NAVOMATIC 400A GUIDE

The Navomatic 400A is an autopilot manufactured by Cessna in the early 1970s.
It is a basic unit but very well-designed and simple to use.

Cockpit Panel
-------------

On the Cessna 337G Skymaster, the cockpit panel is located on the center 
pedestal below the throttle levers. It consists of a pitch wheel, push-pull
heading dial (which switches between lateral modes), and three on-off
switches marked AP ON, NAV and ALT.

Turning On and Off
------------------

The autopilot is turned on and off using the AP ON switch. Note that, unlike
some other autopilots, the Navomatic does not necessarily bring your wings level 
and maintain pitch when it is turned on. It does whatever you have set on the 
various controls. So check them carefully before engaging the autopilot.

Lateral Modes
-------------

The push-pull dial controls the lateral modes. It is pulled out using the 
middle mouse button (alt-click on a Mac) and pushed in using the left mouse 
button.

When the push-pull dial is pulled out, the bank angle of the aircraft is
controlled by the dial. When it is centered, the autopilot levels the wings.
When it is turned left or right, the aircraft turns left or right. When rotated
all the way left or right this produces a standard rate turn. The Flightgear
version is a stepped control where each step is 5 degrees of bank.

Note that the dial has a center detent position which prevents it being pushed
in when it is rotated. Return it to wings level before pushing it in.

When the push-pull dial is pushed in, the autopilot either follows the 
heading bug on the directional gyro, or tracks a radial on the first NAV radio.
The NAV switch on the right side of the panel switches between these two 
modes.

Note that when following a VOR radial using the NAV mode, the radial must
be dialled as the omni bearing on the course deflection indicator (CDI)
AND the heading bug on the directional gyro (DG) must be set to the same
heading. The autpilot corrects for wind drift when following a radial, but
you can give it a little more help by setting the heading bug slightly into
the wind, say 5 degrees in a strong crosswind.

Vertical Modes
--------------

The pitch wheel controls the vertical pitch of the aircraft when the autopilot
is engaged. When at the center detent position, the aircraft will be pitched
for level flight. You may also need to adjust pitch trim (large vertical 
wheel next to the gear up/down lever).

The pitch wheel is rotated with the scroll wheel of the mouse. When rolled 
backward, the aircraft pitches nose upward. When rolled forward, the aircraft 
pitches down. Each step on the wheel is 0.5 degrees of pitch. To return to the 
center detent, click anywhere on the pitch wheel with the left mouse button.

There is no indication of the set pitch because there is no indication on
the real thing!

To hold an altitude, switch the ALT switch to the right. Note that this will
only hold the current altitude if the pitch wheel is at the center detent
position.

If the ALT switch is on and the pitch wheel is moved away from the center
detent, the aircraft adjusts pitch as normal. When the pitch wheel is 
returned to center, the new altitude is held.

ILS Approaches
--------------

The Navomatic autopilots do not have a specific approach mode but the 
Navomatic 400A will track an ILS and glideslope. Navigate to the ILS and ensure
the autopilot is in NAV mode (push-pull dial pushed in and NAV switch on) and
holding a suitable altitude (ALT switch on and pitch wheel centered). A green
light appears on the main panel when the glideslope is captured. There is no
need to wait for the glideslope needle to come down before engaging ALT and
NAV modes -- the autopilot will not climb up to the glideslope.

The autopilot does not disengage automatically, so you must disengage by 
switching the AP ON switch to off before landing.

Flightgear Dialog
-----------------

The position of the panel is awkward (but authentic). You can also control
the autopilot using the autopilot dialog and this is laid out and works
exactly the same as the 3D panel.

General Notes
-------------

The Navomatic 400A manual describes how a VOR radial is intercepted and this
Flightgear simulation simulates that. When the course deflection is
greater than 10 degrees (full scale on the CDI), the autopilot will turn
to intercept at 45 degrees. When the deflection goes below 10 degrees, it 
will fly a smooth arc intercept to the radial. (So expect gentle curves to
the radials, not last minute 30-degree bank turns).

With any autopilot there is a "zone of confusion" when you are very close to
a VOR station, e.g. less than 2nm. Expect course swings if you leave the 
autpilot in NAV mode. It is usually a good idea to pull the push-pull dial
out for wings level if you are passing over a VOR station in NAV mode, 
resetting the omni bearing and heading bug to the new course as required.

To avoid unnecessary changes of course, it is recommended to pull the
push-pull dial out for wings level before changing lateral modes.

The autopilot automatically retrims the aircraft for zero elevator when the
autopilot is disengaged so that joystick users don't experience sudden
dips and climbs when they pick up the controls.

Do not try and engage any mode of the autopilot on the ground, other than
for checking it is operable. Return all controls to center after turning
the autopilot off again.

Recommended configuration for takeoff is push-pull dial out (for wings level),
with pitch wheel rotated backward 4-6 steps for 2-3 degrees upward pitch on
engagement. Alternatively, you can take off with the push-pull dial pushed
in, heading bug set to runway heading and NAV switch off. Note that if you do
not move the pitch wheel before takeoff, the autopilot will level your pitch
when you turn the autopilot on.
