Vertical path non-precision approach safety?

[DE727UPS]

I don't have access to this full article. Anyone with an IFR subscription care to post it?

I bring this up because I've always been trained that following the FMC or GPS derived vertical path will keep you safe to 50 feet above the touchdown zone. Not long ago I was doing a day VMC visual backed up with the localizer into an airport I can't remember. I've gotten used to using the path indication as a substitute for an electronic glide slope in the rare occasion we don't have a full ILS. I tend to crosscheck the outside runway environment picture with the inside VNAV path and VSI descent rate, especially between 500 feet AGL until and I start looking solely outside to begin the flare. So, there I was on path, at about 200 AGL, and my F/O calls out "red over red, a little low". I looked out and sure enough, I was red over red on the VASI and I needed to make a correction. I looked back inside for a flash to confirm my VNAV path and it showed me dead on the proper path while the VASI showed me low. I was a surprised by this but just passed it off to me being an idiot who needs to get out more.

Then, we lost an A300 in BHM on a night non-precision, and it kind of got me doing some research. I clarified what our training guide says, which paraphrased is "use the VNAV path as a reference but outside visual contact with the runway must be maintained". I'm starting to wonder, though, if the VNAV path will keep you out of the trees on runways with high terrain on final approach. I'm thinking that the VASI or PAPI will be all that's keeping you safe and perhaps the path should be ignored, or certainly considered a secondary indication, once you are below the MDA.

Just wondering what others have experienced.

 

[derg]

We have to monitor "Progress: Page 2" for vpath confirmation as the scale on the magenta diamond isn't as precise as a glidepath. You can get off glidepath fairly significantly before it's that apparent.

My merged brethren call that the "Chinese Football" -- I have no clue why.

We're only really authorized to use that with an approach with a valid EOD point, otherwise we're using a DDA (derived decision altitude ~ MDA+50)

 

[DE727UPS]

I didn't know the football was that inaccurate. Will have to start watching Progress Page 2. Thanks.

 

[derg]

I didn't know the football was that inaccurate. Will have to start watching Progress Page 2. Thanks.

I don't want to say it's inaccurate, but I've had almost 80' of deviation on approach and the vertical path indicator barely jiggled.

It's part of the NON-ILS approach procedure here at Southernjets.

I'll bet you with a weekend of programming, they could probably reconfigure the vertical path indicator to expand the scale, much like the localizer once you're established.

 

[Sidious]

I do a localizer backed up by vertical path at my home airport every approach. I've seen the vertical path off by 50-80 feet without showing much difference on my FD. The minimums are 410 at a sea level airport so past that I'm not sure how much the deviation would be. We are usually at least 3 red if you follow that path down though and a little power/pitch correction is needed.

 

[BobDDuck]

I didn't know the football was that inaccurate.

The football (ours looks more like a diamond) is based on putting you at a certain point in space 50 feet above the ground. The vertical is based on GPS data, not altimeter/RA data. Also, you are relying on GPS or IRU data to derive your position. How many times have you parked on a spot that has the lat/long painted next to it and checked your Position Reference page and discovered you were 50+ feet away?

 

[MikeOH58]

I thought a user made glide path will fly you to the start of the pavement. If you have a displaced threshold because of terrain, obviously that is an issue.

 

[TUCKnTRUCK]

Your fms won't ramp the scale, because you can't use GNSS information to replace any component of an ILS or localizer system. (Except DME and marker beacons)

If you don't have waas correction, your fms uses Baro correction for altitude information, therefore your vnav is subject to standard temperature/pressure errors.

Once they let RF capable airplanes start to replace ground based ILS components, this (may) change.

 

[FlyingPoke]

I thought a user made glide path will fly you to the start of the pavement. If you have a displaced threshold because of terrain, obviously that is an issue.

This is my understanding of the system on my aircraft also, but I'm not sure where I got that idea, nor where to verify it.

 

[MikeOH58]

PL21/FMS 3000 is the system I referenced. I am positive the path takes you to the start of pavement. Will look through manuals tomorrow.

