The magnetic compass is the primary direction indicator in an aircraft, but it is prone to a number of errors due to acceleration, turbulence and they are sometimes difficult to read. To solve this problem we use a direction indicator based on a gyro. These are stable, accurate, easy to read and can be coupled to an autopilot and even synchronized to a magnetic compass.
The heading indicator or HI makes it possible to fly accurate turns and headings but it has their own distinctive features and characteristics the pilot needs to be aware of.
During preflight, taxi and cruise the pilot needs to look out for correct instrument indications and verify, if possible, with the real world by looking outside. Any failure during IFR means an emergency should backup instruments not be installed in your aircraft.
Inside the HI you will find a tied gyro which its rotor axis horizontally. Thus it spins in the vertical plane. The rotor is mounted in two gimbal rings. The outer ring is mounted vertically and rotates freely around 360° parallel with the normal axis (vertical) of the aircraft. On this outer gimbal the index card is attached with the headings in older models. Newer models have a vertical indicator card.
The inner gimbal ring is mounted horizontally inside the outer gimbal thus making sure that the inner ring can bank as the aircraft pitches and rolls and this arrangement keeps the rotor in a vertical position.
The gyro is caged when both gimbal ring are mechanically locked or clamped at right angles to each other. Then its possible to adjust or synchronise the heading indicator to the magnetic compass.
The gyro rotor is usually driven by a jet of air directed to a number of grooves on the outside of the rotor. The jet is mounted on the outer gimbal ring and this makes sure that, should the gyro not be in a vertical position, precession will erect the gyro (resulting from air jet force and the direction of rotation). Such as after engine start.
Electrical driven gyro's have improved rigidity, greater freedom in pitch and roll, less drift and a faster erection system. But at the same time they are more expensive too.
There are a number of errors or limitations for this indicator the pilot should know about. These are: gimbal error, drift, apparent drift, suction problems, gimbal limitation. We will address them below.
During steep turns, pitching and rolling of the aircraft the changing relations ship between the two gimbal result in an indication error or drift.
Caused by frictional forces in the mechanics of the instrument and is aggravated by age. This will cause a maximum allowable error of 4° / 15 minutes or 16°/ hour. If the error is beyond that the instrument is u/s (unserviceable).
Caused by rotation of the Earth. Maximum 15°/ hour at the poles and zero on the equator. Not so much of an error but, due to this, the pilot needs to adjust the gyro about every 15 minutes.
To calculate this drift / hour for your current latitude use the next formula: Drift = sin(Latitude) × 15.
The instrument requirements are between 3½ and 4½ inHg. Should for any reason suction be too low the rotor RPM will also be too low and the rate precession will increase. As a result the gyro will deviate because gyro rigidity is lower. The opposite will occur when suction is over specifications plus that the gyro will wear at a much higher rate.
Due to manufacturing limitations there are limits on pitch and roll for the gimbals, normally around 55° after which the gyro topples and you will notice the index card rotating very quickly at that moment.
After engine start the gyro will need about 3 to 4 minutes to get up to speed, you should have it 'caged' during that period, if possible. During taxi, after aligning with the magnetic compass, the indication must increase with right hand turns and decrease with left hand turns.
During flight the HI may not precess more than 16°/hour. In turbulence and during practice and flight training the drift will be higher if the aircraft turns a lot. Just check and reset the HI every 15 minutes with the magnetic compass.