An attitude indicator (AI), also known as gyro horizon or artificial horizon or attitude director indicator (ADI, when it has a Flight Director), is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to Earth's horizon. It indicates pitch (fore and aft tilt) and bank (side to side tilt) and is a primary instrument for flight in instrument meteorological conditions.
Attitude indicators are also used on manned spacecraft and are called Flight Director Attitude Indicators (FDAI), where they indicate the craft's yaw angle (nose left or right), pitch (nose up or down), roll, and orbit relative to a fixed-space inertial reference frame, for which an FDAI can be configured to use known positions relative to Earth or the stars, so that the engineers, scientists and astronauts can communicate the relative position, attitude, and orbit of the craft.
Video Attitude indicator
Use
The essential components of the indicator are:
- "miniature airplane", horizontal lines with a dot between them representing the actual wings and nose of the aircraft.
- the center horizon bar separating the two halves of the display, with the top half usually blue in color to represent sky and the bottom half usually dark to represent earth.
- degree indices marking the bank angle. They run along the edge of the dial. On a typical indicator, there is a zero angle of bank index, there may be 10 and 20 degree indices, with additional indices at 30, 60 and 90 degrees.
A 45 degree bank turn is made by placing the indicator equidistant between the 30 and 60 degree marks. A 45 degree bank turn is usually referred to as a steep turn.
The pitch angle is relative to the horizon. During instrument flight, the pilot must infer the total performance by using other instruments such as the airspeed indicator, altimeter, vertical speed indicator, heading indicator (directional gyro), turn and slip indicator, and power instruments (e.g., an engine tachometer). "Performance = Attitude + Power".
Most Russian-built aircraft have a somewhat different design. The background display is colored as in a Western instrument, but moves up and down only to indicate pitch. A symbol representing the aircraft (which is fixed in a Western instrument) rolls left or right to indicate bank angle.
It was proposed that a hybrid version of the Western and Russian artificial horizon systems be developed that would be more intuitive than either, although this concept never caught on.
Maps Attitude indicator
Operation
Traditional, self-contained attitude indicators use a gyroscope (powered via vacuum or electrically) to establish an inertial platform. The gyroscope is geared to a display that has two degrees of freedom, simultaneously displaying pitch and bank. The display may be colored to indicate the horizon as the division between the two colored segments (typically blue for sky and brown for ground), and is intended to be intuitive to use. The actual bank angle is calibrated around the circumference of the instrument. The pitch angle is indicated by a series of calibration lines, each representing 5° or 10° of pitch depending on design.
Attitude indicators have mechanisms that keep the instrument level with respect to the direction of gravity. The instrument may develop small errors, in pitch or bank during extended periods of acceleration, deceleration, turns, or due to the earth curving underneath the plane on long trips. To start with, they often have slightly more weight in the bottom, so that when the aircraft is resting on the ground they will hang level and therefore they will be level when started. But once they are started, that pendulous weight in the bottom will not pull them level if they are out of level, but instead it's pull will cause the gyro to precess. In order to let the gyro very slowly orient itself to the direction of gravity while in operation, the typical vacuum powered gyro has small pendulums on the rotor casing that partially cover air holes. When the gyro is out of level with respect to the direction of gravity, the pendulums will swing in the direction of gravity and either uncover or cover the holes, such that air is allowed or prevented from jetting out of the holes, and thereby applying a small force to orient the gyro towards the direction of gravity. Electric powered gyros may have different mechanisms to achieve a similar effect. The problem with these leveling mechanisms is that they will respond not only to gravity but to accelerations in other directions due to other causes, such as turns. If the gyro simply immediately oriented itself to the net acceleration vector, then it would be useless. So these adjustment mechanisms act to right the gyro very slowly, such as 2 to 8 degrees per minute, in the hopes that the gyro will stay nearly stable during any brief maneuvering, and that most of the time the aircraft will be flying in a steady, non-accelerating manner so that the gyro has a chance to orient to gravity.
Some attitude indicators can only tolerate a specific range of bank angles. If the aircraft rolls too steeply or achieves an extreme pitch attitude -- while performing aerobatics, for example -- the attitude indicator can "tumble" (or "topple") due to gimbal lock and become temporarily unusable. For this reason, some attitude indicators are fitted with a "caging mechanism" (a device to restore the gyroscope to an erect position). Some attitude indicators can be manually erected once the airplane is in level flight using the caging mechanism. Most modern instruments are designed to tolerate 360 degrees of rotation in pitch and roll without tumbling, although periods of violent aerobatics may tumble nearly any mechanical gyro horizon. Once tumbled, an instrument without a caging mechanism may not be able to re-erect itself until power is removed and the aircraft is in a level pitch and roll attitude for a long enough period that the gyro rotor comes to a stop. However the erection mechanism of most gyros will return it to level very slowly over a period of several minutes.
Attitude and Heading Reference Systems (AHRS) are able to provide three-axis information that can be shared with multiple devices in the aircraft, such as "glass cockpit" primary flight displays (PFDs). Rather than using a spinning rotor for the horizon reference, modern AHRS use 3-dimension ring laser gyroscopes, accelerometers, and magnetometers to detect the airplane's pitch and roll attitude. AHRS have been proven to be highly reliable and are in wide use in commercial and business aircraft. Recent advances in MEMS manufacturing have brought the price of FAA-certified AHRS down to less than $15,000, making them practical for general aviation aircraft.
With most AHRS systems, if an aircraft's AIs have failed there will be a standby AI located in the center of the instrument panel, where other standby basic instruments such as the airspeed indicator and altimeter are also available. These mostly mechanical standby instruments may be available even if the electronic flight instruments fail, though the standby attitude indicator may be electrically driven and will, after a short time, fail if its electrical power fails.
Attitude indicators are subject errors caused by gyroscopic precession. If necessary, the pilot should set the attitude indicator prior to flight.
See also
- Acronyms and abbreviations in avionics
- Korean Air Cargo Flight 8509
- Peripheral vision horizon display (PVHD)
- Turn and slip indicator
References
Source of the article : Wikipedia
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