How to fly GPS approach with vertical profile?

[Chris Tweney]

Thanks to Jan's excellent tutorials I'm pretty confident flying the 737 on the ILS. But I can't seem to figure out how to make the plane track the vertical path on an RNAV/GPS approach. I assume the real 737-300 is capable. Can anyone give me pointers?

 

[Shifty]

The IXEG 737 only has one GPS system. For a GNSS (GPS) approach you require two for Approval. For simulation purposes though as long as your Navigation accuracy is 0.3NM you could do an LNAV/VNAV approach. One thing to have in mind though, If using VNAV Path for the approach,  when you set your MCP alt to your Missed approach alt the Classic will crumble and go into CWS pitch. This was fixed in the NG and allows you to fly to your minimums in VNAV path and have your missed approach alt selected in the MCP for the go around. 

Regards

Craig

 

Edited April 25, 2016 by Shifty

[Shifty]

Just to add to this. With the IXEG VNAV being not 100% finished, I would recommend using level change / Vertical speed for the Vertical profile. You can do this for a GNSS approach in the real world but you have to use the LNAV minimums as opposed to the LNAV/VNAV minimums. We also add 50ft to our minimums because it's a MDA not a DA. Temperature corrections also play a big part in GNSS approach's and caution should be giving to low temps because you will be closer to the ground than the aircraft thinks :-). 

Regards

Craig   

[Chris Tweney]

I see, so it's flown more or less like a localizer-only approach. That'll work!

[jonathan]

On 4/25/2016 at 4:04 PM, Shifty said:

The IXEG 737 only has one GPS system. For a GNSS (GPS) approach you require two for Approval. For simulation purposes though as long as your Navigation accuracy is 0.3NM you could do an LNAV/VNAV approach. One thing to have in mind though, If using VNAV Path for the approach,  when you set your MCP alt to your Missed approach alt the Classic will crumble and go into CWS pitch. This was fixed in the NG and allows you to fly to your minimums in VNAV path and have your missed approach alt selected in the MCP for the go around. 

Regards

Craig

 

Just wondering what should the MCP alt be set at if you want to continue with the VNAV path?

[cmbaviator]

10 minutes ago, jonathan said:

Just wondering what should the MCP alt be set at if you want to continue with the VNAV path?

 

Set it at MDA +50fts

[jonathan]

Just now, cmbaviator said:

 

Set it at MDA +50fts

Cool thanks !

[Litjan]

Airlines´ procedures for this may vary, but this is how we do it:

Use LNAV and VNAV (if desired) to get to the intermediate descent point (usually ca. 10NM for runway, 3000´ AAE). Then set the MCP altitude to a HIGHER altitude (we don´t set it to MDA, because it´s really easy to forget and the plane would level off, screwing up your approach).

When coming up on the intermediate descent point (in LNAV and ALT HOLD), turn down the V/S wheel to get onto the "virtual glideslope" - refer to the vertical deviation indication (just like a glideslope) to stay on the vertical profile. Once ca. 1000 feet below missed-approach altitude, set the MCP to missed approach altitude (to have it ready in case of a go-around).

When approaching the MDA, click on the SPEED button to disengage autothrottle (we rarely disconnect it, as eveyone seems to be doing in their youtube vids - it would not be able to save you when you get too slow, if you completely disconnect it). When pushing SPEED button, the A/T will revert to ARM (dormant, bur ready to help if you need it for go-around). Now align the hardware throttles and gain control of the thrust levers.

At MDA disconnect the autopilot and complete the landing manually. If airport not in sight, hit TOGA buttons for go-around (careful, AP will disengage!!)

Jan

 

[gcharrie]

Thanks for the explanations.

One question : when in VNAV, I tried to force a MCP SPEED (so that we don't use the speed defined by the FMC). I was not able to click on the SPEED button on the MCP (could not engage it)

Is it the normal behaviour ? (I mean we can use only FMC speed while in VNAV)

(Or bug, or wrong usage from me ?)

 

[Litjan]

Correct - in VNAV the speed is determind and set by the FMC - use an alternative pitch mode like V/S or FL CHG to use MCP SPD.

Jan

 

[Shifty]

Unlike the NG the classic doesn't have the Speed intervene option, which allows you to maintain VNAV PATH but gives you control of the MCP speed window. 

 

 

As Jan says different airlines have different procedures. At Jet2 we set MDA +50ft in the MCP, reason being it should level off if nothing else is done and prevent flight into terrain. But once we are 300ft below our missed approach altitude we then set the missed app alt in the MCP. Doing this will drop the aircraft into CWS pitch if in VNAV, so we would select V/S before setting the MCP to the missed app alt. Do I think this is best practice?.....hhhmmm not totally but thats what I get paid to do.....

Regards

Shifty

[cmbaviator]

4 hours ago, Litjan said:

Correct - in VNAV the speed is determind and set by the FMC - use an alternative pitch mode like V/S or FL CHG to use MCP SPD.

Jan

 

 

Jan i flied a rnav approach in VNAV/LNAV but during the descent, i've set to the FAF a speed contraint = Vref +5kts (135kts)  in the FMC leg page but after setting landing flaps, the speed is set to 140 kts instead of 135, it is the seond time that it has done that to me, ot seems that  he maintains the speed constraint value +5kts.

 

i can provide a video this evening if needed

Edited April 28, 2016 by cmbaviator

[Litjan]

Not quite yet - we will revisit this part of the approach in VNAV later on - once that is done and we declare it "good" I will invite all feedback...

Jan

 

[cmbaviator]

12 minutes ago, Litjan said:

Not quite yet - we will revisit this part of the approach in VNAV later on - once that is done and we declare it "good" I will invite all feedback...

Jan

 

fair enough