 

[dasleben]

I can't recall the particular approach at the moment, but I have seen this on the low side. We usually monitor PROG page 2 as well, but it's not necessarily procedure. I'll keep a better look out next time I go fly.

 

[PhilosopherPilot]

On the CRJ the "snowflake" derived vertical path is to the runway pavement beginning, not to the touchdown zone. So if you fly it exactly centered up, you'll be below glide path.

 

[derg]

I thought a user made glide path will fly you to the start of the pavement. If you have a displaced threshold because of terrain, obviously that is an issue.

Well, some of the aircraft, like in all of the Pegasus FMS heads I've seen, will automagically draw you a vertical path to cross over the runway at a 50' TCH while respecting the altitude constraints.

For example, if you're doing the JFK VOR 22L approach, all you've got to do is throw the approach in the box, execute it, hit LNAV, dial in an altitude (we use he nearest 100' of TCH), hit VNAV and speed intervene and it'll fly it just like an ILS approach. However, we have to monitor progress page 2 to insure that we're not more than 50' off the VPATH.

It might be a different number, I'm not sure. I'm sure @Cav, @Cptnchia, @Richman or @PeanuckleCRJ can offer some info as well.

 

[Autothrust Blue]

#jetpilotproblems

 

[derg]

#jetpilotproblems

YOU DAMNED RIGHT!

 

[Autothrust Blue]

YOU DAMNED RIGHT!

"Oh bother, I'd have to turn off LNAV-VNAV..."

 

[dasleben]

"Oh bother, I'd have to turn off LNAV-VNAV..."

VNAV can be a pain below 10k anyway. #beercanjets

 

[PeanuckleCRJ]

Well, some of the aircraft, like in all of the Pegasus FMS heads I've seen, will automagically draw you a vertical path to cross over the runway at a 50' TCH while respecting the altitude constraints.

For example, if you're doing the JFK VOR 22L approach, all you've got to do is throw the approach in the box, execute it, hit LNAV, dial in an altitude (we use he nearest 100' of TCH), hit VNAV and speed intervene and it'll fly it just like an ILS approach. However, we have to monitor progress page 2 to insure that we're not more than 50' off the VPATH.

It might be a different number, I'm not sure. I'm sure @Cav, @Cptnchia, @Richman or @PeanuckleCRJ can offer some info as well.

Works just like that on FIFI, except its a much more simple process. When on any approach except a LOC, you just press APPR and tada!!...Final nav lateral and vertical guidance. LOC we use FPA, but do reference the dummy glidepath that is autogenerated to do 50' TCH.

The path will generate geometric to whatever fix and crossing is on the approach, or a 3 degree path if none input. The donut or brick is very accurate and there is no need to monitor the prog page.

The 737 goes once step further and every approach you press APPR.

 

[Boris Badenov]

I'm going to be a witch in church on this one and ask, once again, what was ever wrong with "dive and drive"? If you have legit, certified, copper-bottomed vertical guidance (ground based or otherwise), use it, obviously. But if you're flying a non-precision approach with no vertical guidance, why guesstimate and hope that the uncertified magic will get you where you want to be? That's an awful lot of button pushing and trust up against little things like "levelling off" and "moving the thrust levers", at least to my way of thinking. Non-precision approaches were designed with this in mind. Planes haven't gotten a whole lot bigger or harder to fly. Edumacate me.

 

[dasleben]

I'm going to be a witch in church on this one and ask, once again, what was ever wrong with "dive and drive"? If you have legit, certified, copper-bottomed vertical guidance (ground based or otherwise), use it, obviously. But if you're flying a non-precision approach with no vertical guidance, why guesstimate and hope that the uncertified magic will get you where you want to be? That's an awful lot of button pushing and trust up against little things like "levelling off" and "moving the thrust levers", at least to my way of thinking. Non-precision approaches were designed with this in mind. Planes haven't gotten a whole lot bigger or harder to fly. Edumacate me.

It may sound more difficult, but at least in Boeing equipment (which are actually certified to do it), utilizing VNAV to provide a continuous descent from the FAF to MDA+50 is much more stable, and requires far less button pushing than a dive and drive approach. You would actually be quite pleased with how nice it is.