KAP 140 Two Axis/Altitude Preselect Autopilot System

The Honeywell / Bendix King KAP 140 autopilot in the WCFC Cessna 172 SP aircraft is a two-axis system (roll and pitch) with altitude preselect. It is a rate-based system, as opposed to an attitude-based system such as found in our Mooney M20P aircraft. Roll rate information is derived from the turn coordinator. Roll axis functions include a basic wing leveler (ROL), heading select (HDG), and VOR/LOC intercept and tracking, which can also track a GPS course.

Pitch information is derived from a pressure sensor and accelerometer. Vertical functions include vertical speed (VS), altitude hold (ALT), glideslope tracking (GS) and altitude preselect (ARM-ALT).

The KAP 140 Autopilot System operates independent of the aircraft’s artificial horizon. Therefore, the autopilot retains roll stabilization and all vertical modes in the event of vacuum system failure. This is one principal advantage of a rate-based autopilot system: it will keep your wings level when you need it most.

Below are personal suggestions as to how one might usefully employ the KAP 140 in flight and several cautionary notes about ways in which misuse or misunderstanding of the system might arise. As always, the manufacturer’s literature, most particularly the limitations and instructions found in the Flight Manual Supplement, which should be included in the aircraft flight manual of each airplane, is the final authority.

PREFLIGHT

A preflight check of the AP should be performed as part of the “radios” or “avionics” portion of the runup. Here is the sequence:

  1. Turn on the AP. The system will perform a self-test, annunciating “PFT” and an incrementing number. When the system check is completed, all of the annunciators will display and the autopilot will disengage.

  2. Engage AP. It will default to ROL and VS.

  3. Check your ability to over-ride ROL control.

  4. Center HDG bug on current heading and engage HDG. Roll bug left and right and observe that the roll servo attempts to bank left and ride to chase the HDG bug.

  5. Use UP/DOWN buttons to command either a climb or descent and observe that the AP attempts to pitch up or down as appropriate. Hold the stick and continue to command either UP or DOWN and observe that after a moment the AP attempts to trim to counteract your force on the stick. This is a test of first the pitch servo and second the pitch trim servo.

  6. Test Autopilot Disconnect / Trim Interrupt switch on pilot yoke to disengage autopilot. Disengage autopilot and re-center the pitch trim as necessary.

INITIAL AUTOPILOT ENGAGEMENT IN FLIGHT

Thus, I believe this would be a practical procedure for initiating autopilot flight if engaging autopilot after takeoff in the climb phase:

  1. Before takeoff, set barometer and set altitude assignment for initial cleared altitude

  2. In the initial climb, press AP to engage autopilot in ROL (wings level) and VS (vertical speed.) We are thus in wings level for lateral and existing vertical speed for vertical. The captured VS will be the vertical speed present at the moment of the AP button press. Please note that the autopilot will attempt to maintain the vertical speed that it senses upon engaging — even if that speed is the product of a temporary phenomenon such as a thermal or a downdraft or a wind shear. Therefore:

  3. Adjust VS with UP / DOWN buttons as necessary immediately after engaging the autopilot.

  4. Set heading bug on desired heading (it should already be there) and press HDG to engage heading. We are now in HDG for lateral and VS for vertical.

  5. If ARM ALT is not annunciated and engaged, manually engage it with the ARM button – and check for annunciation.

  6. Use HDG bug for lateral steering and climb to assigned or cleared altitude, listening for “Approaching altitude” thousand-foot callout and watching for ALT ARM annunciator to switch to ALT annunciation to confirm level off at assigned altitude. If the altitude assignment changes as we are cleared higher, check each time that the ALT ARM annunciator reappears.

  7. Once in cruise, engage HDG or NAV as desired for lateral steering and maintain altitude in ALT mode.

  8. Small (<200 feet) adjustments to altitude may be made with momentary use of the UP / DOWN buttons.

  9. Large (>200 feet approx.) altitude changes may be achieved by disengaging ALT and engaging VS. From ALT mode, increase altitude assignment to target altitude. This will automatically engage ALT ARM – but will not disengage ALT, which must be done manually by disengaging ALT with a single press of the ALT button. The autopilot will not command a climb or descent until you disengage ALT by pushing the ALT button once. The problem is that there is no annunciator to let you know that the ALT hold is still engaged. Do not press ARM if ALT ARM is already annunciated or you will disengage the ARM function. If you do, then manually re-arm. The ARM button toggles the ARM function on and off. Adjust VS with the UP / DOWN buttons.

