J41-FMS/Altitude

[Guest windge]

Hi guys, When i put different altitudes to some waypoints in my FMS (J41), the plane follows the route, but dontfollows the Lateral nav. when the autopilot is ON. for takeoff ive got no config error on the CAP. once i press the Autopilot, do i need to do something else?thxDavid Guindon

[ckyliu 1,749]

I'm a little confused; You said the JS41 follows the route but not lateral nav, but I consider those the same thing. Considering how it's phrased, do you perhaps mean the vertical navigation; the climb and descent stuff?On the LNAV front, to get the AP to follow the route the EADI needs to be in LNAV and MAP mode, so you can see your route drawn on it. You can then enable the NAV mode on the AP when are you are on course to intercept this and it should ARM and then CAP. Alternatively, you can specify a Direct To in the FMS, you can do this by using the button on the FMS labelled with a D struck through with an arrow, remember you still need to set the AP NAV mode afterwards. Once NAV is CAP, be careful not to switch it back in to radio nav mode by pressing the V/L button too many times!As for climb/descents, the JS41 doesn't have a VNAV/Managed Descent mode on the autopilot. You must perform the descent manually, use the IAS hold or VS hold mode, preferably in conjunction with ALT CAP mode. The FMS has a VNAV mode and on the VNAV page it will inform you how far you are from Top of Climb or Top of Descent in both time and distance terms. It will also inform you what VS you should be targetting when you reach ToD. Once you have passed ToD, the VNAV page will constantly update what your VS needs to be in relation to your groundspeed and the calculated descent profile, and it will also tell you your deviation from this. You can then use this information to manage the descent via the AP modes I mentioned in the previous paragraph.Hope that helps,Chris Liu.

[Guest windge]

I'm a little confused; You said the JS41 follows the route but not lateral nav, but I consider those the same thing. Considering how it's phrased, do you perhaps mean the vertical navigation; the climb and descent stuff?On the LNAV front, to get the AP to follow the route the EADI needs to be in LNAV and MAP mode, so you can see your route drawn on it. You can then enable the NAV mode on the AP when are you are on course to intercept this and it should ARM and then CAP. Alternatively, you can specify a Direct To in the FMS, you can do this by using the button on the FMS labelled with a D struck through with an arrow, remember you still need to set the AP NAV mode afterwards. Once NAV is CAP, be careful not to switch it back in to radio nav mode by pressing the V/L button too many times!As for climb/descents, the JS41 doesn't have a VNAV/Managed Descent mode on the autopilot. You must perform the descent manually, use the IAS hold or VS hold mode, preferably in conjunction with ALT CAP mode. The FMS has a VNAV mode and on the VNAV page it will inform you how far you are from Top of Climb or Top of Descent in both time and distance terms. It will also inform you what VS you should be targetting when you reach ToD. Once you have passed ToD, the VNAV page will constantly update what your VS needs to be in relation to your groundspeed and the calculated descent profile, and it will also tell you your deviation from this. You can then use this information to manage the descent via the AP modes I mentioned in the previous paragraph.Hope that helps,Chris Liu.

im trying my best, english is my second langages, ill try to reformulate:Still on ground : I have the LNAV and MAP mode selected, my route planned in the FMS. for example my cruising Altitude is set to 16000, so in the FMS all of the waypoints are at 16000 (except those for the climb and descent). Its when i change some altitudes for certain waypoints that i cant find the way to make them apply. (during flight, the plane still follows FL160 even thought ive done some altitudes changes in the FMS. like i said im on LNAV and MAP mode...without pressing the V/L button :) and naturally the AP is on. NAV is on CAP and ALT is ON. plane is flying is pattern but is not considering the ALT in the FMS except the cruising one.im pretty sure its an easy solution, seems like a link between FMS and maybe the CAP im not doing right, dunno.ps. thanks for the climb/descent hints Skyliu, that was another question i had :-)David Guindon

[ckyliu 1,749]

Okay, basically it's what I stated in my previous post (sorry it was a bit wordy). The NAV button on the autopilot is for LNAV (lateral navigation) only. The FMS (flight management system/computer) has a VNAV (vertical navigation) mode, which you have been using, but it is for information only, you have to carry out the descent yourself. The FMS VNAV cannot be coupled to the autopilot, the idea is you use the calculations on the VNAV page to help plan and manage the descent yourself.As the JS41 does NOT have a VNAV mode on the autopilot, it cannot perform managed descents like a Boeing or Airbus, even though the FMS can calculate them. You must perform the descent yourself, using the autopilot's IAS mode (airspeed hold via pitch) or VS mode (vertical speed hold via pitch), preferably in conjunction with ALT CAP mode (altitude capture, it levels the plane off for you at the altitude you input). Of course, you can disconnect the autopilot altogether and fly the descent manually.NOTE: I think James has explained my a point a little more clearly below, I'm not very good at being concise :(

[Guest Jimmatc]

Hey David,Lateral Navigation (LNAV) refers to the route the aircraft follows over the ground. What you're talking about is Vertical Navigation (VNAV), which the JS41 does not have autocoupled to the autopilot. So, even though you can calculate a vertical path using the FMS (different levels at different waypoints), it's up to you to set the autopilot to climb, descend, or maintain a particular level. As Chris mentioned, VNAV is there as an aid mainly to assist with descents; it will give you a reasonably accurate point at which to commence your descent (TOD).So, to recap: Yes, the FMS has a VNAV function, but the levels displayed therein have no effect on the autopilot. The autopilot will only fly the levels you select!Cheers,James

[Guest windge]

Hey David,Lateral Navigation (LNAV) refers to the route the aircraft follows over the ground. What you're talking about is Vertical Navigation (VNAV), which the JS41 does not have autocoupled to the autopilot. So, even though you can calculate a vertical path using the FMS (different levels at different waypoints), it's up to you to set the autopilot to climb, descend, or maintain a particular level. As Chris mentioned, VNAV is there as an aid mainly to assist with descents; it will give you a reasonably accurate point at which to commence your descent (TOD).So, to recap: Yes, the FMS has a VNAV function, but the levels displayed therein have no effect on the autopilot. The autopilot will only fly the levels you select!Cheers,James

Ha! im happy to know now. I was used to the 747 FMS and i thought the one of the J41 had the same options. Well in a way its cool to know i will have to do more ''pilotting'' compare with the 747.thx for all your great answers, very nice forum here.David Guindon