Avionics and instruments
The compass aligns itself along the magnetic field that surrounds the earth.
Note that The magnetic north pole and the geographic north pole are not located at the same spot.
The name of this instrument describes the 2 essential parts of its characteristics:
- "directional": the instrument serves to see the direction of the plane. In other words, it serves as a compass.
- "gyro": the instrument functions by way of gyroscopes. These are objects that rotate (see the root of the word: "gyrations"). They tend to maintain their position in space.
The advantages with respect to a magnetic compass are:
- it can be set to point to the geographic north pole, not of the magnetic pole as the magnetic compass does.
- it does not have the tendency to lag behind or ahead while turning, as the magentic compass does.
- it is not subject to magnetic influences. The magnetic compass can point in a slightly off direction due to heavy metal objects on or near the aircraft.
The disadvantage is of course that it is a more complex instrument than a magnetic compass.
- It is heavier than a magnetic compass
- It requires a source of power, either electricity or vacuum. If the power source fails, the DG will fail. Because the gyro spins down slowly, it will slowy drift off heading. If the pilot does not notice that the vacuum or elctrical system has failed, he may not realize that the DG has failed. This can lead a pilot off course.
- Gyros precess over time, and must be periodically realigned with the magnetic compass
- Turbulence or maneuvering can "tumble" the gyro, resulting in wildly erratic readings.
The artificial horizon, also known as the attitude indicator, simulates the position of the horizon relative to the plane. The top half of the background of the artificial horizon is blue and represents the sky. The bottom half is brown, and represents the ground. This background moves as the plane pitches forward and backward, and rolls left and right. In the center of the attitude indicator is a small dot, representing the position of the nose of the plane. To the side of this dot are lines representing the position of the wings. These parts of the attitude indicator do not move, so, when the plane pitches backward 10 degrees, the blue background shifts down 10 degrees. Most attitude indicators have major lines for every ten degrees of pitch, drawn across the center of the background. At the top or bottom of the attitude indicator is an arrow pointing out. This arrow points to corresponding markings on a ring on the attitude indicator. This ring does not move with the pitch of the plane, it only moves as the plane rolls left to right. On this ring are generally markings for 10 degrees of roll, 20 degrees, 30 degrees and 60 degrees. It is important to note that the artificial horizon indicates the orientation of the plane, and not the direction of travel. During slow flight, most planes have a high angle of attack, meaning that for straight and level flight, the nose is raised above the horizon. In this case, the attitude indicator would show the nose above the horizon (in the blue) but the airplane would be going straight and level. The artificial horizon is normally controlled by a gyro which is driven by a vacuum generated by the engine. It is possible that this vacuum can fail, especially if there is an engine failure. For this reason, the turn and bank coordinator is generally powered by an electrical generator and can be used as a backup instrument.
Vertical Situation Indicator
The turn and bank coordinator provides two pieces of information -- the bank of the wings, and the degree of coordination between the ailerons and rudder in a turn. As the plane is rolled left and right, the wings on the turn and bank coordinator rotate as well. To make a proper turn, the ball in the middle of the turn and bank coordinator should stay centered. If the wings are rolled without enough rudder applied, the ball will move to the side. There are markings on the side of the turn and bank coordinator that indicate the amount of bank to apply to make a standard rate turn. At this rate, the plane will make one complete turn in two minutes. The gyroscope powering the turn and bank coordinator is normally electrically powered, unlike the attitude indicator, which is normally powered by a vacuum generated by the engine. If one of the two instruments fails, the other one is likely to continue to work. In FlightGear, it is possible to enable automatic turn coordination, in the event you don't have rudder pedals. To do so, start FlightGear with the commandline argument --enable-auto-coordination. * todo: describe how to properly use a rudder to make a coordinated turn
Bendix/King KAP140 Autopilot
The default Cessna 172 has a "Bendix/King KAP140 Two Axis with Altitude Preselect" autopilot. It controls the elevator and the ailerons.
The Pilots Guide can be downloaded here
To activate the autopilot in wings level (ROL) and vertical speed (VS) modes press the AP button. The autopilot will try to keep the wings level by keeping the turn rate at zero. The autopilot will also try to maintain the vertical speed at activation. Use the UP and DN buttons to set the desired vertical speed.
With the autopilot active you can use the HDG button to toggle between wings level (ROL) and heading select (HDG) modes. In heading select mode the autopilot will try to maintain the heading selected by the heading bug on the directional gyro.
Use the ALT button to toggle between vertical speed (VS) and altitude hold (ALT) modes. In altitude hold mode the UP and DN buttons change the altitude by 20 feet per press.
Press the AP button to deactivate the autopilot. The horizontal and vertical modes can not be activated independently.
Please read the Pilots Guide for complete instructions on the use of the KAP140 Autopilot system.