The S-TEC Approach Newsletter December 1995

Installation Secrets
INSTALL ESSENTIALS
by Scott Howard, Technical Rep.
The installation of a new autopilot is more complex and involved than the average avionics installation. It's certainly more than just mounting black boxes connected to A+ and ground.
When you install a new autopilot you are tying into the primary flight controls of the aircraft, interfacing with navigation equipment and connecting to the aircraft electrical system.
For the new autopilot system to work properly and up to its maximum performance capabilities, it is dependent on all the systems it is linked to and their performance. If any of these systems are not performing up to par, the autopilot system's performance could be compromised. Therefore, at the beginning of an installation it is recommended that the performance of these systems be checked by the installer.
Check to see if the control cable tensions meet the manufacturers specifications. Check to assure the control travel is within specified limits and that there is no stiffness or binding in the aircraft flight control system.
When connecting to the DC buss, check for the proper voltage output and be sure there is no AC ripple when the aircraft charging system is operating.
Navigational and other avionics equipment must be delivering optimum outputs to assure that the autopilot which is interfacing with them will perform properly. Be sure and check the outputs against manufacturers specifications at the outset.
When routing the harness avoid tying with transmitter coaxes and any cable carrying high current or alternating current.
ROLL CENTERING
Since all S-TEC autopilots are Rate Based, the interface with the rate gyro must be set properly. This is known as Roll Centering.
The roll centering adjustment for S-TEC Autopilot Systems 40/50/60 is a 4 turn potentiometer located behind the upper left-hand faceplate screw of the panel mounted programmer unit. On the System 65 the access hole is located between the Programmer's ALT & YD buttons. On a system 55 the access hole is between the REV & ALT buttons. Access requires a small slot jeweler's screwdriver.
The roll centering adjustment will compensate for offset in the rate gyro signal of up to +/- 150mvDC. This adjustment must be performed at system installation, after the replacement of a turn coordinator, programmer, or roll computer, or any time symptoms of centering offset appear. These symptoms include:
1. The heading bug is offset from the Lubber Line when tracking in Heading Mode.
2. The STB knob is offset when in level flight in System 40/50 installations. 3. Tracking occurs with needle offset or with the needle lost to one side in NAV (Soft) mode.
4. Needle deflection in NAV mode causes a greater servo response to one side than the other during the ground check.
Since the autopilot is more sensitive to a centering offset in the lower authority NAV (Soft) mode, the roll centering adjustment should be accomplished with the autopilot engaged in this function. Offset may not be as apparent in HDG or STB modes due to higher authority.
SYSTEM 55/60/65 GROUND ADJUSTMENT PROCEDURES -
1. With the aircraft relatively level, turn on electrical power and allow the turn coordinator to run for at least 2 minutes.
2. Tune the NAV Receiver to a non-receiving VOR frequency allowing the OBS needle to center.
3. Center the HDG Bug on the DG or the CRS Arrow in an HSI.
4. Engage the autopilot in NAV mode and allow it to time down into SOFT condition.
5. Adjust the centering potentiometer in small increments to null the control wheel movement allowing time between adjustments so the circuitry can stabilize.
SYSTEM 40/50 GROUND ADJUSTMENT PROCEDURES -
1. Follow steps 1. & 2. above.
2. Engage NAV mode and follow step 5. above.
IN FLIGHT ADJUSTMENTS -
Establish radial tracking in the NAV mode on System 40/50 equipped aircraft and in the NAV SOFT mode in System 60/65/55 equipped aircraft. Adjust the centering potentiometer in small increments to maintain a centered needle allowing time between adjustments for the autopilot to compensate. A CCW (counter clock wise) turn compensates for left needle offset and a CW (clock wise) turn compensates for right needle offset.
Following these basic procedures in the initial phase of a new autopilot installation will assure that the S-TEC system you are installing performs to its full potential making another happy S-TEC User.

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