What is F/F on Boeing 737? (Fuel Flow)

Fuel flow, abbreviated as F/F, is a critical parameter on the Boeing 737 aircraft that measures the rate at which fuel is consumed by the engines. It plays a crucial role in ensuring optimal engine performance and fuel efficiency. Understanding fuel flow is essential for pilots and engineers, as it helps them monitor and manage the aircraft’s fuel consumption during flight.

In this article, we will delve into the concept of fuel flow on the Boeing 737, discussing its significance, factors affecting it, and how it is measured. So, let’s get started!

How Fuel Flow Impacts Performance

Optimizing fuel flow is crucial for achieving optimal performance on the Boeing 737. The fuel flow rate directly affects several key aspects, including:

1. Thrust Output

The amount of fuel flowing into the engines directly affects the thrust output they generate. By managing fuel flow, pilots can adjust the thrust level during different flight phases, such as takeoff, climb, cruise, and descent. This ensures efficient power delivery at each stage and helps maintain the desired speed and altitude.

For example, during takeoff, a higher fuel flow is required to produce the necessary thrust for accelerating and lifting off from the runway. As the aircraft reaches cruising altitude, the fuel flow is adjusted to a more economical level to maintain a steady speed and altitude without excessive fuel consumption.

2. Fuel Efficiency

Fuel efficiency is a major concern in aviation due to its impact on operating costs and environmental sustainability. By closely monitoring and managing the fuel flow rate, airlines can optimize fuel consumption, leading to significant cost savings and reduced carbon emissions.

The Boeing 737 incorporates advanced systems and technologies that enable efficient fuel management. The aircraft utilizes fuel-efficient engines and aerodynamic design features, combined with precise fuel flow control, to maximize fuel efficiency throughout the flight.

3. Range and Endurance

Fuel flow also directly affects the range and endurance of the Boeing 737. By carefully managing fuel consumption, pilots can extend the aircraft’s range and endurance, enabling it to cover longer distances or fly for extended durations without refueling.

For example, during long-haul flights, where the aircraft needs to cover thousands of miles, pilots calculate the optimal fuel flow rate to maximize the range. By adjusting the fuel flow and considering factors such as wind conditions, altitude, and aircraft weight, pilots can plan and execute flights efficiently while ensuring sufficient fuel reserves.

To summarize, fuel flow on the Boeing 737 is crucial for managing performance, optimizing fuel efficiency, and determining the aircraft’s range and endurance. By maintaining an appropriate fuel flow rate, pilots can ensure safe and efficient operations throughout the flight.

Measuring Fuel Flow on Boeing 737

The fuel flow rate on the Boeing 737 is measured in pounds per hour (lbs/hr) or kilograms per hour (kg/hr) depending on the unit system used. The measurement is typically displayed on the aircraft’s engine indication and crew alerting system (EICAS) or engine instrument and crew alerting system (EICAS) display.

The EICAS display provides real-time information on various engine parameters, including fuel flow, enabling pilots to monitor engine performance and fuel consumption. It allows them to detect any abnormal fuel flow readings or deviations from expected values, which could indicate engine malfunctions or inefficiencies.

The fuel flow rate is influenced by various factors, such as:

  • Throttle position: The position of the engine’s throttle controls the fuel flow rate. Adjusting the throttle affects the amount of fuel supplied to the engines, consequently altering the fuel flow rate.
  • Aircraft weight: The total weight of the aircraft, including passengers, cargo, and fuel, affects the fuel flow rate. Heavier aircraft require more fuel to maintain the desired performance, resulting in a higher fuel flow rate.
  • Altitude: As the aircraft climbs to higher altitudes, the air becomes thinner, leading to reduced engine efficiency. This requires adjusting the fuel flow rate to compensate for the decreased oxygen levels and maintain optimal engine performance.

Pilots continually monitor the fuel flow rate to ensure it remains within the expected range throughout the flight. They make adjustments as necessary, considering the flight phase, aircraft weight, altitude, and other relevant factors to maintain optimal fuel flow and overall performance.


Understanding fuel flow is essential for anyone involved in operating or maintaining the Boeing 737 aircraft. By monitoring and managing the fuel flow rate, pilots can optimize performance, improve fuel efficiency, and maximize the aircraft’s range and endurance. The fuel flow rate is measured in pounds per hour (lbs/hr) or kilograms per hour (kg/hr) and is displayed on the EICAS or EICAS display.

Effective fuel flow management contributes to safe and economical flight operations. It allows airlines to reduce operating costs, minimize environmental impact, and ensure efficient fuel consumption. By emphasizing the importance of fuel flow and incorporating advanced systems, the Boeing 737 continues to be a reliable and efficient aircraft for both passengers and operators.

To learn more about the technical aspects of fuel flow on the Boeing 737, you can refer to this article by Boeing. It provides detailed insights into the aircraft’s fuel management system and the factors influencing fuel flow.

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