VORs

[blocks_off]

Hi,

Question based on whether the VORs on the 737-300 auto-tune - example, on the 747-400, 777 series of course we can access the frequencies thru a Nav Rad page and tune in a VOR or set it to auto. Same of course on the 757/767 - either side of the MCP you can set a frequency when in MAN mode.

I ask as I am interested in whether the FMC pos on this aircraft takes into account any land based navigation signals - is it designed in the same way as the 757/767 series where (I understand) the FMC blends those VOR/NDB references into working out the FMC pos and if those aren't available for 12 mins it gives IRS NAV only.

I see this is GPS equipped .. but all the same interested to hear if it is different in this regard.

Thanks

Carl

[Litjan]

Hi Carl,

there is a series of sensors that provide updates to the FMC position.

The basis is the IRS mix position, but it will be updated by the GPS (which is the top priority), also the tuned LOC receiver (lateral update) and two scanning DME receivers that the FMC will autotune (indepenent of the VHF Nav receivers) to update the FMC position.

The FMC will estimate it´s own accuracy (ANP) and throw an UNABLE RNP warning if ANP > RNP. We do not model deteriorating ANP, though - because we always assume that the GPS is available (which in reality mostly is).

Jan

 

[blocks_off]

Thanks Jan,

" We do not model deteriorating ANP, though - because we always assume that the GPS is available (which in reality mostly is). " Understood.

Only one other question - and I never asked when 'vlying' the 737-800 Is there a reason when they only have 2 IRSs?

e.g. 74/75/76 all have 3 and I thought that worse case of relying on IRSs for navigation means you're average position is somewhere in the middle of the triangle ?

- Carl

 

[Litjan]

Hi Carl,

I think it´s simply a question of cost-effectiveness for the desired role.

Two IRS units will also allow "averaging" of the position. In fact the FMC position is an average of all available positions, with a bias (in descending order) towards the GPS, the DME-DME, the LOC, IRS info and also VOR radial information.

The classic was never ment to fly over the oceans or other remote areas and IRS units are crazy expensive...

Jan

 

[blocks_off]

Ok thanks Jan.

I'm a new customer - jumping on board as at the latest version....... The plane is a very nice and immersive experience.

- Carl

[JRBarrett]

Hi Carl,

I think it´s simply a question of cost-effectiveness for the desired role.

Two IRS units will also allow "averaging" of the position. In fact the FMC position is an average of all available positions, with a bias (in descending order) towards the GPS, the DME-DME, the LOC, IRS info and also VOR radial information.

The classic was never ment to fly over the oceans or other remote areas and IRS units are crazy expensive...

Jan

 

You are right about the cost. I work in avionics maintenance in a corporate flight department. A single Honeywell Laseref IV IRS costs over $500,000 new[emoji33]

[avantime]

On 7/6/2017 at 0:19 AM, Litjan said:

Two IRS units will also allow "averaging" of the position. In fact the FMC position is an average of all available positions, with a bias (in descending order) towards the GPS, the DME-DME, the LOC, IRS info and also VOR radial information.

The classic was never ment to fly over the oceans or other remote areas and IRS units are crazy expensive...

When the IRS "averaging" goes wrong, you can end up massively off course. Like this Adam Air 737-300 which got lost for 2 hours, and made an emergency landing on the wrong island 250nm away. The pilots didn't even know where they had landed until they got off the plane.

Here's the official report, It's a fantastic read:

http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/Final_Report_PK-KKE_Release.pdf