What Is Attitude Indicator: 19 Interesting Facts To Know


Attitude Indicator

There are numerous flight instruments with varied functions. The Attitude Indicator is a pivotal flight instrument used in instrument meteorological circumstances.

Attitude Indicator (AI), otherwise called gyro horizon or artificial horizon, navigates the aircraft with respect to the horizon of the Earth. The device operates on the principle of rigidity in space and indicates the direction of the aircraft with clarity. It is powered by a vacuum/gyroscopic system and the tiny airplane and horizon bar simulate the aircraft’s connection to the actual horizon.

Attitude Indicator Diagram

attitude indicator
Attitude Indicator Markings (Pitch and Roll); Image Source: U.S. Dept. of Transportation, FAA, Attitude Indicator, marked as public domain, more details on Wikimedia Commons

Artificial Horizon

Let us understand what Artificial Horizon is, to know about Attitude Indicator in detail.

The Artificial Horizon is a gyroscopic flying instrument that indicates the aircraft’s attitude in relation to the genuine horizon.

The small airplane symbol and the Artificial Horizon symbolizes the flight path to the pilot as to he is flying level or in a banking or turning in an ascending manner. Vertically erect gyroscope during aircraft rolling and pitching, accounts for proper functioning of the Attitude Indicator. The bearings of these devices have very little friction; yet, even this little quantity exerts a constraint on the gyro, causing precession and tilting.

Tilting from upright position to any extent, is minimized through an erection mechanism which is present inside the instrument casing and it exerts a force on the gyroscope. This force causes the spinning wheel to return to its upright position.

AI flight orientation markings; Image Source: U.S. Dept. of Transportation, FAA, AI aircraft orientation, marked as public domain, more details on Wikimedia Commons

Types of Attitude Indicator

One of the six fundamental flying instruments present in each cockpit is the Attitude Indicator. The visualization on the screen displays the aircraft’s relative location to the Earth’s horizon at a look. The aviation industry has seen a significant shift towards electric and digital versions of attitude indicators, although the conventional mechanical gyroscopes can still be found in many aircrafts.

Pilots now have a variety of options for a new attitude indication in the instrument panel thanks to advancements in digital instrument technology:

  • Traditional Attitude Indicator
  • Electric Attitude Indicator
  • Digital Attitude Indicator
  • All-in-one Attitude Indicator

Traditional Attitude Indicator

Many general aviation aircraft use the conventional vacuum/air-based Attitude Indicators.

To spin the gyroscope within the instrument box, a vacuum is drawn or air is blown. Error in the readings of vacuum gyros is a common sight owing to system contamination and leaks. This also affects the accuracy of bank and pitch angles over a period of time. The tumbling issue does not exist in newer models.

The rotor, which is housed in a sealed container, rotates horizontally about the vertical axis. On a gimbal, the housing pivots about the lateral axis, which in turn can pivot about the longitudinal axis. The instrument case serves as the third gimbal required for universal installation. The horizon bar is attached to the gyro through a lever and lodged up on a pivot at the gimbal frame’s rear. This is further connected to the gyro housing by a guide pin.

Maintaining the horizon bar parallel to the natural horizon accounts for gyroscopic stiffness, when the Attitude Indicator is turned on. The tiny aircraft marking symbolizes the aircraft movement in real time and moves with respect to pitch or bank attitude changes. Air drawn through the filter, moves via passageways in the rear pivot and inner gimbal ring, and finally into the housing. Here it faces the rotor vanes via two apertures on opposing sides of the rotor. The air is then drawn into the vacuum pump or venturi tube via four evenly spaced apertures in the lower section of the rotor housing.

Image Source: U.S. Dept. of Transportation, FAA, Venturi vacuum, marked as public domain, more details on Wikimedia Commons

Electric Attitude Indicator

Electric Indicators act as a substitute to the traditional vacuum power failure by providing battery power for operation.

Instead of a vacuum, electric Attitude Indicators employ an electrically powered gyroscope. Caging in electric Attitude Indicators minimize a lengthy erection procedure during maneuvers.

Because of the increasing relevance of the Attitude Indicator, better electric-driven gyros suitable for light plane installation have been developed. Several accessible kinds represent advancements in basic gyro design elements, simpler reading, erection properties, elimination of induced errors, and instrument restrictions. The specifics among different instruments, as well as the instrument display and cockpit controls, will vary depending on the specific design changes.

Digital Attitude Indicator

Pivotal replacement of a vacuum-driven instrument with an electronic equivalent is approved under the rules and regulations set forth by FAA.

