Flight Instruments

rc2017's version from 2017-08-08 09:30


Question Answer
Basic Flight InstrumentsKnown as the 6 pack, the instruments are classified into 2 types. Pitot Static (Air Speed Indicator, Vertical Speed Indicator, Altimeter) Gyro (Turn Coordinator, Attitude Indicator, Heading Indicator)
Dynamic Pressure & Static PressureStatic pressure: is exerted equally in all directions. It doesn’t involve any relative motion.
Dynamic pressure: pressure due to relative motion. Also known as RAM pressure
Total pressure: consists of static pressure plus dynamic pressure
Pitot TubeThe pitot tube provides measurement of total pressure. It’s an open-ended L shaped tube parallel to longitudinal axis of aircraft.

It should be mounted in a position where airflow is not disturbed.
Pitot heaters are provided as a precaution against ice blocking. When aircraft is parked, the pitot tube is usually covered to avoid any potential contamination on ground. As part of preflight check, the tube cover should be taken off and checked.
Static VentStatic vent provides measurement of static pressure. Some a/cs have 2 static vents one on each side of fuselage, for more accurate readings during slip or skid (uncoordinated turn)
What is pitot static systemThe Pitot Static System relies on a Pitot Tube to measure the dynamic pressure due to the forward motion of the airplane through the air, and Static Vents (small hole in the fuselage) to measure the static, outside barometric pressure as the airplane gains or loses altitude.
What is gyroscopicA Gyroscope is a spinning wheel, rotating at a high speed, powered by a Vacuum Pump. It usually maintain its plane of rotation (Rigidity in Space). If a force is applied, The gyro maintains a Precision by turning in the direction of its rotation exactly 90 degrees to its axis.
How does Airspeed Indicator work?The airspeed indicator measures dynamic pressure - the pressure caused by forward movement of the aircraft through air. It is a Pitot static instrument. Pitot tube measures "ram pressure," which is a combination of dynamic and static pressure. The airspeed indicator is really a scale, which compares the static pressure from your static ports to ram press pressure (static + dynamic) from the pitot tube. The two static pressures cancel each other out, leaving the dynamic pressure component. Dynamic pressure is what translates into your airspeed.
Indicated Air SpeedIAS is the airspeed read directly from the airspeed indicator.
Calibrated AirspeedCAS is the actual speed of the aircraft through the air, after being adjusted for position and instrument errors. At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds, with nose high pitch attitudes. When flying at sea level under International Standard Atmosphere (ISA) conditions calibrated airspeed is the same as true airspeed. If there is no wind it is also the same as ground speed.
True AirspeedTrue airspeed is the speed of your aircraft relative to the air it's flying through (non standard pressure & temperature). As you climb, true airspeed is higher than your indicated airspeed. Pressure decreases with higher altitudes, so for any given true airspeed, as you climb, fewer and fewer air molecules will enter the pitot tube. Because of that, indicated airspeed will be less than true airspeed. In fact, for every thousand feet above sea level, true airspeed is about 2% higher than indicated airspeed. So at 10,000 feet, true airspeed is roughly 20% faster than what you read off your airspeed indicator.
Equivalent AirspeedEquivalent Airspeed is Calibrated Air Speed corrected for compression air in the pitot tube.
GroundspeedGround Speed is the actual speed of the airplane over the ground. It's true airspeed corrected for wind. With a true airspeed of 100 knots and a tailwind of 20 knots, you'd be flying a groundspeed of 120 knots.
Vertical Speed IndicatorThe vertical speed indicator (VSI) indicates whether the airplane is climbing, descending, or in level flight. The rate of climb or descent is indicated in feet per minute and is displayed in Hundreds of feet per minute. If properly calibrated, the VSI indicates zero in level flight
Heading IndicatorThe Heading Indicator is a Gyroscopic flight instrument. The heading indicator is used to inform the pilot of the aircraft's heading. Unlike a magnetic compass, the Directional Gyro is not as affected by banks, turns, and speed changes (as it is based on Rigidity and Precision principals)
Turn CoordinatorTurn Coordinator is Gyroscopic based instrument that gives information about the direction and rate of a turn. Additionally, it indicates if the turn is being flown in coordinated flight. If the aircraft is slipping or skidding during a turn, the ball in the bottom portion of the Turn Coordinator will not be centered. During a coordinated turn, the ball will remain centered. If the ball is not centered, the pilot must adjust the turn by using the appropriate rudder and fix the adverse yaw
Attitude IndicatorThe Attitude Indicator is also called the artificial horizon and is a gyroscopic based instrument. It shows the realtime position of the airplane in relation to the actual horizon, whether the plane is climbing or descending, or flying straight and level. The top is blue, representing the sky, and the bottom half is usually brown, representing the ground. As the airplane maneuvers in the air, the pair of wings will show the degree of bank and pitch attitude (nose up in the blue area or nose down in brown area).
Standard Rate TurnA Standard Rate Turn, or Rate One Turn, will give a standard rate of turn of 3 degrees per second. Therefore, a 360 degree turn will be exactly 2 minutes. This allows the pilot to determine by time, the degrees of turn. For instance, a pilot could use a standard rate of turn for 60 seconds, and confidently know they have changed their course by 180 degrees based on 3 degrees per second. In Big Aircraft, the rate of turn is 1.5 degrees per second.
AltimeterThe altimeter displays the altitude of the airplane above mean sea level (MSL) when properly adjusted to the current pressure setting at the local airfield (QNH). It measures the atmospheric pressure from the static port of the aircraft. Therefore it can be said that altimeter displays the Pressure Altitude. The value is expressed in feet. Like a clock, it has 3 hands. An Hour like hand – measuring 10s of thousands of feet, A minute like hand for 1000s of feet and a second like hand for measuring 100s of feet. The altimeter has a knob, that helps the pilot to adjust to the local QNH setting, or adjust to the 29.92/1013.25 subscale when flying over Transition Level. The pressure reading is displayed in a small display meter called as the Kolsman Window. Air pressure decreases with increase in altitude. Generally there is 1 inch of mercury loss for every 1000 feet. Or 1 hectapascals for every 30 feet.
Indicated AltitudeIndicated altitude is the reading on the altimeter when it is set to the local pressure relative to the mean sea level (QNH). It is read directly from what the Altimeter is showing. indicated altitude is simply the altitude you read directly off your altimeter. If uncorrected for pressure changes, your altimeter won't be very useful.
True AltitudeTrue altitude is the vertical distance of your airplane above sea level, how high are you truly above the sea. Commonly expressed as "feet MSL"
Absolute AltitudeAbsolute Altitude is Constantly changing and is the vertical distance measurement of the aircraft directly above the ground. Expressed in "feet AGL" (above ground level)
Pressure AltitudePressure Altitude is the altitude where pressure of 1013.25 millibars or 29.92 inches exists. Pressure altitude and indicated altitude are the same when the altimeter setting is 29.92" Hg or 1013.25 millibars.
Density AltitudeDensity Altitude Density altitude is pressure altitude corrected for non-standard temperature. It has direct impact on the Aircraft's performance on a hot day or a cold day.