Indications of Pitot Tube Blockage
If the pitot tube becomes blocked, the ASI displays inaccurate
speeds. At the altitude where the pitot tube becomes blocked,
the ASI remains at the existing airspeed and doesn't reflect
actual changes in speed.

• At altitudes above where the pitot tube became
  blocked, the ASI displays a higher-than-actual
  airspeed increasing steadily as altitude increases.
• At lower altitudes, the ASI displays a lower-than-actual
  airspeed decreasing steadily as altitude decreases.

Indications from Static Port Blockage
Many aircraft also have a heating system to protect the
static ports to ensure the entire pitot-static system is clear
of ice. If the static polls become blocked, the ASI would
still function but could produce inaccurate indications. At
the altitude where the blockage occurs, airspeed indications
would be normal.

• At altitudes above which the static ports became
  blocked, the ASI displays a lower-than-actual airspeed
  continually decreasing as altitude is increased.
• At lower altitudes, the ASI displays a higher-than an-actual airspeed increasing steadily as altitude decreases.

The trapped pressure in the static system causes the altimeter
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

Figure 3-2. A Typical Pitot-Static System.

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 iii 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 aneroid tries to compress
them against their natural springiness, which tries to expand
them. The result is that their thickness changes is the air
pressure changes. Stacking several aneroid 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
the drum to get the thousands of feet, and then at the pointer
to get the feet and hundreds of feet.