Glideslope intercept on an LNAV+V approach

[RussR]

I've been teaching instruments a lot lately.

When flying an LPV, LNAV/VNAV, or ILS approach, you start on down at glideslope intercept - GS intercept is by definition the FAF for the approach, whether or not you have reached (or passed) the charted FAF.

Due to altimeter errors caused by non-standard temperatures, the actual point of intercept can vary widely from the charted FAF. Especially during the summer, glideslope intercept can occur 1/4 to 1/2 mile before the charted FAF (in the winter, of course, it would be past the FAF). No problem, on an ILS/LPV/LNAV-VNAV, you start down at glideslope intercept and follow glideslope.

But then we've got those oddballs, the LNAV+V and LP+V, where Jeppesen/Garmin publish an advisory glideslope that is not evaluated by the FAA. Since these are still non-vertically-guided approaches, the FAF is still where we are to start descending, just like a VOR or LOC-only approach (or LNAV without the +V).

However, in warm weather, inbound to the FAF, you will first pass through advisory-glideslope intercept, but can't start descending yet. Then you reach the FAF and can start descending, however the advisory glideslope is now below you.

You have basically two choices - do you intercept it from above (which we usually specifically teach not to do with glideslopes), or ignore it?

For CFIs - how do you teach how to use the advisory glideslope on "+V" approaches?

 

[catmandu]

Controlled intercept from above with a greater than normal initial rate of decent, which should be determined ahead of time (part of the 'controlled' part of this technique).

 

[Rockymountain]

I've been teaching instruments a lot lately.

When flying an LPV, LNAV/VNAV, or ILS approach, you start on down at glideslope intercept - GS intercept is by definition the FAF for the approach, whether or not you have reached (or passed) the charted FAF.

Can't go below minimum altitudes even if on advisory as you mention. But just to firm up some terms. Glideslope is something you get with an ILS, with GPS approaches, it is a Glidepath. You also can't get false glide slopes with GPS, so intercepting from above is OK, whereas it is more risky with an ILS. Anyway that is how I see it.

 

[Cooter]

Well if it's descending you early, then that means it may not give you the step down protection based on your displayed altitude. If the error is as significant as you describe it, I would have the students fly it as a step down and disregard the advisory glideslope. Maybe not disregard it totally but I think it interferes with their scan to attempt to chase the needles but then still comply with the step downs. So I wouldn't want that to be the focus.

 

[midlifeflyer]

I'd go with @Rockymountain and @catmandu. You can't go below minimums for the segment.

The reason for the common advice to avoid intercepting from above is mostly about the dangers of following a false glideslope. False glideslope is due to the way an analog, ground-based ILS glideslope works - essentially permitting it to be read out of phase. It's not an issue with a GPS glideslope.

 

[RussR]

Thanks folks. Maybe I wasn't clear enough on my question. I wasn't asking whether it was okay on an LNAV+V to descend and follow the glidepath prior to the FAF - it's not. You must stay at the published intermediate segment altitude until the FAF, which is the location of the published fix.

I was asking whether, since at the FAF you will be above glidepath, you intend to (or teach to) 1) descend quicker than normal until you catch the glidepath from above, or 2) ignore the glidepath entirely.

 

[Cooter]

Thanks folks. Maybe I wasn't clear enough on my question. I wasn't asking whether it was okay on an LNAV+V to descend and follow the glidepath prior to the FAF - it's not. You must stay at the published intermediate segment altitude until the FAF, which is the location of the published fix.

I was asking whether, since at the FAF you will be above glidepath, you intend to (or teach to) 1) descend quicker than normal until you catch the glidepath from above, or 2) ignore the glidepath entirely.

2) is the better option, what's the point of intercepting the glidepath if you will have to leave it again at a subsequent step down fix? That is putting the emphasis in the wrong place. Step downs are not complicated.

 

[mtuomi]

Flying the approach with just the step downs is a lot easier than following the GPS glidepath and especially intercepting it from above. I would just ignore the glidepath and shoot the approach as a normal non-precision.
With multiple stepdowns you'll have to dance the overshoot-and-intercept tango for every fix, and that's not a very good way to fly an approach.

 

[midlifeflyer]

Thanks folks. Maybe I wasn't clear enough on my question. I wasn't asking whether it was okay on an LNAV+V to descend and follow the glidepath prior to the FAF - it's not. You must stay at the published intermediate segment altitude until the FAF, which is the location of the published fix.

I was asking whether, since at the FAF you will be above glidepath, you intend to (or teach to) 1) descend quicker than normal until you catch the glidepath from above, or 2) ignore the glidepath entirely.

