What Is an Encoding Altimeter?

Posted on March 14, 2022 John Symond

When flying a plane, there are several things that a pilot needs to take care of or remain cognizant of, such as their GPS location, uninterrupted communication during the flights, flying on a correct route, etc. Across all these factors, one such important metric is altitude.

Altitude is a vertical distance of an object measured from the mean sea level. For a plane to fly safely and efficiently, its orientation must always be correct. Every aircraft has an altitude indicator that tells the pilot how high or low they are flying from the earth's surface, hence the indicator must remain accurate. Just like pilots, it is also essential for people sitting in control stations to know the aircraft's location, including the altitude. That's where the encoded altimeter comes into the picture.

Aircraft altitude is continuously monitored and transmitted to an attached radar beacon system. This system consists of a rotating disc that changes its position based on how high or low you fly. Photocells sense this disc rotation, and digital signals are then amplified using the encoded altimeter before they are transmitted.

Generally, the altimeter has a dial of a 31/8" size, and it will include multiple pointers and a counter drum. You can find this meter in the instrument panel in the top row of an aircraft cockpit dashboard. On a normal "T" configuration panel, the altimeter is placed to the right of the attitude gyroscope.

Types of Altimeters

There are generally three types of altimeters available in the aviation market. They are as follows:

  1. Three pointer altimeter
  2. Counter drum altimeter
  3. Encoding altimeter

Aviation altimeter measurement ranges vary depending on the type of aircraft. However, most aviation altimeters will come under the first two ranges (-,1000 ft to 20,000ft. and -1,000 ft. to 35,000 ft.). Some aviation altimeters can even measure heights up to 35,000 ft. Most of these are used by corporations, commercial airlines, and military aircraft because they need a more accurate reading for takeoff or landing safety reasons.

Now that you are familiar with the basic types, let us look at each altimeter in detail and understand how they are different from one another.

1. Three Pointer Altimeter

The three-pointer altimeter is generally the most used instrument in aviation when compared to all types of altimeters. It is called a 'three pointer' altimeter because it displays the present altitude by utilizing three pointers: one to display 100 ft increments, another for 1000 ft., and an additional pointer that shows 10,000 ft increments. 

2. Counter Drum Altimeter

The counter drum altimeter is a unique device that provides altitude information using just one pointer and rotating drums that display digits. Its design is the main reason behind the name ‘counter drum altimeter.’ The drum shows 10,000 and 1,000 ft. increments, whereas the pointer shows values from 0 to 999 feet.

3. Encoding Altimeter

An encoding altimeter is equipped with an encoding device that converts altitude data into a digital code which is then transferred to the transponder using wires. The encoding altimeter can either be a three-pointer altimeter or the counter drum type of altimeter. Transponders are standard radio devices on modern aircraft that report aircraft height to the control radar.

4. Blind Encoder

The blind encoder is an ingenious method for using a standard altimeter. The user can see no dial or readout as electronic signals are sent to your aircraft transponder and displayed on screen in real-time. Blind encoders are perfect when you want readings that are quick with high accuracy, and they are generally less costly than an encoding altimeter.

In Conclusion

Altimeters are a type of device that measures altitude. They do this by measuring air pressure in relation to sea level, and they usually use barometric measurements since most altimeters are barometric. The encoder available in the altimeter plays a vital role since it connects an altimeter to a transponder, enabling air traffic controllers to have precise information about your position.

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