737: NPA with VNAV

[FLX/MCT]

Hi guys,

could anyone of you please help me how to fly a non precision APP out of the database with VNAV on the 737? I am pretty familiar with the procedure on the A320.
But I wondered if the 737 would level-off at the preselected altitude - as usuall for VNAV or is this behaviour modified during approach? What altitude do you actually set during approach - the MDA? :confused:
And what's the lateral mode? always LNAV or VOR/LOC when possible?

So far... I am looking forward to your answers


regards,
luke

[alex111]

1. Preselect in the Altitude window in the MCP the next hard altitude from the approach chart which should match the constraints in the FMC (this is just in case there is any constraints missing in the FMC beacuase even if you set MDA directly and the aircraft is in VNAV PATH would still comply with the contraints) then once passed the Final Approach Fix, se the MDA rounded to the nearest hundreds. Make sure your in VNAV PATH, VNAV SPEED will not comply any contraints.

2. VORLOC for Localiser only approaches, LNAV for all others.

Alex.

[despegue]

Alex111,
May I suggest you use extreme caution with this LNAV...
It is still much better in my opinion to use VOR/LOC or HDG SEL. with raw data cross reference for any Non precision approach. Actually, always cross check with your raw data, also in LNAV.
I've had way to many map-shifts on the B737EFIS.
Just GOP. Good Operators Practice.
Mind you, I do think that on the NG with dual GPS, LNAV will be much more accurate and reliable, compared with the Classic.

[FLX/MCT]

Thanks for your answers!
But what happens when the A/C reaches MDA - will it level off or will it continue in VNAV PATH until touchdown?

[Denti]

First, there are two different ways to fly a NPA with FMS guidance. One is LNAV/VNAV, the other is IAN (NG only, at least in our fleet).

Our SOP for LNAV/VNAV is to select MDA in the altitude window and at 500ft above MDA select GA altitude. LNAV/VNAV doesn't level off at MDA except if the FMC procedure contains a level flight segment, otherwise it will continue down to touchdown, SOP for us is to immediately initiate a GA at MDA+50 if no visual contact. Raw Data monitoring is required unless we have two operational FMC, two operational IRSs and two operational GPS receivers (which we dont have on the classic). Of course we can fly all non-ILS approaches except RNAV and GBAS using the oldfashioned method of HDG SEL and VERT SPD, VORLOC where appropriate as well.

IAN is flown similar to a normal ILS except that we cannot use two autopilots (can be flown raw data, FD only or autocoupled, autocoupled is required with RNP 0.15 or lower unless in visual contact). So we select the GA Altitude and just leave it there. It allows flying of every NPA for which we have a procedure in the FMC using the approach mode.

GLS approaches are non-ILS approaches, but precision approaches nonetheless and the handling is exactly the same as for an ILS (of course you have to select the correct entries in the FMC).

All in all it would be easiest for us if we could use IAN on every aircraft and just fly NPA and ILS the same way, however sadly IAN is only available on the newest aircrafts in our fleet, the older ones lack it.

[Blinkz]

The a/c will level at the MDA(or whatever you have set on the MCP) if you don't do anything. In our old outfit the procedure was set the MDA in the MCP 1 mile before ToD. The aircraft will then follow the path down. If you get alt aquire then you will be committed to a GA, however you don't do anything until you get Alt hold. Once on Alt hold you set the MAA and do a standard goaround.

[plain-plane]

1)
Once on approach logic is satisfied: (generally: within 2 nm of decent point or below 2000`AAL) AND you are 300`below MAA you can reset the MCP altitude to the MAA. Then the A/C will then keep following the VNAV path until either you disconnect or it reaches 50`above RWY where it apparently will level off….(never quite had the balls to test the last one in real life):cool:

2)
Or you can set the MCP altitude to your minimums round up to next higher 100`. The A/C will level off at set MCP altitude

[FLX/MCT]

Thank you very much guys for your precise and interesting answers. So the approach logic is the reason that the aircraft won't level off.
Is my assumption right that you can fly NDB, VOR, LOC, GPS and RNAV approaches like that...

Thanks again, you're really great 'teachers' :ok:

[rubik101]

The MDH is set 2 nm prior to the decent point and stays there until such time as you become visual and are assured of a landing. This will have to be above the MDH set in the MCP window. If nothing is seen, then wait for ALT AQUIRE/HOLD, set the MAA and initiate the Go-around.
If you set the MAA 300' below the decent point, which is possible with the system as it operates now, there is nothing to stop you flying the aircraft into the ground without any visual refernce established. The aircraft will not level off once the MDH is changed to an altitude above the present altitude.
There should be no either-or option, 1 or 2.
The only safe way to use the system is as described; leave the MDH until you see the runway, then reset the MAA, recycle the Flight Directors, and land.

[IRRenewal]

But what happens when the A/C reaches MDA - will it level off or will it continue in VNAV PATH until touchdown?

If MAA is set in the MCP, the A/C will level off at the threshold crossing altitude as found on the legs page.

Ask to try it next time you're in the sim.

[RYR-738-JOCKEY]

There seems to be alot of confusion around level off if you do nothing.
There's a specific requirement for VNAV compatible approaches that it ends with either a RWxx point, or a MDxx point. The first one will have an altitude of rwy height + 50, ie threshold height. The latter is normally the MAP with an altitude which coincides with a typical 3' descent from FAF.
Now to the conclusion, VNAV will always level off at the lowest of either VNAV path hard altitudes or MCP selected. More importantly when MDxx or RWxx is no longer active (overflown), the FMC exits it's approach logic and levels off.
In order to approve an operator for VNAV approaches it has to be established that the procedure will not descend you into the ground. Soooo...it levels off, regardless. Either at MCP selcted MDA, or at MDxx point, or at RWxx threshold. Works a treat.