PBN Approaches using DME/DME
 

For those of you flying ancient a/c , e,g. 737 classics/NG , early ATRs, Dash 8 , etc,, : when flying a RNP/PBN approach , if you loose GPS, is it easy or adding significant extra workload to switch to DME/DME nav ( provided there are DMEs around avail of course) , or is it transparent like on the Airbus ( using the MMR) ?

When I flew the Classic, RNP approaches were not allowed with DME/DME.

DME/DME updating was achieved with the Nav boxes, when they were in “auto”, when selected to manual (for an ILS approach for example) then DME/DME updating no longer happened. It was a very simple but very good system. Deselecting nav sources was a matter of 2-3 button presses on the CDU

I don't reckon you can fly PBN approaches without GPS. The best you can expect to get from DME/DME updates is RNP1 which is fine for arrivals and departures but not approaches

Thanks, for the info, appreciated ! but to paraphrase Bob Dylan times are changing , looks like the Swiss are goings all RNP,-GNSS and are installing a couple of extra DMEs to satisfy the safety case in the event of GNSS interferences,(i.e. jamming/spoofing)

Yes we are taking SID and STARS not the final approach ( landing ), which is, as I understand is still GNSS (and ILS of course if there is one) . the Swiss ( GVA) say STARS can be flown DME/DME as well as GNSS so they plan to remove all VORs and NDBs.

You can fly PBN arrivals without GPS as long as it doesn't explicitly state GNSS/GPS required on the chart.

What CM said. The 75/76 will just keep flying the track on the screen.

Echoing the above - RNAV1 arrivals and departures were always fine DME/DME, unless GNSS required was stated. RNP approaches were a no no. If one lost nav accuracy then the aircraft just trundled along the magenta line it thought was right anyway.

It was important to leave the boxes in auto as long as possible, I recall, so as to avoid map shift, and one could not intercept an ILS/LOC/VOR in anything other than HDG to avoid the risk of paralleling the final approach course.

So if you are on a PBN STAR using GPS and it fails, (e.g jammed) and there are DMEs around, the AP will automatically continue the NAV using them , and the whole thing is transparent to you , no need to look for them , ID , channel selection , etc , right ?

You may get any of the following messages ; UNABLE RNP, L GPS, R GPS, GPS depending on what the failure is but eventually the machine will transfer to DME/DME. If you're worried about nav accuracy and want to force it to DMEs you can switch the GPSs off. Bottom right of Prog Page 1 will tell you what mode it's using. DME/DME tuning is automatic (providing the MAN/AUTO switch on the VOR/DME controllers are set to AUTO). As above, they need to stay in AUTO.

Disclaimer - this is 75/76 specific. It's the only type I've flown and have no knowledge of any other Boeing or AB.

Hth

I don't have the books here with me but in general (fee free to correct or be more accurate):

If we are talking about an approach, DME/DME should be OFF. It is ONLY GPS to avoid problems and make sure you are correctly informed when there is a GPS failure or RNP issue. You should discuss backup plans in case this happens, which is pretty much going back to ground based radio updating or using IRS data for your position.

If GPS is in use, DME/DME is automatically ignored and there is no need to switch anything off

What is your source?

B737 Classic & NG FCOM: "When GPS position data is available, radio updating will also occur". I can't find any reference it is actually ignored.

It is true the Supp only talks about "VOR" updates, which I've found normal as DME update is standard OFF. However, some aircraft come in a different configuration, the weirdest one being "if DME is OFF, VOR and LOC is OFF even when selected ON, unless blablabla". The majority of aircraft in our fleet switch back to a standard DME OFF once power down occurs.

Our safe solution: force it all OFF prior to the approach.

As stated above I only comment on 75/76 and hope there’s a Boeing read across. My source is 24 years on type operating non GPS 75, GPS 75 and GPS 76.

hth

True for 75/76, 737 NG different..

Thanks a lot DH , helpful .
@ScepticalOptomist
Is it not the same on the NG ?. Not automatic reversal to DME/DME in case of GPS failure ? ( like on Airbus)