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Old 25th Aug 2010, 11:34
  #109 (permalink)  
Join Date: Dec 2000
Location: UK
Posts: 257

...In your superb photograph ... thank you ... both RMI'S shown seem to be showing a heading of 230 while the heading on the HSI shows 220 with the annunciation TRUE above the HSI...I am thinking that the RMI's are showing magnetic headings ?...

Correct. The RMIs show 230°(M) and the HSI shows 220°(T).

...were you operating on TRUE headings in this case as it seems...


The HSI had earlier been selected to show TRUE data, by moving the RAD/INS switch (out of picture) to INS. This changed the data inputs to the HSI from Magnetic to True, but did not affect the data input to the RMI compass cards. The data input to the RMIs was always magnetic, supplied by one of two compass systems; normally #2 compass system fed the Captain’s RMI card.

The red/black flags visible in the VOR RMI indicate that both the VOR stations selected are out of range.

The flags visible on the HSI indicate:
  • TRUE.......True compass data (not magnetic) is being displayed. (TRUE/MAG)

  • INS..........An INS (not a VOR) is supplying Nav data to this HSI. (INS/RAD)

  • 1............#1 INS (not #2) is supplying Nav data to this HSI. (1/2)

  • HDG.........The steering index shows selected heading (not track). (HDG/TRK)

  • LIN...........The beam bar is displaying linear (not angular) displacement. (LIN/ANG)

... what was the reason for this ?...

When flying long legs over areas of the world with few/no radio facilities, aircraft generally navigate from one Lat/Long waypoint to the next Lat/Long waypoint by using their INS systems, rather than tracking from radio beacon to radio beacon. The North Atlantic Track system is good example of this, as is this route down to BGI. Because there is no useful magnetic/radio information to display in such areas, True (INS) information is usually displayed, which brings with it two main advantages.

Firstly, you get to monitor how well the autopilot is doing, because the beam bar in the HSI should always be centralised if the aircraft is on track.

Secondly, as the aircraft changes course over a waypoint, you get the chance to check the Initial True Track it then takes up corresponds to the ITT pre-calculated on your flight plan. This is an important check (called the “Waypoint Change Drill”) in BA, that you have not mis-programmed the INS waypoints!

This procedure, of navigating in True, is by no means a Concorde procedure, all long range BA aircraft utilise it, and it is in common use in most airlines and aircraft flying long range routes around the World.

Best Regards


Last edited by Bellerophon; 25th Aug 2010 at 21:01.
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