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How Does An Attitude Indicator Work?

Boldmethod

Whether you're flying a Cessna 152 or a Boeing 777, an attitude indicator is one of your most important instruments. But how exactly does it work?

Today we're breaking down how an attitude indicator works, both for round-dial and glass cockpit flight decks.

Traditional Attitude Indicators

A typical round-dial attitude indicator has an internal gyroscope that is spun by your plane's vacuum system. Air is pulled through the attitude indicator's scooped rotor, causing the gyroscope to spin.

Mounted horizontally inside your attitude indicator's casing is a gyro that will spin in place. Mounted to the gyro arms is a card with lines depicting degrees of pitch, and on the outside, degrees of bank.

How Does It Determine Pitch?

The simplest way to think about how your flight attitude is determined is to visualize your plane moving around the gyro, rather than the gyro moving.

A traditional attitude indicator measures your aircraft's pitch and bank by the principle of rigidity in space. The gyroscope will remain in the same position as long as it is spinning (and abrupt attitude changes do not occur).

Solid-State Attitude Indicators

New solid-state avionics don't have the spinning parts used in round-dial attitude indicators. Instead, they have a version of an AHARS or ADAHRS. AHARS stands for Attitude, Heading Reference System, while ADAHRS stands for Air Data Attitude Reference System.

In some planes, you will find an AHRS and ADC, while others may have a combined ADAHRS. Check your Avionic's Pilot Guide to figure out what your plane has.

System Components

In this explanation, we'll use the G1000 NXi installed in a PA28-181 aircraft. There will likely be small differences between your aircraft and this example, but the principles will likely be the same.

The G1000 NXi system is made up of LRUs (Line Replaceable Units). Think of these as the building blocks of the system. Each box is an LRU.

The ADAHRS unit communicates your flight information to the integrated avionics units, as well as the flight displays. You'll also notice that there are multiple linkages to each LRU. This is done for redundancy.

How Does This Determine My Attitude?

The attitude information is processed in the GSU 75 Unit.

Your plane's attitude is calculated using two solid-state gyros. One is set vertically, and the other is set horizontally. Accelerometers within the GSU 75 unit, as well as a magnetometer, help filter out small vibrations and irregularities to make your attitude indicator movements appear smooth.

How Do These Solid-State Gyros Work?

So how do solid-state gyros calculate your attitude information? The answer lies within a component about the size of a quarter, called a Micro ElectroMechanical System, or MEMS for short.

MEMS gyroscopes have a vibrating element that can determine attitude based on the energy transfer of Coriolis acceleration. (AMT-Airframe Volume II 10-56)

Simply put, the vibration element within the MEMS unit acts as a gyroscope and provides attitude information depending on how the frequency of the vibrations change.

MEMS microchips are often combined with accelerometers, GPS units, and magnetometers to form a complete AHRS unit.

Laser Ring Gyros

Older glass instruments might have a laser ring gyro (LRG). These systems use the Sagnac Effect to determine pitch and bank information.

Sagnac Effect is the principle in which light takes longer to travel around an object that is rotating in the same direction as the light is traveling, and less time if the object is rotating in the opposite direction.

As you change your aircraft's attitude, the LRG is rotated, and the wavelength of the lazer light is changed, allowing the AHRS unit to process the change in attitude. An LRG unit is required for each axis of flight.

Putting It All Together

If you're flying a round-dial system, your attitude indicator uses a spinning gyro and the principle of rigidity in space to display your attitude information.

If you're flying a modern glass cockpit aircraft, your attitude information is calculated using solid-state accelerometers, often with the help of a magnetometer.

And finally, if you're flying a transport-category aircraft, your attitude information may be calculated using a laser ring gyro.

Sources:

(Gyroscope Technology and Applications: A Review in the Industrial Perspective, Passaro et. al).

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/amt_airframe_hb_vol_2.pdf

https://www.flight-mechanic.com/principles-of-gyroscopic-instruments/

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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