September 28, 2022
How Do Aircraft Motion Control Systems Work?
AvGeek

How Do Aircraft Motion Control Systems Work?

DALLAS – A commercial aircraft’s motion control systems are essential for helping it fly safely. Here’s a breakdown of how they work. 

A plane’s primary motion control systems are those required for the safe operation of the craft in flight. As an aircraft moves slowly, these components are more sluggish to respond and can feel soft. However, the controls firm up and respond more quickly as the craft’s overall speed rises. 

The first thing to know is that, unlike boats and automobiles, which only move in two dimensions, planes move in three.  Those associated movements are called roll, pitch and yaw. Roll relates to the circular motion, either clockwise or counterclockwise, as a plane moves forward. The pitch concerns the aircraft’s tail or nose moving up or down. Finally, yaw is the right or left motion occurring as the nose turns. 

The three systems described below are also called the aircraft’s primary control surfaces. 

Ailerons 

The ailerons are at the rear edge of the wings, and there is one on each side. They show opposing motion while operating, so as one raises, the other goes down. They increase the lift on one wing while lowering it on the other. That action enables the roll that allows the aircraft to turn. This is the main way a pilot steers a fixed-wing plane. 

Elevator

The elevator, also called a stabilator, is typically on the plane’s rear. It controls pitch, helping the nose move upward or point towards the ground. Airplanes are usually nose-heavy, so the elevator pushes the back end of the plane down to compensate for that characteristic. The elevator is a key part of the aircraft motion control responsible for making the plane climb or make its descent. 

Rudder

The rudder is the plane component controlling the yaw. It works identically to a boat’s rudder, moving the craft left or right. However, its primary function is to counteract a type of drag known as adverse yaw. It occurs when one of the ailerons lowers. This effect can momentarily cause a plane to go in the opposite direction of the one intended by the pilot. 

Wing View from the Lufthansa Airbus A380 Upper Deck. Photo: Chris Sloan/Airways

What Are the Secondary Aircraft Motion Control Systems?


A plane’s secondary aircraft motion control systems allow a pilot to make finer movements during a flight, usually to improve overall performance. 

Flaps

Understanding how the flaps work requires knowing the definitions for the chord line and the angle of attack (AOA). The chord line is an imaginary line between the wing’s leading and rear edges. Then, the angle of attack is the angle between the relative wind and the chord line. 

Flaps move the wing’s rear edges down, and, in turn, affect the chord line. The flaps give the wing a bigger AOA, increasing its lift. However, as the AOA goes up, so does the drag.

Slats

Slats are extendable pieces on the leading wing edges of some fixed-wing aircraft. They give the plane increased lift during its low-speed operations, such as the takeoff, landing and initial climb. The slats activate out and downward from the wing. Some slats also function by bringing panels forward from the wing’s lower surface. 

Spoilers and Speed Brakes

Both speed brakes and spoilers increase drag. However, spoilers also simultaneously reduce lift. A pilot may activate these secondary aircraft motion control systems manually, although some activate automatically. 

Trim Systems

The trim systems adjust the aerodynamic forces on the plane’s control surfaces. Then, the aircraft maintains its set altitude without constant input. Most planes have trim systems to affect the yaw and pitch. However, trim systems for the roll axis are the least common of the three. 

Airbus A220-300 cockpit. Photo: Airbus

The Pros and Cons of Automated Aircraft Motion Control


Today, most planes have automated features, usually collectively referred to as autopilot, that help them fly safely. However, that doesn’t mean pilots can sit back and relax for the whole flight while technology does the work. Although many plane-related duties can happen automatically, many still need a human’s influence. 

That’s true with maintenance, for example. Federal regulations in the U.S. require all planes to get annual inspections. They also need additional checks per every 100 flight hours if carrying any people other than crew members. Maintenance members might use project management software or other tools to help them remember when to perform checks and what to assess, but the job can’t get done without their knowledge. 

An autopilot system controls how the plane moves during distinct flight phases, such as taxi, cruising and approach. However, pilots must go hands-on when the aircraft is at an altitude of fewer than 500 feet due to takeoff or landing. 

One of the advantages of autopilot is that it can correct a plane’s motion more quickly when things like wind gusts disrupt it. The technology can also make it easier for pilots to manage planes’ movements during inclement weather. 

However, things can still go wrong with even the most modern autopilot systems. That happened in 2019 with the Boeing 737 Max planes. One of those planes climbed and dove more than 12 times in six minutes while pilots struggled to troubleshoot the erratic motion. An investigation of these planes revealed that human errors at all phases of the plane’s production and operation ultimately caused the dangerous motion control failures that claimed hundreds of lives in the resultant crashes. 

Most plane passengers never think about all the components working together to ensure they reach their destinations safely. However, as commercial pilots know well, having a thorough understanding of when and how these parts affect the flight is critical for trouble-free travel. 


Featured image: Vueling Airbus A320 wing-view. EC-MUM. Photo: Fabrizio-Spicuglia/Airways

author
Emily Newton is a technology journalist. She is Editor-in-Chief of Revolutionized, an online magazine exploring the latest innovations.

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