effect that temperature has on the metals of the
322 feet. The altimeter should read 322 feet when
properly set to the local altimeter setting (in this
case 29.67). If the altimeter is set to 29.92
The altimeter discussed in the following
(standard day pressure at sea level), the altimeter
paragraphs is a simple one. Several complex
would read about 550 feet. The density altitude
altimeters are discussed later in this chapter, along
is such that the aircraft and engine perform the
with the automatic altitude system.
same as if they were at a standard day altitude
of 1,180 feet. This means the distances required
COUNTER POINTER PRESSURE ALTIM-
for takeoff and landing are longer for higher
ETER. --The purpose of the counter pointer
density altitudes (higher temperatures). The
absolute altitude (AGL) for the aircraft at
height. By studying the dial of the indicator, it
Memphis is zero feet because the aircraft is
is easy to understand the procedure for determin-
touching the surface.
ing the height of the aircraft. A description of the
Look at the altitude for Stapleton Inter-
mechanical operation of this altimeter follows. As
Notice that the difference between pressure
Atmospheric changes cause movement of the
altitude and indicated altitude remains the same
two aneroid diaphragm assemblies (1). These
as at Memphis. However, at the same temperature
assemblies move two similar rocking shaft
the density altitude is much greater at Denver.
assemblies (2) mutually engaged with the main
Several kinds of altimeters are used today.
pinion assembly (3). This movement goes to the
They are all constructed on the same basic
handstaff assembly (4), which operates the hand
principle as an aneroid. They all have pressure-
assembly (5) and drives the counter mechanism
responsive elements (aneroid wafers) that expand
(6) through a disk (7). Because of the special
or contract with the pressure changes of different
design of the hand assembly (5), the counter
flight levels. The heart of a pressure altimeter is
indication is never obscured. An internal vibrator
(8) minimizes friction during the instrument's
of one or more aneroid wafers. The expansion or
contraction of the aneroid wafers with pressure
You make barometric corrections by turning
changes operates the linkage. This action moves
the externally located knob (9). The knob engages
the indicating hand/counter to show altitude.
the barometric dial (10) and the main plate
Around the aneroid mechanism of most altimeters
assembly (11) that supports the entire mechanism.
is a device called the bimetal yoke. As the name
You make adjustments so the reading on the
implies, this device is composed of two metals.
barometric dial corresponds to the area baro-
It performs the function of compensating for the
metric conditions in which the aircraft is flying.
Figure 6-9.-Counter pointer pressure altimeter.
Figure 6-8.-Simplified aneroid mechanism.