Para visual display (PVD)

[Hardy Heinlin]

Good morning,

are there any real world PVD users aboard and have they ever used the PVD for a manual rollout?

The books are vague on this subject, partially even contradictory.

We know the PVD is inhibited for an autoland rollout. I suspect it can be used only for takeoff, only for this. Not even for a manual rollout.

But what if it can be used for a manual rollout: Can the PVD be armed inflight or only during the flare below 5 feet RA, or even later after touchdown?

When arming the PVD, the PVD barberpole first goes through a selftest, 1 sec left, 1 sec right, 1 sec stop -- it's obviously not the best time doing this barberpole rotation test during rollout.

In PS1 the PVD works only for takeoff.


Cheers,

|-|ardy

[amadeo24]

Hi Hardy, maybe this link helps:

http://www.pprune.org/tech-log/466815-paravisual-display-pvd-747-400-a.html

[Raj Hoonjan]

Hi Hardy

The following video is of a BA 777 auto landing at LGW:

http://www.youtube.com/watch?v=ZRWlkFKENLA

See from 5min 30sec into the video.  The PVD activates after the AP has been disconnected during the rollout.

Hope it helps
Raj

[Hardy Heinlin]

Thanks, guys.

This 777 video proves it can be armed in flight.

I just learned the selftest is inhibited above 40 kt. Everything's clear now.


Cheers,

|-|ardy


Edit: Ah, not 100% clear yet. I think the selftest is inhibited above 40 kt if on the ground. When I arm it in flight, I'd like to adjust the brightness, too. I can't do this if it won't unshutter for the quick selfttest.

[John H Watson]

Re 777's and 744's

http://www.pprune.org/tech-log/466815-paravisual-display-pvd-747-400-a.html#post6772269

[Hardy Heinlin]

Ha!

I hear the PVD shutter noise for a fraction of a second when the FLARE mode engages.

The armed PVD wants to unshutter at 5 feet, and the FLARE mode which is engaged at 5 feet, too, will promptly reshutter it.

It's a matter of milliseconds. I'll reduce the PVD threshold to 4 feet ...


|-|


Or the 5 feet are used only for takeoff, while for landing a ground signal is used, i.e. sort of a 5 feet hysterisis. The books say "on ground", but ... "on ground"  could also mean "RA < 5 feet" when they talk about the PVD. I assume it really means "on ground" in this case.

[Hardy Heinlin]

Two years later, same forum thread, but a different PVD question:

I'm wondering if the PVD descriptions from the manufacturers are simplified. Some of them just say the PVD indicates the deviation from the centerline. When I simplify my PVD model like that (I tried it years ago), the indication is pretty useless: If the barber pole rotates ever so slightly as soon as you are off the centerline, you will immediately kick the opposite pedal to get back to the centerline. And while your heading is changing, the deviation indication initially remains the same. Only when you approach the centerline, the pole rotation slows down. This effect is too slow. You will overshoot. The thing will be useless.

Well, if I make the model less sensitive, the deviation indication reacts too late. And that thing will be useless also.

That's why I added closure rates and intercept heading angles to the steering command (similar to flight director commands which also calculate with closure rates and angles etc. pp.). And bingo, the indication really supports the steering, is intuitive, and does not lead to pilot induced oscilations around the centerline. I can't imagine the real PVD does NOT use these intercept and closure rate parameters in addition to the plain localizer deviation data. Also, why should it be computed by the FCC if it were a simple localizer receiver/deviation indicator?

One could still call it a deviation indicator if the word "deviation" also implies "intercept heading"-deviation, and not only radio beam deviation.

The PVD shutters when on the wrong runway. This feature is not lost if a large LOC deviation compensated by a good intercept heading makes the pole rotation stop, e.g. deviation 30 meters and intercept heading 40°. The allowed offset is not unlimited. You must be within 2° / 3.2° (hysterisis). So it will unshutter only when you are on the correct runway. I mean, it doesn't tell you the wrong runway by rotating the pole, but by shuttering the display.

I used the PVD myself in Lufthansa 744 sims many years ago. Sadly, I can't remember how it reacted. It just worked intuitively.


Cheers,

|-|ardy

[Hardy Heinlin]

OK, I just learned that a certain AMM also mentions these parameters:

airplane yaw rate
cross body acceleration
localizer deviation
localizer deviation rate

So it is in fact a true steering director, and not just another simple CDI like that simple LOC diamond on the PFD.


|-|

[John H Watson]

It's not clear from the postings here what the 744 PVD does on touchdown.

From PPRuNe.

I've just seen a YouTube video of a 777 autolanding. The PVD comes alive during rollout after A/P disengagement. It wasn't armed after touchdown. If the system is switched off completely after T/O (memo message/s removed), how did it rearm itself with an in-air inhibit?  

Different aircraft, different logic.

On the 777, unlike the 744, you can rearm the PVDs once airborne (and you get the memo messages again), they will then unshutter on touchdown.

A pilot in a big sim has also confirmed that you can't arm a 744 PVD in the air.

Rgds
JHW