Heading Indicator & Directional Gyro: 9 Important Concepts


What is Heading Indicator?

An airplane has many gyroscopic instruments. One of the three fundamental gyroscopic devices is the Heading Indicator. In the section below, we take an attempt at explaining what is a Heading Indicator.

The Heading Indicator functions mechanically with the accompaniment of the magnetic compass. Straight flight and accurate turns to headings are tough to achieve in foul air or a turbulent wind flow due to several errors in the magnetic compass. A Heading Indicator comes to the rescue in such situations as it is unaffected by the pressures that make interpreting a magnetic compass problematic.

what is a heading indicator
C172 Heading Indicator; Image Source: The original uploader was Cleared as filed at English Wikipedia., C172 heading indicatorCC BY-SA 3.0

Acceleration, deceleration, and high-altitude curvature of the Earth’s magnetic field are a few of the multiple factors that generate inaccuracies in the magnetic compass. A Heading Indicator is a vital assistance in such situations. It does not frequently oscillate while leading or behind a turn, and it is easy to interpret in turbulence or during maneuvers.

What is the difference between the Heading Indicator and the magnetic compass?

The pilot uses a Heading Indicator to determine the plane’s current heading, or direction of flight, based on 360 degrees about magnetic north. Its calibration necessitates utilizing the aircraft’s magnetic compass, which indicates its heading about magnetic north.

In most airplanes, the magnetic compass serves as the principal indication of direction. However, it is difficult to read in turbulence and prone to acceleration and turning mistakes, making it challenging to fly by properly. The Heading Indicator contains a gyroscope and is mandated to be aligned with the magnetic compass during flight. However, it does not get affected by any mistakes pertaining to the same acceleration and turning mistakes as the magnetic compass. This allows for precise maneuvers and a steady direction.

Directional Gyro

The Directional Gyro is one of the six essential instruments, used in the navigating commercial and general aviation aircrafts.

Directional Gyro is the older ancient name of the Heading Indicator itself.

The fastest-moving component of a piston-powered aircraft is the Directional Gyros, commonly known as heading indications or direction indicators. It is an essential aircraft navigation instrument, and its spinning rate is close to 24,000 rpm. It can be mistaken as a compass at first sight.

Heading Indicator Diagram

directional gyro
Image SOurce: U.S. Dept. of Transportation, FAA, DG interior, marked as public domain, more details on Wikimedia Commons

Heading Indicator Purpose

The heading indication sets up for a straight and level, unaccelerated flight with respect to the magnetic compass. This, however, suffers from a variety of inaccuracies. One of them is caused by the downward slope of the Earth’s magnetic field, known as “dip,” in other words.

Dip error accounts for inaccuracies in the magnetic compass during banking or while accelerating or decelerating. Hence it serves no purpose beyond unaccelerated, straight, and level flight. The Heading Indicator is not impacted by the Earth’s magnetic field and prevents the inaccuracies of a magnetic compass.

The Heading Indicator displays the same information as the magnetic compass but without inaccuracies. Before each flight, the compass sets the Heading Indicator’s heading on the ground itself.

How does the Heading Indicator work?

The Gyro is contained in a vertical plane inside the Heading Indicator. It employs space rigidity for operation. The airplane icon in the instrument’s middle indicates the aircraft’s direction.

The rotor rotates in a vertical plane, and a compass card is attached to it. The dots on the card maintain the exact location in space relative to the Gyro’s vertical plane, owing to the stiffness of the rotor in space. It is divided into five-degree increments for clear and precise heading information. Numbers are positioned at every 30 degrees, whereas N, S, E, and W are the denotations for the cardinal directions respectively.

The gyroscope inside the Heading Indicator contributes with an erection mechanism for its attachment to the aircraft yawing plane. Erection Mechanism refers to the concept that the plane is defined by the aircraft’s longitudinal and transverse axes. The gyroscope can be powered either electrically or from a suction pump powered by the engine. Pressure pumps can occasionally be witnessed in high-altitude aircraft.

