S-TEC Autopilot
Physical S-TEC 55X autopilot control unit for the Six-Pack glareshield — the autopilot X-Plane 12's default Skyhawk flies, with its mode buttons, annunciators, VS knob and X-Plane mapping.
Overview
Most real 172SPs with steam gauges have a Bendix/King KAP 140. X-Plane 12's default Skyhawk flies an S-TEC 55X instead, so that's what this panel replicates: a KAP 140 replica would have an altitude preselect and a BARO key that do nothing in the simulator.
The S-TEC 55X is a two-axis autopilot with heading, NAV, approach, reverse course, GPS steering, altitude hold and vertical speed modes. It sits on the glareshield; the heading bug it follows is set on the Heading Indicator.
Panel Layout
| Control | Type | X-Plane 12 |
|---|---|---|
| HDG | Push button | sim/autopilot/heading |
| NAV | Push button | sim/autopilot/NAV |
| APR | Push button | sim/autopilot/approach |
| REV | Push button | sim/autopilot/back_course |
| ALT | Push button | sim/autopilot/altitude_hold |
| VS | Push button | sim/autopilot/vertical_speed |
| VS knob | Rotary encoder | sim/autopilot/vertical_speed_up / _down |
| AP disconnect | Push button (on the yoke) | sim/autopilot/servos_off_any |
| GPS / NAV | Toggle switch | sim/autopilot/hsi_select_gps / hsi_select_nav_1 |
The GPS / NAV switch picks what the CDI and the autopilot follow. It is reconciled against sim/cockpit2/radios/actuators/HSI_source_select_pilot (2 for GPS, 0 for NAV1), so the switch and the simulator can't disagree for long.
The 55X's two-key shortcuts (HDG + NAV to intercept, ALT + VS) aren't bound: each key does its own mode.
Annunciators
The 55X's display annunciators are LEDs. Most light when their mode is armed or captured; GPSS and RT only once captured.
| Legend | Dataref (sim/cockpit2/autopilot/) |
|---|---|
| RDY | otto_ready |
| AP | autopilot_on |
| HDG | heading_status |
| NAV | nav_status |
| APR | approach_status |
| REV | backcourse_status |
| ALT | altitude_hold_status |
| VS | vvi_status |
| GS | glideslope_status |
| GPSS | gpss_status = 2 |
| RT | rate_status = 2 |
| TRIM UP | pitch_mistrim = 1 |
| TRIM DN | pitch_mistrim = −1 |
The selected vertical speed (vvi_dial_fpm) shows on a 5-digit seven-segment window.
Construction
The bezel is 3D-printed in PLA and painted to match the glareshield. The buttons and the GPS / NAV switch sit on one MCP23017 I²C expander and the 13 annunciator LEDs on a second (GPIO 21 SDA, 22 SCL). The VS knob uses GPIO 25 and 26, and the MAX7219 for the VS window data 23, clock 18 and chip select 5. One ESP32 drives the whole unit. In the app it's part of the Cessna 172 (six-pack) Blueprint.
Status
Wiring and X-Plane mapping defined. Bezel design not yet started.