This shouldn't be a big deal in flight sim, but im interested in the realworld procedures:

Do pilots actually tune in navaids like VORs, when the nav radios are not used otherwise? I saw in the FMC, that if a VOR is tuned in, the computer creates a radio position.

If I understood the system correctly, the real 737 doesn't feature a autotune system, unlike the 767 for example, where the FMC automatically triangulates the aircrafts position, if possible. But the 737 has GPS receivers, does this make VOR tuning obsolete? Should I tune the VORs along my path, or is satellite and inertial navigation precise enough?

And a related question: Can I get a working map view on the ND, when both IRS and GPS are off? After all, tuning in nearby VOR's should allow the FMC to triangulate my position. I haven't figured out how to use only the radio position when the other systems are failing, though...

I saw a video on youtube once taken in the cockpit during flight. They specifically mentioned that the nav radios were updated automatically by the FMC, however I know this doesn't happen with the NGX. It looked like an older video, so it may have been a Classic, not an NG. Although, thinking about it......it was a Classic. Hard to imagine that a feature like that wouldn't be included in the newer NG's. Possibly an option and the airline PMDG modeled after didn't have this?

I saw a video on youtube once taken in the cockpit during flight. They specifically mentioned that the nav radios were updated automatically by the FMC, however I know this doesn't happen with the NGX. It looked like an older video, so it may have been a Classic, not an NG. Although, thinking about it......it was a Classic. Hard to imagine that a feature like that wouldn't be included in the newer NG's. Possibly an option and the airline PMDG modeled after didn't have this?

 

I think the FMS is updated by the GPS receivers as a primary source. VORs, LOCs, and other navaids are secondary sources to update the position.

Autotune isn't available on the NG. It's been a while since I've checked, so I can't remember whether a valid signal from those boxes will update the FMC or not.

 

However, it is good practice to tune any along route navaids, or if there are none, nearby navaids. This will increase your situational awareness, particularly if you were to have a dual FMC failure.

From the 737 technical website:

 

http://www.b737.org.uk/navigation.htm

 

Radio Position: This is computed automatically by the FMC. Best results are achieved with both Nav boxes selected to AUTO (happens automatically on NG), thus allowing the FMC to select the optimum DME or VOR stations required for the position fix. Series 500 aircraft have an extra dedicated DME interogator (hidden) for this purpose and NG's have two. Radio position is found from either a pair of DME stations that have the best range and geometry or from DME/VOR or even DME/LOC.

The NAV STATUS page shows the current status of the navaids being tuned. Navaids being used for navigation (ie radio position) are highlighted (here WTM & OTR).

 

I think if you are flying a GPS/RNP approach, you should tune your NAV radios to 108.00 to prevent any pickup of a navaid.

I think if you are flying a GPS/RNP approach, you should tune your NAV radios to 108.00 to prevent any pickup of a navaid.

 

You will have to turn off VOR updates on NAV OPTIONS page (FCOM SP.4.8).

 

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Letter m near RTM and IMSN shows that these frequencies are tuned manually on radio stack. Highlighted WTM & OTR shows navaid used for update.

 

The FMC tunes navaids automatically and independently of what you do with the navaids.

 

DME updating should be off for RNAV/RNP approaches.

The FMC tunes navaids automatically and independently of what you do with the navaids.

 

DME updating should be off for RNAV/RNP approaches.

Is DME updating turned off automatically when you selected a RNAV/RNP approach into the FMC? Or do you have to do it manually, and how would you do that?

 

Is DME updating turned off automatically when you selected a RNAV/RNP approach into the FMC? Or do you have to do it manually, and how would you do that?

They are defaulted to off in the planes I fly, but to turn DME updating off, you need to go to NAV OPTIONS.

Alright, I give it a try when I get home. Thanks!

The Primary Data source of the FMC navigation function is the ADIRU, Because the ADIRU drifts over time, The FMC navigation fuction uses data from other sensors to correct the ADIRU data that it receives.

 

The FMC uses independent measurements from the navigation sensors filtered with ADIRU position, heading and velocity data to produce an accurate calculation of the airplane’s position in the horizontal plane.

 

The ADIRU must be in the NAV mode to provide valid data to the FMC.

 

These are the navigation sensors to update the FMC position on priorty

1. ADIRU/GPS

2. ADIRU/DME/DME

3. ADIRU/DME/VOR

4. ADIRU/DME/LOC

5. ADIRU ONLY.

 

The First priorty for FMC position updating is GPS data from the GPS receivers.

 

The next priorty is a pair of DME station autotuned by the FMC.

The FMC will autotune the DME interrogators but VOR must be manually tuned by the flightcrew.

The FMC sends four navaid frequencies to the navigation control panels. The frequencies then go to the DME interrogators.

 

In an airport terminal area when the airplane is on a localizer approach, the FMC uses localizer deviation and DME distance to update the FMC position.

 

On the FMC NAV OPTION page:

The DME UPDATE ON/OFF prompt LSK 3L is default selected to ON.

The VOR UPDATE ON/OFF prompt LSK 4L is default selected to ON.

The GPS UPDATE ON/OFF prompt LSK 3R is default selected to ON.