SIRS NAVIGATION LTD
COMPONENT MAINTENANCE MANUAL
B. Balancing the Compass Card (130)
NOTE: The compass card is balanced during manufacture so that the N-S axis
dips 0
°
to 1
°
at its North-seeking end, i.e. S on the card. This condition
applies to latitude 51
°
North where the vertical force of the Earth's
magnetic field is 0.44 oersted. The E-W axis of the card must be
horizontal ± 1
°
.
(1) Use the bubble protractor and magnetic needle to help adjust the
balancing bowl 3254 LMT to the correct angle for the latitude
concerned.
(2) Fill the bowl with alcohol. Invert the compass card in the alcohol to prevent
air being caught in the dome. Turn the compass card through 180
°
and
assemble it to the support stem.
(3) Examine the level of the compass card, with the adjustable pointer. If the
compass card is not level, identify the 'high spot’ and follow the procedures
detailed in paragraphs (5) to (6).
(4) If the balance is satisfactory, follow the procedure detailed in paragraphs 3.B.
(5) and (7) then continue with procedure 3.C.
(5) Remove the compass card from the balancing bowl and dry in a jet of clean,
dry air.
(6) Apply a small quantity of balancing paste to the inner surface of the compass
card at the 'high' area of the card. Surface dry with a jet of clean, dry air.
(7) Weigh the compass card. It must not weigh more than 2.8g.
C. Check Heading Errors (Ref. Fig. 7001)
(1) Put the balancing bowl on the sighting vane attached to the divided test -
head.
(2) Align the centre of the ‘stem sapphire’ with the centre line engraved on
the lower limb of the sighting vane and attach the balancing bowl.
PAGE 7002
APR 10/03
34-25-21
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