Instrument Rating Ground School: Flight Instrument Systems and Gyroscopic Principles

Flight Instrument Systems

FAA Instrument Requirement (§91.205)

According to FAR 91.205(d), For IFR flight, the following instruments and equipment are required:

1. Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.
2. Two-way radio communication and navigation equipment suitable for the route to be flown.
3. Gyroscopic rate-of-turn indicator, except on the following aircraft:
4. Slip-skid indicator.
5. Sensitive altimeter adjustable for barometric pressure.
6. A clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation.
7. Generator or alternator of adequate capacity.
8. Gyroscopic pitch and bank indicator (artificial horizon).
9. Gyroscopic direction indicator (directional gyro or equivalent).

G - generator/A. R - 2 Radios A - Attitude B- Ball C- clock A - Altix R - Rate of+ D - D. GIRO

FAA Instrument Requirement (§91.205)

Additionally, per 91.205(e) If VOR navigation equipment is required under paragraph (d)(2) of this section, no person may operate a U.S .- registered civil aircraft within the 50 states and the District of Columbia at or above FL 240 unless that aircraft is equipped with approved DME or a suitable RNAV system. When the DME or RNAV system required by this paragraph fails at and above FL 240, the pilot in command of the aircraft must notify ATC immediately, and then may continue operations at and above FL 240 to the next airport of intended landing where repairs or replacement of the equipment can be made.

Gyroscopic Flight Instruments

Several flight instruments utilize the properties of a gyroscope for their operation. The most common instruments containing gyroscopes are the turn coordinator, heading indicator, and the attitude indicator.

Gyroscopic System Operation

Gyroscopic instruments, a convenience for VFR flight, are an absolute necessity for IFR flight. The three gyroscopic instruments in your aircraft are the attitude indicator, heading indicator, and turn coordinator. On most small airplanes, the vacuum system powers the attitude and heading indicators, while the electrical system powers the turn coordinator. Gyroscopic instrument operation is based on two fundamental concepts that apply to gyroscopes - rigidity in space and precession.

Heading indicator Vacuum relief valve 33 P .08 6 E 12 15 O Vacuum pump 0 4 6 SUCTION 2 8 O 10 Suction gauge Attitude indicator 1 Vacuum air filter Schematics of a typical vacuum system Overboard vent line STRY PAS

Gyroscopic Flight Instruments - Principles

Plane of Rotation Plane of Force FORCE Plane of Precession Rigidity in Space Precession

Attitude Indicator

The attitude indicator, or artificial horizon, is a mechanical substitute for the natural horizon. It is the only instrument that gives you an immediate and direct indication of the airplane's pitch and bank attitude.

During your instrument training, the attitude indicator will become the central part of your scan; you will use the attitude indicator to make precise adjustments to the aircraft's pitch and bank without outside visual reference.

Bank index Gimbal rotation -20 TEST Roll gimbal Horizon reference arm Gyro Pitch gimbal

Attitude Indicator: Working

Its operating mechanism is a small brass wheel with a vertical spin axis, spun at a high speed by either a stream of air impinging on buckets cut into its periphery, or by an electric motor. The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll about the gyro as it remains fixed in space.

A horizon disk is attached to the gimbals so it remains in the same plane as the gyro, and the aircraft pitches and rolls about it.

Bank index Gimbal rotation -20 TEST Roll gimbal Horizon reference arm Gyro Pitch gimbal

Attitude Indicator: Working Details

On early instruments, this was just a bar that represented the horizon, but now it is a disc with a line representing the horizon and both pitch marks and bank-angle lines. The top half of the instrument dial and horizon disc is blue, representing the sky; and the bottom half is brown, representing the ground. A bank index at the top of the instrument shows the angle of bank marked on the banking scale with lines that represent 10°, 20°, 30°, 45°, and 60°. A small symbolic aircraft is mounted in the instrument case so it appears to be flying relative to the horizon. A knob at the bottom center of the instrument case raises or lowers the aircraft to compensate for pitch trim changes as the airspeed changes.

10° 20° 30° + + 45° 60° 2 0 0 -FIO 20- 20 +

Attitude Indicator: Working Principles

A small symbolic aircraft is mounted in the instrument case so it appears to be flying relative to the horizon. A knob at the bottom center of the instrument case raises or lowers the aircraft to compensate for pitch trim changes as the airspeed changes.

