Understanding Altimeters

Understanding Altimeters


In its purest form, an altimeter is simply an absolute pressure gauge. This means that it is displaying the pressure being exerted by the atmosphere at its current location.

The earth is surrounded by an atmosphere. This atmosphere is the air that we breathe. The atmosphere is held in place by the earth’s gravity. The atmosphere has a specific weight. The weight of the atmosphere is approximately 14 pounds of weight for every square inch of earth when measured at sea level on an average day.

An accurate method of measuring this weight is to use a barometer. A barometer is a reservoir filled with mercury. The reservoir has two openings; one opening is exposed to the atmosphere and the other empties into a glass tube. The atmosphere pushes down on the mercury within the reservoir causing the mercury to fill up the glass tube. How far the mercury goes up into the glass tube is directly proportional to the weight of the atmosphere pushing it. This is why barometric pressure is normally expressed in terms of “Inches of Mercury (InHg)”.

At sea level, on an average day, the barometric pressure is 29.92 InHg. However, this will vary constantly depending on the weather. Stormy weather tends to pull the atmosphere away from the earth’s surface causing lower pressure. Hot, dry weather pushes the atmosphere down causing higher pressure.

The weight of the atmosphere also changes depending on altitude. The closer to sea level that you are, the more air there will be, consequently the atmosphere will weigh more. As you go higher in altitude, the less dense the atmosphere will be, therefore less weight or pressure is exerted. An altimeter measures this change in atmospheric weight as expressed in terms of pressure or feet of altitude.


All About Altimeters

All About Altimeters


General Information

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:

  1. Three pointer altimeter
  2. Counter Drum Altimeter
  3. Encoding Altimeter

Typical altimeter ranges are:

  1. –1000 Ft. to +20,000 Ft.
  2. –1000 Ft. to +35,000 Ft.
  3. –1000 Ft. to +50,000 Ft.
  4. –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.


Encoding Altimeter

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.


Blind Encoder

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.


TGH Aviation Looks to The Future

TGH Aviation Looks to the Future
Auburn, CA , October 29, 2015


Back in 1957, a small aviation company was born, originally named The Gyro House. Fast forward almost 60 years later, The Gyro House has evolved into what is now known as TGH Aviation with almost 40 full time employees under its belt.

Aviation industries always have to keep their eyes on innovation. With that in mind, TGH Aviation is looking to the future with some exciting developments in place today, and in the future!

First, TGH has promoted long time employee Hilary Coury to manage both the Sales and Marketing Departments. Mrs. Coury has worked with TGH for over 6 years and brings a lot of experience and expertise to the position! Mrs. Coury is aggressively making strides to expand the role TGH Aviation plays in today’s aviation landscape making sure that customers, old and new, know what the growing capabilities are and that TGH Aviation is the first company you think of for avionics repair.

As Mrs. Coury has expanded her duties, she has also expanded the marketing department, hiring two new employees to help usher TGH into a new era. The new Marketing Coordinator, Christine Short, brings with her decades of experience marketing for radio stations across the country. Mrs Short has been tasked with expanding name recognition of TGH Aviation through social media, email and print. Elizabeth Tucciarone is fielding a completely new position for TGH, Online Marketing Coordinator. Ms. Tucciarone is responsible for creating a completely new, from the ground up ecommerce site that will feature all of the avionics merchandise sold through TGH Aviation. Ms. Tucciarone has almost a decade of experience creating compelling imagery for web and print.

In addition to exciting new management, TGH recently sought and received approval from Senior Aerospace/Ketema to service their entire line of fuel system components. An increasing demand forGPS enabled equipment inspired TGH to also incorporate functional testing, evaluation, and repair for Garmin’s 430/530 GPS/Nav/Comm units. Typically, a flat rate fee is required when dealing directly with the OEM for these panel-mounted units. TGH tackled the opportunity to resolve simple fixes more cost-effectively, with the purchase of innovative test equipment. Similar strategies were utilized when TGH recently established repair services for the KRA10A/KRA405B radar altimeters, and TDR90 transponders.

As TGH Aviation looks to the future, their laser sharp focus on their customers continues to be priority number 1.   As one of the most trusted and respected Part 145 Repair Stations in the industry today, TGH strives to create a great customer experience each and every time. The company offers nearly 20,000 service capabilities, including the repair of primary flight instruments, avionics, aural warning systems, fuel flow transmitters, and their related indicators and refueling sensors. For a complete list of capabilities, go to for more information.

Contact: Christine Short
Tel. 530-823-6204

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