What is STS in Aviation? (Speed Trim System)

The Speed Trim System (STS) in aviation refers to a feature found in many aircraft that automatically adjusts the aircraft’s pitch trim to enhance stability and control during different phases of flight. The primary purpose of the speed trim system is to assist pilots in maintaining the desired pitch attitude without excessive control inputs, particularly during changes in airspeed or configuration. The STS continuously monitors the aircraft’s airspeed and adjusts the pitch trim to maintain a predetermined speed stability.

The speed trim system is designed to compensate for aerodynamic changes that occur during flight, such as changes in the center of gravity and the aircraft’s sensitivity to pitch. It operates by moving the horizontal stabilizer, which influences the aircraft’s pitch attitude. The STS helps improve safety and efficiency by reducing pilot workload and enhancing the overall handling characteristics of the aircraft.

How Does the Speed Trim System Work?

The speed trim system works by adjusting the horizontal stabilizer’s position based on various inputs, including airspeed and configuration. When the aircraft experiences a change in speed, the STS automatically moves the stabilizer to maintain the desired pitch attitude. For example, if the airspeed increases, the STS will trim nose-down to decrease the aircraft’s pitch angle, and vice versa.

The speed trim system uses inputs from different sensors to continuously monitor the aircraft’s parameters. These sensors include:

  • Angle of Attack (AOA) sensors
  • Accelerometers
  • Airspeed sensors

These sensors provide crucial information to the STS, allowing it to adjust the aircraft’s pitch trim accurately. By monitoring the airspeed, the speed trim system helps prevent an excessive pitch-up or pitch-down condition, enhancing stability and minimizing the risk of a stall or loss of control.

The Benefits and Limitations of the Speed Trim System

The speed trim system offers several benefits to both pilots and aircraft operations:

1. Increased Safety: By automatically adjusting the pitch trim, the STS improves stability and control, reducing the risk of pilot-induced oscillations and loss of control situations. This enhances overall flight safety and provides pilots with an additional layer of protection.

2. Reduced Pilot Workload: The speed trim system assists pilots by alleviating the need for constant manual pitch trim adjustments. This allows the pilots to focus more on other critical tasks, such as navigating, monitoring systems, and communicating with air traffic control. Reduced workload can also help prevent pilot fatigue during long flights.

3. Improved Handling: The STS enhances the aircraft’s handling characteristics by maintaining the desired pitch attitude during changes in airspeed or configuration. This results in smoother and more predictable flight performance, providing a better experience for both crew and passengers.

Despite its numerous advantages, the speed trim system also has some limitations:

1. Sensor Reliability: The accuracy and reliability of the sensors used in the STS are critical for its proper functioning. If any of the sensors fail or provide incorrect information, it can affect the system’s performance and potentially compromise flight safety. Regular maintenance and sensor calibration are essential to ensure the system’s reliability.

2. Environment-specific Limitations: The STS’s performance can be affected by external factors such as extreme weather conditions, icing, or turbulent airflow. These conditions may introduce additional variables and create challenges for the system to maintain the desired pitch trim accurately. Pilots must be aware of these limitations and adapt their flying techniques accordingly.

3. Pilot Understanding and Training: It is crucial for pilots to have a thorough understanding of the STS and its operation. Proper training ensures that pilots can effectively utilize the system and understand its limitations. An understanding of how the STS interacts with other aircraft systems also contributes to safe and effective flight operations.


The Speed Trim System (STS) significantly contributes to the overall safety and efficiency of aviation operations. By automatically adjusting the aircraft’s pitch trim, the STS enhances stability and reduces pilot workload. It helps maintain the desired pitch attitude during changes in speed or configuration, improving flight handling and passenger comfort. However, proper training, regular maintenance, and awareness of its limitations are crucial for safe and effective utilization of the speed trim system.

For more information about the Speed Trim System and its role in aviation, you can visit FAA’s Airplane Flying Handbook.

For More: What is RPL in Aviation? (Repetitive Flight Planning)