737NG use of VNAV

[M.82]

Hi guys, only a question:

From FCTM Rev 9 (page 5.41)

"VNAV should be used only for approaches that have one of the following features:
1) a publish GP angle on the LEGS page for the final approach end of the runway.
2) an RWxx waypoint coincident with the approach end of the runway.
3) a missed approach waypoint before the approach end of the runway (MXxx).

These features permit construction of normal glide path. VOR APROACHES WITH THE MISSED APPROACH POINT ON THE LEGS PAGE BEYOND THE RUNWAY THRESHOLD AND CIRCLING ONLY APROACHES DO NOT HAVE THESE FEATURES."


So, we should fly those approaches which have the missed aproach point beyond the runway threshold with V/S only?


Thanks:ok:

[flite idol]

Not if you can comply with 1 or 2.

[OK465]

VOR APROACHES WITH THE MISSED APPROACH POINT ON THE LEGS PAGE BEYOND THE RUNWAY THRESHOLD AND CIRCLING ONLY APROACHES DO NOT HAVE THESE FEATURES."

Not if you can comply with 1 or 2.

I'd be careful with this and #2. There appears to be a contradiction here.

If you look at KPWA, the VOR approaches to both 17L & 35R have the missed approach point at the VOR which is beyond both thresholds at the mid-field point.

But both approaches are 737 FMS coded with RW17L & RW35R waypoints and 54' TCH's (MSL altitudes) associated with the RWxx waypoints but no GP angle.

So that above info from the FTCM about these type approaches not having any of those coded features is suspect.

With VNAV selected, I would be careful not to end up in VNAV SPD (A/T idle) to mins instead of VNAV PATH. How the approaches would actually fly is suspect then also. Have someone try one of these in the sim first if you've got PWA in the database.

Maybe terpster or FPOBN can shed some light on this??

[M.82]

I m no shuere if VNAV changed from VNAV PATH to VNAV SPEED in this approach,

Here you have a RW17 and RW35 waypoint in LEG page, but you also have a miss apporach point after the end of the runway.

I guess that the aircraft will fly a glide angle of 2.81 after MCLAN (FAF) and if you don't see the runway at 1.1 nm (VDP) with 1640 Ft you should fly with that altitude until PWA VOR with ALT HOLD then the miss app proc.

If you have a RWxx waypoint, the FMC also have a glide angle.

[OK465]

If you have a RWxx waypoint, the FMC also have a glide angle.

You would think so. But that's what is curious about these two approaches.

Should they even have RWxx waypoints?? They don't adhere to the FCTM guidance about the 3 unavailable 'features' and.....

The FMS version I'm familiar with does NOT display an angle on the legs page even though RWxx and the MAP beyond it are both on the legs page. The VNAV function MAY use an FMC calculated geometric path to RWxx TCH from the FAF but that angle is NOT displayed on the legs page like you might expect inside the FAF with a coded GS angle.

These are VOR (not VOR or GPS) approaches that can be loaded from the FMS. Like I say, it's curious that they both HAVE RWxx waypoints.

Someone who actually deals with the coding or a current 737 NG guy may need to weigh in on this. I'm out of my league without actually flying them in a sim a couple of times and trying VNAV. :) :confused:

It's an interesting question you've asked.

[Capn Bloggs]

Possibly not relevant: different aeroplane and FMS, but our Jepp database has coded 3° CDAs for approaches that have the RW waypoint and others that do not have the RW waypoint. My understanding is that the RW waypoint is used if the final approach track lies within 0.14nm of the threshold (or something like that).

Some of these approaches have the "3°" displayed amoungst the approach waypoints; others don't. Dunno why, but they all give a 3° CDA for the FMS to follow.

[Denti]

@OK465, i don't know if its about the coding or the FMS. However in the companies i've flown the 737 and in the short time i'm on it (just 11 years now) we always had a coded descent path that was on the legs page. However i cannot remember a single approach we use where the missed approach point is behind the runway.

That said, VNAV is only a backup mode nowadays, IAN is the primary mode and that one works like an ILS. The MCP speed window is open and the approach mode follows the FMC generated final approach course and glide path (FMA modes FAC and GP).

[B737NG]

In case you have a GPS or R-NAV Approach you have no further overlay.

Saying that means it is not a VOR/DME or NDB/DME Approach that is flown in LNAV/VNAV +50 feet to the MDA. If the Approach is a approoved RNAV approach you have in the applicable cases a note on the Chart that the proceedure is published and as approoved Operator you can use the given mimima and do not have to add the +50 feet as per recommendation.

I hope it helps to clear that. If not go to the Website of the FAA and search for it, if it is in Europe then search the EASA pages and you get more detailed information, the wording however can be confusing to avaoid legal liability.

[MrHorgy]

We have to have EITHER a RWXX point, or a missed approach point, as well as the GP angle coded into the box. If we don't it's V/S all the way.

[M.82]

Ja, no problem OK465


Thanks for you help guys!
:ok:

[FlightPathOBN]

This is certainly odd....the missed approach waypoint MXxxx

some years ago, there was a movement to code in the missed approach point as a waypoint, so that a pilot could see the distance to the MAP....

while it sounded like a good idea, the logistics of adding and maintaining this many waypoints, negated any benefits.

with the 424 coding, at least all the ones I have coded, the RWxx is required. I did several approach procedures into China where the runways were brand new, and not in any Navdatabase, the FMC would disco if there was not a specific RWxx point.

There was also significant issue on a few of them, as the Smiths box would not accept runways above 10,000 feet, so even creating waypoints with very small offsets didnt work.