Just Flight Standard VC10 Operation Manual page 79

We can now check the status of the alignment process. We can do this by moving the CDU mode selector to
DSRTK/STS. On the right-hand display are two digits. The first digit is the one that we're interested in. This tells
us the state of the CDU alignment, from 9 being 'no alignment' to 0 being 'perfectly aligned'. These numbers are
known as the API or Accuracy Performance Index.
When it goes below 5, the READY NAV light illuminates to indicate that they are sufficiently aligned for navigation.
Ideally we'd wait for both systems to reach 0 to ensure the INS is as accurate as possible, but this can take up
to 17 minutes.
Once the READY NAV light comes on, rotate the MSU mode selectors to NAV.
During the flight the gyros will 'drift'. For every hour of flight they can drift off course by up to 220 metres.
To correct for drift and maintain a high API, we can use DMEs to update our position. During our departure we
pass two VOR/DMEs, TRN and MAC. These are waypoints 3 and 5 respectively. Tune the TRN VOR into NAV 1
and tune the standby frequency to MAC. As you fly from TRN to MAC you can flip the frequencies. Once we're
out in the North Atlantic tracks, however, there are no VOR/DMEs in range so drift will be inevitable. Only when
we exit our Oceanic track can we pick up a VOR or two, starting with YBC.
VC10 Professional – Operations Manual
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