User:SoundGod3/Draft of Instrument Panel

The Instrument Panel, in aviation terminology, is a collection of flight instruments that give the pilot of an aircraft information on speed, attitude, altitude, engine performance, and navigation in order to safely operate the vehicle. Instruments are mounted on a forward-looking dashboard in the cockpit of the aircraft, similar to that of a car. The instruments in the panel can vary widely between aircraft types, often depending on age and intended purpose. For instance, older aircraft such as a 1912 Curtiss Jenny may have rudimentary flight instrumentation: a magnetic compass, engine RPM indicator, airspeed indicator, and fuel indicator. A semi-modern training aircraft such as a 1975 Cessna 172M will have more instruments availaible for the instrument-rated pilot, such as a Turn Coordinator, HSI, VOR Display/Heading Indicator, Attitude indicator, and so on. High-performance modern aircraft such as a Boeing 737 will have these instruments and more specialized ones implemented in a glass cockpit display.

As can be seen, the number and complexity of the instruments vary by the intended design of the aircraft. The higher the performance of the vehicle, the more weather conditions will be encountered over the lifetime of the vehicle, and the more instruments need to be referenced to safely operate it. This article will cover the instrumentation found in IFR certified airplanes, which can be found to some degree of implementation in every aircraft flying today.

Uses of the Instrument Panel

The instrument panel functionality is largely dependent upon the meteorological conditions the aircraft is operating under.

• Under Visual Meterological Conditions (VMC), the pilot has the outside world to reference attitude, speed, and altitude, as well as access to visual navigation references. (see Visual Flight and VFR for details.) The instrument panel is used in this situation primarily to monitor the engine performance, and as a backup or aid to controlling and navigating the aircraft. The Curtiss Jenny is a good example of a VFR-only aircraft, as the instruments included (see above) cannot give information on speed, attitude, or altitude; all of these must be found through exterior references.
• Under Instrument Meterological Condtions (IMC), the ability to use exterior references to maintain control and navigate the aircraft may be severely hampered. In these situations, without spatial references, the instrument panel is the primary source of information for controlling and navigating the aircraft. Any aircraft flying through cloud layers, inclement weather, poor visibility (such as fog), or in Class A Airspace must operate under these conditions. (see Instrument Flight, IFR, and SVFR for details.)

Primary Flight Instruments

The primary flight instruments on the panel are used to maintain positive control over the aircraft. By positive it is meant that the airplane will respond in the manner intended to the pilot's actions. These six instruments, sometimes known as the Big Six, are the most critical to safe operation under IMC.

Gyro Instrumentation

Attitude Indicator (AI)

Main Article: Attitude Indicator

The Attitude Indicator replaces the visual horizon with an artificial one, so that the pilot can control the airplane's pitch and bank effectively without seeing outside. Since these attributes are crucial to maintaining the altitude, speed, and heading of the aircraft, this is a very important instrument. The AI uses a gyroscope, which, under the principle of rigidity in space, resists external forces applied to it. The aircraft changes attitude around the instrument, giving indication of the relative position of the aircraft to that of the gyro, which when turned on lies in the same reference plane as the earth's horizon.

Heading Indicator (HI)

Main Article: Heading Indicator

The Heading Indicator determines the heading of the aircraft with respect to magnetic north (0°). The HI uses it's internal gyroscope to affix an attitude in space, the reference to magnetic north is set before flight on the face of the indicator, and the aircraft yaws around the gyroscope. The deviation in yaw attitude from initial rest is then indicated as the heading difference from magnetic north.

Turn Coordinator (TC)

Main Article: Turn Coordinator

The Turn Coordinator, or Turn and Bank Indicator, is an instrument that provides information on the "quality" of a turn. Aside from providing bank information (and the 3°standard rate turn guides), the turn coordinator also includes an Inclinometer, or Slip-Skid indicator, which gives information on how coordinated the turn is (how much rudder must be applied to cause the aircraft lift vector to be perpendicular to the cockpit floor.) Coordinated flight prevents certain stalls and spins, and prevents passengers from feeling lateral forces during turns.