Vaccuum Pressure Pump System for Heading Indicator; Image Source: U.S. Dept. of Transportation, FAA, Vacuum Pump system, marked as public domain, more details on Wikimedia Commons
Venturi Vaccuum Pump for Heading Indicator; Image Source: U.S. Dept. of Transportation, FAA, Venturi vacuum, marked as public domain, more details on Wikimedia Commons

The heading indication will wander with time. This is called Real Drift and it requires regular adjustment concerning the magnetic compass. On the other hand, an inaccuracy causes that Apparent Drift in the Directional Gyro is subject to the effect of the Earth’s rotation and longitudinal position. The latitude nut must produce a true wander in order to overcome this apparent wander of earth rotation. It is positioned on the inner gimbal to operate on the Gyro in the local vertical with its weight.

Apparent Drift can also be subject to transport wander, which accounts for aircraft movement and confluence of meridian lines towards the poles. It is the change in the course along a great circle (orthodrome) flying path.

How to read Heading Indicator?

We must understand the concept of Drift to comprehend the process of reading a Heading Indicator. The aircraft typically undergoes two types of Drift- Mechanical and Apparent.

Time-to-time realignment of the Heading Indicator in reference with the magnetic compass shall take care of both Mechanical and Apparent Drift. After realigning with the magnetic compass, uncaging of the specific older Heading Indicators becomes essential. Advanced planes feature HI gyros that align automatically without any mechanical intervention.

The Heading Indicator has mechanical elements like friction that causes unalignment with the magnetic north due to gyroscopic precession. This is known as Mechanical Drift.

Furthermore, because the plane is traveling over a revolving globe, the variation in a line in the space of the plane to the north over time results in Apparent Drift.

What does it mean to slave a gyroscope in aircraft avionics components?

The presence of a slaving knob in the Heading Indicator allows the pilot to realign it with the magnetic compass, compensating for both Mechanical and Apparent Drift.

The compensation of the Drift is done every ten or fifteen minutes by the slaving knob. These are also referred to as the Flux Gate System. By removing the need for manual adjustment every ten to fifteen minutes, these ‘slaved gyros’ minimize pilot effort.

Manual alignment of the Heading Indicator with the magnetic compass requires the following steps:

  • Fly straight and level to a reference point, chosen directly ahead of the airplane.
  • Maintain the stability of the nose with the reference point, followed by reading the magnetic compass heading.
  • Maintain alignment between the airplane’s heading and the reference point and correct the Heading Indicator with the readings obtained from the magnetic compass.
  • Ensure consistent heading of the airplane towards the reference point throughout the operation
  • Repeat the procedure for any mistake.

Which instrument shows real heading of aircraft?

IFR activities necessitate the use of Directional information. A magnetic compass might be used for this.

A magnetic compass, while highly reliable, has so many intrinsic flaws, such as magnetic dip. Hence it has been augmented with gyroscopic heading indications. Therefore, Heading Indicators indicate an aircraft’s true heading.

The aircraft’s heading can be viewed over the instrument glass against the tiny aircraft symbol, which displays Directional information concerning 360°.

Electric Heading Indicator

Electric Heading Indicators are Horizontal Situational Indicators (HSIs) regarding how they are powered for operation. They are also termed Electric Gyro.

Horizontal Situational Indicators (HSIs) are devices that integrate navigation and heading into one. HSIs are typically electrically powered and align with the flux gate. Like an attitude indicator, a Heading Indicator’s Gyro is mounted in a double gimbal, but its horizontal spinning axis allows sensing the rotation about the aircraft’s vertical axis.

Why do pilots push and pull the heading HDG knob in a flight?

The HDG knob controls the autopilot’s heading control. Hence the Push and Pull operation of the HDG knob can be understood as given below:

  • Push HDG – managed navigation: The aircraft follows a pre-decided flight route per the FMS flight plan.
  • Pull HDG – chosen heading: It returns to its original position. Clockwise rotation of the knob will make the aircraft turn right, and the left turn is subject to anti-clockwise rotation.

Any changes in aircraft direction are started and controlled by the Heading (HDG) knob when the autopilot is in Heading Mode rather than Navigation (NAV) mode. When in NAV mode, pilots frequently use the HDG knob to set the Heading Bug on the current course as a reference.

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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