King Air 90 Series Maintenance Manual (Rev D2)
61-23-00 (Rev D0)
AUTOFEATHER SYSTEM - DESCRIPTION AND OPERATION
(LJ-362 and After; LW-1 and After, if Installed)
1. Description
WARNING: Stabilized ground operation within the propeller restricted RPM range can generate high propeller stresses
and result in fatigue damage to the propeller. This damage can lead to a reduced propeller fatigue life,
propeller failure and loss of control of the airplane. The propeller restricted RPM range is defined in the
airplane flight manual. Contact the airplane or propeller manufacturer for corrective actions if a propeller
restriction or limitation is violated.
A. The automatic feathering system provides a means of immediately dumping oil from the propeller governor to enable the
feathering springs to start feathering the propeller blades as soon as engine torquemeter oil pressure drops below 6.5 psi at
power settings of 90 percent N1 or greater. The system is actuated by placing the autofeather arm switch in the left subpanel to
the ARM position. This closes the circuit from a 5-ampere circuit in the right subpanel to the power switches mounted on a
bracket in the control pedestal; however, the autofeathering system will remain inoperative as long as the power levers are
retarded below the 90 percent N1 position. When the power levers are advanced to the 90 percent N1 position, a mechanical
actuator connected to each power lever will close its respective switch and complete the circuit to the high and low pressure
switches mounted adjacent to the torque pressure transmitter just forward and above the left-hand exhaust outlet on each
engine. These pressure switches monitor torquemeter oil pressure. The high pressure switch is a single-pole, double-throw
switch that actuates under pressures exceeding 12 psi (Ref. Figure 1).
B. When actuated, the LH high pressure switch provides power from its "C" contact through the contacts of the unenergized RH
arming relay to the coil of the RH arming light relay. This actuates the arming light relay and completes the circuit to the
AUTOFEATHER ARMED light in the annunciator panel. The RH high pressure switch is similarly hooked into the annunciator
panel through the LH arming and arming light relays. The arming and arming light relays for each system are mounted on the
inboard aft side of the first bulkhead forward of the firewall in each nacelle. When the torquemeter oil pressure of an engine
drops below 12 psi, the high pressure switch on that engine opens the circuit to the arming relay and AUTOFEATHER ARMED
light for the opposite engine. The high pressure switch closes the circuit through its "B" contact to the coil of its respective
arming relay, which will remain de-energized as long as torquemeter oil pressure exceeds 6.5 psi because of the open circuit at
the "C" contact of the low pressure switch. When torquemeter oil pressure drops below 6.5 psi, a ground return for the arming
relay is provided through the "B" contact of the low pressure switch on the affected engine. Current through the relay then
energizes the solenoid dump valve on the overspeed governor. The solenoid dump valve immediately reduces governor oil
pressure to zero, permitting the propeller blades to immediately move to the feathered position on the engine in which
torquemeter oil pressure dropped below 6.5 psi. At the same time, the autofeathering system of the other engine is disarmed
through the hookup between the high pressure switches and arming relays of the right and left systems. For testing purposes,
the autofeather arm switch is equipped with a TEST position by which the power switches in the pedestal can be bypassed to
check out the system with the power levers retarded to below the 90 percent N1 position.
Copyright © Textron Aviation Inc. Page 1 of 2
Retain printed data for historical reference only. For future maintenance, use only current data. Print Date: Fri Jul 19 04:38:56 CDT 2024
King Air 90 Series Maintenance Manual (Rev D2)
61-23-00 (Rev D0)
Figure 1 : Sheet 1 : Autofeather Electrical Schematic
Copyright © Textron Aviation Inc. Page 2 of 2
Retain printed data for historical reference only. For future maintenance, use only current data. Print Date: Fri Jul 19 04:38:56 CDT 2024
