B747-400 Standby RMI Flags In TRUE HDG
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B747-400 Standby RMI Flags In TRUE HDG
Hi, folks
I noticed a few weeks ago on an Atlas 744 Freighter that when I selected the Magnetic Heading Reference switch to TRUE heading (from NORM), that the VOR and compass flags dropped into view on their single RMI. At the time, I assumed this was normal.
Last week, I read in their maintenance manual that the RMI uses both MAG and TRUE (a few other airline's AMMs also specified this, but haven't had the chance to confirm it on their aircraft).
The AOM doesn't go into much detail on this issue and the wiring schematics don't help.
What happens on your RMI-equipped 744's? (BA? TG? etc) Is it an option to have the RMI operate in TRU?
Perhaps the RMI only uses True Hdg if the switchover to TRU is automatic (at high latitudes)?
Thanks.
Cheers.
NSEU
P.S. I hear that Thule in Greenland has a VOR which uses TRU reference, so there is no reason why RMI's won't work at high latitudes (The FMC doesn't like it for position updating, however, according to one bulletin).
I noticed a few weeks ago on an Atlas 744 Freighter that when I selected the Magnetic Heading Reference switch to TRUE heading (from NORM), that the VOR and compass flags dropped into view on their single RMI. At the time, I assumed this was normal.
Last week, I read in their maintenance manual that the RMI uses both MAG and TRUE (a few other airline's AMMs also specified this, but haven't had the chance to confirm it on their aircraft).
The AOM doesn't go into much detail on this issue and the wiring schematics don't help.
What happens on your RMI-equipped 744's? (BA? TG? etc) Is it an option to have the RMI operate in TRU?
Perhaps the RMI only uses True Hdg if the switchover to TRU is automatic (at high latitudes)?
Thanks.
Cheers.
NSEU
P.S. I hear that Thule in Greenland has a VOR which uses TRU reference, so there is no reason why RMI's won't work at high latitudes (The FMC doesn't like it for position updating, however, according to one bulletin).
Last edited by NSEU; 22nd May 2010 at 02:07.
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Tried it out the other day on an -400F originally delivered from the Boeing factory to CI.
Over Japan, so 7 degrees mag variation. When the sw was flipped from NORM to TRUE, the RMI compass card ( installed CA's side only) rotated the 7 degrees, so card reads true heading ok, but both VOR needles flagged, as you experienced. But when the VOR /ADF needle was selected to ADF, the nav flag withdrew and it did point to the AM radio station correctly.
Maybe the assumption in airplane logic is that VOR's are not referenced to true, though a few are, as you point out?
Over Japan, so 7 degrees mag variation. When the sw was flipped from NORM to TRUE, the RMI compass card ( installed CA's side only) rotated the 7 degrees, so card reads true heading ok, but both VOR needles flagged, as you experienced. But when the VOR /ADF needle was selected to ADF, the nav flag withdrew and it did point to the AM radio station correctly.
Maybe the assumption in airplane logic is that VOR's are not referenced to true, though a few are, as you point out?
The ADF points a relative bearing - relative to the nose, that is, so it works on top of the card regardless of the reference the card is using.
The VOR uses a signal phase difference to work out on which magnetic radial from the station the aircraft is position on. The original VOR equipment had you select a radial with an Omni-Bearing Selector (OBS), and would then show you position relative to that radial. To provide that information to a needle on the RMI, and have the needle point to a magnetic number reference on the card is simple, but the system would have to take into account the magnetic variation at the VOR station to adjust for a card in true - and it doesn't have this information, hence the flag. It would be a "bad" reference as using the information to plot a position would require a pilot to make adjustments (and you couldn't trust the average pilot to make those adjustments correctly.)
The VOR uses a signal phase difference to work out on which magnetic radial from the station the aircraft is position on. The original VOR equipment had you select a radial with an Omni-Bearing Selector (OBS), and would then show you position relative to that radial. To provide that information to a needle on the RMI, and have the needle point to a magnetic number reference on the card is simple, but the system would have to take into account the magnetic variation at the VOR station to adjust for a card in true - and it doesn't have this information, hence the flag. It would be a "bad" reference as using the information to plot a position would require a pilot to make adjustments (and you couldn't trust the average pilot to make those adjustments correctly.)
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Thanks, gentlemen. I really didn't expect anyone to try this in the air 
Next time Atlas flies in, I'll try and repeat my checks to see if I wasn't imagining that Heading Flag.
I'd be interested to hear from other 744 operators who still have these instruments, especially BA. Our fleet no longer has these.
I have this vague recollection that there are differences between automatic switchover and manual selection, but can't be sure.
Cheers.
NSEU
Next time Atlas flies in, I'll try and repeat my checks to see if I wasn't imagining that Heading Flag.
I'd be interested to hear from other 744 operators who still have these instruments, especially BA. Our fleet no longer has these.
I have this vague recollection that there are differences between automatic switchover and manual selection, but can't be sure.
Cheers.
NSEU
