Trim Tab Control Systems

Trim tabs are provided on the control surfaces to aid in trimming the aircraft during flight. Lateral trim is obtained through operation of a trim tab on the left aileron. A ground adjustable tab is located on the right aileron to compensate for any inherent imbalance about the longitudinal axis of the aircraft. Noseup and nosedown trim is obtained through operation of the trim tabs on the elevators, one trim tab on each elevator control surface. Left and right trim is obtained by operation of the rudder trim tab. During autopilot operation, operation of the ELEV TRIM switch located on either of the control wheels will cause the autopilot to disengage. The autopilot elevator servo will function only when the elevator tab switch is placed in the NORMAL position. All trim tab actuators are driven by 115-volt, single-phase ac motors, except during emergency operation when the elevator trim tab actuator is driven by a 28-Vdc motor.

2.9.2.1 Trim Tab System Controls and Indicators

Trim tab system controls and indicators are shown inFigure 2-50.

Figure 2-48. Rudder Pedal Adjustment Control

Figure 2-49. Control Boost Switch Panel

2.9.2.1.1 Aileron and Emergency Elevator Trim Tab Switch

An aileron and emergency elevator trim tab switch is located on the trim control panel on the flight control pedestal.

It is a recessed, five-position (EMER NOSE UP, EMER NOSE DOWN, OFF, LOWER LEFT WING, LOWER RIGHT WING) toggle switch, with all switch positions other than the OFF (center) position spring loaded to return to the center position upon release of the switch. When the switch is held in the LOWER LEFT WING or LOWER RIGHT WING position, the trim tab on the left aileron control surface is actuated by a tab motor to trim the aircraft laterally. With the ELEV TAB power-selector switch positioned to EMER and the emergency elevator trim tab switch held in the EMER NOSE UP or EMER NOSE DOWN position, the elevator trim tabs are actuated by a tab motor to raise or lower the nose of the aircraft. When the switch is in the OFF (center) position, the electric motors that actuate the trim tabs are deenergized.

The emergency elevator trim tab switch is operative only when the ELEV TAB power selector switch is placed in the EMER position. With the ELEV TAB power selector switch in EMER, 28-Vdc power is supplied to the elevator trim tab control relays, through the emergency elevator trim tab switch, from the essential dc bus through the ELEVATOR EMER TAB CONTROL circuit breaker on the copilot lower circuit breaker panel. When the emergency elevator trim tab switch is placed in the EMER NOSE UP position, it will energize the elevator tab down relay that connects 28-Vdc power to the elevator trim tab dc actuator and will raise the nose of the aircraft. When the switch is placed in the EMER NOSE DOWN position, the elevator tab up relay is energized, connecting 28-Vdc power to the elevator trim tab dc actuator and will lower the nose of the aircraft. The emergency elevator tab actuator receives 28-Vdc power from the essential dc bus through the ELEVATOR EMER POWER circuit breaker on the copilot lower circuit breaker panel.

Figure 2-50. Trim Tab System Controls and Indicators

Two control relays in the aileron trim tab power circuit are energized by the trim tab control switch. The relays eliminate the necessity to route the 115-Vac power required to operate the aileron trim tab actuator through the trim tab control switch. When the switch is placed in the LOWER LEFT WING position, it will energize the tab down relay that connects 115-Vac power to the aileron trim tab actuator and lowers the left wing. When the switch is placed in the LOWER RIGHT WING position, it will energize the tab up relay that connects 115-Vac power to the aileron trim tab actuator and lowers the right wing. The aileron tab motor receives 115-Vac power from the essential ac bus through the TRIM TAB AILERON circuit breaker on the pilot side circuit breaker panel. The relays are actuated by 28-Vdc power from the essential dc bus through the AILERON TAB CONTROL circuit breaker on the copilot lower circuit breaker panel.

2.9.2.1.2 Elevator Tab Switches

Dual NOSE UP, NOSE DOWN, and center off elevator trim tab switches are located on the outboard handgrip of each control wheel. The dual switches on the pilot or copilot control wheels must be operated simultaneously on their respective wheels to provide both power and ground to the control relays. The two sets of dual switches are connected in parallel and either set of switches can control the tab when the ELEV TAB power selector switch is positioned to NORMAL. When either the pilot or copilot control wheel dual switches are in the NOSE UP or NOSE DOWN position, dual relays are actuated to apply power to the elevator trim tab actuator. With the ELEV TAB power selector switch in NORMAL, 115-Vac power from the essential ac bus, through the TRIM TAB ELEVATOR circuit breaker on the pilot side circuit breaker panel, is applied to the actuator. The elevator tab switches on the control wheels are inoperative when the ELEV TAB power selector switch is placed in the EMER or OFF position.

