Question on LNAV button

[enrico68 3]

Hello everyone, I have been studying the Jetstream for a few weeks, AOM, Introduction, Tutorial, still there is that button, LNAV, that I do not understand. When I fly to intercept a VOR radial for example on an airway, I press V/L, enter the correct frequency in the RMU, and set the desired radial through the CRS button. Then I press NAV on the autopilot, and the aircraft turns to intercept it.

The LNAV button, instead, is a mistery to me. When and why do you activate it? When I push the LNAV button while in flight tracking a VOR radial, nothing seems to happen. I would appreciate if anyone can help me understand the use of that LNAV button. Thanks,

and a big thank you to PMDG for a great aircraft, and a very detailed AOM and documentation, you PMDG people know what you're doing, that is why I own all your airplanes, besides the 747 V 2.0 and the 777, just waiting for them to be released.

 

Enrico :wink:

 

PS. I have attached an image of the LNAV button

[Mickel 42]

LNAV follows the active flight plan from the FMC, not a VOR, if I'm understanding your problem correctly. When you press the full map button (other end in the pic), do you have a flight plan showing on the nav display?

[PHXMD-11 86]

 

 

When I push the LNAV button while in flight tracking a VOR radial, nothing seems to happen. I would appreciate if anyone can help me understand the use of that LNAV button. Thanks,

 

You need to program the flight plan into the FMS. Follow the tutorial for step-by-step instructions.

 

The selector below the FULL/MAP button needs to be selected to LNAV for the aircraft to follow the route in the FMS>

[enrico68 3]

Hello Mickel,

 

let me see if I have got that right: the LNAV button is there to be pressed when you want to see the flight plan from the FMS displayed on the EHSI monitor, but, in order for the autopilot to follow it, you still have to press on the NAV button on the autopilot. Is this right?

 

Enrico

[enrico68 3]

Hi PHXMD-11

 

I just quickly went over the part of the tutorial, where it says to press LNAV and MAP, to bring up the moving map, with the FMS flightplan. It is clear now, I'll redo the tutorial flight again, but I think I got it, many thanks

 

Enrico

[enrico68 3]

Hello again, after reading the tutorial one more time, and trying a few flights, I finally understand the LNAV function. This is my first time dealing with a FMS, although a simpler one, compared to the Boeing FMC or airbus MCDU. Let's just say the Jetstream to me is my intro to the world of the big irons. It is an awesome aircraft, a lot of fun to fly, it sure needs to be studied, and in that respect the tutorial and the AOM that come with it are very well organized; besides, as the Jetstream does not have the VNAV function, you have to monitor your route and speed, in order to meet the altitude requirements of the flight plan. A lot of fun to fly it, it just reads PMDG all over it,

 

Enrico

[Mickel 42]

Excellent. It's well worth persevering with. It has the most hours in my log book. ;-)

 

Mike

[enrico68 3]

Helo Mike,

 

I hear ya! I recommend it to anyone, this Jetstream is just too enjoyable, I can tell I  get better and better with every flight, it is a learning experience. I am very glad I bought it. By the way, thanks for your support, I appreciate it 

 

Enrico :smile:

[Steven_Caffey 23]

You seem to have got it, but I'll post this for the benefit of anyone else who may be confused by this. Think of it this way, the picture in the OP is of a Display control panel. All it does is control what information is shown on the display. Getting the autopilot to do something is controlled by the autopilot control panel.

[scandinavian13 5,192]

 

 

All it does is control what information is shown on the display. Getting the autopilot to do something is controlled by the autopilot control panel.

 

Not quite...

 

The V/L and LNAV buttons not only select the information displayed on the nav display, they also select the NAV mode.  While, yes, you need to select NAV on the autopilot panel in order to follow a NAV track, the source of this NAV track information is selected using the V/L and LNAV buttons.

[ckyliu 1,749]

Just to expand on what Kyle said, the first press will just change the display mode temporarily, a second press will change the actual navigation source selection.

 

There is VNAV functionality within the FMS but it is very basic and not coupled to the autopilot or even the EADI. It will however calculate you a TOD, required descend rate and your deviation from the FMS calculated vertical profile, based on altitude restrictions input.

[scandinavian13 5,192]

Just to expand on what Kyle said, the first press will just change the display mode temporarily, a second press will change the actual navigation source selection.

 

When currently using NAV on the AP, yes.  When not using the AP in NAV mode, I believe all you need is a single press.

 

Either way, the method you mentioned is very useful for remaining on FMS direction, but being able to set the ILS course without switching to V/L completely.

[enrico68 3]

After a few tests, I can confirm what scandinavian and ckyliu say.

 

When you are flying  in NAV mode (for example in LNAV,with the autopilot coupled to the FMS), and you want to say set the ILS course before the approach, you just press once on V/L. That way the autopilot will still stay in LNAV mode, and in the meantime, you can work on your ILS course selection. When it is time to say follow a radial, you then have to press twice on V/L to couple the autopilot to the radial selected.

 

If you are not in NAV mode, instead, and want to track a radial, you press only once on V/L, set the desired radial, and then press on NAV.

 

I believe the only exception is when you are transitioning from say LNAV to Approach. In this case, when you arm the autopilot for the approach, by clicking on the APR button once, the airplane will automatically bring up the V/L screen and transition the autopilot to LOC/G-S tracking.

 

I have flown quite a few scenarios, with NAV on/off, LNAV or VOR/LOC, and that is how the Jetstream autopilot behaves. Tonight I'll try flying a DME arc to see how well the autopilot chases the arc. I have the correct waypoints from Navigraph, and the Jeppesen charts; my feeling is that the Jetstream is not capable of recreating a perfect arc, and it might zigzag more then curve, but let's see. If it does that, I'll add manually a few more waypoint, closer to each other, this way helping the Jetstream follow the arc. If anyone of you has already flown a DME arc and has any suggestions, please let me know,

 

Enrico  :smile:

[scandinavian13 5,192]

 

 

Tonight I'll try flying a DME arc to see how well the autopilot chases the arc. I have the correct waypoints from Navigraph, and the Jeppesen charts; my feeling is that the Jetstream is not capable of recreating a perfect arc, and it might zigzag more then curve, but let's see. If it does that, I'll add manually a few more waypoint, closer to each other, this way helping the Jetstream follow the arc. If anyone of you has already flown a DME arc and has any suggestions, please let me know,

 

Not gonna happen - arcs like that are what's called Radius to Fix (or RTF) when it comes to flight data (what the FMS reads).  Being able to fly this via autopilot is somewhat of a recent development, as far as aircraft capability.  Several major airlines don't have this equipage in their aircraft.  The J41 does not have this either.  DME arcs were hand-flown procedures, and in most cases, still are.

[Steven_Caffey 23]

Not quite...

 

The V/L and LNAV buttons not only select the information displayed on the nav display, they also select the NAV mode.  While, yes, you need to select NAV on the autopilot panel in order to follow a NAV track, the source of this NAV track information is selected using the V/L and LNAV buttons.

 

Ah yes, that's right. I've gotta get back to flying this plane, been a while.