Papi versus rnav glideslope

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pstan

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Stan
the rnav 19 approach at PQI shows very close to a 3 degree glideslope from the FAF to the threshhold. Yet the papi is set to 3.7 degrees if I remember correctly.

Would this be due to close in obstacles? And if so, does the visual segment from the da to the runway need to be clear of obstacles?

Stan
 
pstan said:
the rnav 19 approach at PQI shows very close to a 3 degree glideslope from the FAF to the threshhold. Yet the papi is set to 3.7 degrees if I remember correctly.

Would this be due to close in obstacles? And if so, does the visual segment from the da to the runway need to be clear of obstacles?

Stan
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It is most likely due to obstacles in the visual segment of the approach. From the approach chart, one can conclude that the approach is not clear of obstacles on a 20 to 1 slope and the TCH is higher than usual at 55 feet.

There are TERPS criteria for an approach that limits the height of obstacles in the visual segment, but the details are complicated.
 
The lowest PAPI light is supposed to turn red at least 1 degree above the obstacle free plane. Just put in a set and had to get a tree cut to perserve a 3 degree glide slope.
 
This exemplifies the reason the Garmin book says that LNAV+V glide path information is advisory only, and the pilot remains responsible for staying above obstacles when below the MDA.
 
That and being below MDA, you better damn well be visual.
 
Ron Levy said:
and the pilot remains responsible for staying above obstacles when below the MDA.
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As this PQI approach shows, seems you can have obstacles below the mda/da not just for Lnav+V, but also lnav, lnav/vnav and lpv approaches.

Stan
 
pstan said:
As this PQI approach shows, seems you can have obstacles below the mda/da not just for Lnav+V, but also lnav, lnav/vnav and lpv approaches.

Stan
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If you mean obstacles that penetrate the guidance approach path, that's not true for an LPV or any approach with a visual descent point depicted on the chart AFaIK. OTOH, for an LNAV+V it's certainly possible for obstacles to exist above the guidance path past the point where the MDA intersects it.
 
gismo said:
If you mean obstacles that penetrate the guidance approach path, that's not true for an LPV or any approach with a visual descent point depicted on the chart AFaIK. OTOH, for an LNAV+V it's certainly possible for obstacles to exist above the guidance path past the point where the MDA intersects it.
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Is that the case? Doesn't MDA to TDZ need to be clear of obstacles at some reasonable glidepath?

:dunno:
 
dmccormack said:
Is that the case? Doesn't MDA to TDZ need to be clear of obstacles at some reasonable glidepath?

:dunno:
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If that's true, what's the "reasonable glidepath" angle and how would you know what it is?. You're supposed to be visual from the MDA on down. And I'm pretty sure that the clear zone is a requirement for a VDP so it follows that at least some of the NP approaches without a VDP have one or more obstacles preventing the VDP designation. Of course I'm stating all this from memory, when I get a chance I'll check the TERPS for the official answer.
 
pstan said:
As this PQI approach shows, seems you can have obstacles below the mda/da not just for Lnav+V, but also lnav, lnav/vnav and lpv approaches.
Click to expand...
While that's true as far as it goes, even below the DH on an LNAV/VNAV or LPV approach, you are guaranteed obstacle clearance as long as you stay on the GS because it keeps you above the 40:1 OCP. For plain LNAV approaches, there's no such guarantee of obstruction clearance below MDA even when following the +V verttical guidance, as seen on this approach to PQI.
 
dmccormack said:
Is that the case? Doesn't MDA to TDZ need to be clear of obstacles at some reasonable glidepath?
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Normally, yes, but not always, as seen in this approch where a 3.7 degree glide path is necessary to provide safe clearance of the close-in obstructions even though a 3-degree glide path is provided in LNAV+V. I suspect that 597 MSL tower about 1500 feet from the threshold is driving this. It may not penetrate the 40:1 OCP, but it is probably too close for comfort on a 3-degree glide path to the normal TDZ 1000 feet down the runway.

Note also that the TCH is 55 feet, rather higher than normal. This suggests that the 3-degree RNAV glide path takes you farther down the runway than normal in order to clear the obstacle, but the VASI will bring you down to the "normal" touchdown zone.
 
gismo said:
If that's true, what's the "reasonable glidepath" angle and how would you know what it is?. You're supposed to be visual from the MDA on down. And I'm pretty sure that the clear zone is a requirement for a VDP so it follows that at least some of the NP approaches without a VDP have one or more obstacles preventing the VDP designation. Of course I'm stating all this from memory, when I get a chance I'll check the TERPS for the official answer.
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I wasn't clear -- the OP said that VNAV approaches did not assure obstacle clearance from MDA in.

That don't seem right...
 
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Ron Levy said:
While that's true as far as it goes, even below the DH on an LNAV/VNAV or LPV approach, you are guaranteed obstacle clearance as long as you stay on the GS because it keeps you above the 40:1 OCP. For plain LNAV approaches, there's no such guarantee of obstruction clearance below MDA even when following the +V verttical guidance, as seen on this approach to PQI.
Click to expand...

Thanks, better explanation to my poorly worded question.
 
