0121-17 – S-Tec Autopilot Yaw Damper Computer Assembly (14/28 Volts)
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Overview Section
The S-Tec Autopilot Yaw Damper Computer Assembly, Part Number 0121-17 (a variation within the 0121 series), is a crucial component of the S-Tec (now Genesys Aerosystems) autopilot system. This assembly functions as the central processor for the Yaw Damper (YD), a supplementary autopilot system designed to improve the aircraft's dynamic stability. The YD achieves this by sensing and counteracting undesirable motions like adverse yaw and Dutch roll, especially critical in multi-engine, swept-wing, or pressurized turboprop aircraft to enhance safety and passenger comfort.
This specific unit, designated with the base number 0121 and the dash number -17, is a remote-mounted sensor/computer/servo amplifier. The S-Tec system is known for its use of a highly accurate accelerometer instead of a traditional rate gyro, leading to improved reliability by reducing reliance on mechanical moving parts. The unit supports the standard aircraft electrical buses, compatible with both 14 VDC and 28 VDC systems, offering broad compatibility across the GA (General Aviation) fleet. The assembly holds TSO (Technical Standard Order) C9c approval.
Key Features Section
Yaw Damper Function: Provides automatic, corrective rudder control inputs to effectively damp yaw oscillations (Dutch roll) and significantly improve ride quality.
Accelerometer Technology: Utilizes a solid-state accelerometer for superior yaw rate and lateral acceleration sensing, leading to a more robust and maintenance-friendly system than older mechanical gyro designs.
Dual Voltage Compatibility: Engineered to operate reliably on either the 14 VDC or 28 VDC aircraft electrical system without modification.
Integrated Design: Combines the sensor, computer processing unit, and servo amplifier into one compact unit, which simplifies remote mounting and reduces installation complexity.
Multiple Operating Modes: Can be activated independently, or automatically engage when the main autopilot (roll/pitch axes) is selected ON.
Lightweight and Reliable: The solid-state design ensures low power draw and a high Mean Time Between Failure (MTBF).
Specific -17 Configuration: The dash number -17 indicates a specific software version, wiring interface, or set of performance characteristics tailored for a defined group of aircraft models under a corresponding Supplemental Type Certificate (STC).
Technical Specifications Section
Part Number: 0121-17 (Specific dash number variation)
Component Type: Yaw Damper Computer Assembly / Amplifier
OEM Manufacturer: S-Tec (Genesys Aerosystems/Moog)
Sensor Type: Accelerometer (Solid-State)
Voltage: 14 VDC or 28 VDC (Dual compatibility)
Power Consumption: Typically 1.0 A Average, 3.0 A Maximum (System-dependent)
TSO Approval: Approved to TSO C9c
Application: Provides commanded output to a dedicated S-Tec rudder servo actuator (sold separately).
Aircraft Compatibility Section
High-Performance Aircraft: Primarily installed via Supplemental Type Certificate (STC) in various high-performance piston twins and light turboprops where active yaw stability is required.
Common Applications: Includes various models of Beechcraft (Baron, King Air), Cessna (400 series twins, Caravans), and Piper (Seneca, Navajo, Meridian).
Note: The -17 dash number is crucial. Installation is strictly governed by an FAA-approved STC, which specifies the exact software and hardware requirements. The specific dash number must match the requirements outlined in the aircraft's Equipment List and the S-Tec Installation Manual.
Applications Section
Dynamic Stability: Actively suppresses the Dutch roll phugoid (a combination of yawing and rolling) to maintain flight efficiency and reduce crew workload.
Turn Coordination: Assists the main autopilot in providing precisely coordinated turns by automatically applying necessary rudder input to nullify adverse yaw.
Comfort and Safety: Significantly enhances passenger comfort by eliminating irritating side-to-side oscillations, while ensuring stable flight characteristics at all airspeeds and altitudes.
Why Choose the 0121-17?
The 0121-17 is a core component of a certified flight control system. The integrity of the yaw damper is vital for the safety and efficiency of aircraft prone to stability issues. Using an uncertified or incorrect unit risks generating spurious or destabilizing rudder commands, or failing to engage when needed. The TSO-certified S-Tec unit, with its precision accelerometer technology and specific -17 software, guarantees the required electronic stability, processing accuracy, and reliability necessary to maintain the aircraft's certified handling qualities and overall airworthiness.
The 0121-17 specifically addresses the challenge of adverse yaw and Dutch roll. Adverse yaw is the tendency of an aircraft to yaw opposite the direction of a turn immediately upon aileron input, forcing the pilot to apply coordinated rudder. Dutch roll is a coupled, lateral-directional oscillation inherent in many aircraft designs. The system measures the lateral acceleration (sideways slip) and applies a command to the rudder servo that is proportional to the slip but filtered (via a washout filter) so it does not interfere with steady coordinated turns. The precise calibration of this filter and the gain scheduling are contained within the -17 unit's software, making the dash number a key identifier for correct aircraft application.
Note: The 0121-17 is a major avionics component. It must be installed, wired, and tested by an FAA-certified avionics technician in strict accordance with the relevant STC and the S-Tec Component Maintenance Manual (CMM). The final check always involves a flight test to verify the proper damping and trim operation under various flight regimes.
Documentation
S-Tec / Genesys Aerosystems Installation Manual: Required for detailed wiring diagrams and mounting instructions.
Supplemental Type Certificate (STC): Defines the approved aircraft models and installation limitations for the -17 configuration.
FAA TSO C9c: Technical standard for the performance of the yaw damper system.