RNAV approach problems

JvanE

I tried a few RNAV approaches but although the first 2 went fine the last 3 all went bad. As soon as I hit ACT APPR on the FMS the mode changes to LNAV HDG SEL, the plane flights towards the magenta 'approach' line, all good. But then it keeps on moving in that direction and overshoots the NAV path towards the airport. It just won't intercept the correct lateral path! VNAV works okay, no problems there.

I tried entering the hdg myself on the FMS > NAV page (the entry starts blinking when I click on ACT APPR) but it doesn't matter, the plane simply overshoots the correct path.

Any clues to what I may be doing wrong? The odd thing is that it did work 2 times...

Is LNAV HDG SEL the mode I should see? Shouldn't that mode contain APPR somehow? If so, how come it doesn't? I arm the appr after the intersection, the waypoint before the FAF. Should I arm earlier on?

EDIT
Just noticed rondon9898 posted a video on RNAV approaches. I will have a look at it now.

JvanE

Okay, problem solved. Turns out you only have to ARM the APPR. You won't see any indication on the PDF of this (or anywhere else: the FMS will only show ACT APPR now) but as soon as you intercept the final course the mode will change from LNAV to LNAV APPR. Seems you should not hit ACT APPR at all. At least not on my PC...! I performed three landings in a row now this way and it worked like a charm.

BTW I descended from cruise using VNAV, setting the IAF as TOD target, and could use VNAV all the way to the threshold: I didn't have to select DH or anything just before the approach to perform a perfect RNAV approach. I only had to select a lower ALT during cruise in order to enable VNAV. From there on the FMS doesn't care what you dial in as ALT anymore (as long as you do not select ALT SEL) and it

JvanE

Okay, here is how it works for me.

1. Make sure all NO LINKs (above the runway entry) are removed from your flightplan. This way LNAV will lead you all the way to the destination runway in one go without you having to do anything for it. (You can of course use DTO if you want to skip parts of the plan.)
2. During cruise set TOD using the first waypoint with an altitude constraint.
3. Just before TOD set the constraint ALT (although any lower ALT will do) and enable (arm) VNAV. Do NOT enable ALT SEL!
4. As soon as you start descending (VNAV PATH) you don't have to bother with ALT anymore: VNAV will fly you all the way down to the airport regardless of what ALT you set. It will also automatically keep all constraints (which is why I simply picked the first constraint in step 1).
5. When you are on course to intercept the final track towards the runway select ARM APPR on the FMS's NAV page. You won't see any indication of this on the displays but the FMS will now show ACT APPR. If you do NOT arm the approach, VNAV will switch to PITCH HOLD at the FAF!
6. LNAV will automatically change to LNAV APPR when you are on course for the landing (usually at the IF waypoint).

That's it. It's quite simple once you figured it out.

JvanE

In addition: if in step 5 the FMS already shows ACT APPR (instead of ARM APPR) leave it as it is: apparently sometimes the approach is armed automatically. Don't know why and how but it just happened to me. ACT APPR only shows up whenever the approached has been armed.

jpgmultimodal

Hello JvanE,

Just flew to ENTO Rwy 18 VNAV from ESOK DCT VATEX FL80 and followed the STAR VATE2S 3000 Ft by IBSAT then DTO UNVID.

Just arm the APPR, no need to ACTIVATE. The APPR does not ARM automatically, you have to ARM it yourself but it ACTIVATES automatically.

From EROBU, spot on the virtual G/S (VNAV PATH) but the altitudes were below the published altitudes 7 Nm -190 Ft to 2 Nm -90 Ft but on the ground the difference was only -20 Ft which is not too bad for the baro VNAV.

It appears to work OK.

JP

JvanE

@jpgmultimodal said:

It appears to work OK.

It does indeed, see my last posts. And yes, activation is automatic once you armed the approach: that’s what I meant to say with step 6.

helico55

sometimes arm app not appear and not act app as well.
why not appear ?

FraPre

For that you would really have to have seen the flights and your handling. Were ARM APPR and ACT APPR really not offered, or maybe they activated themselves alone, which is also provided from defined points in the event that the crew does not do this ?!

helico55

From what distance of the airport i must to arm approach ?

FraPre

The ARM APPR function is possible from a direct distance of 50 NM from the destination airport; it does not appear on the NAV page beforehand