DME Updating on the 737NG

[STBYRUD]

Hello folks, my company has been receiving some brand new 737s since a couple of months and I made a curious observation - on all of them (in contrast to the older ships in the fleet) the DME updating is turned off by default - I have a couple of theories, but no solid evidence as to why this is the case. Usually I would turn it back on again, but I would like to know the reason behind this... One captain claimed that this was because the 'spurious accuracy' would deteriorate the ANP (yet VOR and LOC updating are both still on) - since the FMC is 90% or so biased towards the GPS position anyway this shouldnt matter one bit - throwing in another position source (and according to my ATPL knowledge the most accurate after GPS) should improve the overall accuracy and redundancy of the system.

Any thoughts?

[ImbracableCrunk]

DME updating is turned off for RNP approaches I've done, so ANP sounds like a good answer to me. I'm guessing the default is user preference.

[c100driver]

Yes the DME update default ON or OFF is a customer selectable option.

Operators that do a lot of RNP and RNP AR work usually default DME off, so your Captain is correct. Our RNP AR approaches mandate DME off.

It will not VOR update because it requires DME as it is a VOR/DME update. Not 100% but I think that is the same for LOC as well.

[STBYRUD]

In the documents I looked through there is only a generic reference that certain RNP procedures may require DME updating to be turned off, but in general it remains a valid fallback option. The question is - no one was able to tell me what to do on the old airplanes where its on by default - should we turn it off then?

[Denti]

DME/DME updating alone (without GPS updating) is enough for approaches down to RNP 0.3, at least we were certified for that with our old -300s without GPS updating. So i guess switching it off is probably only required below that, however with certification for approaches down to RNP 0.15 for RNAV approaches and down to CAT I for GPS/GLS approaches we do not have any requirement to switch DME/DME updating off. Usual ANPs with DME/DME enabled are 0.02NM.

[ImbracableCrunk]

The question is - no one was able to tell me what to do on the old airplanes where its on by default - should we turn it off then?

Turn it off if you're going to do an RNP approach or departure. If not, don't worry about it being on.

[BOAC]

Stbyrud - not sure who you work for, but normal airlines have technical departments and ops managers that will answer this query for your company ops. Assuming (hoping) you have such, have you tried them?

[aterpster]

Denti:

DME/DME updating alone (without GPS updating) is enough for approaches down to RNP 0.3, at least we were certified for that with our old -300s without GPS updating. So i guess switching it off is probably only required below that, however with certification for approaches down to RNP 0.15 for RNAV approaches and down to CAT I for GPS/GLS approaches we do not have any requirement to switch DME/DME updating off. Usual ANPs with DME/DME enabled are 0.02NM.

The U.S. stopped allowing DME/DME for conventional RNAV IAPs after a couple of serious incidents where the DME geometry was poor, at least at final approach altitudes.

Conventional RNAV procedures indeed use RNP 0.30 alerting and integrity, but unlike RNP AR there are wider primary obstacle protected areas, as well as secondary areas.

In an aircraft certified for RNP AR, DME updating should be automatically disabled anytime an RNP AR IAP is activated.

[zoom33]

Still wondering

Same for us, some of our new 737ng has dme update selected off by default in fmc.
If it is for rnav rnp operation it doesn't make sense, vor update must be set off also in this case. Dme/dme update is more precise than vor/vor
A captain told me that new NG has only one dme receiver that is why it is off by default. ( arguing with gps it is not nedded ... )
He told me also that it is all automatic and with vor update only it will also use the dme if avalaible.
Any enlightenment about this subject ?
I am often overflying areas with gps jamming so I am concerned. Generally jamming last for 10 min and our rnp increase but still around 0.5

[A and C]

Zoom 33

I very much doubt that the 737NG has only one DME box as if this was to become unserviceable it would ground the aircraft.

Care is needed when quoting the radio fit of aircraft as different customers order different options, on another thread some one said the B787 was no longer fitted with ADF, this may well be true for one airline but I can quote the flight manual for another airline that has two ADF's fitted.

[zoom33]

I just checked MEL and we have two DME.

[galdian]

From memory - and maybe different for different operators - the DME units agility tune, one DME receiver continuously tunes and uses multiple DME sources.

In my old company MEL was "two fitted, one required", I don't think there were any further qualifying statements.

Cheers.

[aterpster]

The early 767, at least, had scanning DME sets. I don't recall the particulars other than they would scan for 10 DME ground stations.

[mischo1990]

Hmmm in our company we have to turn off VOR updates on RNAV approaches i still didnt find anywhere WHY?

[imriozer]

Zoom33,

There is no vor/vor updating... not accurate enough.

[FlyingStone]

For all RNP (AR) approaches, VOR updating has to be disabled as it is not nearly accurate enough for approach operation. Some authorities also prohibit DME/DME update during RNP (AR) operation, so you have to select DME update to OFF.

DME UPDATE default to OFF is a customer option, and the aircraft still has 2 DME receivers nevertheless. The purpose of this is to reduce the crew workload in preparation for RNP (AR) approaches, which are becoming more and more common. Furthermore, if you select DME UPDATE to OFF, the VOR and LOC updates are inhibited if the GPS updating is working - even though it is selected ON in the FMC.

[vickers vanguard]

Modern DME UNITS

Folks, pretty much all modern DME units are multiple channels ( typically 3 ). This means that the unit tunes and locks to 3 DME stations at the same time. Typically, channel 1 is paired with the onside VHF nav, while channel 2 and 3 are tunned automatically by the onside FMS. Modern FMSs will tune the nearest 2 DME stations based on entered, last know position, or GNSS position if received , so the FMS can come up with an Rnav solution. The geometry of the tuned stations is taken into account ( i.e elevation vs height of airplane.....often it's not used on ground). This Rnav solution depending on the maker of the FMS, will either be blent with other nav sources with each one having its own weight ( aka importance ), or just kept as a back up when GNSS quits. All Rockwell collins FMS work this way, the older ones would blend nav sensors, while the newer ones will use a priority based selection skeme, with GNSS first, DME/DME second, VOR/DME third, and IRS last. Those FMS (Rockwell )that are designed to shoot RNP AR approaches, will jump from GNSS to IRS directly, and will not use DME/DME or VOR/DME for position. Consult the Faa Advisory circular for RNP AR, and it's all in there. Some avionics might use DME/DME for backup during an RNP-AR, but not Vor/ Dme.
Important Note : even before the advent of RNP AR ops, with FMS that would blend several nav solutions ( GPS, Dme/Dme, Vor/ Dme, and IRS), VOR//Dme updating needed to be turned off to comply with US RNAV dep and arrivals procedures.
Hope that helps