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when the 100-foot pointer is between the 900- and 1,000-foot position.
3.10.2. Counter-Drum-Pointer Altimeter. Aside from the familiar circular scale and 100-foot pointer
the counter-drum-pointer presentation differs somewhat in appearance from the present three-pointer
altimeter. Starting at the left of the instrument illustrated in Figure 3.16 and reading from left to right,
there are two counter windows and one drum window (white). The numerals presented in the counter
windows indicate 10,000s and 1,000s of feet, respectively. The drum window numbers always follow the
pointer number, thereby indicating 100s of feet.
3.10.2.1. Two methods may be used to read indicated pressure altitude on the counter-drum-pointer
altimeter: (1) read the counter-drum window, without referring to the 100-foot pointer, as a direct digital
readout of both thousands and hundreds of feet; (2) read the two counter indications, without referring to
the drum, and then add the 100-foot pointer indication. The 100-foot pointer serves as a precise readout
of values less than 100 feet.
3.10.2.2. The differential air pressure which is used to operate the counter-drum-pointer altimeter is
processed by an altitude transducer where it is converted to electrical signals that drive the indicator. The
transducer is also used to send digital signals to a transponder for purposes of automatic altitude
reporting to Air Route Traffic Control Centers (ARTCC). A standby system is available for use if an
electrical malfunction occurs. In the standby system, the altimeter receives static air pressure directly
from the pitot-static system. When the instrument is operating in the standby system, the word
STANDBY appears on the instrument face. A switch in the upper right-hand corner of the instrument is
provided to return the instrument to its normal mode of operation. This switch may also be used to
manually place the instrument in the STANDBY mode.
Figure 3.16. Counter-Drum-Pointer Altimeter.
100 AFPAM11-216 1 MARCH 2001
3.11. Altimeter Errors. The pressure altimeter is subject to certain errors which fall in five general
categories.
3.11.1. Mechanical Error. Mechanical error is caused by misalignment of gears and levers which
transmit the aneroid cell expansion and contraction to the pointers of the altimeter. This error is not
constant and must be checked before each flight by the setting procedure.
3.11.2. Scale Error. Scale error is caused by irregular expansion of the aneroid cells and is recorded on
a scale correction card maintained for each altimeter in the instrument maintenance shop.
3.11.3. Installation or Position Error. An installation or position error is caused by the airflow around
the static ports. This error varies with the type of aircraft, airspeed, and altitude. The magnitude and
direction of this error can be determined by referring to the performance data section in the aircraft
technical order. An altimeter correction card is installed in some aircraft which combines the installation
or position and scale errors. The card indicates the amount of correction required at different altitudes
and airspeeds.
WARNING: Installation or position error may be considerable at high speeds and altitudes. Apply the
corrections as outlined in the technical order or on the altimeter correction card.
3.11.4. Reversal Error. Reversal error is caused by inducing false static pressure in the static system. It
normally occurs during abrupt or large pitch changes. This error appears on the altimeter as a momentary
indication in the opposite direction.
3.11.5. Hysteresis Error. Hysteresis error is a lag in altitude indication caused by the elastic properties
of the material within the altimeter. This occurs after an aircraft has maintained a constant altitude for an
extended period of time and then makes a large, rapid altitude change. After a rapid descent, altimeter
indications are higher than actual. This error is negligible during climbs and descents at slow rate or after
maintaining a new altitude for a short period of time.
3.11.6. Setting the Altimeter. The barometric scale is used to set a reference plane into the altimeter.
Rotating the barometric pressure set knob increases or decreases the scale reading and the indicated
altitude. Each .01 change on the barometric scale is equal to 10 feet of altitude. The majority of
altimeters have mechanical stops at or just beyond the barometric scale limits (28.10 to 31.00).
Attempting to adjust outside this range may cause damage to the instrument. NOTE: Altimeters not
equipped with mechanical stops near the barometric scale limits can be set with a 10,000-foot error.
Therefore, when setting the altimeter, ensure the 10,000-foot pointer is reading correctly. Check the
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