Bendix/King KAP140 Autopilot

The Bendix/King KAP 140 Two Axis/Altitude Preselect Autopilot System is the autopilot of the default Cessna 172, controlling the elevator and ailerons.

Quick Guide

Normally the autopilot boots as soon as the electrical system has power. It is commonly wired to an electrical avionics bus and secured by a dedicated breaker; if it stays dark, check power, avionics bus switches and the breaker.

1. To activate the autopilot in wings level (ROL) and vertical speed (VS) modes press the AP button. The autopilot will try to keep the wings level by keeping the turn rate at zero. The autopilot will also try to maintain the vertical speed at activation. Use the UP and DN buttons to set the desired vertical speed.
2. With the autopilot active you can use the HDG button to toggle between wings level (ROL) and heading select (HDG) modes. In heading select mode the autopilot will try to maintain the heading selected by the heading bug on the directional gyro.
3. Use NAV button to toggle between navigation mode (NAV) and wings level (ROL) mode. NAV mode is flying to NAV1 or GPS. That is one of the Heading modes in KAP140 that direction when heading bug OBS operated. Please be careful.
4. Toggle other mode and approach (APR) mode when APR button pushed and following marker beacon, VOR, GPS and ILS (localizer and glide slope) for automatic approach. This mode is recommended for instrument approach.
5. The REV button enables the back course mode having the autopilot flying away from the runway. This mode is like APR mode except that the direction is away from the localizer (LOC) and that glide slope (GS) is not used.
6. Use the ALT button to toggle between vertical speed (VS) and altitude hold (ALT) modes. In altitude hold mode the UP and DN buttons change the altitude by 20 feet per press.
7. The ARM button enables altitude preselect by the rotary knob using procedure below, pushing it again disables altitude preselect.
   1. Input the current atmospheric pressure using the BARO button and rotary knob
   2. Check that the display is showing altitude and set your desired altitude, using the rotary knob.
   3. Set your desired vertical speed using UP and DN button.
   4. Press the ARM button that is enable ARM mode.
8. The BARO button sets the atmospheric pressure. When the BARO button is pushed, enter desired atmospheric pressure using the rotary dial/knob. When pushed long, it switches the setting display to hPa (and back) (hint: you can do InHG<>hPa conversion easily with this)
9. Press the AP button to deactivate the autopilot. The horizontal and vertical modes can not be activated independently.

Please read the Pilot's Guide for complete instructions on the use of the KAP140 Autopilot system.

Limits

• Not certified for use below 200ft AGL, below 80 or above 160 knots IAS or when alternate static port is active
• Only activate when flaps are retracted.
• Do not override the autopilot with flight controls, instead deactivate it temporarily to make manual adjustmens
• Do not activate the autopilot near the ground (takeoff, landing) or at low speeds: it will mess up and may crash you into the ground.
• It also has no autoland capability, so it can't land you based on ILS signals.
• Don't activate it when in an unstable or mistrimmed flight attitude. Tough it will try(!) to stabilize the plane, this is unsafe.
• It will not rescue you out of a stall, so watch the airspeed regularly. If the plane slows down, the autopilot will increase pitch to maintain the VS/ALT mode ordered, putting you into a stall. Disengage the autopilot and manually stabilize your flight.
• Altitude catching only works if you are flying towards the desired setting. For example, if you are at 1500ft, entered 2000ft as target and then descend, it will not intercept but fly you into the ground. Similarly, if you enter an altitude below you and accidentally climb, you will eventually reach the planes service ceiling and stall. The "ALT ARM" mode does not mean "bring me to that altitude".
• Like with altitude interception, intercepting a VOR radial or ILS will not work, if the heading bug was not aligned or if you try to intercept from a custom angle (ie. engaging NAV mode from ROL).

Altitude alert (beeping sound)

A nice feature for assisting manual flights is the altitude alert. This is the aural beeping alert when you get near the selected altitude preset:

• When getting near 1000ft of the selected altitude, it starts to beep five times and show a steady "ALERT" right below the altitude.
• When intercepting the altitude, the ALERT annunciator vanishes if you get 200ft near, and will shortly flash up when crossing the selected altitude to signal it "catched on".
• When now exceeding the +-200ft band, it will alert by flashing and beeping.

