737 VOR/LOC button for VOR approach (both FMC fail)

[zuxrat]

Good afternoon everyone!

I would be so grateful if someone could explain me one moment with Boeing 737:
imagine we have both FMS fail so we need to revert to conventional navigation. We navigate via VOR and during flying SID (or any other route via VOR) we can use VOR/LOC so autopilot can maintain our preset radial. Right?

And now goes the questions: we start the VOR approach for runway with no FMC (only raw data).

1. Do we need to set to the same active freq on NAV1 and NAV2 for VOR we use for approach (for me it’s strange, we can choose VOR2 on EFIS)?

2. I found nothing in FCTM connected with VOR approach using raw data. The only recommendation for non-precision approach is to use the HED SEL or LNAV to maintain the radial, but why can’t we use VOR/LOC? It seems much easier. I noticed that Boeing has no recommendations to use VOR/LOC for VOR approach. Yeah, usually we have FMC so we only check the VOR needles and the route is set in FMC, but in case of FMC failures we don’t have all of this.


do I understand something wrong?

[Check Airman]

No 737 experience, but I’ve had to fly raw data in a jet before. If the AP tracks the VOR on approach like it does en route, you’re better off not using it.

[Banana Joe]

1. Do we need to set to the same active freq on NAV1 and NAV2 for VOR we use for approach (for me it’s strange, we can choose VOR2 on EFIS)?

You don't need to have the VOR frequency set on both sides, you need to have it set on the AP side for tracking in VOR/LOC. The same applies to ILS operations, unless you need to engage both APs for an autoland. With the Navigation Display in Expanded or Full ROSE VOR mode, the deviation bar is relation to the frequency and course set on the relative side.

2. I found nothing in FCTM connected with VOR approach using raw data. The only recommendation for non-precision approach is to use the HED SEL or LNAV to maintain the radial, but why can’t we use VOR/LOC? It seems much easier. I noticed that Boeing has no recommendations to use VOR/LOC for VOR approach. Yeah, usually we have FMC so we only check the VOR needles and the route is set in FMC, but in case of FMC failures we don’t have all of this.

Recommendation is the key word. You're free to use VOR/LOC if you wish and if your SOP allows you to. It's a tool and if tracking is unsatisfactory you use HDG SEL or fly manually.

No 737 experience, but I’ve had to fly raw data in a jet before. If the AP tracks the VOR on approach like it does en route, you’re better off not using it.

It's not always the case, but I had the AP tracking the selected radial 3 degrees to the left. But that's why it's a tool that needs to be monitored. Like all the other modes.

[FlyingStone]

1. You don't need the same frequency tuned, but if VOR/LOC AFDS mode is used, then roll guidance will differ on PFD/EADI for each pilot, which isn't desirable, if it can be avoided.

2. There are multiple reasons why Boeing doesn't recommend using VOR/LOC as a roll mode during VOR approaches. Primary one would be the fact that a large majority of 737 these days would be NG/MAX with dual GPS installation, and a large share of those will be with dual FMCs as well, so LNAV is clearly the main prize in terms of accuracy and low workload. Note that normal procedures assume all installed equipment is operative.

Second, not all VOR approaches are constructed with VOR placed at the airport. If you have a VOR approach, where you cross the VOR halfway through final approach, behaviour of the VOR/LOC mode when you cross the station might be less than ideal and it may even rever to CWS R if signal is lost for more than couple of seconds.

I agree with Banana Joe, if your Operations Manual allows the use of VOR/LOC, use it, but as with any automation, monitoring its performance is vital.

[zuxrat]

Thank you for good explanation!