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The source of
the pressure for operating the airspeed indicator,
the vertical speed indicator (VSI), and the
altimeter is the pitot-static system. The major
components of the pitot-static system are the
impact pressure chamber and lines and the static
pressure chamber and lines, each of which are
subject to total or partial blockage by ice,
dirt, and/or other foreign matter. Blockage
of the pitot-static system adversely affects
instrument operation. [Figure 17-10]
Partial static system blockage is insidious
in that it may go unrecognized until a critical
phase of flight. During takeoff, climb, and
level-off at cruise altitude the altimeter,
airspeed indicator, and VSI may operate normally.
No indication of malfunction may be present
until the airplane begins a descent.
If the static reference system is severely restricted,
but not entirely blocked, as the airplane descends,
the static reference pressure at the instruments
begins to lag behind the actual outside air
pressure. While descending, the altimeter may
indicate that the airplane is higher than actual
because the obstruction slows the airflow from
the static port to the altimeter. The VSI confirms
the altimeter’s information regarding rate
of change because the reference pressure is
not changing at the same rate as the outside
air pressure. The airspeed indicator, unable
to tell whether it is experiencing more airspeed
pitot pressure or less static reference pressure,
indicates a higher airspeed than actual. To
the pilot, the instruments indicate that the
airplane is too high, too fast, and descending
at a rate much less than desired.
If the pilot levels off and then begins a climb,
the altitude indication may still lag. The VSI
indicates that the airplane is not climbing
as fast as actual. The indicated airspeed, however,
may begin to decrease at an alarming rate. The
least amount of pitch-up attitude may cause
the airspeed needle to indicate dangerously
near stall speed.
Managing a static system malfunction requires
that the pilot know and understand the airplane’s
pitot-static system. If a system malfunction
is suspected, the pilot should confirm it by
opening the alternate static source. This should
be done while the airplane is climbing or descending.
If the instrument needles move significantly
when this is done, a static pressure problem
exists and the alternate source should be used
during the remainder of the flight.
With conventional instrumentation, the design
and operation are similar regardless of aircraft
or manufacturer. By comparing information between
the six conventional instruments, pilots are
able to diagnose common failure modes. Instrument
failure indications of conventional instruments
and electronic flight displays may be entirely
different, and electronic systems failure indications
are not standardized. With the wide diversity
in system design of glass cockpits, the primary
display and the backup display may respond differently
to any interruption of data input, and both
displays may function differently than conventional
instruments under the same conditions.
It is imperative for pilots to obtain equipment-specific
information in reference to both the aircraft
and the avionics that fully prepare them to
interpret and properly respond to equipment
malfunctions of electronic flight instrument
displays. Rapidly changing equipment, complex
systems, and the difficulty or inability to
simulate failure modes and functions can impose
training limitations. Pilots still must be able
to respond to equipment malfunctions in a timely
manner without impairing other critical flight
tasks should the need arise.
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