2.9.2.1.3 Elevator Tab Power Selector Switch

An elevator tab power selector switch is located on the flight control pedestal. It is a three-position (NORMAL, OFF, EMER) toggle switch used to select the source of electrical power for operation of the elevator trim tabs. When the switch is in the NORMAL position, 115-Vac power is supplied from the essential ac bus, through the ELEVATOR TRIM TAB circuit breaker on the pilot side circuit breaker panel, to a trim tab actuating motor relay for autopilot or control wheel handgrip switch operation of the elevator trim tabs. In the NORMAL position, the elevator trim tabs can be controlled only from the control wheels. When in the EMER position, the elevator trim tabs can be controlled only from the emergency elevator trim tab switch located on the pedestal. During emergency operation, 28-Vdc power is supplied from the essential dc bus through the ELEVATOR EMER POWER circuit breaker, located on the copilot lower circuit breaker panel, to a trim tab actuating motor that will drive the elevator trim tabs either up or down when the respective elevator trim tab control relay is energized by actuation of the trim tab control switch on the pedestal. When the ELEV TAB power selector switch is in the NORMAL position, the ELEVATOR TRIM TAB CONTROL relays are powered by 28-Vdc from the essential dc bus, through the ELEV TAB CONTROL circuit breaker located on the copilot lower circuit breaker panel. When the elevator tab power selector switch is in the EMER position, the elevator trim tab control relays are powered by 28-Vdc from the essential dc bus, through the ELEVATOR EMER TAB control circuit breaker located on the copilot lower circuit breaker panel. When the ELEV TAB power selector switch is placed in the OFF position, all circuits to the elevator trim tabs are deenergized.

2.9.2.1.4 Rudder Trim Tab Switch

A rudder trim tab switch is located on the trim tab control panel of the flight control pedestal. It is a three-position (NOSE LEFT, OFF, NOSE RIGHT) switch that controls operation of the rudder trim tab motor. The NOSE LEFT and NOSE RIGHT positions are spring loaded to return to the OFF (center) position upon release of the control switch. When the switch is in NOSE LEFT or NOSE RIGHT position, 115-Vac power from the essential ac bus, through the RUDDER TRIM TAB circuit breaker on the pilot side circuit breaker panel, energizes the rudder trim tab motor to position the rudder trim tab and trim the aircraft.

2.9.2.1.5 Rudder Trim Tab Position Indicator

A rudder trim tab position indicator is located on the pilot instrument panel. The indicator is connected to a transmitter mounted on the rudder trim tab actuator housing and indicates to the pilot the degree of rudder trim tab positioning

relative to the rudder control surface. This indicator is energized by 28-Vdc power from the main dc bus, through the TABS & FLAPS POSITION INDICATOR circuit breaker in the aft fuselage junction box. For aircraft 165313 and up, this indicator is energized by 28 Vdc from the main DC bus through the RUDDER TAB circuit breaker in the aft fuselage junction box. The indicator dial face is calibrated from 0_ to L and 0_ to R in increments of 5_ of rudder trim tab travel from the neutral 0_ marking. The needle on the indicator shows the exact angle between the rudder trim tab and rudder surface, and shows the direction in which the trim will act.

2.9.2.1.6 Aileron Trim Tab Position Indicator

An aileron trim tab position indicator is located on the pilot instrument panel. This indicator is connected to a transmitter mounted on the left aileron trim tab actuator, and indicates to the pilot the degree of left aileron trim tab positioning relative to the aileron control surface. This indicator is energized by 28-Vdc power from the main dc bus through the TABS & FLAPS POSITION INDICATOR circuit breaker in the aft fuselage junction box. For aircraft 165313 and up, this indicator is energized by 28 Vdc from the main DC bus through the AILERON TAB circuit breaker in the aft fuselage junction box. The indicator dial face is calibrated from the neutral position of 0_ to 20_

up and 0_ to 20_ down in 5_ increments of left aileron trim tab travel. The needle on the indicator shows the exact angle between the aileron trim tab and the left aileron surface and the direction in which the trim will act.

2.9.2.1.7 Elevator Trim Tab Position Indicator

An elevator trim tab position indicator is located on the pilot instrument panel. The indicator is connected to a transmitter mounted on the elevator trim tab rotary actuator housing and indicates to the pilot the degree of elevator trim tab positioning relative to the elevator control surface. This indicator is energized by 28-Vdc power from the main dc bus through the TABS & FLAPS POSITION INDICATOR circuit breaker in the aft fuselage junction box.

For aircraft 165313 and up, this indicator is energized by 28 Vdc from the main DC bus through the ELEVATOR TAB circuit breaker in the aft fuselage junction box. The indicator dial face is calibrated from the neutral position 0_ to 25_ up or 25_ down in 5_ increments of elevator trim tab travel. The needle on the indicator shows the exact angle between the elevator trim tabs and the corresponding elevator surface, and the direction in which the trim will act.

Note

Trim tab travel is controlled by limit switches set at 6_ nose down and 25_

nose up, and by mechanical stops set at 8_ nose down and 27_ nose up.