Fake 'RNAV approaches'

1st Official Post

    Whenever I fly the Q400 I like to fly into airports with an ILS. It's just so easy to do. ;) However, a lot of airports don't have an ILS and when they do, they often don't work correctly. I just flew into Key West and suddenly it occured to me that when you set the airport destination as target altitude in the Q400 and then descent using VNAV, following the flightplan route with LNAV, you are sort of doing a perfect 'RNAV approach' into the airport...! Why didn't I think of that before?! (Or why did I forget about that?!) The flightplan, with its automatic FAF, is always flown PERFECTLY by the AP and you never get into problems with it like you can with ILS (missing ILS, LOC not working, plane going left right left right all the time). Added bonus is that it's very easy to fly into ANY airport this way: now I don't have to look for (and wish for a working) ILS anymore! :) I think I will be using this method from now on.

    I wonder if anyone else is using these options to fake an RNAV approach?

    Quote from Jet-Pack

    Just press the approach button with the nav source on FMS in the q400, lj45 or c90 and it'll fly the rnav approach, without the need to decrease the selected altitude.

    In the other aircraft with the advanced autopilot you also get rnav approaches when no ils is available! (Just press the approach button :) )

    Wow, didn't know that! Nice! :)

    Copying real rnavs is not so easy, the routes might go in OK but when flown, the route can explode into spaghetti at the last minute or the route can switch to earlier waypoints. The DIY routes following real examples might involve speed limited short radius turns or reversals over themselves at a lower altitude, these do not work well with the current FS2 route software.

    Copying real routes is rewarding because it is a realistic, often beautiful simulation and because it does not involve flying through (not between) mountains.

    Quote from Jet-Pack

    Just press the approach button with the nav source on FMS in the q400, lj45 or c90 and it'll fly the rnav approach, without the need to decrease the selected altitude.

    I tried this a few times and it works but I do wonder what annunciations I should be seeing on the PFD...? I usually descend in VNAV (and use LNAV for direction). When I get near the FAF I usually am not already at the altitude I set (2000 above the airport elevation with ALT SEL enabled). When I press APPR the only thing that seems to happen is that ALT SEL is being turned off. I don't see any other indication. When I press APPR again things go wrong (LNAV changes to HDG or VNAV changes to PITCH MODE).

    Is it correct that pressing APPR in this situation simply only disables ALT SEL which let's the plane descend on the magenta 'VNAV line'? Is that what's supposed to happen? If so, what is the difference between pressing APPR or simply disabling ALT SEL...? Or isn't there any difference? In that case I might as well NOT enable ALT SEL when I set that 2000 ft altitude and fly in VNAV without doing anything until I land.

    Short version... ;) What should happen when I press APPR 5 nm before the FAF when I am still flying with LNAV and VNAV?

    EDIT

    Just did another flight. This time I was at 2000 ft before I reach the point where I would want to enable APPR. To my surprise ALS SEL turned off automatically but the annunciation didn't change to ALT: it stayed on VNAV. I dropped below 2000 ft a bit but then it suddenly stayed where it was for a while until it intercepted the descending magenta altitude line again...

    Maybe all this different behaviour has to do with the fact that I am staying with FMS1 as source now instead of switching it to another source?

    EDIT 2

    What the heck! I had another look at the cockpit of the Q400 and see the source is ILS1...? While I didn't change the source? I am getting confused here: strange things are happening. ;)

    Edited once, last by J van E (July 31, 2018 at 11:08 PM).

    • Official Post

    The source currently only changes when you move the aircraft to another location.

    If you show approach guides it becomes clear what the default route is doing, there is a level off segment before the final approach fix. The autopilot just follows this profile in VNAV. On the PFD it may look the same but internally there are some differences. Q400 isnt the easiest for this. A320 destinguishes better and enhances situation awareness more i find.

    Quote from Jet-Pack

    The source currently only changes when you move the aircraft to another location.

    If you show approach guides it becomes clear what the default route is doing, there is a level off segment before the final approach fix. The autopilot just follows this profile in VNAV. On the PFD it may look the same but internally there are some differences. Q400 isnt the easiest for this. A320 destinguishes better and enhances situation awareness more i find.

    Ok, thanks. I can't remember moving my aircraft but I'll keep an eye on it. And very good to know about the level off segment (nice!): I already wondered why the magenta line didn't drop down further while I was at my assigned altitude: I thought the route was going down to the airport in a straight line...! I might indeed turn on those guides for once to see what's going on.

    And I know the Q400 isn't the easiest but that's exactly why I love it. ;)

    EDIT

    Got it under control now! Turns out that VNAV dips a bit below my set altitude of 2000 when flying on APPR but it settles down nicely on the magenta line anyway after that. Because of this dip I immediately started to press buttons and stuff before because I thought things were going wrong but things are going fine after all. Loving these RNAV approaches!

    Edited once, last by J van E (August 1, 2018 at 7:28 PM).