MD-11 strange VOR navigation behavior ?

[cniedzi 0]

Hi Captains,Yesterday I noticed a strange VOR navigation behavior. I have flown an EHAM 18C approach and I have tried to sequentially track two radials of VOR SPL using autopilot: 300 and then 002. So, initially I have flown HDG 225 and set FMC as shown in following figures:http://forums.avsim.net/user_files/194256.jpg http://forums.avsim.net/user_files/194257.jpgThe first radial was correctly tracked, so then I have armed the second radial – 002:http://forums.avsim.net/user_files/194258.jpg http://forums.avsim.net/user_files/194259.jpgThe problem was the a/c did not start to track the radial 002; instead there was a message “VOR1 CRS” on PFD and the a/c continuously has flown previous crs.http://forums.avsim.net/user_files/194260.jpgIs it not possible to fly sequentially two radials of the same VOR station or do I have done something wrong? When one arms HDG mode before arming the second VOR radial, there aren’t any problems. Additionally, you can successfully fly two radials of two different VORs.R.Czarek

[tcas_climb 45]

Hello R.According to that FMA, AP1 is controlling VOR1 CRS, so try pushing the autoflight button again to switch to AP2 and see if that will switch to VOR2 CRS

[tcas_climb 45]

Or you could try arming VOR2

[xkoote 57]

Well from those pics it seems that VOR 2 is already armed. Secondly I doubt that AP 1 or 2 has to do with VOR 1 or 2. My doubt is in the fact that the AP has ARM and/or Intercept logic on a second VOR whilst the first VOR is being tracked or in CRS mode.XanderEDIT: If you have doubts regarding the AP selected as mentioned. Try the same but with the offside VOR armed first. VOR 2 on course 300 and VOR 1 on course 002, with AP1 controlling. Just to see what happens.

[cniedzi 0]

>>EDIT: If you have doubts regarding the AP selected as>mentioned. Try the same but with the offside VOR armed first.>VOR 2 on course 300 and VOR 1 on course 002, with AP1>controlling. Just to see what happens.The same issue... But when we play with two different VORSs, everything is fine using only one AP.R.CZarek

[mfrantz 45]

Czarek,In VOR tracking mode, when you are overflying the VOR station you will notice that on the PFD FMA the roll mode changes from VOR1 (or VOR2) to VOR1 CRS (or VOR2 CRS). This happens because when over the VOR station the received signal is very sensitive and not reliable for tracking, so to avoid problems the VOR signal is ignored and the radial course is followed applying an internal HDG (actually TRK) mode. This applies for some distance immediately before and after the VOR station. After that, normal VOR tracking resumes and the FMA changes to VOR1 (or VOR2).What happens in your case is that this temporary internal course tracking mode (VOR1 CRS) is activated before entering the capture zone of the second armed VOR radial, and when this course tracking mode is exited the aircraft is not on an intercept heading to the other armed VOR radial, so it will never enter the capture zone.Maybe we should make some adjustments to the length of the VOR CRS mode and/or the capture criteria. I will investigate this.

[cniedzi 0]

Thx Michael,Fully understood; great explanation:)R.Czarek