four lights. Controls for dial lighting are external
AAU-24/A ALTIMETER. --The AAU-24/A
to the altimeter.
altimeter (fig. 6-26) contains a precision pressure-
sensing device, counter, and pointer drive
To overcome the effects of stop and jump
mechanisms. It also contains a combination
friction in altimeter mechanisms, the altimeter has
counter drum and pointer for altitude display. The
counter displays two digits, showing multiples of
an internal, electrically operated mechanical
vibrator.
10,000 feet and 1,000 feet respectively, and moves
intermittently. The drum shows multiples of 100
The block diagram, shown in figure 6-27,
feet and moves continuously. The pointer travels
shows altimeter operation. Outside air pressure
one revolution for each increment of 1,000 feet
from the aircraft static-pressure system goes to
of altitude. The pointer scale is from 0 to 9; each
the altimeter case. Two evacuated pressure-sensing
step representing an increment of 100 feet. Each
element assemblies within the case expand
100-foot step is split into two increments of 50
as the case pressure decreases with increasing
feet each.
altitude. Expansion or contraction of each
The barometric setting (baroset) knob is
element sends movement to a rocker shaft
assembly via a link-arm configuration. Pivots
located in the lower left corner of the bezel. It
for the rocker shaft assemblies couple to a
protrudes a maximum of 0.73 inch in front of the
temperature compensating ring that changes the
bezel. The baroset knob works with a four-digit
movement of the rocker shafts. This movement
counter, designated IN Hg, to set the altitude
compensates for temperature changes. A dif-
indication to the prevailing barometric pressure.
ferential gear assembly, coupled by segment gears
It is adjustable from 28 to 31 inches of mercury.
to the rocker shafts, provides an output move-
There is a locking screw next to the baroset knob.
ment. This movement is proportional to the
This screw is used only during calibration
average expansion or contraction of the two
procedures to align the barometric pressure
sensing element assemblies.
(IN Hg) indication with altitude indication.
The output gear of the differential gear
Two sets of internal lights, one red and one
assembly drives a gear train mechanism. This gear
white, provide dial lighting. Each set consists of
Figure 6-26.-AAU-24/A altimeter dial face.
6-24
