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to remain at the altitude where the blockage occurred. The
VSI remains at zero. On some aircraft, an alternate static
air source valve is used for emergencies. [Figure 3-2] If
the alternate source is vented inside the airplane, where
static pressure is usually lower than outside static pressure,
selection of the alternate source may result in the following
erroneous instrument indications:
1. Altimeter reads higher than normal,
2. Indicated airspeed (IAS) reads greater than normal,
and
3. VSI momentarily shows a climb. Consult the Pilot’s
Operating Handbook/Airplane Flight Manual (POH/
AFM) to determine the amount of error.
Effects of Flight Conditions
The static ports are located in a position where the air at
their surface is as undisturbed as possible. But under some
flight conditions, particularly at a high angle of attack with
the landing gear and flaps down, the air around the static
port may be disturbed to the extent that it can cause an error
in the indication of the altimeter and ASI. Because of the
importance of accuracy in these instruments, part of the
certification tests for an aircraft is a check of position error
in the static system.
The POH/AFM contains any corrections that must be applied
to the airspeed for the various configurations of flaps and
landing gear.
Pitot/Static Instruments
Sensitive Altimeter
A sensitive altimeter is an aneroid barometer that measures
the absolute pressure of the ambient air and displays it in
terms of feet or meters above a selected pressure level.
Principle of Operation
The sensitive element in a sensitive altimeter is a stack of
evacuated, corrugated bronze aneroid capsules. [Figure 3-3]
The air pressure acting on these aneroids tries to compress
them against their natural springiness, which tries to expand
them. The result is that their thickness changes as the air
pressure changes. Stacking several aneroids increases the
dimension change as the pressure varies over the usable
range of the instrument.
Below 10,000 feet, a striped segment is visible. Above this
altitude, a mask begins to cover it, and above 15,000 feet,
all of the stripes are covered. [Figure 3-4]
Another configuration of the altimeter is the drum-type.
[Figure 3-5] These instruments have only one pointer that
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
3-4
A crosshatched area appears on some
altimeters when displaying an altitude
below 10,000 feet MSL.
1,000 ft. pointer
Altimeter setting window
Static port
Barometric scale adjustment knob
Aneroid
Crosshatch flag
Altitude indication scale
10,000 ft. pointer
100 ft. pointer
Figure 3-3. Sensitive Altimeter Components.
30.0
29.9
29.8
I00
FEET
I
6 5
4
7
9
2
8
3
CALIBRATED
TO
20,000 FEET
ALT
F0
Figure 3-4. ThrFeeig-Puorein 3te-r4 .A Tlthimreeet-epro. inter altimeter. Figure 3-5. Drum-Type Altimeter.
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
A preflight check to determine the condition of an altimeter
　
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