Flight Instruments

deck. Due to the venturi effect of the air flowing around the 100 ft. pointer fuselage, the air pressure inside the flight deck is lower than the exterior pressure. 10,000 ft. pointer When the alternate static source pressure is used, the Aneroid wafers 1,000 ft. pointer following instrument indications are observed: 1. The altimeter indicates a slightly higher altitude than actual. 2. The ASI indicates an airspeed greater than the actual airspeed. 3. The VSI shows a momentary climb and then stabilizes if the altitude is held constant. Each pilot is responsible for consulting the Aircraft Flight Manual (AFM) or the Pilot’s Operating Handbook (POH) Crosshatch flag Static port to determine the amount of error that is introduced into the A crosshatched area appears on some altimeters when displaying system when utilizing the alternate static source. In an aircraft an altitude below 10,000 feet MSL. not equipped with an alternate static source, an alternate method of introducing static pressure into the system should Barometric scale adjustment knob a blockage occur is to break the glass face of the VSI. This Altimeter setting window most likely renders the VSI inoperative. The reason for choosing the VSI as the instrument to break is that it is the Figure 8-2. Altimeter. least important static source instrument for flight. until the pressure inside the wafers is equal to the surrounding Altimeter static pressure. Conversely, if the static pressure is less than The altimeter is an instrument that measures the height of the pressure inside of the wafers, the wafers are able to expand an aircraft above a given pressure level. Pressure levels which increases the volume. The expansion and contraction are discussed later in detail. Since the altimeter is the only of the wafers moves the mechanical linkage which drives the instrument that is capable of indicating altitude, this is one of needles on the face of the altimeter. the most vital instruments installed in the aircraft. To use the altimeter effectively, the pilot must understand the operation Principle of Operation of the instrument, as well as the errors associated with the altimeter and how each affect the indication. The pressure altimeter is an aneroid barometer that measures the pressure of the atmosphere at the level where the altimeter is A stack of sealed aneroid wafers comprise the main located and presents an altitude indication in feet. The altimeter component of the altimeter. An aneroid wafer is a sealed uses static pressure as its source of operation. Air is denser wafer that is evacuated to an internal pressure of 29.92 at sea level than aloft—as altitude increases, atmospheric inches of mercury ("Hg). These wafers are free to expand pressure decreases. This difference in pressure at various levels and contract with changes to the static pressure. A higher causes the altimeter to indicate changes in altitude. static pressure presses down on the wafers and causes them to collapse. A lower static pressure (less than 29.92 "Hg) The presentation of altitude varies considerably between allows the wafers to expand. A mechanical linkage connects different types of altimeters. Some have one pointer while the wafer movement to the needles on the indicator face, others have two or more. Only the multipointer type is which translates compression of the wafers into a decrease discussed in this handbook. The dial of a typical altimeter in altitude and translates an expansion of the wafers into an is graduated with numerals arranged clockwise from zero increase in altitude. [Figure 8-2] to nine. Movement of the aneroid element is transmitted through gears to the three hands that indicate altitude. In Notice how the static pressure is introduced into the rear of the Figure 8-2, the long, thin needle with the inverted triangle sealed altimeter case. The altimeter’s outer chamber is sealed, at the end indicates tens of thousands of feet; the short, wide which allows the static pressure to surround the aneroid needle indicates thousands of feet; and the long needle on wafers. If the static pressure is higher than the pressure in the top indicates hundreds of feet. aneroid wafers (29.92 "Hg), then the wafers are compressed 8-3