PRNAV based on DME/DME
 

I had an interesting dilemma the other day and haven't yet been able to tie down an explanation, so I thought I would turn to the collective knowledge of the Tech Log.

Boeing 737-800
Failure of DME1 which was on the Hold Item List.

MEL 34-13 says, inter alia, Both required for RNAV1 (PRNAV) operations when the procedures are based on DME/DME.

A number of our Navtech STAR plates are adorned with the words RNAV 1 [and] Loss of RNAV1 capability, request vectoring. eg ENHD

Others such as EPMO have the words RNAV 1 required on both STARS and SIDS and where aircraft are not able RNAV 1 (PRNAV) radar vectoring is provided.

My questions therefore is: How does one determine whether DME/DME is required for the PRNAV? At both ENHD and EPMO we couldn't be sure, so simply requested vectoring which was happily provided.

Over to the floor.

Many thanks for constructive replies.

In your 737-8AS the following quotes apply from FCOM:

"FMC logic selects the GPS position as the primary update to the FMC position. If all GPS data becomes unavailable, the FMC reverts to radio or IRS updating.
The dual frequency–scanning DME radios are automatically tuned by the FMC. The stations to be tuned are selected based upon the best available signals (in terms of geometry and strength) for updating the FMC position, unless a specific station is required by the flight plan. Radio position is determined by the intersection of two DME arcs.
If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based on IRS position information only. The two VHF Nav radios are used by the FMC for localizer updating during an ILS approach and by the crew for navigation monitoring."

And:
"The FMC and CDU are used for enroute and terminal area navigation, RNAV approaches and to supplement primary navigation means when conducting all types of instrument approaches.
The dual FMC installation is certified as a “sole source” navigation system.
Airplanes equipped with two FMCs are certified to operate outside radio navaid coverage. The second FMC serves as a backup, providing complete navigational functions if the other FMC fails."

So, unless you determined that (using a RAIM check) there is a GPS out at destination at your time of arrival, GPS updating will be used for position updating therefore not requiring DME.

Easy answer isn't it? If the plate says DME REQUIRED you can't fly it using other RNAV methods, GPS or whatever else your plane might be using.

What does your AFM say? If it says that PRNAV is available with GPS updating alone you're good to go IMHO.

If your plate says 'DME' required, and I have one FMS, I am perfectly ok.

ENHD, except for the omnidirectional procedures all published procedures in the AIP seems to be RNAV 1 based om GNSS.

For US operators, AC 90-108 says:So, look at your OpSpecs and any applicable NOTAMs if you do not have GNSS.

A sensible/practical solution might be to check the ANP. If it shows < 1 nm, I'd go for it.

As 172_driver stated in a nutshell.

The entire concept of PBN is that it is the performance of the system not the components of the system that is important.

ANP less than or equal to RNP is really all that is required.

In relation to the OP question the of the DME MEL. My take is that a second FMC would via scanning DME or agility tuning (as fitted) would still meet the RNP, however a backup for a missed approach may be required be that RADAR or Dead Reckoning.

AFM dictates what you can and cannot do with the airplane.
Chart says what is required for STAR/SID/...

The two do not necessarily say the same or require the same.

Thus a RNAV 1 chart may (or not) say DME is required.
If DME is required the MEL items prohibits you flying such procedure.
If no reference to DME required on RNAV 1 chart you are good to go as your other systems installed guarantee you RNAV 1 compliance.

An RNAV 1 procedure based on DME/DME essentially means that when the procedure was created, the following was undertaken:

1. If RNAV 1 is required for SID:
   • DME/DME and GNSS is assumed as the navigation sensor.
   • Greater of the following two types of areas is applied
     • GNSS criteria or
     • DME/DME criteria
   • Classified into Straight Departure and Turning Departure (Turn>15degrees)
   • It is assumed that aircraft climb straight until reaching 120m(394ft) after takeoff.
   • Procedure Design Gradient (PDG) for aircraft Nominal = 3.3% (200ft/NM) (Origin of the climb is assumed to be 5m above Departure End of Runway(DER))
   • Obstacle Identification Surface (OIS) Nominal = 2.5%
     • Normally no obstacle penetrates this surface.
     • In case of penetration:
       • Avoid the obstacle by turn, or
       • Raise PDG
2. If RNAV 1 is required for arrivals:
   1. If RNAV 1 is required for STAR:
      • DME/DME and GNSS is assumed as the navigation sensor.
      • Greater of the following two types of areas is applied; GNSS criteria or DME/DME criteria.
   2. If RNAV 1 is required for APPROACH (*Published as RNAV(GNSS) RWYxx APCH):
      • GNSS is assumed as the navigation sensor.
   3. GNSS Area width is derived not from navigation accuracy, but from RAIM Alert Limit (IMAL).
      • ATT (Along-Track Tolerance) = IMAL
      • XTT = IMAL + FTT(Flight Technical Tolerance)
      • Area semi width = 2 x XTT
        • 8.0NM (IAF outside 30NM, Missed Approach WP, DEP)
        • 3.0NM (IAF within 30 NM from ARP, IF , Missed Approach WP, DEP)
        • 1.2NM (FAF)
        • 1.0NM (MAPt)

The fact that an RNAV 1 approach is based on DME/DME thus just means that the accuracy tolerance is higher then for GPS.
The reason your MEL refers to BOTH required when procedure is based on DME/DME, is that it cannot guarantee GPS accuracy thus bases itself on most accurate signal (being DME/DME) being lost for navigational purposes.
Then, if RAIM would be out of tolerance, you would fly blind...

Thank you all. A helpful range of answers.

c100driver and 172_driver, my question was specific to flying P-RNAV STARs and SIDs with an aircraft that is equipped to fly RNAV/GNSS but is temporarily prohibited from flying DME/DME P-RNAV due to the MEL. In those circumstances, I'm not sure the ANP would reveal anything.

As Skyjob has pointed out, it seems a P-RNAV procedure that (for some reason) still requires DME/DME arcs will state those requirements.

Skyjob, until I started looking in earnest I had not noticed any PRNAV1 SID or STAR plates that mentioned a DME requirement. Many such plates do not even show a VOR or DME for obvious reasons.

However, I flew a TOSVI2U RNAV1 into LROP the other day. Interestingly this plate actually lists the VORs that must be serviceable for P-RNAV DME/DME operations. I have also seen other P-RNAV procedures which simply include the note 'DME'.

In the absence of such notes, I think you're right - good to go.

I do agree with Skyjob's idea of ascertaining a RAIM check, or you might be relying on a GPS position that is possibly corrupt. Apart from that, if the performance of the navigation system as a whole is sufficient for RNAV1 I cannot see any safety implications? We rely on GPS for RNAV approaches (RNP0.3), post RAIM check, so why not PRNAV.

The MEL is in my opinion not a straight forward document and various interpretations are often needed to comply. The note of MEL 34-13 (.."Both required for RNAV1 (PRNAV) operations when the procedures are based on DME/DME"..) is impossible to answer for the line pilot, sitting on the flight deck facing the MEL item.

That's my reasoning to come to my conclusion that if ANP is less than RNP1.0, I would not object to using the procedure.

You are taking a sensible pragmatic approach! :ok: