A.L.1 PART 1 - DESCRIPTIVE
before the controls revert automatically to Manual, but even if no
control movement is made, accumulator pressure will not be main-
tained for a long period due to seepage through the hydraulic com-
ponents. With some types of hydraulic failure immediate reversion
to Manual will result.
21. Controls and Indicators
(a) Pre-mod. 452 aircraft
Two ON/OFF switches (44) are on the instrument panel and control
electrically the hydraulic cocks, one for the aileron circuit and the
other for the elevator circuit. When the engine is running and a switch
is selected ON hydraulic pressure is fed to that circuit and to the pawl
which engages in the jack piston rod.
(b) Post-mod. 452 aircraft
When mod. 452 is embodied a revised wiring and switching arrange-
ment is applied to the aileron and elevator pawl release units so that
Manual is automatically selected if any pawl disengages even though
momentarily. The aircraft can therefore be flown only with Power
correctly engaged or in Manual. The two ON/OFF switches of pre-
mod. 452 aircraft are replaced by two ganged MANUAL-Off-
POWER selector switches, which are spring-loaded to the central
(off) position. Deflection upwards to POWER isolates the fail safe
circuit to allow initial engagement or re-engagement. Deflection
downwards immediately selects Manual.
(c) All aircraft
(i) Two magnetic indicators (43), one for the aileron circuit and the
other for the elevator, are mounted beside the switches and show
black when the piston rod locking pawls are correctly engaged
and white when the appropriate pawl is disengaged or incorrectly
engaged, or alternatively when electric power to the indicator is
(ii) An audio warning over the pilot's headset warns the pilot if the
pump is losing pressure. Further warning is given by the red
warning light and the reading of the hydraulic pressure gauge.
(d) Manual emergency selector buttons (Mod. 502)
When Mod. 502 is embodied, two yellow and black striped push-
buttons are fitted at the bottom left hand corner of the instrument
panel to permit the emergency selection of Manual of either ailerons
or elevators or both, should the aircraft electrical system have failed.
When either button is pressed an electrical supply from an indepen-
dent dry battery is connected to the appropriate selector cock and
the associated controls then revert immediately to Manual.
NOTE.-These buttons must not be pressed if normal electrical power
is available as in this event it is likely that the control circuit
fuse will be blown.
22. Elevator feel and trim
(a) The air loads on the elevator are resisted entirely by hydraulic jack
effort, no load being felt on the control column. To provide control
feel, a spring is fitted in the control circuit between the control column
and the variable incidence tailplane. The spring gives an artificial feel
to the elevator control circuit by imposing a force proportional to stick
deflection but not airspeed (i.e. air loads).