VOR ----- (dashed)

[United744]

Hi,

I noticed that if you completely shutdown the aircraft, and power it up again that the VOR radios apparently start in a parked or standby mode? They don't appear to autotune or anything until you "wake them up" by entering a frequency or ident. Why is this?

[Hardy Heinlin]

Hi,

autotuning begins in flight.

Before the IRS is aligned, the FMC cannot even be sure where the aircraft is, hence it cannot know which stations on the planet provide a good geometry for the present aircraft position. The FMC autotunes only those stations that are in a reasonable, useful relative position.

[John H Watson]

Before the IRS is aligned, the FMC cannot even be sure where the aircraft is,

I don't think the 744's FMC is that fussy  :mrgreen:  I have the feeling it's happy to use GPS or Last Position.

Speaking of not being fussy, I've noticed that the 744's FMC will pre-tune stations for a SID for an airport on the other side of the world (can't remember if the IRU's were aligned, though).

Rgds
JHW

[Hardy Heinlin]

Yes, but by "being sure" I mean GPS is still a backup, not the primary nav, i.e. when all IRUs fail, the FMC will stop navigating.


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[United744]

@John: I think it has the letter P for "Procedure" when tuned for departure/approach automatically.

[Hardy Heinlin]

See page 341.

[John H Watson]

Yes, but by "being sure" I mean GPS is still a backup, not the primary nav, i.e. when all IRUs fail, the FMC will stop navigating.

Navigating yes, but  autotuning seems to be different from navigating. The FMC still knows it's current position from GPS and Last Position, so still has the potential to tune suitable stations.

Unfortunately, the indications at my home station are not a good example, because:
1) It doesn't like tuning the local DME/VOR because it's too close (but can be forced to do so by inhibiting lots of other stations)
2) The next best station is a military station (DME only), which is a valid station and the FMC can use this, but won't display the frequency on the NAV RAD page (not a VOR). i.e. you have to look elsewhere for signs it is autotuning this station.
3) The military station and the next best station are out of radio range (on the ground) so there are no DME range indications or audible idents.

My sentence about (P)rocedure tuning was simply to make a comment about FMC un-fussiness. On the ground, at least, the observations are that FMC will still command an autotune frequency.

Rgds
JHW

[the mad hatter]

i can put that to the test as I have the FMC x2 ad all the other black art boxes to go with that now to wire it :-)

[Hardy Heinlin]

On the ground, at least, the observations are that FMC will still command an autotune frequency.

I think we discussed this a few years ago, based on your observations. If I recall correctly, it was unclear which condition exactly removes the dashes. There was no clear pattern. But it was clear that autotuning (A) must start at least when airborne for position updating. So we left it there.

If you manually enter something valid into the dashes, the dashes disappear. Then you delete that manual entry, it changes to "A" and you can't get the dashes back.


Cheers,

|-|ardy

[John H Watson]

Unfortunately, I didn't know about the military airbase taking up a tuning slot until relatively late, so my tests seemed confusing.

e.g. If I delete all manual stations, I end up with "BIK"(Bindook) for the L VOR (autotuned) and dashes for the right. What do the dashes indicate? No station, or a slot taken up by the "RIC" (Richmond) Tacan? The ND displays showed BIK and RIC on the left and right side respectively, so in this case, I assume the latter.

Then if I pull all the FMC and CDU CBs for over a minute ("FMC EICAS messages in view), when I reset the CB's, the same stations appear on the Nav Rad page. Is there a longer term memory or did the FMC auto-retune BIK and RIC? Or did the VORs simply repopulate the FMC with the previously tuned frequencies?

If course, there may be lots of other variables. Each DME can tune 5 stations at the same time. What you see on the Nav Rad page and what you see on the ND may not always correspond to the frequencies used by the FMC.

Unfortunately, I won't have the chance to run through the tests again.

Rgds
JHW

[Will]

If I were building the 747, I would make autotuning start once the aircraft's position is known, and a route is entered in the FMC. I wouldn't wait until the aircraft is airborne. That seems a little late.

[Hardy Heinlin]

It's all a bit vague. Perhaps I should make a simplified compromise: allow one autotune event on the ground whenever there is a route edit event. That is, do not autotune while taxiing.


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[Will]

Sounds reasonable. Again, imagining that I were the designer of the system, I'd autotune once after each route edit while on the ground, for optimal departure information, and I'd retune if the signal were to be lost before departure. (Like maybe if the aircraft taxis into a zone of no reception, or if a station goes offline somehow.) But without exceptions like those, I don't see a need to continually auto-tune on the ground. But that's just me, thinking of what a practical solution would be... I obviously don't know what the real 747 does.

[John H Watson]

I don't think there is any point re-tuning on the ground due to signal strength. Sometimes I even got poor reception from the airport VOR with direct visual sighting. The aircraft is not going to pick up non-airport stations outside line of sight anyway.

If the position is known (GPS or Last), but no route entered, I don't see why the FMC wouldn't simply pick stations within range (excluding cone of confusions stations, such as the local VOR/DME and those stations manually inhibited).

I would only change the tuning (of the in-range stations) on the ground if it was part of the SID or Route. If the SID or Route is changed, then the tuning should change (no limit on the number of changes). Once in the air, the aircraft can refine the tuning for best geometry and to exclude failed stations not Notam-ed.

[Hardy Heinlin]

Muaha.

False alert.

P tuning on the ground already works in PSX. Just checked. The dashes disappear when a SID is entered and executed.


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