Instrumentation

The Pressure Altimeter
5
Blockages and Leaks
If the static source becomes blocked, the
altimeter will not register any change in height
- the height at which the blockage occurred
will still be indicated regardless of any climb
or descent. On many aircraft, an alternative
source of static pressure will be available.
Should the static line fracture in a pressurized
aircraft, the altimeter will show the (lower)
cabin altitude rather than aircraft altitude
A fracture in the static line within an
unpressurized aircraft will normally result in
the altimeter over-reading, due to the pressure
in the cabin being lower than ambient due
to aerodynamic suction. See Chapter 2
Emergency Static Source.
The Pressure Altimeter 5
Figure 5.9 Static feed blocked
If the aircraft is CLIMBING then the altimeter
will UNDER-READ.
If the aircraft is DESCENDING then the altimeter will OVER-READ.
The amount of the error will increase as the aircraft moves away from the height at which the
blockage occured.
Density Altitude
Density altitude can be defined as the altitude in the standard atmosphere at which the
prevailing density would occur, or alternatively, as the altitude in the standard atmosphere
corresponding to the prevailing pressure and temperature. It is a convenient parameter in
respect of engine performance figures.
Preflight Altimeter Checks
In the UK, the apron is the
designated location for preflight
altimeter checks (the
apron being the loading and
unloading and/or parking
area). Apron elevation
is displayed in the flight
clearance office of the
aerodrome concerned and
is also published in the AGA
section of the UK Air Pilot.
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