(d)
Perform leakage test and restore systems to normal after any one of the above tests is performed.
(3)
The tests are to be made after the airplane has been on the ground at least 4 hours, except for digital air data computers.
(4)
The systems must be continuously energized for at least 30 minutes prior to tests, except for digital air data computers.
(5)
Head correction must be applied to pneumatic, servoed/pneumatic, or servoed altimeter test readouts to compensate for any difference in height between test set and altimeter being tested. If height difference exists, use appropriate test chart plus altimeter head correction test chart (Fig. 502). Determine height difference and read head correction for pressure altitude applied to altimeter. Subtract correction from altimeter reading if altimeter is above test set. Add correction to altimeter reading if altimeter is below test set. Height corrections are explained as follows:
(a)
Pneumatic altimeter - Height difference between altimeter and test set.
(b)
Servoed/pneumatic or servoed altimeter - Height difference between air data computer and test set.
(6)
Completely test one air data computer at a time if No. 1 and 2 are installed. Any or all of the air data indicator instrument scale error tests may be performed when air data computers are tested one at a time.
Feb 20/89 34-12-0 Page 501
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(7) If two air data computers or their associated servoed indicator are to be tested, check that proper pitot and static connections are being used.
B. Equipment and Materials
(1)
Air pressure and vacuum source, 5 to 40 inches of mercury absolute, and gauge for measuring differential pressure, accurate within ±0.010 inches of mercury. Range should be at least 10 inches of mercury.
(2)
Air pressure and vacuum source, 5 to 30 inches of mercury absolute.
(3)
Pressure gage accurate to within ±0.032 inches of mercury.
(4)
Stopwatch.
(5)
Mercury thermometer, accuracy ±0.5°C.
(6)
Appropriate gauge saver restrictors.
(7)
Adequate lengths of hose to couple pressure source with pitot tubes and vacuum source with static ports or drains.
(8)
Pitot static probe adapter P/N AD-953-7 and flush static port adapter 33410LH-125-4, Nav-Aids Limited, 2955 Diab Street, Quebec, Canada H4S1M1, or equivalents.
(9)
Decade resistance boxes, General Radio 1432-F.
C. Prepare to Test Air Data Pressure Instruments
(1)
To prevent damage to equipment and instruments, check following precautions before applying pressures to pitot static system.
(a)
Apply or release vacuum or pressure at rate of climb or descent less than 3000 feet per minute for static system and 300 knots per minute for pitot system between test points. (Appropriate gauge saver restrictors should be used.) At each test point, pressure should be reduced slowly to desired level without overshoot.
(b)
Pressure in pitot lines should always be greater than or equal to pressure in static lines. Differential pressure should never exceed 10.00 inches of mercury or fall below zero.
(c)
The digital air data computer will be damaged if static pressure exceeds pitot pressure more than 2 inches of mercury.
(d)
Absolute pressure applied to static system should never exceed ambient absolute pressure when any instrument is connected to static system.
(e)
Captain's, first officer's, and alternate static systems must be exposed to same absolute pressure conditions simultaneously during any test unless otherwise instructed.
(f)
Do not change setting of static source selector valve while vacuum is on static system. Check that selector valve remains in NORMAL position unless test procedures specify otherwise.
(g)
Check that pitot static probe heaters remain off during tests.
(h)
Inspect each pitot static probe head both before and after these tests for evidence of damage to circular leading edge of its barrel.
(i)
Do not apply pressure in excess of 31 inches of mercury absolute to static lines.
CAUTION: IF THE ABOVE PROCEDURES ARE NOT OBSERVED, EQUIPMENT AND INSTRUMENTS WILL BE DAMAGED.
(j)
Check that instrument vibrator circuit breakers are closed (if installed) during testing.
(2)
Disconnect cabin differential pressure indicator from static pressure source and cap pressure source.
(3)
Ensure that flight recorder (if installed) is either operating or that its magazine is removed and recorder pressure bar is up.
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