Understanding Control Surface Trimming in Fly-By-Wire Aircraft

In modern aircraft that use fly-by-wire (FBW) systems, the control surfaces such as ailerons, elevators, and rudders still need trimming, but the process is more automated and efficient compared to traditional mechanical systems. This article delves into the purpose of trimming, the distinction between FBW systems and fully powered control systems, and how trimming in FBW aircraft works.

Purpose of Trimming

Trimming in aircraft serves the critical purpose of adjusting the neutral position of control surfaces to maintain level flight without the need for continuous pilot input. This helps reduce pilot workload and enhances flight comfort. Effective trimming ensures that the aircraft remains stable and controllable, even under varying flight conditions.

FBW Systems vs. Fully Powered Control Systems

FBW Systems: In aircraft equipped with fly-by-wire systems, the pilot's inputs are translated into control surface movements by the flight control computers. These systems can automatically adjust the trim settings based on real-time flight conditions, reducing the need for manual trimming. Many FBW systems include automatic trim features, which continually adjust the control surfaces for optimal performance. For example, if the aircraft experiences a change in load or speed, the system can automatically trim to maintain stable flight.

Fully Powered Control Systems: In aircraft with fully powered control systems, hydraulic systems control the movement of the control surfaces without any mechanical linkages. The control stick directly activates hydraulic valves and servo cylinders, moving the control surfaces. These systems do not typically have manual trim tabs. Instead, the neutral or zero point of the control surface can be adjusted to achieve the desired trim. In some aircraft, such as the T-38, the pitch trim is adjusted by moving the springs that hold the stick in a specific position when not moved by the pilot. In the B-52 and many airliners, the pitch trim may adjust the position of the horizontal stabilizer without trim tabs.

Automation and Pilot Control

While automation is a significant feature in FBW systems, pilots retain the ability to manually adjust trim if necessary. This is particularly useful in specific flight scenarios or if the automatic system does not respond as desired. This manual control ensures that pilots can intervene when needed, maintaining full control and stability during flight.

Efficiency and Safety

The integration of trimming functions within FBW systems enhances efficiency and safety. By reducing the need for continuous manual input, pilots can focus on other aspects of flight, leading to smoother operations and a lower risk of pilot error. Automation and pilot control together contribute to more reliable and safer flight operations.

Key Differences and Considerations

It's important to recognize the differences between FBW systems and fully powered control systems. While both systems require trimming, the methods differ. FBW systems automatically adjust the trim settings based on flight conditions, while fully powered control systems allow for manual adjustment of the neutral or zero point of the control surfaces.

For instance, when dropping bombs in modern aircraft, there's a need for quick leveling of the wings. This is often managed through the pitch trim function, which is integrated into the FBW system. Similarly, in older aircraft, there was no need for trim tabs on control surfaces, but trim was available for all surfaces, often achieved by adjusting the position of control springs or the horizontal stabilizer.

In summary, while control surfaces in FBW aircraft still require trimming, the process is often more automated and efficient. Pilots can rely on both automatic and manual trim functions to maintain control and stability during flight, enhancing both efficiency and safety.