Flight Instruments

3. Absolute altitude—the vertical distance of an aircraft Vertical Speed Indicator (VSI) above the terrain, or above ground level (AGL). The VSI, which is sometimes called a vertical velocity indicator (VVI), indicates whether the aircraft is climbing, 4. Pressure altitude—the altitude indicated when descending, or in level flight. The rate of climb or descent is the altimeter setting window (barometric scale) is indicated in feet per minute (fpm). If properly calibrated, the adjusted to 29.92 "Hg. This is the altitude above VSI indicates zero in level flight. [Figure 8-5] the standard datum plane, which is a theoretical plane where air pressure (corrected to 15 °C) equals 29.92 "Hg. Pressure altitude is used to compute density Principle of Operation altitude, true altitude, true airspeed (TAS), and other Although the VSI operates solely from static pressure, it is a performance data. differential pressure instrument. It contains a diaphragm with connecting linkage and gearing to the indicator pointer inside 5. Density altitude—pressure altitude corrected an airtight case. The inside of the diaphragm is connected for variations from standard temperature. When directly to the static line of the pitot-static system. The area conditions are standard, pressure altitude and density outside the diaphragm, which is inside the instrument case, altitude are the same. If the temperature is above is also connected to the static line but through a restricted standard, the density altitude is higher than pressure orifice (calibrated leak). altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This Both the diaphragm and the case receive air from the static is an important altitude because it is directly related line at existing atmospheric pressure. The diaphragm receives to the aircraft’s performance. unrestricted air, while the case receives the static pressure via the metered leak. When the aircraft is on the ground or in level A pilot must understand how the performance of the aircraft flight, the pressures inside the diaphragm and the instrument is directly related to the density of the air. The density of case are equal, and the pointer is at the zero indication. the air affects how much power a naturally aspirated engine When the aircraft climbs or descends, the pressure inside produces, as well as how efficient the airfoils are. If there are the diaphragm changes immediately, but due to the metering fewer air molecules (lower pressure) to accelerate through action of the restricted passage, the case pressure remains the propeller, the acceleration to rotation speed is longer higher or lower for a short time, causing the diaphragm to and thus produces a longer takeoff roll, which translates to contract or expand. This causes a pressure differential that a decrease in performance. is indicated on the instrument needle as a climb or descent. When the pressure differential stabilizes at a definite ratio, As an example, consider an airport with a field elevation the needle indicates the rate of altitude change. of 5,048 feet MSL where the standard temperature is 5 °C. Under these conditions, pressure altitude and density altitude are the same—5,048 feet. If the temperature changes to 30 °C, the density altitude increases to 7,855 feet. This Diaphragm means an aircraft would perform on takeoff as though the field elevation were 7,855 feet at standard temperature. Conversely, a temperature of –25 °C would result in a density 2 I 3 altitude of 1,232 feet. An aircraft would perform much better VERTICAL SPEED under these conditions. UPTHOUSAND FT PER MIN 0 4 Instrument Check IDOWN 3 2 Prior to each flight, a pilot should examine the altimeter for proper indications in order to verify its validity. To determine the condition of an altimeter, set the barometric scale to the Direct static pressure current reported altimeter setting transmitted by the local airport traffic control tower, flight service station (FSS), or Calibrated leak any other reliable source, such as ATIS, AWOS, or ASOS. The altimeter pointers should indicate the surveyed field elevation of the airport. If the indication is off more than Figure 8-5. Vertical speed indicator (VSI). 75 feet from the surveyed field elevation, the instrument should be referred to a certificated instrument repair station for recalibration. 8-7