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0121-15 – S-Tec Autopilot Yaw Damper Computer Assembly (14/28 Volts)
Available now for sale or exchange on the Avionics Market.
Overview Section
The S-Tec Autopilot Yaw Damper Computer Assembly, Part Number 0121-15 (often referenced generically as the 0121 series), is a crucial component of the S-Tec (now Genesys Aerosystems) autopilot system. This assembly functions as the brains of the Yaw Damper (YD), a supplementary autopilot system designed to improve the aircraft's stability by sensing and counteracting adverse yaw and Dutch roll motions, particularly in multi-engine, swept-wing, or pressurized turboprop aircraft.
This specific unit, designated with the 0121 base number and a dash number (e.g., -15), is a remote-mounted sensor/computer/servo amplifier. A key innovation of the S-Tec system is the use of a highly accurate accelerometer instead of a traditional rate gyro, which substantially reduces moving parts and improves performance by sensing both skid and slip in a single sensor. The dual voltage capability (nominally 14 or 28 VDC) makes the unit versatile across a wide range of aircraft platforms. The unit is typically TSO (Technical Standard Order) approved to TSO C9c.
Key Features Section
Yaw Damper Function: Provides automatic rudder control input to dampen yaw oscillations (Dutch roll) and improve passenger comfort.
Accelerometer Technology: Utilizes a solid-state accelerometer sensor for yaw rate sensing, eliminating the need for a mechanical rate gyro and enhancing reliability.
Dual Voltage Compatible: Operates on standard aircraft electrical buses, supporting both 14 VDC and 28 VDC systems (power consumption is approximately 1.0 amps average).
Integrated Design: The assembly often includes the sensor, computer, and servo amplifier in one compact, remote-mounted unit, simplifying installation.
Multiple Operating Modes: Can operate automatically when the main autopilot (roll/pitch axes) is engaged, or independently via a panel-mounted ON/OFF/AUTO switch.
Lightweight and Compact: The solid-state design results in a significantly reduced size and weight (approximately 3.8 lbs total system weight including the servo actuator) compared to older systems.
Optional Yaw Trim: Can interface with an S-Tec autopilot to provide yaw trim functionality, helping center the skid-and-slip ball.
Technical Specifications Section
Part Number: 0121-15 (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: 1.0 A Average, 3.0 A Maximum
Weight (System): Approximately 3.8 lbs (includes servo)
TSO Approval: Approved to TSO C9c
Application: Interfaces with the aircraft's rudder control through an S-Tec servo actuator (sold separately).
Aircraft Compatibility Section
High-Performance Singles/Twins: Commonly installed under Supplemental Type Certificate (STC) in various high-performance piston and light turbine aircraft where yaw stability is critical.
Beechcraft: Used extensively in Baron, Bonanza, and King Air models (requires specific STC).
Cessna: Used in various twin-engine and turboprop models.
Piper: Used in Seneca, Navajo, and Malibu/Meridian models.
Note: Installation of this avionics unit is strictly governed by an FAA-approved Supplemental Type Certificate (STC), which defines the compatible aircraft models, wiring, and flight manual supplements. The specific dash number (-15) must match the requirements outlined in the aircraft's Equipment List and Installation Manual.
Applications Section
Turbulence Damping: Automatically dampens yaw motions induced by turbulent air, providing a smoother ride for passengers and crew.
Roll Coordination Support: Works with the main autopilot to coordinate turns, especially in high-speed flight, by providing immediate rudder input to cancel adverse yaw.
High-Altitude Stability: Essential for improving the stability and handling of aircraft at high cruising altitudes where air density is lower.
Why Choose the 0121-15?
The 0121-15 is a flight control system component critical to the precision and safety of the aircraft's autopilot and stability systems. Using a component that is not TSO-certified or fails to meet the required electrical specifications can lead to: 1) Uncontrolled Rudder Input: Causing dangerous flight conditions; 2) System Incompatibility: Failure to integrate correctly with the roll/pitch axes of the main autopilot; or 3) Reliability Issues: Due to vibration-sensitive mechanical components (which this unit avoids with its accelerometer design). The certified S-Tec unit guarantees the required precision, low weight, and electronic stability for safe, reliable operation as a critical part of the autopilot/yaw damper system.
The advanced performance of the S-Tec Yaw Damper is due to its reliance on the accelerometer to measure lateral acceleration (side forces) rather than just rotational velocity (yaw rate). This distinction allows the system to directly sense the slip or skid condition of the aircraft, which is the direct cause of the uncomfortable and inefficient motion known as Dutch roll. The computer assembly takes the lateral acceleration signal, processes it through complex compensation algorithms (e.g., washout filters and gain schedules) to determine the necessary rudder servo command to return the skid-slip ball to the center, thereby ensuring coordinated flight and optimal aerodynamic efficiency.
Note: The 0121-15 is a major avionics component. It must be installed, wired, and tested by an FAA-certified avionics technician in accordance with the relevant STC and the S-Tec Component Maintenance Manual (CMM). The final step is a flight test to ensure the Yaw Damper gain and trim are set correctly for the specific aircraft.
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.
FAA TSO C9c: Technical standard for the performance of the yaw damper system.
