(4)  
Evacuate system to about 5 psi vacuum (10 psia).

(5)  
Check that warning horn sounds intermittently,

(6)  
Press ALTITUDE HORN CUTOUT switch and check that horn stops blowing.

(7)  
Remove vacuum source electrical power, if no longer required.

H30038
501  Cabin Altitude Warning Switch Installation 
Feb 15/77  Figure 401  31-26-31 
Page 401 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

PITOT STATIC SYSTEM -DESCRIPTION AND OPERATION
EFFECTIVITY
GJ     ALL EXCEPT B-2524
1. General
A.  The pitot static system consists of four combined pitot static probes; two alternate static ports; and connecting tubing to air data instruments, air data equipment, and drain trap and test fittings (Fig. 1 and 2) . These instruments measure dynamic (pitot) and ambient (static) air pressures to determine airplane vertical speed, airspeed, mach number, and altitude.
B.  Heater circuits are provided for anti-icing of the pitot static probes. Refer to Chapter 30, Pitot Static Tubes and Temperature Probe Anti-Icing System.
2.  Pitot System (Fig. 2)
A.  Four pitot tubes are used to sense dynamic air pressure. Two probes are mounted on each side of the fuselage below control cabin window No.
B.  The captain's pitot source is from the upper left pitot static probe and the first officer's from the upper right pitot static probe. These pitot pressures are applied to the appropriate instruments and equipment as shown in Fig. 2.B. Auxiliary No. 1 and No. 2 pitot pressures from the lower right and lower left pitot static probe feed various indicators and equipment.
3.  Static System (Fig. 2)
A.  Static or ambient pressures are sensed through two ports on the combined pitot static probe. Each static port is connected to a port on the opposite side of the airplane. The four static pressure systems feed instruments and equipment as shown in Fig. 2.B. Static pressure is also available from alternate static ports at station 406 on both sides of the airplane. These alternate static ports, when selected, feed instruments and equipment normally connected to captain's and first officer's static systems.
C.  The static source selector valves provide a second source of static pressure for the captain's and first officer's instruments and equipment. Each valve is controlled by the applicable static source selector switch located on the captain's or first officer's side panel. Static air pressure is used from the appropriate static source when the switch is in NORMAL position and from the alternate static source when the switch is in ALTERNATE position.
D.  Provisions for a tail-cone static test system are installed in the empennage assembly. 
5C8
Feb 20/89 34-11-0 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

4.  Pitot Static Probe
A.  The pitot static probe is designed to accurately sense pitot (total) and static (altitude) pressure over the complete operating range of the airplane. The pitot static probe consists of a probe and pressure sensing ports. A strut projects the probe several inches out from the fuselage and isolates the probe from the effects of fuselage variance. A baseplate contains the electrical and pressure fittings. Dowl locator pins are also installed on the baseplate to ensure accurate alignment of the installed probe.
B.  Pitot pressure is sensed at an opening in the forward end of the probe. Static pressure ports are located in the contoured midsection of the probe; the contoured surface provides aerodynamic compensation to correct the static pressure position error caused by the airplane. The contour configuration and port locations minimize errors due to mach number and angle of attack.
C.  An anti-icing heater is installed in each probe to prevent ice accumulation on the probe during flight. The heater is electrically connected to two terminal pins shielded in the baseplate of the probe. The heater is self-regulating coaxial type. The temperature-compensating characteristic provides maximum power for in-flight operation, while automatically reducing the power in still-air conditions to prevent overheating. If the heater is energized at rated voltage during ground operation, the pitot static probe will become hot, and those areas which require high heat concentration may become discolored. This discoloration is normal and has no adverse effect on the unit. Maximum performance, accuracy and life will be obtained from the pitot static probe by minimum amount of operation of the anti-icing heaters in still air.
D.  Do not expose the pitot static probe to contaminants; avoid solvents, lubricating oil and grease. Do not insert any object into the static ports, which could flare out the edge of the ports. If necessary to use any fluids on the pitot static probe, use fluids that evaporate without leaving harmful residue, such as water.
　
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