The orbiter nose wheel is steerable after nose wheel touchdown
at landing. The nose wheel is electrohydraulically steerable through
the use of the general-purpose computers and the commander's or
pilot's rudder pedals, in conjunction with the orbiter flight
control system in the control stick steering mode. In addition,
the nose wheel may also be steerable through the use of the commander's
or pilot's rudder pedals in the direct mode.
Nose wheel steering is advantageous to allow positive lateral
directional control of the orbiter during the rollout phase of
a mission in the presence of high crosswinds and blown tires.
Recent modifications of the nose wheel steering system have been
incorporated to allow a safe high-speed engage of the nose wheel
The first in the series of changes was to redefine the NWS switch
on panel L2. The forward position of the switch now activates
the direct mode of nose wheel steering, the center position activates
the GPC mode of nose wheel steering, and the aft switch off position
deactivates all nose wheel steering. A flexible handle extension
was also added to the switch handle.
The commander selects and activates the GPC mode while performing
the pre-entry checklist of cockpit switches while his eyes and
attention are inside the cockpit. No other flight crew action
is required in order to engage active nose wheel steering at the
time of nose wheel touchdown. Other added features provide assurance
that the nose wheel will be positioned straight ahead at the moment
of nose wheel touchdown, and the final enable signals in the GPC
mode are sequenced at nose wheel touchdown and begin active nose
wheel steering at that time.
During derotation and nose wheel touchdown, the flight crew is
looking out the windshield and at the head-up display. If the
commander chooses to select the direct mode of nose wheel steering
or deactivate all nose wheel steering by selecting off without
visual verification of the actual switch position, he can sweep
his left hand over the switch (forward motion for direct or aft
motion for off ) and be certain of the position of the NWS switch
without looking at the switch.
Only hydraulic system 1 supplies hydraulic system supply pressure
to the nose wheel hydraulic actuator for steering in either the
GPC or direct mode. If hydraulic system 1 supply pressure is unavailable,
the commander or pilot can apply the left and right main landing
brakes by applying toe pressure on the rudder pedals differentially,
which allows directional control by differential braking. The
nose wheel steering actuator limits the motion of the nose wheel
to plus or minus 10 degrees and also prevents nose wheel shimmey.
When the NWS switch is set to either GPC or direct , the nose
wheel steering system fault detection logic detects (1) the interruption
of hydraulic system 1 supply pressure; (2) an open or short in
the nose wheel steering servovalve circuitry; (3) an open or short
in the position feedback, rate position error; (4) an open or
short in the command transducer; and (5) broken linkage or loss
of electrical power. At this time, the NWS fail C/W light on panel
F3 is illuminated, and the nose wheel steering reverts automatically
to the caster mode.
General-Purpose Computer Mode. The NWS switch on panel L2, positioned
to GPC, enables the nose wheel steering solenoid control valves,
which supplies hydraulic system 1 pressure to the nose wheel steering
servovalves. In addition to the GPC mode selection of the NWS
switch, the flight control system roll/yaw CSS push button switch/light
indicator on panel F2 or F4 must be depressed to enable GPC mode
steering. When either push button switch/light indicator on panel
F2 or F4 is depressed, a white light is illuminated within the
When the commander or pilot positions the rudder pedals in the
GPC roll/yaw CSS mode, the rudder pedals command position is appropriately
scaled within the GPC's software and transmitted to a summing
network, along with accelerometer inputs from within the flight
control system. The accelerometer inputs are utilized to prevent
any sudden lateral deviation of the orbiter's velocity vector
direction. From this summing network, a nose wheel steering command
is sent to a comparison network as well as to the steering servo
The three new steering position transducers in the unit added
on the nose wheel strut receive redundant electrical excitation
from the new steering position amplifier (added to the middeck
ceiling), which receives redundant electrical power from data
display unit 2 on the flight deck.
Each of the three new transducers transmits nose wheel position
feedback to a redundancy management mid-value-select software,
which then transmits a nose wheel position signal to the comparison
network. The orbiter nose wheel commanded and actual positions
are compared for position error and for rates to reduce any error.
Absence of an error condition will enable nose wheel steering
after weight on the nose gear is sensed in the software. Weight
on the nose gear requires weight on all three landing gear and
a nose-down orbiter attitude. The enable signal permits hydraulic
system 1 pressure to be applied to the nose wheel steering actuator.
If hydraulic pressure is below 1,350 psi, the actuator remains
in the shimmy damp mode, and a failure is annunciated to the NWS
fail C/W yellow light on panel F3. If hydraulic system 1 pressure
is above 1,350 psi, the actuator is configured to the GPC CSS
mode to position the nose wheel utilizing the commander's or pilot's
If more than one of the three new position transducer feedback
signals are lost, the comparison logic switches hydraulic system
1 supply pressure off to the nose wheel steering actuator, and
pressure sensing immediately inhibits nose wheel steering in the
GPC CSS mode automatically. Nose wheel steering will then revert
to a nose wheel caster mode and the NWS fail yellow C/W light
will be illuminated on panel F3. This redundancy in the GPC CSS
mode provides immediate protection against any undesired lateral
nose wheel response during nose wheel steering.
If nose wheel steering has failed, the NWS switch on panel L2
positioned to off will extinguish the NWS fail C/W yellow light
on panel F3, unless the GPC CSS mode comparison circuits caused
the light to be illuminated. When the NWS switch is positioned
to off, hydraulic system 1 supply pressure to the nose wheel steering
system is removed, allowing the nose wheel to caster.
Direct Mode. When the NWS switch on panel L2 is positioned to
direct, the nose wheel steering solenoid control is enabled after
weight is sensed on the nose gear. This supplies hydraulic system
1 supply pressure to the nose wheel steering servo system.
When the commander or pilot positions the rudder pedals in the
direct mode, the rudder pedals command position is shaped to provide
a less sensitive rudder pedal command in the midrange and provides
steering commands directly to the nose wheel steering servoactuator,
bypassing the GPC software altogether.
The two additional steering position transducer/amplifier boxes
are from Honeywell, the NWS box was modified by Sterer, and the
DDU 2 modification was made by Collins.
A new additional nose wheel steering system design is expected
to be available in late 1988 or early 1989.