VOR Station Elevation in NPA

[mmrassi]

Hi

737 FCOM 1 (Supplementary Procedures) mentions not to use AFDS VORLOC if the VOR station elevation is more than 5000ft. Can anyone explain me the reason and tells if there is a similar recommendation for other planes?

[Green Guard]

strange, as the higher elevation of VOR station, the less error when ~20 NM or less

[aterpster]

Green Guard:


strange, as the higher elevation of VOR station, the less error when ~20 NM or less


That would be correct with DME but not with VOR radials.


Along that line, DME slant range is disregarded in TERPs approach design because such errors are insignificant at approach altitudes.

[Denti]

There are airlines out there that still use the VORLOC mode for VOR tracking? Interesting...

[Captaintcas]

Yes, Denti, we do.
How else to follow a VOR? Do you go in heading sel.?! And follow up raw data?!
Lnav?! Then you risk map shift and input error at low altitude or terrain... So no, not allowed and not done in my outfit flying B737.

[cosmo kramer]

mmrassi,
737 is 3 different aircrafts. Are you perhaps flying Classic? In my 737NG supplementary procedures, no such limitation is mentioned.

Captaintcas,
I think most airlines recommend LNAV for VOR tracking (in case of VOR approach, of course cross checked with raw data in the background). For departure, cruise and arrival, we rely on Require Navigation Performance vs. Actual Navigation Performance and associated caution messages (without raw data cross check). Most of the procedures and route structure require P-RNAV anyway (Europe). I have a feeling you are flying classic too? ;)

Personally, if I track a VOR radial not part of a VOR approach, it will be with autopilot and FD off, for fun/practice (and not with AFDS VOC/LOC).

[underfire]

VORLOC...wow, blast from the past!

In regards to the abv 5000' question, there is an increased radius of turn at higher altitudes that causes problems for the curve algorithm in the FD and autopilot software...

EDIT: Cosmo, where are you flying RNP DEP?

[mmrassi]

Thanks for all related and irelated answers.

I fly 735 which has GPS position update but we still see that there is a statement like this regarding use of VORLOC.
The last answer from our friend "underfire" was quite interesting and I appreciate if you give me more details regarding your idea.

[cosmo kramer]

Despite everything said in the quotes, the system requires me to enter at least 10 charters to post a message... here by done (in excess).
underfire
Cosmo, where are you flying RNP DEP?

cosmo kramer
Most of the procedures and route structure require P-RNAV anyway (Europe)

[cosmo kramer]

underfire
Cosmo, where are you flying RNP DEP?
Just to clarify where RNP and ANP comes into the equation. The FMC will show you what lateral value you have to comply with to fly RNAV-X/P-RNAV. Should these values be exceeded you will get the "UNABLE REQD NAV PERF-RNP" message. In the respective QRH checklist you will find the following option:

On a procedure or airway without an RNP alerting requirement:
Verify position.

...hence, you won't need to do a raw data check as long as the FMC is not alerting you to do so, and even if a VOR is part of your departure, as such you can fly it in LNAV and do not need to tune it and fly it in VOR/LOC....

....which was in response to Captaintcas:
How else to follow a VOR? Do you go in heading sel.?! And follow up raw data?!
Lnav?! Then you risk map shift and input error at low altitude or terrain

On the LEGS PAGE of the FMC at the bottom left, you can keep an eye on your navigation performance written as RNP/ACTUAL, hence:

For departure, cruise and arrival, we rely on Require Navigation Performance vs. Actual Navigation Performance and associated caution messages (without raw data cross check).

Yes, Boeing doesn't really care about the definitions of RNAV-X/B-RNAV/P-RNAV vs. RNP (it seems). Should we as pilots care? As long as we know what lateral limits are applicable and appropriate response should they be exceeded.