I thought a number of people answered that question, particularly @catmandu. I agree with him but would add that "controlled" also means, to use the FAA's standard for descent from the MDA, "at a normal rate of descent using normal maneuvers." IOW, if I needed 700 FPM rather than 500 FPM to get down and capture it, sure. If I needed 2,000 FPM, I'd ignore it. Chances are, if you ignored it, and use the typical higher rate of descent many use for nanoprecision approaches, you'd probably catch up anyway.

I would teach that as a component of pilot decision making. With no wrong answer. other than busting minimums

 

[Cooter]

I thought a number of people answered that question, particularly @catmandu. I agree with him but would add that "controlled" also means, to use the FAA's standard for descent from the MDA, "at a normal rate of descent using normal maneuvers." IOW, if I needed 700 FPM rather than 500 FPM to get down and capture it, sure. If I needed 2,000 FPM, I'd ignore it. Chances are, if you ignored it, and use the typical higher rate of descent many use for nanoprecision approaches, you'd probably catch up anyway.

I would teach that as a component of pilot decision making. With no wrong answer. other than busting minimums

Why are you interested in capturing the glide path at all?

 

[luvflyin]

Are there approaches that have different LNAV and LNAV+V minimums? Or LP and LP+V?

 

[midlifeflyer]

Why are you interested in capturing the glide path at all?

Just a preference, depending on the situation.

The purpose of the advisory glidepath is to provide a "normal" stable, consistent descent rate which will bring you to a position for an equally "normal" consistent descent rate down to the runway. I personally prefer that to the old "dive and drive" if conditions permit, just as you may prefer the opposite. As I said, a component of aeronautical decision making.

 

[Cooter]

Just a preference, depending on the situation.

The purpose of the advisory glidepath is to provide a "normal" stable, consistent descent rate which will bring you to a position for an equally "normal" consistent descent rate down to the runway. I personally prefer that to the old "dive and drive" if conditions permit, just as you may prefer the opposite. As I said, a component of aeronautical decision making.

I think recapturing a glide-path that is not providing the right information is a destabilizing influence. Based on the OPs scenario, it will require a higher than necessary VSI to recapture it and once captured it isn't providing the advisory step-down clearance that it's intended to. I'm also not a fan of dive and drive, but those aren't the only two options.

 

[midlifeflyer]

Are there approaches that have different LNAV and LNAV+V minimums? Or LP and LP+V?

I don't think so. The +V is advisory only. It just identifies a stable descent to the runway. It doesn't change the existing minimums.

 

[MauleSkinner]

Keep in mind that if the glide path is high or low at the FAF due to ambient conditions, it's also going to be high or low crossing the threshold. Why not just fly half a dot above, or whatever it happens to be?

The other thought that comes to mind is that Garmin doesn't do the "+V" without WAAS...are those glidepaths barometric or do they use WAAS? If the latter, they're not subject to temperature.

Same question for the L/VNAV glidepaths, for that matter.

 

[luvflyin]

I don't think so. The +V is advisory only. It just identifies a stable descent to the runway. It doesn't change the existing minimums.

I'd say it's just a matter of mindset then. Instead of saying "I'm going to fly the VNAV+V Approach" it's "I'm going to fly the VNAV Approach and if I capture the glidepath then I'll use it to assist me in a stabilizing the remainder of the Approach. There's a "gotcha" that could get you on one of these +V's if your're use to following the needle down an ILS. You can be below DA as long as you made the decision to and began the Missed at DA. On a +V you have to abandon the needle a little above the MDA to make sure you don't go below it.

 

[iamtheari]

What you have to fly is obvious and I don't think I saw anyone arguing about that. But as far as what to do about the glideslope needle, my thought is this: If the needle goes down before you reach the stepdown fix, ignore the needle the rest of the way in. You can interpret it as an indication that you need to descend slightly faster than normal after the fix to get to the next charted altitude before crossing the next fix, but given that the conditions that led to the needle going down early will probably persist through the rest of the approach, you will probably still end up "behind" the needle all the way to the MAP. Why bother diving to catch the needle when you'll just lose it again at the next stepdown?

 

[RussR]

Are there approaches that have different LNAV and LNAV+V minimums? Or LP and LP+V?

No. +V does not indicate a line of minimums. It is merely advisory and is added by Jepp/Garmin. it is not an official part of the procedure.

Keep in mind that if the glide path is high or low at the FAF due to ambient conditions, it's also going to be high or low crossing the threshold. Why not just fly half a dot above, or whatever it happens to be?

The other thought that comes to mind is that Garmin doesn't do the "+V" without WAAS...are those glidepaths barometric or do they use WAAS? If the latter, they're not subject to temperature.

Same question for the L/VNAV glidepaths, for that matter.

+V is flown using WAAS. So the glidepath itself is unaffected by temperature, however the location of the aircraft along the glideslope for any indicated altitude does vary by temperature.

