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OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-34-00
Page 1
October 11/01
Revision 5
2. Description of Subsystems, Units and Components:
A. Pitot-Static System:
The pitot-static system samples the atmospheric environment to provide
indications of airplane airspeed and altitude. (See the illustration in Figure
3 for a depiction of the system.) Three pitot tubes are mounted on the
exterior of the airplane and aligned forward to sense dynamic air pressure
generated proportional to airplane velocity. As the airplane moves through
the atmosphere, more air molecules are encountered than if the airplane
were stationary. The increase is proportional to speed, and is sensed as
pressure. The hollow pitot tube heads, positioned into the airplane
slipstream, direct the increased pressure into the pitot system for
measurement by the airspeed indicators. The metal heads of the tubes are
heated electrically to prevent blockage by frozen precipitation. Internally,
the pitot tubes are plumbed with nylon tubing. The increased dynamic air
pressure in pitot tubes is compared with static air pressure sensed by other
tubes attached to static ports mounted flush on the airplane fuselage. (The
static ports are essentially vents, since they are not aligned into the
slipstream.) By measuring the difference between static air pressure and
the increased dynamic air pressure, an accurate determination of airspeed
can be made.
There are four static air pressure tubes and ports: three are paired with and
plumbed to the associated pitot tube (pilot to DADC #1, copilot to DADC
#2, and standby to the standby airspeed and altimeter). The fourth static
tube and port is connected to the cabin pressure indicator and controller.
Cabin pressure is measured and controlled within structural limits by
comparing the air pressure within the airplane to the atmospheric pressure
outside the cabin (for the cabin differential pressure). Unlike the pitot tubes,
the static tubes have dual connections, with each tube plumbed to a static
port on either side of the airplane. Dual ports are necessary in order to
obtain an accurate static pressure sampling. If a static tube were not
connected to a port on either side of the airplane, any untrimmed flight
condition around the vertical axis, such as a skid or slip, would induce an
increase in the sampled static pressure due to the port being slightly
aligned into the airplane slipstream.
NOTE:
On some airplanes the customer has chosen to install
an optional secondary cabin pressure indicator. For
those airplanes, a switch on the aft portion of the
copilot side console, labelled NON-ESSENTIAL
PITOT/STATIC CONTROL, is used to isolate the
secondary cabin pressure indicator from the primary
indicator. When the secondary indicator is isolated
with the switch, an independent verification of cabin
altitude is available.
B. Standby Airspeed Indicator:
The standby airspeed indicator provides an emergency source of airspeed
information in the event of failure or malfunction of the primary flight
displays. The standby indicator displays current airspeed, maximum
airspeed for altitude (VMO) and Mach number. On airplanes Serial Number
OPERATING MANUAL
2A-34-00 PRODUCTION AIRCRAFT SYSTEMS
Page 2
October 11/01
Revision 5
(SN) 1000 to 1059 (except SN 1001 and 1034) not having Aircraft Service
Change (ASC) 66, airspeed is indicated with a white pointer that rotates
clockwise around the face of the instrument proportional to airplane speed
and VMO by a “barber pole” striped pointer. Pitot/static connections provide
the input for airspeed information. Although the indicator has a drum-type
readout of Mach number in the upper center of the instrument, the ARINC
429 connections necessary to derive Mach speed information from the
DADC are not installed. The OFF flag covering Mach readout will be
displayed at all times on the instrument, indicating that this feature is not
available.
On airplanes SN 1060 and subsequent, and prior airplanes having ASC 66,
the standby airspeed indicator is not powered (except for lighting circuits)
and obtains speed information directly from the pitot/static system. On
these airplanes, airspeed is indicated by a rotating white pointer, Mach
number is read on an additional scale positioned on the outside perimeter
of the airspeed scale and VMO / MMO is denoted with a red band on both
the airspeed and Mach number scales. A knob on the lower face of the
instrument is provided to set a movable airspeed reference bug (VREF).
See the illustration in Figure 1.
On all airplanes, the indicator is illuminated internally using 5V DC power
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