Ron Levy said:
While that's true as far as it goes, even below the DH on an LNAV/VNAV or LPV approach, you are guaranteed obstacle clearance as long as you stay on the GS because it keeps you above the 40:1 OCP. For plain LNAV approaches, there's no such guarantee of obstruction clearance below MDA even when following the +V verttical guidance, as seen on this approach to PQI.
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We had this discussion on another thread, the 40 to 1 surface applies to the surface that is evaluated for obstacles on the missed approach segment, not the approach segment.

In an LNAV approach, the 250 ft ROC is provided up to the MAP, which is usually the threshold. So if you have an MDH higher than 250 ft, there is the possibility of obstacles all the way up to the runway threshold. Besides the 7 to 1 surface that is used to evaluate the descent portion of an LNAV approach to the MDA (or step down to the MDA), the approach visual segment is evaluated using two surfaces, 34 to 1 and 20 to 1. If there are obstacles that penetrate the 34 to one slope, the visibility requirement can not be below 3/4 mile. If there are obstacles below the 20 to 1 surface, a VDP can not be published, the minimum visibility required is 1 mile, and either the obstacle has to be lighted and charted or the approach requires a note denying the use at night.

The main point is that you use see and avoid obstacles in the visual segment.
 
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John Collins said:
We had this discussion on another thread, the 40 to 1 surface applies to the surface that is evaluated for obstacles on the missed approach segment, not the approach segment.
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The OCP's you mentioned still apply for approaches with approved vertical guidance, like the LPV and LNAV/VNAV, even though not for truly nonprecision approaches like the LNAV. They are also not a factor for LNAV+V, since those are not "official" vertical guidance. And that's my point -- if you're flying LNAV+V, you have no assurance of obstruction clearance below the MDA even if you're on the +V vertical guidance. OTOH, if you're flying LNAV/VNAV or LPV, you do have such assurance on the glide path all the way to the touchdown zone.

But I apologize for getting the OCP ratios wrong.
The main point is that you use see and avoid obstacles in the visual segment.
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On LNAV, yes, and that was my point, but not on LNAV/VNAV or LPV any more than on an ILS. That's covered in this case by displacing the GS/runway intercept point for the LNAV/VNAV and LPV further down the runway, as indicated by the unusually high 55-foot TCH for the 3-degree glide path. If you fly the 3-degree glide path all the way down, you'll touch down long. However, if you fly it until you see the VASI, and then follow the VASI down, you'll clear the obstruction and touch down normally, even though you'll be below the GS after you start following the VASI.
 
Ron Levy said:
The OCP's you mentioned still apply for approaches with approved vertical guidance, like the LPV and LNAV/VNAV, even though not for truly nonprecision approaches like the LNAV. They are also not a factor for LNAV+V, since those are not "official" vertical guidance. And that's my point -- if you're flying LNAV+V, you have no assurance of obstruction clearance below the MDA even if you're on the +V vertical guidance. OTOH, if you're flying LNAV/VNAV or LPV, you do have such assurance on the glide path all the way to the touchdown zone.
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What does the acronym OCP stand for?

It is true that the LNAV+V is not "official" in the sense that it is not defined in the TERPS although it is in the TSO, and there isn't necessarily any assurance of obstacle clearance in the visual segment other than the 250 ft ROC and when a VDP is present the visual segment is clear on a 20 to 1 and if there is a gray feather to the runway on the NACO charts the visual segment is clear on a 34 to 1. So sometimes you do and sometimes you don't.


Ron Levy said:
On LNAV, yes, and that was my point, but not on LNAV/VNAV or LPV any more than on an ILS. That's covered in this case by displacing the GS/runway intercept point for the LNAV/VNAV and LPV further down the runway, as indicated by the unusually high 55-foot TCH for the 3-degree glide path. If you fly the 3-degree glide path all the way down, you'll touch down long. However, if you fly it until you see the VASI, and then follow the VASI down, you'll clear the obstruction and touch down normally, even though you'll be below the GS after you start following the VASI.
Click to expand...

True.
 
Ron Levy said:
if you're flying LNAV+V, you have no assurance of obstruction clearance below the MDA even if you're on the +V vertical guidance. OTOH, if you're flying LNAV/VNAV or LPV, you do have such assurance on the glide path all the way to the touchdown zone.
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Ron, with respect to the Lnav/vnav or LPV approaches ONLY, where is this information about assurance of no obstructions below the DA? Can we read it ourselves somewhere?

You've been pretty clear, but I just want to inquire about one other thing:

When an APV or LNAV/VNAV approach is being designed up, is there any option for the designer to increase the required DA and vis limits, and allow obstacles below the DA?

Stan
 
pstan said:
Ron, with respect to the Lnav/vnav or LPV approaches ONLY, where is this information about assurance of no obstructions below the DA? Can we read it ourselves somewhere?

You've been pretty clear, but I just want to inquire about one other thing:

When an APV or LNAV/VNAV approach is being designed up, is there any option for the designer to increase the required DA and vis limits, and allow obstacles below the DA?

Stan
Click to expand...

There can be obstacles in the visual segment of LNAV/VNAV and LPV approaches and this can cause an increase in the DA, TCH, Visibility requirement, or the GS angle. This is not a simple subject to discuss as there are multiple considerations. An example requiring an adjustment to a DA is where an obstacle penetrates the OCS in the visual segment, the DA location is adjusted further from the runway threshold back to the point where the OCS matches the obstacle height, then the DA is selected above this location.
 
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