Inertial Navigation System; Mode Selector Unit - Just Flight Standard VC10 Operation Manual

INERTIAL NAVIGATION SYSTEM

The VC10 received various upgrades and improvements throughout its life. One of the major improvements to the
aircraft was the fitting of an Inertial Navigation System (INS). The INS made the job of navigation much easier by
allowing the aircraft to automatically follow up to nine waypoints in sequence, with little or no input from the crew.
The system relies on gyroscopic technology to monitor changes in speed, direction and orientation. It then uses
that data to calculate the distance travelled from its original position. While it lightened the workload of the crew,
the system wasn't perfect. For every hour of flight, the system's accuracy would drop and the reported position
could be off by around 220 metres. Over the course of a long flight, 'Gyro Drift' was enough to send an aircraft
off course and the only way to fix the issue was to recalibrate the system mid-flight. This usually involved passing
over a specific VOR point and resetting the INS position. The system was widely used in all kinds of vehicles,
from aircraft to spacecraft, submarines and ships. INS has now been largely replaced by GPS.
The system fitted to the VC10 was built by US company Delco and was widely used on aircraft such as the
original Boeing 747. The VC10 has two systems fitted for redundancy. Both panels are viewable by pressing
[Shift]+[2] and [Shift]+[3]. The controls for each system come in two parts – a Mode Selector Unit (MSU) and a
Control Display Unit (CDU).

Mode Selector Unit

1. Mode selector knob
2. Ready NAV indicator light
3. Battery indicator light
The mode selector knob has five positions:
OFF
The unit is powered off.
STBY (standby)
This mode operates in two different ways depending on how it was switched to. From the OFF position, the
system begins powering up the internal gyros and the unit is brought up to operating temperature. At the same
time, the Accuracy Performance Index, or API for short, is measured. Switching the CDU's mode selector to
DSRTK/STS, you can view the current API level. The scale begins at the number 9 and decreases to 0, with
9 being the equivalent of 'no clue where I am' and 0 being 'I'm right here, on this tiny spot'. The system is
considered to be ready when the API is 5 or lower. If STBY is selected from the NAV position (described as
'Downmoding'), the gyros are kept running and alignment is maintained. In either situation, position data can be
entered into the system.
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