VOR/DME approach?

[G-BPEC]

Just wondering if it is possble and practical to fly a VOR DME approach in LNAV/VNAV mode, for example using the altitude restrictions on the chart as speed/alt restrictions on the VNAV coloumn at the various points on the chart (entered on LNAV as P/B/D waypoints)?

I realise that all the waypoints and retrictions are probably already entered by the airline, of course, but theroetically (sp?) ???

Is this done often? If so, how close to the ground is it done on the approach?

Thanks
'EC

[ 25 July 2001: Message edited by: G-BPEC ]

[Track]

We fly non-precision ( VOR/DME and NDB ) approaches 9 out of 10 times using LNAV. It's even a recommended procedure in our company. Only when a circling is needed we put the MDA in the altitude window and use VNAV, otherwise its not allowed. For most airports the whole procedure including procedure turns and the missed approach is already in the databse, if not then we construct it ourselves. Of course we always monitor LNAV with raw data. On the 737-800 with it's GPS system, LNAV is always right on track.

We also do published GPS approaches using LNAV and VNAV. In this case the minimums are very close to CAT I ILS minima.

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Track

[minuteman]

Right; you've got to ask yourself here - what approach are you flying? Of course a VOR/DME approach with minima/OCA(H) based on tracking a certain VOR radial to a certain DME. While LNAV/VNAV are pretty accurate navigation systems it doesn't say on the chart that the VOR radial is to be superceded by your FMC data. While GPS approaches are indeed published and GPS is indeed extremely accurate, the method by which it gets its data can be removed at any time, or blocked by anyone (as happens quite often in the UK.) Don't get me wrong - I think GPS is great! Much better accuracy than my IRSs!
So then, what happens the next time someone hits a mast or a hill or what have you as a result of map shift etc (I know not too likely) and the FDR shows LNAV/VNAV engaged? What approach were they flying?
I reckon fly your VOR/DME approach in VOR/LOC /equiv. plus V/S to MDA in your altitude window. Modifying V/S then allows you to compensate for height restrictions. By all means check LNAV etc if you wish.
Or you could always hand fly it with raw data - much more fun!!!

[None]

My company is in the final process of training everyone for the LNAV/VNAV approaches. I recently flew one in VFR to JFKs 22L. The non-flying pilot stays in raw data to make sure we stay on course. The GPS/FMS system has done very well. I'm not fond of the pitch down at VNAV path intercept however. Otherwise, VNAV seems to do a great job determining a glidepath that complies with all of the altitude restrictions on the approach. Additionally, there is no pitch change required at autopilot disconnect. It is trimmed up and all set. As previous replies above have said, raw data back-up is probably best for the moment.

[safety_worker]

Check out a similar discussion on 'bluecoat'.
Consider checking your FMC capabilities / approvals for LNAV-VNAV combined ops.

[CAT MAN]

what system are you using??? What about map shift? How does VNAV cope with stepped descents???

[None]

The VNAV approaches are in the FMS data base under the originally named approach (VOR DME Rwy 22L). When you call up the approach, all of the stepdown fixes are there on the legs page, with their associated altitude constraints. Once cleared for the approach, and on an intercept heading, select LNAV (or LOC if a LOC approach). Once established and at or above the depicted altitude (and cleared for approach), select VNAV and select the TDZ elevation in the altitude select window. This lets the VNAV determine the proper angle and lets the jet fly it. In the case of the JFK VOR DME 22L, it is published to expect a VNAV angle of 3.14 degrees. Read the restrictions on the plate; VNAV is only for authorized operators. Of course this is only for the jet I fly and its associated systems.

I do not see map shifts with FMSs that have GPS & IRS as back-ups. If it was a straight FMS with a AHRS system, much different story, and the PNF absolutely must be in raw data the entire time.

[springbok449]

TR3 quite agree, flying a final approach ie. intercepting an ILS localiser or VOR localiser from LNAV and descending on the profile in VNAV well...

[A and C]

Some interesting comments above about GPS "dropping out" this is not an issue as the LNAV will revert to DME/DME or VOR/DME but i would allways back up the LNAV with raw data.

As for GPS failure just how often will it "drop out" ?
I have been flying in europe with IFR aproved GPS units for about 10 years now and in that time have had the system "flag" twice and then for less that 3 min ,i think that to much anti GPS rubish is put out by the crystal palace at gatwick in the past but i am told that a GPS aproach is soon to be aproved at LGW because that some of the american airlines dont have ADF fitted and so if the ILS is off line would have to divert.

Comments please !.

[None]

The FMS I fly with typically has dual GPS, tripple IRS, and then the DME-DME update you referred to. I can't imagine the odds of losing all of those simultaneously (not with the many back-up electrics on the jet I fly). So, I consider the VNAV approach acceptable as is, based on my experiences with these systems.

We have dual ADFs installed on our jets. It would be interesting to hear for a fact that there are companies who choose not to outfit their international fleets with at least one ADF. I'm always amazed how far out I can receive the NDB at Prins Christian Sund in Greenland, and Nain NDB north of Goose. Both have been solid in excess of 250nm. I think NDBs provide improved SA when on vectors as well (not to mention a source for news during cruise).

[waypoint5944]

Hi!
I also have some FMS app questins.
Q1>I really don't understand the word
"VNAV is only for authorized operators" on the approach plate. What does that authorized operators mean? Does that mean the specific aircraft or system?
Q2>The MDA.
When we fly the vnav app, we actually go below the published MDA during missed app.
Is it legal?
Is it O.K. for authorized operators?
Q3> Can it be legal to fly the PBD drawn points by LNAV for non-precision app when that published app is not on the FMS?
Good day!!

[None]

A1: I suppose it means the pilot, the navigation system, and the aircraft are FAA/JAA approved. This is why we have been trained/checked on the VNAV approaches this year, but still do not have authorization to execute them (other than as a visual approach)on the line. There are still some who haven't been through the training.

A2: That's what the training program teaches. The specific words on the plate say, "Only authorized operators may use VNAV DA(H) in lieu of MDA(H)."

A3: That is not approved for our operation.

I think the objective of the VNAV/LNAV approach is to let you off at the missed approach point with a stabilized descent. It does this very well. All that remains is to make sure you disengage the autopilot (if used) before you get more than 50 feet below the DA/MDA. The jet is trimmed and on a perfect glidepath.

[Capt H Peacock]

I should be very careful if I were you. There's the obvious issue of map shift, but apart from anything else the FMC's attempts to chase the next height/speed would in my view lead to an unstable and uncomfortable profile.

If you fly it with the flight system use VLOC and V/S or HDG SEL if that's all you've got. Better still fly the aircraft manually and enjoy one of the last pleasures of the job, a well flown and planned non-precision approach. Remember to adjust the altitudes on the chart for temperature as well, 10° below ISA = 4% higher! 300ft/nm always works well for the 3° g/p and if you can, draw up a little table of height/distance from a suitable DME is you have one.

Try doing the Kamikaze onto 13L at JFK with LNAV/VNAV and I assure you the airplane will have you on the edge of your seat!

Go on, your a pilot, fly the bloody thing!