The turn coordinator, like the two previous instruments, uses a gyroscope to determine the bank attitude of the aircraft. However, for safety reasons to be explained shortly, the gyroscope is powered by the aircraft electrical system instead of the vacuum system.

Gyro Considerations

The Gyro Instruments develop a condition known as gyroscopic precession, which causes external forces applied to it (friction from the gyro frame) to be "felt" at right angles to the force application. Errors, which must be accounted for in pilotage, develop from this phenomenon. The gyro instruments, with the aforementioned exception of the turn coordinator, are powered by the aircraft engine vacuum system, which blows compressed air through vanes in the gyro rotors to make them spin at high speed.

Pitot-Static Instrumentation

Altimeter (ALT)

Main Article: Altimeter

The Altimeter displays the altitude (height) of the aircraft in Feet MSL. This is an important distinction, because there is a difference between ft. Mean Sea Level (MSL)(or True Altitude and ft. Above Ground Level (AGL), or Absolute Altitude . AGL measures the aircraft height above the ground, which may vary depending on the underlying terrain (i.e, an aircraft overflying Boston at 6000 ft. AGL would only be 720 ft. AGL over Denver in level flight.) MSL depicts the aircraft height above sea level, corrected for local atmospheric pressure, which ideally remains constant despite the terrain.

The altimeter, as found in modern aircraft, is essentially a precision aneroid barometer. It contains a stack of disk-shaped pressure vessels called aneroid cells, pressurized to sea level atmospheric pressure at STP. As the aircraft ascends, outside air sent through a static port decreases in pressure at a predictable rate- about $1.093\times 10^{-3}$ in. Hg per foot altitude (near sea level), and vice versa for descents in altitude. As the aneroid cells expand and contract, a mechanical linkage transferrs the motions to a readable diaplay. The display is similar to that of a clock; it has three hands that revolve clockwise as altitude is gained. The longest hand corresponds to 10 ft. changes, the short hand to 1000 ft. changes, and a third hand or altitude bug to 10,000 ft. changes.

There are two major considerations that pilots must take into account when using an analog altimeter. The first is that the atmospheric pressure fluctuates across the surface of the planet as time goes on. The pressure sealed in the aneroid cells correspond to the standard pressure at sea level, or 29.92 in Hg (10132 mb). As weather patterns move through a local area, the outside pressure will increase or decrease, and thus the local reference pressure will not be equal to that of the disks. This can cause inaccurate readings that may display the aircraft as being thousands of feet in altitude above or below it's true altitude. To correct this, ATC and local Aerial weather stations broadcast the current local atmospheric pressure adjusted to sea level. This information is then set in the Kollsman Window of the altimeter to correct for the discrepancy.

The second consideration is the effect of Outside Air Temperature (OAT) on the aircraft. As temperature rises, the density of the air mass is lessened (see Ideal gas law.). An altimeter reading corrected for both pressure and temperature is called Density Altitude. This is a useful measure when calculating aircraft performance. Hot, humid days, for example, adversely affect the climb rate and takeoff speed V2 of an airplane. If one were to factor in a high airport elevation, the lift that is achieved at V2 by the aircraft wings may not be sufficient to safely depart the runway. Density altitude is thus an important figure for determining go/no go situations for pilots.

Airspeed Indicator (AS)

Main Article: Airspeed indicator

The Airspeed Indicator measures the forward velocity of the aircraft with respect to the air mass it is traveling in. The speed that is called Indicated Airspeed, or IAS. This differs by the speed of the air mass from Ground Speed (GS), and by temperature and pressure variances from True Airspeed (TAS). These are similar to the discrepancies found in the altimeter.

The airspeed indicator measures the dynamic pressure difference between ram air, or air input from the pitot tube by the motion of the aircraft, and static air pressure from the interior of the aircraft static port. On all altimeters made after 1976, the ouptut is measured in nautical miles/hour, or knots(kts). Older altimeters may use kts, statute mph or respective country equivalents.

The IAS is an important figure in aviation. A given wing, for example, stalls at the same IAS and angle of attack (AOA) despite outside conditions. Also, velocities such as V1, V2, and Vref, respective of weight for a given airplane, are given as IAS. TAS is also important, especially if there are timing constraints on a given flight plan, to calculate how much time it would take to travel from one point to another.