The long and thick needle shows increments in 100 feet. The short and thick needle shows altitude in 1000 feet increments. The longest and thinnest needle with the triangle at the end shows the altitude in ten thousand feet increments. It shows the derived altitude with three needles. The altimeter indicates the pressure difference between the set ground pressure or sea level pressure to the currently measured static air pressure. In standard conditions the pressure is 1013 hPa or 29.29 inches of mercury. The correct pressure setting is given by local weather stations, automatic terminal information service (ATIS) or automated weather observing system (AWOS). Rotate the pressure setting knob in the lower left corner to adjust the reference pressure of the altimeter, indicated on the right hand side in hekto-pascals. Bottom right - A steep turn to the right with 60° bank angle and 5° nose up pitch. Bottom center - A steep turn to the left with 45° bank angle. Top right - A climbing right turn with 5° pitch up and 20° bank angle to the right. Top center - A descent with 10° nose down pitch. Top left - A climb with 10° nose up pitch. The image above shows the the following situations in flight. The white dashes at the top of the instrument indicate the bank angles 0°, 10°, 20°, 30°, 60° and 90° for left and right turns. White diagonal lines at the on the brown earth part show the 22.5° and 45° bank angles. Thin white horizontal lines mark the pitch angles -5°, -10° and -15° and thin horizontal black lines mark the pitch angles +5° and +10°. You can also see several lines marking the pitch angles and bank angles to be able to read these angles in degrees. Attached to them is a symbolic picture with a white horizontal line to represent the horizon, a blue sky part above and a brown earth color for the ground. The gyros remain in a fixed attitude while the aircraft rotates around it. Our Cessna 172 is uses a vacuum driven system. The gyros can be driven by electrical motors or by a small vacuum pump attached to the engine, depending on aircraft panel configuration. The attitude indicator consists of three gyroscopes that maintain their attitude relative to the ground reference. For the simple case of flying straight and level we can still give some good speed estimates for the airspeed at which the aircraft will stall. And because of that fact you can't rely solely on the airspeed to stay away from a stall. The airspeed at which a stall happens varies depending on the aircraft weight, the maneuvers that we fly, the flap configuration and many other factors like the current engine power and propeller wash onto the wings. Then we either start descending or if we still try to maintain altitude we will get into a stall situation and the wing will loose even more lift relatively quickly. ![]() If we try to fly slower and slower and we will eventually reach a point where the wing cannot generate enough lift and we can no longer fully compensate the gravitational pull. We usually only need enough lift to compensate the weight of the aircraft when we want to fly straight and level. The more airspeed that we have the more lift the wing can theoretically produce. This is the indicated airspeed and is typically measured in knots, giving us a speed in KIAS, knots indicated airspeed. The airspeed indicator uses the pitot and static pressure from small openings in the fuselage and on the wing to mechanically compute the speed of the oncoming air.
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