The Flap System
The flap system, as previously stated, is an electrically operated and hydraulically actuated system utilising the open centre hydraulic system discussed in the Hydraulic Power systems section of this course.
The flaps are operated by the FLAPS position switch which is mounted on the lower right hand side of the instrument panel next to the centre pedestal.
FLAPS selector switch
The selector switch has three detent positions for easy selection of the flaps by the pilot, these positions are:
- Takeoff - 10 degrees
- Approach - 20 degrees
- Landing - 30 degrees
The flaps can be selected right down to the APPR position up to an IAS of 200 KIAS. Beyond the APPR position flap selection must be delayed until 180 KIAS.
The flaps themselves are made up of two fowler flap segments on each wing, being actuated by mechanical linkages which are driven by a hydraulic actuator.
The outboard flap segments are mechanically linked to the main inboard sections and are designed to move at a reduced rate, i.e. when fully extended the inboard flap segments will be at 30 degrees deflection whilst the outboard segments will only be at 20 degrees.
The flap system works in the same manner as the landing gear system.
With the flaps UP, selecting T/O, APPR, LAND or any intermediate position the flap control valve and the bypass valve are energised together.
The bypass valve closes stopping hydraulic pressure from returning to the reservoir causing hydraulic pressure to build.
Simultaneously the flap control valve opens allowing the hydraulic pressure to flow in the flap system causing the hydraulic flap actuator to move the flaps in the selected direction.
Once the flaps reach the desired position, electrical limit switches de-energise the flap control valve to the neutral position and open the bypass valve which allows hydraulic fluid to flow back to the reservoir.
Retraction of the flaps occurs in the same manner only the flap control valve directs the hydraulic fluid to the opposite side of the flap actuator.
When the flaps are selected the HYD PRESS ON light will illuminate indicating that the hydraulic system is pressurised and the flap position pointer should be observed for movement as well as visually checking outside for movement of the flaps.
Flap Position Pointer
If the HYD PRESS ON light fails to illuminate the bypass valve has not closed, therefore the hydraulic system will not pressurise.
If the light comes on and fails to extinguish after the flaps have reached their selected position, the bypass valve failed to open causing hydraulic pressure to remain in the system. The usual procedure here would be to pull the flap circuit breaker, but please refer to the Abnormal procedures section of the flight manual.
A landing gear warning horn is provided and is used to indicate aurally to the pilot of an abnormal condition.
The warning horn is controlled by the power levers and/or the flap selector and will sound intermittently when either:
- The gear is not down and locked and either power lever is retarded below approx 1/8 inch from the flight idle detent, or
- The landing gear is not down and locked and the flap selector is positioned beyond the APPR position.
The warning horn can be silenced by pressing the HORN DISABLE button on the gear control panel except when the flaps are selected beyond the APPR setting with the gear not down and locked.
The horn will be reset once the power levers are advanced further than 1/8 inch forward of the flight idle detent after being silenced by the HORN DISABLE button.
The HORN DISABLE button
The horn will also sound if the landing gear switch is positioned to the UP position with the aircraft on the ground.
Note: It is strongly suggested that this not be tried however and is not required to be checked during the preflight walk around procedure.
The warning horn can be tested prior to flight by pressing and holding the PRESS TO TEST button on the annunciator panel with the battery switches on.
Retard the power levers individually to the flight idle detent and then forward of the 1/8 inch position again to ensure that both power lever switches are working correctly.
The flap selector switch will also activate the horn whilst holding the PRESS TO TEST button by lowering the flap selector beyond the APPR flap setting.
Correct function of the landing gear warning horn is a safety item and must be checked prior to flight.
PRESS TO TEST button located on the annunciator panel