SECONDARY FLIGHT
CONTROLS
FLAPS
General
The single-slotted flaps are electrically con-
trolled and hydraulically actuated on all air-
planes except SNs 23-003 through 23-029,
which have mechanically positioned flap control
valves. The left and right flaps are interconnect-
ed by cable to prevent flap asymmetry.
Position switches mechanically connected to
each flap provide flap position information to
the landing gear and stall warning systems.
Except on model 23 airplanes, a warning horn
sounds if the flaps are extended beyond 25º and
the gear is not down and locked.
Flap Selector Switch
The flap selector switch is located on the right
side of the throttle quadrant. A schematic of the
flap system is shown in Figure 15-8. On SNs 23-
003 through 23-089 (unless modified by AMK
73-5), there is a single flap actuator.
Flap Position Indicator
A FLAP position indicator is located on the cen-
ter instrument panel. Flap position signals are
electrically transmitted to the indicator which is
DC powered through the TAB & FLAP PN
circuit breaker. The indicator will read UP with
loss of electrical power, regardless of actual
flap position.
Operation
When the flap selector switch is placed in the
DN position, a hydraulic control valve is posi-
tioned to direct pressure to extend both flap actu-
ators. The flaps may be stopped at any interme-
diate position by placing the selector switch to
the center, neutral position. This removes electri-
cal power from the control valve, which moves
to the neutral position, trapping fluid between
the control valve and the actuators to hold the
flaps in the selected position. Full flap extension
is 40º.
When extended, the flaps are protected from
excessive airloads by a 1,650-psi relief valve in
the downline. When the relief valve opens,
hydraulic pressure is relieved.
If the flap selector switch is left in the DN posi-
tion, the control valve will remain energized,
hydraulically maintaining the flaps in that posi-
tion. If hydraulic pressure is lost under these
conditions, a check valve near the control valve
traps pressure between the control valve and the
actuators, holding the flaps in the down position
(SNs 24-297 and subsequent, and SNs 25-181
and subsequent).
Placing the selector switch in the FLAP UP posi-
tion electrically energizes the control valve to
direct pressure to the retract side of both actuators.
The flaps on airplanes with the Howard
Raisbeck MKII wing are modified for greater
lift, and maximum extension is reduced to 38º.
They operate as described above. Optional pre-
select flaps may also be installed on these
airplanes.
Operation (Preselect Flaps)
When the flap selector switch is placed in the
desired position (10º, 20º, DN), the flaps are
extended by hydraulic pressure. If increased air-
speeds impose excessive airloads on the flaps, a
relief valve in the downline opens, allowing par-
tial retraction of the flaps. However, the flaps
will return to the selected position when the air-
load is reduced.
If the flaps move in either direction from the pre-
selected position, a series of switches activates
the flap control valve to hydraulically return the
flap to the preselected position.
When the selector switch is moved toward the
FLAP UP position, an intermediate stop is
encountered at the 20º position to facilitate
retraction in a go-around situation. Further
movement of the selector switch toward FLAP
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
15-12
FOR TRAINING PURPOSES ONLY
