What is A/T on Boeing 737? (Autothrottle)

The Autothrottle (A/T) system is an integral part of the Boeing 737 aircraft, playing a crucial role in controlling the thrust output of the engines. It is an automated system that works in conjunction with the Flight Management Computer (FMC) to maintain the desired airspeed during different phases of flight. In simple terms, autothrottle takes care of the engine power settings so that pilots can focus on other aspects of flying.

As the name suggests, the autothrottle system automatically adjusts the throttle levers to achieve the selected speed, reducing the workload for the pilots. It ensures that the aircraft maintains a constant speed during cruise, safely accelerates during takeoff, and decelerates smoothly during landing. By controlling thrust output, the A/T system also helps in improving fuel efficiency and reducing engine wear and tear.

How Does Autothrottle Work?

The Autothrottle system on the Boeing 737 is equipped with multiple modes to cater to different flight phases. These modes include:

  • Takeoff Mode
  • Climb Mode
  • Cruise Mode
  • Descent Mode
  • Approach Mode
  • Go-Around Mode

In takeoff mode, the autothrottle system assists the pilots in achieving the desired takeoff thrust. It takes inputs from the flight management system and adjusts the throttle levers accordingly. This allows for a smooth acceleration and rotation during takeoff.

During climb mode, the autothrottle system maintains the desired climb speed set by the pilots. It continuously adjusts the throttle levers to match the required thrust for the given climb profile. This helps in optimizing performance and fuel consumption.

Once the aircraft reaches the desired altitude, the autothrottle system enters the cruise mode. In this mode, it works to maintain a constant cruise speed, adjusting the thrust as necessary to compensate for external factors like wind and weight changes. By automatically fine-tuning the throttle levers, the A/T system ensures a smooth and efficient cruising experience.

During descent mode, the autothrottle system assists in a controlled descent by adjusting the thrust to maintain the desired speed. It helps the pilots in managing the aircraft’s energy state and descent profile, ensuring a safe and accurate approach to the destination.

Approach mode is activated as the aircraft prepares for landing. The autothrottle system works in conjunction with the flight management system to maintain a pre-selected approach speed. It adjusts the thrust to ensure a stable approach and helps in reducing the workload for the pilots during a critical phase of flight.

In case a go-around is necessary, the autothrottle system automatically adjusts the thrust to the appropriate level, allowing for a quick and safe climb out from the runway. This mode provides pilots with an added safety net during missed approaches or emergency situations.

Benefits of Autothrottle

The Autothrottle system offers several advantages and benefits for both pilots and the aircraft:

1. Workload Reduction: The A/T system reduces pilot workload by automatically controlling the engine power settings, allowing them to focus on other critical tasks such as navigation, communication, and overall flight management.

2. Consistent Speed: The autothrottle system ensures that the aircraft maintains a consistent speed throughout different flight phases, enhancing safety and passenger comfort.

3. Fuel Efficiency: By optimizing the engine thrust output, the A/T system helps in improving fuel efficiency, leading to cost savings for airlines and reduced environmental impact.

4. Engine Protection: The autothrottle system prevents excessive engine wear and tear by regulating the thrust output within safe limits. This helps in extending the engine life and reducing maintenance costs.

5. Safety Enhancements: The A/T system provides an added layer of safety by automatically adjusting the thrust during critical flight phases such as takeoff, landing, and go-around. It assists pilots in maintaining the desired speed and contributes to overall flight stability.

It is important to note that while the autothrottle system is a valuable tool, pilots are still responsible for monitoring its operation and making appropriate manual adjustments when required. In cases of system malfunctions or abnormal situations, pilots are trained to take control and override the automated system.

In conclusion, the Autothrottle (A/T) system on the Boeing 737 plays a vital role in ensuring safe and efficient flight operations. By automatically adjusting the throttle levers, it assists pilots in maintaining desired speeds during different flight phases. The A/T system offers several benefits, including reduced pilot workload, consistent speed control, improved fuel efficiency, engine protection, and enhanced safety. However, it is essential for pilots to remain vigilant and be proficient in manual throttle control to deal with any unexpected situations.

For More: What is EDP on Boeing 737? (Engine Driven Pump)