Aircraft Braking Systems

Aircraft are among the most complex engineering achievements in the history of aviation. Just as crucial as the takeoff and cruise phases, the landing process and safe ground maneuvers are operationally critical. To ensure controlled and safe touchdown on the ground, advanced engineering mechanisms have been developed that integrate and synchronize multiple braking systems. These systems are designed to effectively reduce the aircraft’s high speed and enable a safe stop on the runway.

Commercial passenger aircraft can reach speeds of approximately 240 to 280 km/h during landing. For such large, heavy aircraft to come to a complete stop within an average runway length of 1.5 to 3 kilometers, highly powerful and reliable braking technologies are required. Adverse weather conditions, short runways, high payload capacity, or emergency situations further increase the critical importance of braking systems and elevate performance requirements to the highest level.

Classification of Braking Methods

For aircraft capable of traveling thousands of kilometers in flight, the landing phase is a multidimensional engineering challenge requiring the synchronized operation of aerodynamic, mechanical, and digital subsystems. In modern aircraft, the braking process is not limited to the pilot pressing the brake pedal; it also involves the deployment of aerodynamic surfaces, redirection of engine airflow in reverse, and the use of advanced computerized control systems—all integral components of the braking operation.

Aerodynamic Braking (Air Resistance / Lift Reduction)

Aerodynamic braking utilizes the drag generated by airflow as soon as the aircraft’s landing gear makes contact with the runway to reduce speed. The most commonly used aerodynamic components in this context are spoiler systems.

Operation of the Spoiler Mechanism: Spoiler panels mounted on the upper surface of the wings are activated automatically or by pilot command upon touchdown, lifting upward to disrupt airflow. This process creates two primary effects:

• Lift Reduction: The deployment of spoilers reduces the wing’s lift force, allowing the aircraft to settle more firmly onto the runway. This enhances tire-to-surface contact and improves mechanical braking performance.
• Increased Air Resistance: Spoiler deployment disturbs the airflow, significantly increasing friction and aerodynamic drag, thereby naturally reducing the aircraft’s speed.

Aerodynamic braking is an efficient method as it requires no energy consumption; however, it is insufficient on its own to bring the aircraft to a complete stop. Therefore, it is typically used as a supplementary aid during the initial deceleration phase.

Reverse Thrust System

Although jet engines are primarily designed to propel the aircraft forward, after landing this thrust can be redirected in the opposite direction to serve a braking function. This method is known as reverse thrust and is widely used in jet-powered aircraft.

Working Principle: During normal operation, engines direct airflow rearward to generate forward thrust. In reverse thrust mode, movable deflector doors at the engine exhaust activate and redirect the airflow forward. This generates a reverse force that decelerates the aircraft. This method:

• Provides significant braking contribution during the initial landing phase,
• Enhances operational safety on short runways,
• Increases fuel consumption but plays a critical role in emergency stops.

In turboprop aircraft, a similar braking effect is achieved by reversing the pitch angle of the propellers. However, to prevent engine overheating, reverse thrust systems are typically used only for short durations.

In-Flight Speed Control: Spoilers and Airbrakes

Spoilers are not only used during landing but can also be actively deployed during flight for speed control, particularly when a reduction in speed or controlled descent is required. Some aircraft are additionally equipped with body airbrakes. Airbrake systems, commonly found in military jets and certain specialized civil aircraft, extend outward from the fuselage to create drag perpendicular to the airflow.

Comparison of Spoilers and Airbrakes:

• Spoilers reduce lift to enhance landing safety while also acting as speed brakes.
• Airbrakes are designed solely to reduce speed and have no direct effect on lift.

Wheel Brakes

From the moment of runway contact, the primary braking load is carried by the wheel brakes on the landing gear. In modern commercial aircraft, carbon disc brakes are the most commonly used system for this purpose.

Technical Features:

• Carbon Discs: Known for their high heat resistance, lightweight construction, and long service life.
• Hydraulic Systems: Brake force is transmitted via hydraulic pressure. The pressure applied by the pilot on the pedal is transferred through the hydraulic circuit to the brake mechanism.
• Heat Management: Brake discs can reach temperatures of 300–500 °C during braking. Effective heat management is critical to maintaining brake performance.
• Anti-Skid System: Prevents wheel lockup and skidding on the runway surface, functioning similarly to the ABS system in ground vehicles.
• Auto-Brake: Allows selection of different braking intensities (LOW, MED, MAX). This system automatically initiates braking during landing, reducing pilot workload.
• Brake-to-Vacate (BTV): The aircraft pre-selects its exit point on the runway and optimizes its braking profile accordingly, improving runway utilization efficiency.

Auxiliary Systems and Emergency Braking

In some aircraft, additional braking methods are employed based on specific operational requirements:

• Parachute Braking: Especially in military jets, speed-reducing parachutes are deployed after landing on short runways.
• Temperature Sensors: Brake disc temperatures are monitored after landing. If certain temperature thresholds are exceeded, takeoff is prohibited for safety reasons.

Coordinated Operation of Braking Systems

After touchdown, the coordinated activation of braking systems is essential:

1. Upon runway contact, spoilers automatically deploy.
2. The pilot activates the reverse thrust system.
3. Once the aircraft is fully on the ground, wheel brakes engage at maximum effectiveness.
4. Digital control systems such as anti-skid, auto-brake, and BTV process real-time data to optimize braking force, maximizing both safety and performance.