737 RNAV approach question

[taneb 2]

Hey everyone, I have mastered many things in the NGX, but there's one thing that still has me scratching my head. It's on the RNAV approach. Now, before you say so I am not asking for an RNAV tutorial, but what I am doing wrong.

 

If you get the chance, try this approach and see if you can get it right. So I'm on VATSIM, and I'm making an approach into Denver and DEN_APP tells me to expect the RNAV Z approach for rwy 16L, and so everything is going as planned. I have it all in my fmc, and LNAV VNAV is on, but I can't seem to get on the "glide slope" after LEETS, which is the FAF. It is a very quick approach from the IF (QUIKE) which requires you to to make some steep descents in a short amount of time. Once you get to the FAF, you are supposed to be at 7000. My altimeter is set for the minimums of 5600, and I pass the FAF at 7000, and then it starts to go down with the "three degree glide path" of the approach. But I guess the glide path is not steep enough, because I reach my minimums where I am about to disconnect the autopilot, but I'm already way too high. Is there a way to adjust the glide path, or is there anything I'm doing wrong here?

Could anyone try this approach and see if they are able to do it, because I've tried it many times now and I am starting to think that it's impossible.

 

Thanks for the help

 

Taneb

[Stearmandriver 2,205]

Are you saying you set the mode control panel altitude to 5600, the published minimums on the approach? If so, that's likely the problem. The plane is shallowing its descent to level off at 5600, so if you wait until reaching 5600 to disconnect you're naturally high.

 

On any approach flown in LNAV / VNAV PATH, you set touchdown zone elevation in the mode control panel (round down). So on this approach, set 5300. This will cause the plane to mimic behavior on an ILS and fly the glidepath all the way to the ground. You naturally intervene at or before minimums and either disconnect and land or go around. If you go around, remember to set your missed approach altitude in the MCP. My airline does it following the Gear Up call, ie: "positive rate, gear up, set missed approach altitude".

 

Give that a try and report back.

[JoeDiamond 67]

Another way to do it, and the way my company does it, is when cleared for the approach and in VNAV PATH set the MCP altitude select to the DA rounded up the nearest 100 feet.  Once inside the FAF and at least 300' below the missed approach altitude set the MCP altitude to the missed approach altitude.  As long as you are at least 300' below the missed approach altitude the vertical mode will stay in VNAV PATH and the airplane will continue to descend.

 

If you were to forget to set the missed approach altitude the airplane would level off in VNAV ALT when it reached the MCP altitude.  In that case you would be too high to land and would end up going missed, but you would not bust any minimums.

[spesimen 135]

i tried this approach and found that i was a bit high coming in also, it looked like if i waited until minimums to start maneuvering i would be too high.

 

one thing i noticed is that the FMS data for the current AIRAC lists the runway elevation as 5484, i think this is  correct (it is 50ft + the 5434 field elevation that is listed on the charts). however, in FSX the default denver scenery has a flat elevation of 5325. so the path will naturally be about 109 feet higher than it should be. i'm not sure if it's possible to override that elevation number somehow, i was unable to put a different value into it in the FMS, im guessing since it's supposed to be fixed to the actual runway height. elevation is sort of wacky at denver since there is quite a bit of variance in the actual height of the various runways and FSX has to use the same elevation for all of them. i'm also not sure if that 109 feet discrepancy would fully account for how high i was but it might! i think it might be possible to test it by altering the runway info in the airac data.

 

cheers

-andy crosby

[Stearmandriver 2,205]

Actually the approach chart for this approach lists the TDZE as 5357, so yeah if the scenery is in the 5300s and the AIRAC is not, that could put you high too.

 

C'mon Joe... whose company invented this stuff? :-) Just kidding; either technique is valid and yours is how my old company did it. Only danger there is you run the risk of the plane capturing the current altitude if you don't scroll through it fast enough. Either way will keep the plane from starting a level off at the DA / DDA though, which is what you really don't want.

 

I won't have the chance to try this until tomorrow though, and now I'm curious. Are you guys staying in VNAV PATH all the way down, or does one of those steep intermediate descents force you into VNAV SPD from which you never recover?

[ALF1 15]

I do it Joe's way. But it is nice to have the input from both of you.

[JoeDiamond 67]

C'mon Joe... whose company invented this stuff? :-) Just kidding; either technique is valid and yours is how my old company did it

 

I'm not saying that either method is better or worse than the other, merely making the readers aware of another technique.

 

As for which way is the correct way?  It's the one that comes from the people who sign my paychecks. 

[Stearmandriver 2,205]

I'm not saying that either method is better or worse than the other, merely making the readers aware of another technique.

 

As for which way is the correct way?  It's the one that comes from the people who sign my paychecks. 

Amen to that, man.

