747 ND VOR Raw Data?

[NSEU]

744 Forum: Hangar 7 » VOR "cone of confusion" on EFIS

A simming forum, but well-respected.

Would someone be able to confirm that the Navigation Display VOR display on the 744 is manipulated by the FMC or EIU's to produce a constant deviation over the cone of confusion?

I've always thought of the display as being raw data. Anyone flown over a VOR recently, with the VOR display in view (on the 744)? Does the deviation bar waver or drop out when the aircraft is directly over the station? What happens to the pointers on the compass rose?

I can understand the FMC computing a steady path over a VOR station for LNAV (for A/P use), but this other stuff is new to me.

Thanks.
Rgds
NSEU

[STBYRUD]

Can't speak for the 747-400, but on the 737NG (which essentially has the same flight instruments) the deviation bar disappears when crossing the station, don't really recall what the needle does...

[Coffin Corner]

I didn't think you could directly track a raw data VOR by using the VOR rose and CDI in a 744? From what I have been told the only way to track a VOR is to do it via the FMS by setting an inbound course and flying it in LNAV?

p.s. Q400 rated, not 744 rated (obviously)

[NSEU]

From what I have been told the only way to track a VOR is to do it via the FMS by setting an inbound course and flying it in LNAV?

Autopilot-wise, yes. There is no VOR selection on the APFD Mode Control Panel on the 744. The radial would therefore have to be tracked manually if the FMC was not available.

I see some responses to this message have been removed for reasons unknown, but in essence...

Most pilots will be flying with MAP mode displayed, so won't normally see the VOR display. The VOR pointers waver and then disappear as the aircraft flies over the station, so we would expect the course deviation bar to do the same.

Hopefully someone could check this out for me next time they go flying (in the 744), perhaps noting altitude and duration (time) of oscillation/disappearance?

Thanks
Cheers
NSEU

[bigduke6]

We'll see what it looks like tomorrow, if I can actually go straight over (or near enough as needed) a VOR.

[main_dog]

I see some responses to this message have been removed for reasons unknown

NSEU, you may be referring to my post... I withdrew it in consideration of the fact that having been on the ship less than two years, there's a heck of a lot more about it that I don't know than I do know. Thus like Duke I wanted to go flying first and see it firsthand before making an a$$ of myself . Anyway here's the post, with the proviso that I'm not sure yet!

Like most pilots on the 400 (or any EFIS jet really) when I do fly over a VOR I am usually in map mode, so I'm not sure about the deviation bar. The VOR pointer needle however will start wavering increasingly as you approach the station, only to disappear as you overfly it, and reappear on the other side.

It stands to reason that if you had displayed VOR on your ND, the deviation bar would similarly waver, disappear and finally reappear on the other side of the station, as it is just another form of presenting the same raw information the needle pointer does.

As far as I'm aware the pointer/deviation bar are pure raw data and in no way manipulated by the FMC.

MD

PS Indeed you can't Coffin, all you can do is let the aircraft fly it in LNAV provided it is doing a good job, or else follow the radial yourself "manually" by adjusting the heading (using HDG SEL).

[bigduke6]

At FL300, approaching one station, got very minor CDI scalloping starting at around 9 DME slant range. At 5.1 DME had several full scale CDI oscillations, then the wording (no actual pointer flag, just either TO or FROM on bottom right of ND) "TO" disappeared momentarily, then came back, then switched over to "FROM", with several more full scale CDI deflections.

So essentially raw data.

[NSEU]

Excellent. Thanks for the feedback, BigDuke

I had strong doubts about the VOR display being manipulated by the FMC. Unless there has been some unusual modification carried out, it sounds like someone has misinterpreted what someone else has told him

Cheers
NSEU

[main_dog]

Finally overflew a VOR today (only once in 8 hours flying! Navigation ain't what it used to be), at FL380.

The deviation bar started wavering noticeably at about 9/10 miles, then disappeared completely together with the TO sign at 7.2 DME slant range. For a period of about 20/25 seconds, except for the occasional flicker the deviation bar and VOR pointer remained out of sight (interestingly this happened absolutely simultaneously with the standby RMI's VOR flag appearing, and I'm sure the FMC has no input there).

At approximately 7.2 DME outbound the deviation bar reappeared (wavering wildly at first, then steadying), together with the VOR pointer and a FROM sign.

Based on this I'm completely convinced the presentation is 100% raw data. Having said that there are more customer options on a Boeing than you can shake a stick at, perhaps it's an optional feature?

MD

[eckhard]

I can confirm that the behaviour of the VOR display observed by main_dog and bigduke6 is the same as that seen by me over the last few years flying and simming (FFS) the 747-400.

I'm sure that the VOR display is pure raw data (and the LOC display is as well, for that matter).

[NSEU]

Thanks, gentlemen.

I doubt there is an option for FMC manipulation. It wouldn't make sense. Raw data is presented on the other displays to allow you to check that the FMC data is correct.

Cheers
NSEU