troubleshoot this system is by substitution. Loaner units may be available. Contact a Duncan Aviation Component Customer Account Rep for more information
Older King Airs had black plastic bulkhead connectors, which were common failure items. PL-9 is the connector most often used for autopilot-related wiring. Check both sides of the firewall, as these are 3-part connectors.
King Airs are also known for autopilot computer corrosion issues because the computers are located immediately below the brake reservoir. Brake fluid is highly corrosive and can cause fatal damage.
Don’t forget that the King Air is famous for its avionics AC fuses located on J-Box #1, mounted on the forward firewall accessible through the copilot avionics bay door.
Best to check them with a Digital Volt Meter for continuity, as it is difficult to spot a blown fuse through its little window.
And have spares on hand, as you can bet the last person used up all the spares.
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