[Stearmandriver 2,205]

Tried this approach last night in the sim. Worked fine. Ended up at the threshold slightly above the ILS g/s but certainly in a position to land. It's not unusual for RNAV glidepaths to not be entirely coincident with the ILS, especially in the sim, but I definitely wouldn't call it "way too high".

 

Only thing is, my AIRACS is a couple cycles out of date. Guess it's possible a revision borked up the approach data.

[Driver170 42]

The SOPs i follow and a very well known UK and Europe operator is set the MAA at 1000’ AAL (White altitude reference bar on altimeter). If the MDA/DA is 900’ AAL or higher, then the MAA must be set earlier to avoid ALT ACQ. This point must be agreed by the flight crew prior to the approach. In the unlikely event that the MAA is below 1300’ AAL then the MAA must be set 300’ below MAA.

Mind..

 

FINAL APPROACH SEGMENT

 

The final approach segment for a GNSS approach will begin at a named waypoint normally located 9.3 km (5.0 NM) from the runway threshold.

 

And..

 

Descent gradient/angle

 

The optimum descent gradient/angle is 5.2 per cent/3°, however where a higher gradient/angle is necessary, the maximum permissible is 6.5 per cent/3.7°. The descent gradient/angle is published.

[taneb 2]

Sorry guys, haven't been on the forum for a couple of days. I think you are right, I will use some of your suggestions and report back. I am new to RNAV, so I was just trying to be sure.

[Driver170 42]

http://www.icao.int/APAC/Meetings/2013_APRAST3/5%20-%20CFIT%203%20CASA%20AC%20008A%20APV%20Final.pdf

[Guest]

Hey everyone, I have mastered many things in the NGX, but there's one thing that still has me scratching my head. It's on the RNAV approach. Now, before you say so I am not asking for an RNAV tutorial, but what I am doing wrong.

 

If you get the chance, try this approach and see if you can get it right. So I'm on VATSIM, and I'm making an approach into Denver and DEN_APP tells me to expect the RNAV Z approach for rwy 16L, and so everything is going as planned. I have it all in my fmc, and LNAV VNAV is on, but I can't seem to get on the "glide slope" after LEETS, which is the FAF. It is a very quick approach from the IF (QUIKE) which requires you to to make some steep descents in a short amount of time. Once you get to the FAF, you are supposed to be at 7000. My altimeter is set for the minimums of 5600, and I pass the FAF at 7000, and then it starts to go down with the "three degree glide path" of the approach. But I guess the glide path is not steep enough, because I reach my minimums where I am about to disconnect the autopilot, but I'm already way too high. Is there a way to adjust the glide path, or is there anything I'm doing wrong here?

Could anyone try this approach and see if they are able to do it, because I've tried it many times now and I am starting to think that it's impossible.

 

Thanks for the help

 

Taneb

RNAV approaches are good up to a specific end/on final...then you're responsible for the landing.  Thats where your going wrong.  It's usually the last waypoint on the RNAV then it's yours.  Just checked your approach and yes after LEETS @ 7,000-RNAV stops and you fly the rest of the approach and landing without aid, just visual visual.  Gotta set your baro correctly-(Radio / 5690)-also.  Use the ILS for approach if you use VATSIM.  They-(ATC Controllers)-usually take you out of the descent path anyway to compensate for traffic, wind change etc, and it's easier to navigate

[taneb 2]

i tried this approach and found that i was a bit high coming in also, it looked like if i waited until minimums to start maneuvering i would be too high.

 

one thing i noticed is that the FMS data for the current AIRAC lists the runway elevation as 5484, i think this is  correct (it is 50ft + the 5434 field elevation that is listed on the charts). however, in FSX the default denver scenery has a flat elevation of 5325. so the path will naturally be about 109 feet higher than it should be. i'm not sure if it's possible to override that elevation number somehow, i was unable to put a different value into it in the FMS, im guessing since it's supposed to be fixed to the actual runway height. elevation is sort of wacky at denver since there is quite a bit of variance in the actual height of the various runways and FSX has to use the same elevation for all of them. i'm also not sure if that 109 feet discrepancy would fully account for how high i was but it might! i think it might be possible to test it by altering the runway info in the airac data.

 

cheers

-andy crosby

Hey I think you are right. Could you please shed me some insight on how to change the elevation data in the airac?

thanks

[spesimen 135]

Hey I think you are right. Could you please shed me some insight on how to change the elevation data in the airac?

thanks

 

i am not very well versed in scenery editing but i believe with the freeware ADE program you can just go in and edit stuff like that. i don't think setting it up is trivial, there is some SDK files you need or something if i recall correctly, but that may not be necessary just for messing with the altitude.

 

it's a tricky thing to solve because all of the runways at that airport have fairly different real world elevations at the touchdown zone, i think the default scenery was set to that because the runways from the south are at that elevation, and FSX doesn't really support non-flat runways. i dunno, maybe poke around the scenery design forum i think there is a lot of info there and maybe some walkthroughs

 

cheers!

-andy crosby