This feature just needs a calibrated baro setting and a selected altitude, thus it is also active when using the modes utilizing altitude preset described below.

Example workflow

This example tries to show you how to deal with the autopilot to achieve common tasks. There are more advanced techniques to achieve with the autopilot, but they are out of scope for this quick introduction. Please refer to the 'KAP 140 Pilot's Guide' which is linked below.

For better familiarization it would be good to follow trough the following guide inside flightgear.

Note  Be aware of procedure differences when you have an Horizontal Situation Indicator (HSI) installed in the aircraft: In this case, the KAP 140 will not have an automatic 45°-Intercept for NAV/APR/REV modes when switching from HDG-mode and you need to select the desired intercept course manually by turning the heading bug to the desired intercept course.

Before-takeoff preparations

• For the HDG/NAV mode it is important to calibrate the Directional Gyro (DG) to the magnetic compass (with running motor, so the DG has power): read the magnetic compass and rotate the DG so its upper bearing mark reflects that course (use the red heading bug if necessary, it makes it easier at the beginning). If your DG was not calibrated, this will not be the magnetic course and you probably will not get where you wanted to.
• For the ALT mode it is important to calibrate the barometric setting of the autopilot to the one of the altimeter: read the altimeters baro setting from the Kollsman window (the small window at the altimeter), press the "BARO" key at the autopilot and rotate the right "altitude-preselect knob" so the autopilot shows the same setting. If you miss to calibrate the baro setting, you will under-/overshoot altitude presets
• You should run through the HDG and VS modes on ground as preflight check to make sure the autopilot is operating the aileron, elevator and trim wheel
• If you intend to follow the runway heading after takeoff, now is a good time to adjust the heading bug.
• If you intend to climb to a specific altitude and let the autopilot intercept it, you should now rotate the altitude preselect knob of the autopilot to the desired altitude.

After Takeoff: Hold heading, continue climbing

It is not advised to engage the autopilot immediately for takeoff as it will mess up with lower speeds. Establish a smooth climb rate and trimmed flight first.

When you achieved a stable climb rate after takeoff, engage the autopilot by pressing "AP". It will engage in ROL/VS mode: keeps the wings level and maintain climb at the current rate. The moment you engage the autopilot it will show "ROL", "VS", and at the right side the currently set climb rate for some seconds. Always visually confirm the autopilot shows the modes you expect!

If not done yet, rotate the red heading bug at the DG to the desired heading before switching to HDG mode.

Once you press the HGT button, the autopilot will switch from ROL to the HDG mode, and the plane now follows the desired course. When you change the heading bug, the plane will bank and follow the new heading (it will do this at standard turn rate).

Climb to Altitude, then hold it (intercept altitude)

The autpilot now should be in HDG/VS mode. You should check the vertical speed the autopilot automatically set to the climb rate you had when engaging it. Using the buttons "UP" and "DN" you can adjust the vertical speed setting to the desired value.

To let the autopilot intercept the desired altitude, you need to enter it using the knob on the right side. Turn in the desired altitude. Doing this should automatically "arm" the autopilot (it shows "ALT ARM"), telling you that it waits to reach the entered altitude. If it was not arming automatically, you can arm it by manually pressing "ARM".

The plane will now climb to the altitude you requested. When you get at 1000ft near your setting, it will beep to let you know its close. Once you reach the altitude it will reduce the climb rate to zero, leveling out at the desired altitude: this is indicated by the "ALT ARM" vanishing. If you are not at the altitude you entered, you probably forgot to calibrate the baro setting.

