IFH 5-4

A crosshatched area appears on some altimeters when displaying an altitude below 10,000 feet MSL. 10,000 ft. pointer Altimeter setting window Static port Barometric scale adjustment knob Aneroid Crosshatch flag Altitude indication scale 100 ft. pointer 1,000 ft. pointer Figure 3-3. Sensitive altimeter componements. Figure 5-3. Sensitive altimeter components. 30.0 29.9 29.8 I00 FEET I 45 6 7 9 2 0 8 3 CALIBRATED TO 20,000 FEET ALT Figure 3-4. Three-pointer altimeter.Figure 5-4. Three-pointer altimeter. I 4 5 6 7 9 2 0 8 3 0 6, 5 0 0 ALT M B 2 9 9 2 I N H G Figure 3-5. Drum-type altimeter. Figure 5-5. Drum-type altimeter. makes one revolution for every 1,000 feet. Each number represents 100 feet and each mark represents 20 feet. A drum, marked in thousands of feet, is geared to the mechanism that drives the pointer. To read this type of altimeter, first look at the drum to get the thousands of feet, and then at the pointer to get the feet and hundreds of feet. A sensitive altimeter is one with an adjustable barometric scale allowing the pilot to set the reference pressure from which the altitude is measured. This scale is visible in a small window called the Kollsman window. A knob on the instrument adjusts the scale. The range of the scale is from 28.00 to 31.00 inches of mercury ("Hg), or 948 to 1,050 millibars. Rotating the knob changes both the barometric scale and the altimeter pointers in such a way that a change in the barometric scale of 1 "Hg changes the pointer indication by 1,000 feet. This is the standard pressure lapse rate below 5,000 feet. When the barometric scale is adjusted to 29.92 "Hg or 1,013.2 millibars, the pointers indicate the pressure altitude. The pilot displays indicate altitude by adjusting the barometric scale to the local altimeter setting. The altimeter then indicates the height above the existing sea level pressure. Altimeter Errors A sensitive altimeter is designed to indicate standard changes from standard conditions, but most flying involves errors caused by nonstandard conditions and the pilot must be able to modify the indications to correct for these errors. There are two types of errors: mechanical and inherent. Mechanical Altimeter Errors A preflight check to determine the condition of an altimeter consists of setting the barometric scale to the local altimeter