Ground School - Instruments

Instruments & Controls
*Control Surfaces* As a result of the four forces of flight, and movement about the three axis, control surfaces are built into the airplane. These control surfaces allow the pilot to climb, descend, turn, and maintain control the aircraft. Movement of the control surfaces is done in the cockpit by pilot or auto pilot by changing the position of the control yoke. Control surfaces are basically moveable units of stabilizing structures that deflect air. This deflected air results in movement of the aircraft dependent on the control surface use. The surface that stabilizes Yaw movement is the Vertical Stabilizer. The movable unit is the Rudder. The Horizontal Stabilizer stabilizes the airplane in Pitch. It’s movable unit is the Elevator. Roll is stabilized by the wings. The movable portion of the wing that effect Roll is the Aileron. The other movable unit of the wing is the Flap. This unit has effect in Pitch since it increases the lift of the wing which tends to move the nose up.
	*Aircraft Instruments* Instruments seen on the aircraft panel above can be divided into three types: A) Engine, B) Navigation, C) Flight Instruments. The instruments below are flight instruments. They are mechanical devices that give the pilot information about the airplane’s speed, attitude, altitude, and direction. It’s these instruments that permit planes to be flown in low visibility, or Instrument Conditions, without any reference to visual keys outside the airplane such as the ground or horizon. The instruments can further be divided up into Pressure Measuring Instrument and Gyroscopic Instruments. The pressure instruments are further divided into Pitot and Static pressure, and gyroscopic instruments divided into Vacuum and electrically driven.
Flaps Aileron Wing Rudder Vertical Stabilizer Elevator Horizontal Stabilizer	Pitot pressure Static Pressure Vacuum Pump Pressure Electrical
The Airspeed Indicator measures the speed of the airplane as it goes through the air. On a day without any wind, the indicated airspeed would be the same as the speed of the airplane going over the ground. However if the plane is going into a headwind of 15 MPH at an indicated airspeed of 135 MPH, the ground speed would be 120 MPH. Likewise the plane had a 15 MPH tail wind, the ground speed would be 150 MPH. The instrument operates with Ram or impact air entering the pitot tube. It also uses static air pressure in the case as a reference for the ram air pressure. The impact air pushes on a diaphragm which moves the indicating needle on the instrument face. The instrument is color coded to alert the pilot to the various operating speeds of the aircraft.	The Attitude Indicator, or sometimes called the, Artificial Horizon, is a vacuum driven gyroscopic instrument. It relays information to the pilot as to the flight attitude of the plane, whether it is climbing, descending, or banking. The white line going across the face of the instrument represents the horizon, blue the sky, black, the ground. The power for the instrument comes from the suction through a closed system generated by a vacuum pump. The air flowing through the instrument case moves vanes attached the gyroscope, causing the gyroscope to spin and the instrument to operate.	The Altimeter is an aneroid barometer which converts a barometric pressure reading measured in inches of mercury into an altitude measured in feet. It measures the altitude of the aircraft above sea level when the altimeter is set to the current barometric pressure setting. If the elevation of an airport is 670 feet above sea level, when set correctly, the altimeter should read 670 feet while the airplane is still on the ground. As the plane climbs the decreasing pressure of the static air relaxes a bellows in the instrument. When the bellows moves, a linkage connected to the hands of the instrument increases the indicated altitude. When the plane descends, the bellows expands due to the higher pressure, with a decrease in indicated altitude.
The Turn Coordinator is actually two instruments in one. The miniature airplane is an electrically driven gyroscope that initially measures the rate of roll of a bank. Once the bank is stabilized, the instrument measures rate of turn of a bank. The two white marks below the wingtips of the airplane are timing marks. When the wingtip is aligned with the either mark, depending on the direction of the bank, it will take 2 minutes to make a complete 360 degree turn. The black ball below the miniature airplane is in a fluid filled tube called and inclinometer. It is very much like a common carpenters level. The only power driving this portion of the instrument is centrifugal force acting on the airplane during a turn. When the ball stays in the middle during a bank, the turn is coordinated. If the ball falls to the side of the dipped wing during a turn, the plane is in a slipping turn. If the ball is opposite of the dipped wing, the plane is in a skidding turn.	The Directional Gyro is another vacuum driven gyroscope. It looks much like a compass. A major difference it has with the compass is that it doesn’t rely on the earth’s magnetic field to operate. When the gyroscope is spinning it has a principle of remaining rigid in space. That is the spinning wheel will resist any change in position. The DG takes advantage of that principle. When an airplane is turning the gyroscope will resist moving with the turn. The energy used to resist the turn instead moves the compass card which will indicate the heading of the airplane. DG’s are used because they are not effected by magnetic disturbances or have turning errors inherent to the compass. They are susceptible to gyroscopic precession which are errors due to the mechanical friction imposed on the spinning gyroscope.	The Vertical Speed Indicator measures the speed at which the plane climbs or descends. Like the altimeter, it measures the pressure changes of the static air. Where the altimeter traps the static air in the case to maintain the indicated altitude, the *VSI* allows the pressurized air in or out of the instrument at a controlled rate. The rate the air comes in or goes out of the instrument is translated into feet per minute. On the ground high static pressure is inside the instrument. As the plane climbs, the high pressure leaks out a calibrated hole in a bellows. As the bellows contracts, a linkage moves the indicator hand upward showing a climb. As the plane descends, the bellows expands by the intake of the higher static pressure. The indicating hands then show a descent. The rate the air moves in or out of the instrument determines how much the needle moves.