Enroute climb / descend

In case you want to change the currently maintained altitude, you can do so by two means:

1. Small adjustments can be made by pressing just the UP/DN buttons. Each press will adjust the set altitude for 20ft. Note that we are in ALT mode, so we adjust absolute altiutde with UP/DN knobs, not climb rate!
2. For larger adjustments you just rotate the altitude preselect knob to the new desired altitude. The autopilot will show "ALT ARM". When you are ready to climb or descend, put the autopilot to VS mode by pressing the ALT button. Now push UP/DN buttons to climb or descend, and everything else will behave like already described above. You can adjust the desired climb rate anytime. When intercepting, the autopilot will automatically enter ALT mode again (which changes the behaviour of UP/DN buttons to absolute corrections too!).
3. For other cases you can also use the manual ALT mode: Put the autopilot in VS mode by pressing "ALT", adjust the climb rate to your liking and as soon as you feel the altitude is right, you can press "ALT" again. The AP will show "ALT" immediately, indicating it is holding this altitude. If your climb rate was a little higher, you probably overshoot the desired altitude, but the autopilot will soon return to it.

Note that you can use these techniques with ROL, HDG and NAV modes.

VOR interception (NAV mode)

For longer trips it is nice to let the autopilot track a VOR radial. The difference to the HDG mode is that the plane will compensate for wind drift as it seeks to keep the CDI-needle centered.

To use the NAV mode you have to tune NAV1 to the VORs frequency and select the desired radial with the OBS knob of your CDI1 (which links to NAV1). Now you can intercept the radial, and you have two options to do that:

1. From HDG mode: select the radial also on your DG heading bug (the plane will try to follow that now). Then engage by pressing "NAV". The autopilot will show "NAV ARM" and turn the plane to an 45° interception angle, but it will remain in HDG mode. Once you are close enough, the NAV-mode will kick in ("NAV ARM" vanishes and NAV shows) - the plane now follows the radial.
2. From ROL mode (all angle intercept): It works like HDG mode, but once you push the NAV knob, the plane will intercept the radial at the angle you are currently flying. To initially bring the plane to the desired angle, use either HDG-mode with the heading bug, or fly manually, then engage ROL mode by pressing HDG.

Both modes will show a flashing "HDG" annunciator to remind you that you have to set the DG heading bug to the radial in both cases (the autopilot computes the needed course from that)!

ILS assisted approach (APR mode)

The NAV and APR modes are really similar, but the APR mode does additionally follow also the glideslope signal from an ILS, and the ILS has a fixed radial (so it ignores the OBS knob setting of the CDI!).

The interception works exactly as in the NAV mode described above. If you engage the APR mode knob the plane will start to intercept the signal, showing "APR ARM" and continue with the currently selected lateral mode (ROL or HDG with 45° intercept angle). Remember to set the DG heading bug to the desired approach course ("HDG" will flash to remember you of that).

As soon as the plane intercepts the localizer (that drives the CDI needle left/right), it will behave like in NAV mode. "APR ARM" will vanish and switch to "APR", showing you are in approach mode now.

When the APR mode engages, the GS mode will try to intercept the vertical glideslope beam. As long as you stay below the beam, it will show "GS ARM". When the vertical glideslope is intercepted, the plane will start to follow it downwards.

Attention! This mode is dangerous, because it will drive you into the ground if you don't disengage the autopilot. It is not meant to land you automatically, just to guide you close to the runway. As soon as you are near the runway you should disengage and land manually.

Flying opposite direction of VOR or ILS (REV mode)

Using the REV mode allows you to fly away from an VOR or ILS localizer signal (note that glideslope is ignored). It works the same as NAV or APR, just in the opposite compass direction. This is helpful if you want to fly straight away from the runway at start or to fly away from a VOR that you tuned the inbound course into the OBS.

As APR and NAV, you can intercept the radial or localizer either from ROL or HDG mode. In either case, dial in the Front Inbound course into the OBS and the DG heading bug, and not the direction you want to fly. Course reversal is done from the autopilot automatically.

Related content

• Joystick Autopilot Bindings Snippets for joystick.xml file that allow control of most of the autopilot functions using the joystick.

Caution These joystick bindings only work with the older KAP versions, not the new one with the preflight check simulated!

External links

• Bendix/King KAP 140 Pilot's Guide download link (PDF, 6.8 MB), Honywell, rev. 3, Nov 2005.
• Bendix/King KAP 140 Pilot's Guide, Honeywell, rev. 3, Nov 2005.