All About Altimeters
The altimeter provides the basic function of indicating to the pilot the altitude of the aircraft above mean (average) sea level (MSL). The indicator is normally a 31/8” size dial face with multiple pointers or a combination of pointers and counter drum. Location of this indicator is typically in the top row of instruments near the center of the instrument panel. In a standard “T” configuration panel the altimeter is just to the right of the attitude gyroscope.
Types of Altimeters
The various types of altimeters include:
- Three pointer altimeter
- Counter Drum Altimeter
- Encoding Altimeter
Typical altimeter ranges are:
- –1000 Ft. to +20,000 Ft.
- –1000 Ft. to +35,000 Ft.
- –1000 Ft. to +50,000 Ft.
- –1000 Ft. to +80,000 Ft.
Most general aviation altimeters will fall into the first two ranges. Ranges above 35,000 Ft. are typically corporate jets, commercial aircraft, and military aircraft.
Three Pointer Altimeter
The three-pointer altimeter is the most common type of instrument used in general aviation. It is named as such because it utilizes three pointers in order to display the current altitude. One pointer is used to display 100 Ft. increments. A second pointer is used to display 1000 Ft. increments and the third pointer displays 10,000 Ft. increments. The Technical Information Section of this document provides instructions on how to properly read a three-pointer altimeter.
Counter Drum Altimeter
The counter-drum altimeter is named as such because it displays altitude utilizing a single pointer and a rotating drum that displays digits. The drum displays ten thousand and one thousand foot increments. The pointer displays from 0 to 999 feet.
An encoding altimeter can be of either the three-pointer or counter drum type of altimeter with an encoding module built into it. The encoding module takes the altitude information and converts that data into a digital code. This code is then sent via a set of wires to the aircraft transponder. A transponder is a radio device that reports the aircraft altitude to ground control radar.
The blind encoder is a very special type of altimeter. This unit has no dial or read out that is visible to the user. It has only an electronic output to the aircraft transponder. The use of a standard altimeter in conjunction with a blind encoder is often more economical than purchasing an encoding altimeter.