  10. Listen and watch for “approaching altitude” alert and for ARM ALT annunciator to change to ALT, as in #5 above.

  11. Use HDG or NAV for lateral steering as desired. Note, however, that this autopilot will not perform GPS steering. It will not fly a heading from digital information. It simply chased the course deviation needle on the DG or HIS and therefore requires that the OBS be set to the desired course and the HDG bug be set to the desired course (not heading). The OBS informs the autopilot of the intended course (not heading) and the HDG bug prevents the autopilot from flying in circles as it chases the CDI back and forth the track the course by limiting intercept angles.

The transition from level flight in ALT mode to vertical speed mode (VS) for a climb or descent has two possibly confusing and non-obvious steps. Any time you “reset” the altitude, the “Arm Alt” annunciates and Altitude Capture is engaged – but ALT is not necessarily disengaged. You must press the ALT button once to toggle altitude hold off and allow the vertical speed to be programmed and the autopilot to command a climb or descent to the new assigned altitude. Note that if you change the altitude assignment, Arm Alt engages and if you, at that time, press “Arm” you will in fact disengage the Arm Alt function. Think of these buttons as toggle buttons, toggling ALT and ARM ALT on or off.

APPROACH USE

When the Approach Mode (APR) is engaged, this mode will provide auto-beam capture and tracking of VOR, GPS, LOC and Glideslope (GS) on an ILS. This autopilot will not track a GPS-derived glideslope for an LPV approach. It requires a LOC signal.

From an initial approach configuration, probably HDG or NAV for lateral and ALT for vertical control:

  1. If flying a full approach with course reversal, track outbound using either NAV (VOR or RNAV) or REV (LOC) or HDG (manual steering) as you prefer. You must execute the PT manually in HDG. ALT should be engaged unless a further descent is required.

  2. When inbound to the LOC or final approach course, whether upon completing a course reversal or receiving vectors to final, we can either fly in HDG mode to intercept the final, or we can somewhat automate the process by arming the AP to intercept the LOC or final approach course when encountered. This may be accomplished in one of two ways, each of which requires a different technique:

    1. Inbound to the LOC or final approach course, we can arm the autopilot to intercept the LOC or final approach course by engaging either HDG or ROL. Plus APPR. (Obviously, the proper approach navigation must be set in NAV 1, the GTN 650.) Cessna 172 SP KAP-140 autopilot guide v5 03 09 2014, G. Scheer page 3 of 43) 4)

    2. HDG + APPR will command an intercept angle of 45 degrees to the final approach course that cannot be adjusted because the heading bug must be set to the final approach course. The OBS must be set to the final approach course, although with a localizer I don’t think it really matters other than good practice.

    3. ROL + APPR will fly wings level, thus more or less maintaining present heading, to intercept the final approach course. Again, HDG bug must be set to the final approach course. This will allow us to fly something other than a 45 degree intercept, such as the standard 30-degree vector to final. If the heading needs to be changed, we must disengage the AP, change heading, and re-engage the autopilot. (We could engage heading, move the heading bug, wait for the airplane to change heading, then move the HDG bug back to the final approach course, but that seems even more cumbersome and fraught with potential for confusion.)

    4. When we reach the final approach course, the other annunciations will extinguish leaving only APPR, which informs us that we are now tracking the final approach course (or LOC) inbound.

  3. Once established on final, assure that APPR is engaged. APPR must be engaged to capture the glideslope. Note that this AP will capture the GS from below, as proper, or above. (not recommended as good practice.)

  4. King does not recommend using the ALT ARM function to command a level off at MDA or DA. It is too imprecise and the consequence of relying on the AP, should it be mis-programmed or should it fail, are dire. A non-precision approach must be flown using VS to command descent, since there is no “lateral-only” mode for this AP. I am not certain if the VS mode functions normally in the APPR rather than ALT ARM mode.

VACUUM or GYRO FAILURE USE

One of the virtues of these rate-based autopilots that derive lateral information from the turn coordinator is that they will continue to function if the attitude indicator fails, a classic accident scenario in IMC. The 172 SP has dual vacuum pumps, which should mitigate the chance of a vacuum failure, but the AI could still fail mechanically. In this case the AP will still work to keep the wings level and will still maintain altitude and perform VS climbs and descents. A problem occurs if the DG fails because the HDG bug is required for every lateral mode except ROL. The HDG, NAV, APPR, and REV modes all require the heading bug, which will be deceptive if the DG fails. For lateral control, we will fly in ROL only, toggling the AP off and on to make manual heading changes.

REFERENCES

This article was written by George Scheer (george.scheer@gmail.com) and is reproduced here with his permission.