TB 9-4920-457-24
Table 4. Altimeter Accuracy Check - Continued
Test instrument
Pneumatic pressure standard indications
(ft)1 2
indications
(ft)
Min
Max
35,000
34,700
(34,475)
[34,720]
35,300
(35,525)
[35,280]
40,000
(39,400)
[39,700]
(40,600)
[40,300]
45,000
(44,325)
[44,680]
(45,675)
[45,320]
50,000
(49,250)
[49,660]
(50,750)
[50,340]
55,000
[54,640]
[55,360]
1Values
in parentheses are for TIs with 50,000 ft altimeters.
2Values
in brackets are for TIs with 55,000 ft altimeters.
3Wait 5 minutes, then record pneumatic pressure standard indications at these points.
NOTE
No hysteresis check is specified for 55,000 ft altimeters.
(11) Slowly open VACUUM VENT valve until TI indicates 18,000 ft (25,000 ft for
50,000 ft altimeters). After 10 minutes pneumatic pressure standard indication will be
within 70 ft (150 ft for 50,000 ft altimeters) of indication recorded in (10) above for 18,000 ft
(25,000 ft for 50,000 ft altimeters).
(12) Slowly open VACUUM VENT valve until TI indicates 16,000 ft (20,000 ft for
50,000 ft altimeters). After 10 minutes pneumatic pressure standard indication will be
within 70 ft (150 ft for 50,000 ft altimeters) of indication recorded in (10) above for 16,000 ft
(20,000 ft for 50,000 ft altimeters).
(13) Slowly open VACUUM VENT valve until altimeter indicates lowest reading
with valve open completely. After 5 minutes altimeter will indicate within 50 ft (60 ft for
50,000 ft altimeters) of indication recorded in (8) above.
(14) If all indications are within tolerance, annotate correction chart and place in
holder on TI faceplate. The correction chart will show standard indications when TI
indicates nominal values.
b. Adjustments
(1) Loosen locking screw, located left of barometric adjustment knob (screw may be
covered with black paper dot).
(2) With pneumatic pressure standard indicating 0 ft altitude, adjust barometric
adjustment knob until altimeter indicates 0 ft altitude.
(3) Pull adjustment knob out and adjust barometric indicator to 29.92 in. Hg
(1013.3 mb). Push knob down.
NOTE
It may be necessary to repeat (2) and (3) above until no further
adjustments are required.
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