Home Boeing Boeing 737 What is LRRA on Boeing 737? (Low Range Radio Altimeter)

What is LRRA on Boeing 737? (Low Range Radio Altimeter)

The Low Range Radio Altimeter (LRRA) is an important component of the Boeing 737 aircraft. It is a radio altimeter system that provides crucial information about the aircraft’s height above the ground during various stages of flight. This system is vital for safe landings and takeoffs, especially in low visibility conditions such as fog or bad weather.

The LRRA measures the vertical distance between the aircraft and the ground directly beneath it. It uses radio waves to accurately determine this distance, providing pilots with real-time altitude information. By knowing the precise height above the ground, pilots can make better decisions during critical phases of flight, ensuring safe operations.

How Does the Low Range Radio Altimeter Work?

The LRRA operates on a frequency of 4.3 GHz, which falls within the microwave frequency range. It consists of an antenna mounted on the aircraft’s belly, a transmitter, and a receiver. The antenna emits radio waves towards the ground, and upon reflection, the receiver picks up these signals. The time it takes for the radio waves to travel to the ground and back up to the aircraft is then measured.

Based on the measurement of the time taken by the radio waves, the LRRA calculates the distance between the aircraft and the ground below. This information is then displayed to the pilots on a dedicated instrument called the radio altimeter indicator. The indicator provides both visual and auditory cues, enabling pilots to maintain a safe altitude throughout the flight.

Importance of Low Range Radio Altimeter

The Low Range Radio Altimeter plays a critical role in various flight phases, ensuring the safety and efficiency of Boeing 737 operations. Here are some key reasons why the LRRA is so vital:

1. Precise Altitude Awareness: The LRRA provides pilots with accurate and reliable altitude information, especially during critical phases such as takeoff and landing. This allows them to make well-informed decisions regarding descent rates, approach angles, and landing flare techniques.

2. Terrain Awareness: By constantly monitoring the height above ground, the LRRA helps pilots maintain situational awareness and avoid potential conflicts with terrain or obstacles. This is particularly crucial during approaches to airports where terrain and obstacles may be present.

3. Enhanced Safety in Low Visibility Conditions: In poor visibility conditions, such as fog or heavy rain, the LRRA becomes even more important. It enables pilots to accurately judge their distance from the ground and make adjustments accordingly, ensuring a safe landing or takeoff.

4. Enhanced Autopilot Functionality: The LRRA also plays a key role in the autopilot system of the Boeing 737. It provides essential altitude information to the autopilot, allowing it to maintain a precise altitude during different phases of flight, including climb, cruise, descent, and approach.


The Low Range Radio Altimeter is a crucial component of the Boeing 737 aircraft, providing pilots with accurate and real-time altitude information during critical flight phases. By using radio waves to measure the vertical distance between the aircraft and the ground, the LRRA enhances safety, situational awareness, and operational efficiency. Pilots rely on this system to make informed decisions and ensure the smooth and secure operation of the aircraft.

For More: What is CCU on Boeing 737? (Cargo Control Unit)

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