The heading indicator (HI) is one of the basic ‘six pack' instruments of the main cockpit and it informs the pilot about direction. Located just underneath the artificial horizon, the HI displays headings in 5-degree increments and uses a rotating gyro to operate. Because it is supplemented by the magnetic compass, the HI has many advantages over the traditional compass, yet the instrument must be reset every 10 to 15 minutes to stay accurate.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is another name for the heading indicator?
Another name for the heading indicator is directional gyro.
How does the heading indicator affect pilot navigation?
The heading indicator affects pilot navigation by informing the pilot of the aircraft's heading and provides a stable, reliable reference for aircraft direction. Because the magnetic compass has dip errors and oscillates in flight, the heading indicator is used in conjunction with the magnetic compass to maintain a steady course and to keep cross-country navigation steady. The pilot uses the heading indicator to navigate, to maintain heading, and to set up clean turns. The line on the heading indicator indicates the direction the aircraft is pointing with reference to magnetic north.
Under IFR and low-visibility conditions, the heading indicator provides a stable, reliable heading reference and gives real-time information about the aircraft's nose direction, permitting accurate turns when outside references are lost. To guarantee accuracy, the pilot must calibrate the instrument every fifteen minutes, because apparent drift - caused by Earth rotating at fifteen degrees per hour and by mechanical friction - makes the heading indicator stray from the correct heading. By periodically resetting the gyro to the magnetic-compass reference, the pilot keeps the navigation system precise and the aircraft safely on course.
How does a directional gyro work?
A directional gyro works on the basis of a rotating mechanism. The directional gyro uses a gyroscope spun up to 24,000 rpm by a vacuum pump driven from the aircraft's engine, exhibiting rigidity in space so the gyroscope resists change to its position. The gyroscope axis points toward the center of the earth while weights keep the axis vertical, aligning with gravity direction. This vertical orientation lets the gyro maintain direction reference on aircraft yawing planes, defined by longitudinal and horizontal axes. Pulling the knob releases the gimbals, allowing aircraft to turn around the card. Pushing the knob disengages the gimbals, allowing the pilot to set gyro and card to current heading, maintaining readout accuracy.
A directional gyro works because a rotating mechanism stays steady, offering true reference while the aircraft turns around it. Unlike the magnetic navigational instrument that swings unpredictably, the gyro's steadiness permitted me to focus on additional flying parameters. To keep the display truthful, we modify the gyro's signal to equalize with the marked compass direction.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What powers the heading indicator?
Most training aircraft power the heading indicator with a vacuum system driven by an engine-driven pump that creates suction. The vacuum pump provides suction for the heading indicator gyro, and the gyro spins at a rate of nearly 24,000 rpm. A suction gauge on the panel indicates the vacuum system. The vacuum system causes the gyro to spin by sucking air through.
What is a vacuum directional gyro? A vacuum directional gyro is simply a heading indicator whose gyro is spooled up by air from a vacuum pump. The gyro is usually driven by suction from a vacuum pump.
What is an electric directional gyro? An electric directional gyro is a heading indicator whose gyroscope receives direct current from the electrical system. An electric motor spins the gyroscope.
How many degrees of freedom does a directional gyro have?
The directional gyro has two degrees of freedom. It spins in the horizontal plane, so its axle defines the spin axis while the instrument sits in a double gimbal. The inner gimbal allows movement about two axes, giving the gyro wheel freedom to move about two axes. The outer gimbal allows only one axis of freedom, the vertical, so yaw sense plus the rotational degree of freedom of the gyro flywheel equals two degrees of freedom.
What is the main component of the directional gyro indicator?
The directional gyro indicator's main element is a rigid, high-speed gyroscope that keeps its spin axis fixed in space. Gimbals allow aircraft to rotate around this rigid wheel while the display remains steady. This gyroscopic rigidity makes the spinning wheel stay fixed in space, so the instrument can show heading changes without being affected by the aircraft's turns, acceleration, or deceleration.
The display consists of a circular compass card calibrated in degrees, with cardinal points denoted on the circular card. The card is driven by signals that come from a flux valve. The flux valve is the direction-sensing device of the system: it continuously senses Earth's magnetic field, amplifies lines of magnetic force into a signal, and sends that signal to the compass card so the card displays compass rose direction in 5-degree increments.
What is the difference between a vertical gyro and a directional gyro?
A vertical gyro keeps its gyro axis in a vertical state by weights, so its spin axis stays aligned with gravity and serves as a pitch-and-roll reference. A directional gyro uses a gyro wheel that is free to spin on a horizontal gimbal; once the instrument is set to the current compass heading, the rigid-in-space spin axis continues to point toward the original heading, letting the airplane's yaw motion reveal itself as a compass-card drift. Thus, the vertical gyro tells the pilot how the nose moves up, down, or banks, while the directional gyro tells the pilot how the nose swings left or right in the horizon.
What is a slaved gyro compass?