220 20 20 ~20 20 - 10 -- -40 20 - · STBY PWR TEST . Climbing left bank Straight climb Climbing right bank Pointer 10° 20° Bank scale MW 20 45ª - 20 -10 20 - - 20 60 10 10 90° 10 20- - 20 STBY PWR TEST PUZZ TO CAGE Level left bank Artificial horizon Adjustment knob C 20 -20 10 -10 . STBY PWR TEST . · STBY PWR TEST . · STBY PWR TEST . Descending left bank Straight descent - 20 - · STBY PWR TEST . STBY PWR O TEST · STBY PWR TEST . STBY PWR TEST . TO Level right bank 20 0 /20 - Descending right bank £ 30° 20 -

Attitude Indicator: Erecting Mechanism

Before the gyro can spin in the horizontal plane, it must erect itself. While the aircraft is taxiing, gravity provides the force to level the gyro. On a vacuum-driven attitude indicator, this is accomplished through the action of pendulous vanes.

Precession Port A Port A Applied force Precession Exhaust air equal in all directions gyro erect Gyro precesses, increasing exhaust from port A Precessing force at port A erects gyro, exhaust air again equal at all ports Figure 10-101. The erecting mechanism of a vacuum-driven attitude indicator.

Attitude Indicator: Errors

Errors in modern attitude indicators are usually very minor, resulting in less than 5 of bank error and 1 bar width of pitch error in a 180 turn. These errors occur because the pendulous vanes act on the attitude indicator's gyro in an undesirable way during turns. Since the gyro is parallel to the horizon, and the G-force inside the airplane is toward the bottom of the airplane (not directly down the gyro's axis of rotation), the vanes open asymmetrically and cause the gyro to precess. The same action which erects the gyro during taxi tries to line up the gyro with the G-force from the turn. Errors in both pitch and bank indication are usually at a maximum as the aircraft rolls out of a 1800 turn, and cancel after 360 0 of turn.

Bank index Gimbal rotation VEST Roll gimbal Horizon reference arm Gyro Pitch gimbal

Attitude Indicator: Precession Errors

Acceleration and deceleration also may induce precession errors, depending on the amount and duration of the force applied. During acceleration, the horizon bar moves down, indicating a climb, which, unfortunately reinforces the same illusion a pilot can experience during acceleration. Instrument Cockpit Check: The ICC for attitude indicator is that, after engine start the gyro must stabilize and be erect within approximately 5 minutes. Then set the miniature airplane with reference to the horizon.

Stlow Bank Pitch Bank index Gimbal rotation VEST Roll gimbal Horizon reference arm Gyro Pitch gimbal

Heading Indicator

The heading indicator is fundamentally a mechanical instrument designed to facilitate the use of the magnetic compass. Errors in the magnetic compass are numerous, making straight flight and precision turns to headings difficult to accomplish, particularly in turbulent air. A heading indicator, however, is not affected by the forces that make the magnetic compass difficult to interpret

Main drive gear Compass card gear Gimbal rotation DD 12 15 Gimbal Adjustment gears Gyro Adjustment knob

Heading Indicator Operation

The operation of the heading indicator depends upon the principle of rigidity in space. The rotor turns in a vertical plane and fixed to the rotor is a compass card. Since the rotor remains rigid in space, the points on the card hold the same position in space relative to the vertical plane of the gyro. The aircraft actually rotates around the rotating gyro, not the other way around. As the instrument case and the aircraft revolve around the vertical axis of the gyro, the card provides clear and accurate heading information.

Main drive gear Compass card gear Gimbal rotation DD 12 15 Gimbal Adjustment gears Gyro Adjustment knob

Heading Indicator: Errors

The errors include:

1. Precession caused by friction. Because of precession caused by friction, the heading indicator creeps or drifts from its set position. Among other factors, the amount of drift depends largely upon tahe condition of the instrument. If the bearings are worn, dirty, or improperly lubricated, the drift may be excessive.
2. Because of precession caused by friction, the heading indicator creeps or drifts from its set position. Among other factors, the amount of drift depends largely upon the condition of the instrument. If the bearings are worn, dirty, or improperly lubricated, the drift may be excessive.

Main drive gear Compass card gear Gimbal rotation DD 12 15 Gimbal Adjustment gears Gyro Adjustment knob

Heading Indicator Instrument Cockpit Check

Instrument Cockpit Check: The ICC for attitude indicator is that, after engine start align the HI with the magnetic compass heading.

Every 15 minutes of cruising flight, you must align the heading indicator with the magnetic compass.

Main drive gear Compass card gear Gimbal rotation DD 12 15 Gimbal Adjustment gears Gyro Adjustment knob

Turn Indicators

Attitude and heading indicators function on the principle of rigidity, but rate instruments such as the turn-and-slip indicator operate on precession. Precession is the characteristic of a gyroscope that causes an applied force to produce a movement, not at the point of application, but at a point 90° from the point of application in the direction of rotation.

Gimbal rotation Horizontal gyro Gimbal Gyro rotation Gimbal rotation Gyro rotation Canted gyro Standard rate turn index Standard rate turn index Inclinometer Inclinometer Turn coordinator Turn-and-slip indicator