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TM 55-4920-416-13
NAVAER 17-15C-539
Section IV
n.  Slowly close the PRESSURE DECREASE CONTROL
u.
Set VACUUM and PRESSURE selector valves to
VALVE 4 until the master panel AIRSPEED INDICATOR and
OFF.
the MACH INDICATOR under test begin to read.
o.  Adjust the PRESSURE DECREASE CONTROL
4-72.  CALIBRATE MACHMETER.  Adjust VACUUM and
VALVE 4 to the desired check point selected in step e. above.
PRESSURE Control Valves 1, 2, 3 and 4 to obtain an
p.  Re-adjust the VACUUM DECREASE CONTROL
ALTIMETER and AIRSPEED INDICATOR as selected from
VALVE 1 if necessary to restabilize the ALTIMETER within 40
Table 4-2 in paragraph 4-71, step e. Enter the readings of the
feet of the control altitude.  Also re-adjust the PRESSURE
instrument under test on a calibration card.
DECREASE CONTROL VALVE 4 if necessary to re-stabilize
the AIRSPEED reading.
4-73. CASE LEAK TEST, MACHMETER. Operate Test Set
and adjust all control valves as in paragraph 4-71 to obtain an
ALTIMETER setting of 10,000 ft. and a MACHMETER setting
NOTE
of 1.2.
a.  Close needle valves on both hose assemblies to the
When calibrating MACH INDICATORS at
instrument being tested.
high simulated altitudes (above 40,000
b.  Observe the MACHMETER being tested; the pointer
feet) it may be necessary to stabilize the
should not move more than .02 Mach in 60 seconds.  A
ALTIMETER reading at a position with the
reading change in excess of this indicates a leak that will
VACUUM DECREASE VALVE 1 at its
necessitate removal and replacement of the instrument.
maximum position and the VACUUM
c.  Slowly open the needle valves on the hose
INCREASE VALVE 2 at a position that will
assemblies to the instruments being tested.
yield the simulated altitude in a manner
d.  Return all instruments to normal in accordance with
similar to paragraph 4-71. The pressure
the shut-off procedure at the end of paragraph 4-71.
settings are then obtained in a manner
e.  Remove test hose connections in accordance with
similar to those obtained in paragraph
q.  When both the ALTIMETER and the AIRSPEED
CAUTION
indications are stabilized at the proper points, note the MACH
reading on the instrument being tested and compare with the
During all operations and testing MACH
reading in Table 4-2.
INDICATORS, extreme care must be
r.  Proceed through all the required check points, being
exercised when applying pressure to the
careful to stabilize the ALTIMETER within 40 feet each time.
AIRSPEED INDICATOR so that the limit of
s.  After completing all the check points, turn the power
the INDICATOR does not exceed 670.
switch to OFF.
When making these tests the master
t.  Slowly open all VACUUM and PRESSURE control
instruments on panel are not protected
valves until all the instruments return to normal.
by relief valves.
Failure to observe
extreme precaution will result in serious
damage to master instruments.
21

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