LNAV/VNAV glidepaths, while designed for Baro-VNAV systems, are flown using WAAS in the Garmin GPSes so the same situation applies.

Thanks to those who brought up stepdown fixes. The procedures that prompted this question do not have stepdowns in final so I was not considering that. I agree, if there are stepdowns, then it would be silly to increase workload even more by approaching the glidepath from above, adjusting configuration to capture it, then adjusting again to level off prior to each fix (as you would have to do when its warm).

This is one reason why, while I understand the intent of the +V approaches, I do feel that they really complicate training. An IR student is already pretty swamped by material, then you introduce all the weirdness of the +V. There's a glidepath that looks just like the LPV, but you can't intercept it prior to the FAF; you can follow the glidepath if you want to, but you don't have to; watch out for final stepdown fixes (that you can ignore on an LPV); you're flying a glidepath, but remember you can't go below DA like on an LPV, etc.

Obviously I'm still struggling with how best to present them to instrument students.

 

[coloradobluesky]

If the glideslope doesnt allow you to fly the approach within the charted stepdowns, DON'T USE IT! Cobbing together some jury rigged approach with wrong guidance is a recipe for disaster. Just fly it without the glideslope! Following the rules when tempted to do something else is why captains get paid and why private pilots get to live to fly another day. Gotta make those right decisions in the face of temptation to do it an "easier" way!

 

[iamtheari]

Obviously I'm still struggling with how best to present them to instrument students.

I'm not an instructor--barely done with being an instrument student, even--but this is my thought on what to tell a student: "The +V is the GPS's indication that the glideslope needle is a crutch and possibly a lie. Maybe someday it will help you, but to pass the test and to be a safe instrument pilot it's probably better to pretend the glideslope needle just isn't there whenever you see a +V on the GPS."

 

[midlifeflyer]

Obviously I'm still struggling with how best to present them to instrument students.

I think you just describe what it is (an optional capability provided by some manufacturers) , their purpose (to guide the pilot from the FAF to the runway on a stable approach path) and their limitations (they are not tested like an ILS glideslope or LPV glidepath and it is the pilot's responsibility to use the altimeter to ensure compliance with altitude restrictions, so the can choose to use them or not. That's just rote. The higher learning is the correlation that, "gee it's just like the other equipment. Things aren't perfect and I have to make some decisions when things look wrong."

 

[Harold Rutila]

I'm not sure I agree with the premise of this post. In areas where altimeter error is great enough to warrant a temperature correction, the FAA has established cold temperature operations procedures on those approaches. In warm weather, there is no such risk. There is also no restriction on following an "advisory glideslope" outside of the FAF so long as the airplane is at or above the relevant minimum crossing altitude at the next fix. I have not found an instance in 100+ hours of instrument instruction this past summer where the advisory glideslope takes an airplane below the minimum crossing altitude at the final approach fix, nor any intermediate fix.

Specifically, while flying the Dallas Executive Airport (RBD) RNAV Runway 35 approach, using an Avidyne IFD and its "advisory glideslope" functionality, the "advisory glideslope" allows us to comply with all of the crossing restrictions on the chart while simultaneously avoiding the "dive-and-drive" methodology often employed on non-precision approaches. I did this approach multiple times in July in 105° degree heat with no problems noted. This also had to do with the fact that the speed of the plane I was flying (a Grumman) did not require a glidepath to be portrayed outside of the intermediate fix HOBMI at any point. Even if it did, the angle between HOBMI and the FAF CUDAD is 4.91 degrees, and the "advisory glideslope," to the extent it would exist outside of HOBMI, would exist in a way that allows the airplane to follow it without undershooting the crossing altitude at HOBMI.

Something else that is relevant to this discussion is how the FAA has not yet come up with an explanation regarding how it treats these "advisory glideslopes." I was calling them "artificial glideslopes" until earlier this summer when another CFI-I pointed this fact out to me. It appears that all of these systems including ILS glideslopes, LPV glideslopes, and these "advisory glideslopes" are collectively referred to by the FAA simply as a "glideslope." Avidyne, on the other hand, does use the "advisory glideslope" terminology.

Quotes used around "advisory glideslope" are to further emphasize that the glideslope in question is not ground-based or LPV-based.