It is implied that digital attitude indications may be used in a glass cockpit installation, altering how pilots scan the instrument panel by merging many traditional gauges into a single unit. Because to the use of solid-state electronics, digital versions are free of pitch and bank constraints. In addition, the systems have battery backup power in the event that the aircraft’s electrical system fails.

All-in-one Attitude Indicator

A glass cockpit, similar to that of a business jet, can be installed in a small general aviation aircraft.

The all-in-one primary flight displays typically consist of:

 The digital panels are lighter and need less maintenance, which helps to cut expenses while also improving the overall flying experience.

How to read Attitude Indicator?

Angular movement around the longitudinal axis indicate bank whereas the same about the lateral axis to indicate pitch (nose up, level or nose down) in the Attitude Indicator. It takes use of the gyro’s stiffness. It is gimbaled to allow movement around the lateral axis, which indicates pitch attitude, and rotation about the longitudinal axis, which indicates roll attitude. Once activated, the indicator remains in a fixed position regardless of the aircraft’s attitude.

Attitude Indicator Parts

The following are the main points of interest to the pilot:

  • The tiny wings connected to the casing remain parallel to the plane’s wings.
  • The horizon bar that divides the ball’s top (bright) and bottom (dark) halves.
  • The degree markings on the dial’s top perimeter. The first three are 10 degrees apart on both sides of the center, followed by 60-degree bank markings and 90-degree bank arcs.

Cessna 172 Attitude Indicator Markings

Attitude Indicator: How it Works?

This section describes the working of the Attitude Indicator through detailed explanation on how every component works individually and contribute to the process as a whole.

Alignment of the wings with the horizon bar is taken care of by the adjustment knob. This allows the pilot’s height to be adjusted. Preferably, the adjustment should be performed when the vehicle is on the ground level.

If the wings are higher than the horizon bar, the aircraft is climbing. Wings that are lower than the horizon bar suggest a good flight. The small airplane wings (attached to the casing) symbolize the aircraft’s wings. The blue section of the ball rolls in an opposite direction to the aircraft’s turn. Its upper portion represents the sky.

Attitude Indicator Bank Angle | Attitude Indicator Degrees

  • A typical turn rate is defined as 15° of bank angle.
  • The width of the symbolic aircraft’s wings and the dot in the middle of the wings signify a pitch shift of around 2°.
  • Pitch from an artificial horizon line is shown in degrees (long lines: 10°, small lines: 5°).
  • The roll from the bank index is displayed in degrees (10°, 20°, 30°, and 60°).
  • It is intended for a 360° roll and an 85° pitch.

Pendulous Vanes Attitude Indicator

A vacuum-driven attitude indicator’s pendulous vane mechanism maintains the gyroscope upright. If the gyro is moved for whatever reason, the vanes and air jets generate a precession force that causes the gyro to erect anew, restoring it to the vertical position.

Attitude Indicator Errors | Attitude Indicator Failure

Erroneous signal readings in the Attitude Indicator can be caused by surpassing of the design suction limits or interruption of the free rotation of the gyro at design speed. Some mistakes are caused through production and maintenance.

Out of balance Components, Clogged Filters, Incorrectly-set Valves and Pump Failure:

Proper installation and inspection might help to reduce such mistakes.

Friction and worn components create other mistakes that are inherent in the instrument’s structure. These flaws, which result in irregular precession and the failure of the instrument to retain correct indications, grow with the instrument’s life.

Design related Errors:

These are caused by the instrument’s usual operation.

Presence of a skid while turning deviates the pendulous vanes from their vertical orientation, resulting in inward processing of the gyro. After returning to straight-and-level, synchronized flight, the small aircraft does a turn in the opposite direction as the skid.

Error in markings:

These inaccuracies are generated by centrifugal force moving the pendulous vanes, causing the gyro to process toward the inside of the turn. In a 180° sharp turn, the inaccuracy is highest.

The erecting mechanism quickly corrects this precession mistake, which is typically 3° to 5°.

Acceleration and Deceleration Errors:

Precession errors are also caused by acceleration and deceleration, depending on the amount and extent of force applied.

The horizon bar descends with acceleration, suggesting a rise. Control performed to rectify this signal will result in a lower pitch attitude than indicated by the instrument. Deceleration causes the opposite mistake.

Other mistakes, such as “Transport Precession” and “Apparent Precession,” are related to the rotation of the planet and are important to pilots and navigators who fly at high speeds and over great distances.

Attitude Indicator Troubleshooting

The common errors have generalized solutions for overcoming them throughout the operation.

  • To eliminate slow movement caused by precession problems, replace worn-out bearings.
  • To eliminate carbon contamination from pump vanes, replace or change the filter.
  • In the event that the limitations are exceeded, stabilize the gyro to center it and restart the erecting process.

Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts. With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously. Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93

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