A slaved gyro compass combines the best features of a directional gyro and a direct reading compass. It senses magnetic north through a magnetic flux detector called flux valve. The slaved gyro utilizes Earth's magnetic field to provide constant correction required by the gyroscopic element. This magnetic flux eliminates the need for manual realignment every ten to fifteen minutes and reduces pilot workload. The slaved gyro compass is capable of operating any number of remote compass indicators, so it is also known as a remote indicating compass or gyro-magnetic compass.
In the new digital cockpit the slaved state is toggled by a dataref that allows turning on or off the slaving electric gyros. Old installations used a slaving control and compensator unit that had a pushbutton, providing the pilot a means of selecting either slaved gyro or free gyro mode. Whether old or new, the magnetic slaving transmitter is mounted remotely in a wing tip to avoid magnetic interference, and the slaving meter shows a full deflection to one side whenever the aircraft is in a turn and the card rotates, indicating clockwise or counter-clockwise error until correction torque realigns the gyro.
What are the errors of the directional gyro?
Two main errors on the directional indicator gyro are drift and gimballing error. Real drift is slow and continuous and is due to friction occurring in the rotation axis of gimbals. Friction in the gimbals causes the gyroscope to lose rigidity and begin to precess. Apparent drift is predicted by ‘sin Latitude’, because the earth rotates 15 degrees per hour; apparent drift is greatest over the poles. Gimbal error is the mechanical error introduced by the cardan design of a gyro DI. It occurs during large turns at steep angles and disappears when the aircraft returns to level flight. Heading indicator errors also include caging, alignment, suction, and parallax errors, as well as failure to set the directional gyro correctly.
What causes directional gyro failure? Directional gyro failures are most commonly caused by bearing failure. Heading drift in the directional gyro is a pre-indicator of failure, and abnormal sound or vibration from the instrument indicates failure. Gyros often give clues of imminent failure, so pilots monitor for anomalies and malfunctions.
How is a directional gyro aligned to the correct heading?
The directional gyro is aligned to the correct heading by pilot input. To keep the directional gyro (DG) pointing to the true heading, the pilot must realign it while on the ground using a known reference like the runway heading. Before takeoff, the pilot sets the DG to the runway heading. A knob on the front of the instrument is pushed in to lock the gimbals, letting the pilot rotate the gyro and card until the number opposite the lubber line agrees with the magnetic compass.
In flight, drift caused by earth-rate and aircraft movement over the curved surface of the earth makes the reading wander, so the common heading indicator does require periodic adjustment. The normal procedure is to realign the direction indicator once every 10-to-15 minutes. When correction is needed, the pilot again pushes the knob, locking the gimbals and allowing the card to be rotated until the lubber-line number matches the current magnetic compass reading. Releasing the spring-loaded knob instantly disengages the gimbals so the gyro again free-spins with the aircraft.
How do you replace a directional gyro?
To replace a directional gyro, begin by unpacking carefully. Remove the directional gyro from the shipping container. Position the replacement unit in the standard 3 1/8 inch (7.94 cm) panel cutout used by the RCA15 Series, then slide the instrument forward until the bezel seats flush. Directional gyro installation requires inserting the gyro into the instrument panel cutout. Secure it with brass instrument screws. The screws are normally tightened until snug, as over-torque will distort the case and bind the gimbals.
Installing replacement gyro is done in reverse order of removal: disconnect the pneumatic or electrical lines, loosen the captive nuts, withdraw the old unit, fit the new unit, reconnect the lines, and perform a leak and slippage check. Directional gyro is installed according to the aircraft manufacturer's instructions. These instructions specify hose routings, torque values, and the instrument resets before performing maneuvers of any kind.
If the gyro is shot, your choices are four: get it repaired, get it overhauled, get an exchange unit, or buy new. Most malfunctioning gyros can be repaired and returned to service. Overhauling the gyro involves replacing the 4 gimbal bearings with 4 new bearings featuring a titanium carbide coating. Gyro overhauls incorporate relevant service bulletins. Operators include the relevant SB as part of the next gyro overhaul, because service bulletins replacing gimbal bearings are recommended and extend life characteristics. During reassembly, replace the clean and inspected bearings in their recesses. Bearings must fit their recesses with no more than finger pressure. Add one drop of gyro instrument oil to each bearing.
I started replacement of the gyro only after assuring the aircraft's electrical infrastructure is powered down. I unplug the multi-pin coupling, take out board screws, and elevate the former unit without harm to the dashboard or adjoining devices. I clear the mounting region and adjust the new gyro into place. I reconnect the electrical coupling and tighten panel screws.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the history of the heading indicator?
Early development of the heading indicator began when inventor Lawrence Sperry tested attitude indicators in 1916 as part of the military's push for instrument flight during World War I. Sperry constantly conceived innovative flight instruments and personally tested them, establishing the foundation for gyroscopic flight instruments.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
The late 1920s saw the adoption of gyroscopic flight instruments, leading to attitude and heading indicators introduced in the early 1930s. In 1929 aviator Jimmy Doolittle made the first takeoff-to-landing instrument flight using a Sperry artificial horizon, demonstrating the practical application of these instruments. By World War II attitude indicators were common in military aircraft, proving their reliability under demanding conditions.