 

[RussR]

The cold weather correction is because you are lower than you think, and that poses the risk of running into the ground while your altimeter says you're still at a safe altitude. Correct, there is no similar risk in high temperature conditions. However, high temperature very much still affects altimetry. Recall the familiar figure from the Pilots Handbook of Aeronautical Knowledge:



When it's warm, our true altitude is higher than indicated. So that's usually good from a safety standpoint. Following a glideslope/glidepath, though, it creates some issues. Chiefly because the glideslope (whether ILS or GPS-based) is NOT affected by temperature. Here's a fancy picture:



On a standard day, the airplane intercepts glidepath right at the FAF at 3000. But on a warm day, the airplane is actually higher than 3000 feet, although the altimeter still reads 3000. Therefore, the airplane will intercept this same glidepath farther out from the runway - it's just geometry. If the pilot follows this glidepath down, then when the airplane crosses the FAF it therefore MUST be lower than the 3000 ft indicated altitude it started at - maybe 2900 ft*. Again, the airplane is actually at a true altitude of 3000 ft at this point - but the indicated altitude is less - 2900. When it gets to the stepdown fix COTOX, it will be lower than 1800 - but not by as much since altimeter errors reduce the closer in elevation you get to the reporting station (the airport).

If the pilot is flying an LPV, LNAV/VNAV (using WAAS), or ILS, this won't be an issue (and COTOX would be ignored anyway). But flying an LNAV+V or LP+V, the aircraft would cross the FAF and the SDF indicating below the published altitude and would at the least be a bust on a checkride.

A similar situation was the cause of some problems at LAX some years ago. Aircraft were intercepting the ILS glideslope 15 miles or more from the runway, well outside the FAF and flying it down. There were lots of intermediate stepdown fixes before the FAF on these approaches and on hot days the aircraft's indicated altitude would be below the published altitude. This was a problem because of aircraft flying underneath them to other airports, and caused numerous losses of separation and pilot violations.

* 100 feet is not an unreasonable error for hot days and being 1500-2000 feet about the airport elevation.

 

[RussR]

Specifically, while flying the Dallas Executive Airport (RBD) RNAV Runway 35 approach, using an Avidyne IFD and its "advisory glideslope" functionality, the "advisory glideslope" allows us to comply with all of the crossing restrictions on the chart while simultaneously avoiding the "dive-and-drive" methodology often employed on non-precision approaches. I did this approach multiple times in July in 105° degree heat with no problems noted. This also had to do with the fact that the speed of the plane I was flying (a Grumman) did not require a glidepath to be portrayed outside of the intermediate fix HOBMI at any point. Even if it did, the angle between HOBMI and the FAF CUDAD is 4.91 degrees, and the "advisory glideslope," to the extent it would exist outside of HOBMI, would exist in a way that allows the airplane to follow it without undershooting the crossing altitude at HOBMI.

I am not familiar with the Avidyne IFD. Were you flying an "LNAV+V" procedure, or was it calculating its own advisory glidepath? The LNAV+V is that advisory glidepath coded by Jeppesen or Garmin (for Garmin GPSes). I especially ask because HOBMI to CUDAD as published is certainly not 4.91 degrees - 1000 feet over 7.5 nm is about 1.26 degrees by my math.

 

[luvflyin]

No. +V does not indicate a line of minimums. It is merely advisory and is added by Jepp/Garmin. it is not an official part of the procedure.



+V is flown using WAAS. So the glidepath itself is unaffected by temperature, however the location of the aircraft along the glideslope for any indicated altitude does vary by temperature.

LNAV/VNAV glidepaths, while designed for Baro-VNAV systems, are flown using WAAS in the Garmin GPSes so the same situation applies.

Thanks to those who brought up stepdown fixes. The procedures that prompted this question do not have stepdowns in final so I was not considering that. I agree, if there are stepdowns, then it would be silly to increase workload even more by approaching the glidepath from above, adjusting configuration to capture it, then adjusting again to level off prior to each fix (as you would have to do when its warm).

This is one reason why, while I understand the intent of the +V approaches, I do feel that they really complicate training. An IR student is already pretty swamped by material, then you introduce all the weirdness of the +V. There's a glidepath that looks just like the LPV, but you can't intercept it prior to the FAF; you can follow the glidepath if you want to, but you don't have to; watch out for final stepdown fixes (that you can ignore on an LPV); you're flying a glidepath, but remember you can't go below DA like on an LPV, etc.

Obviously I'm still struggling with how best to present them to instrument students.

I'd think that introducing them after they've done some ILS/LPV's and some VOR/LNAV's might be a good idea. That way they can make the comparisons of how they relate to each of those.

 

[luvflyin]

The cold weather correction is because you are lower than you think, and that poses the risk of running into the ground while your altimeter says you're still at a safe altitude. Correct, there is no similar risk in high temperature conditions. However, high temperature very much still affects altimetry. Recall the familiar figure from the Pilots Handbook of Aeronautical Knowledge:

View attachment 55957

When it's warm, our true altitude is higher than indicated. So that's usually good from a safety standpoint. Following a glideslope/glidepath, though, it creates some issues. Chiefly because the glideslope (whether ILS or GPS-based) is NOT affected by temperature. Here's a fancy picture:

View attachment 55958

On a standard day, the airplane intercepts glidepath right at the FAF at 3000. But on a warm day, the airplane is actually higher than 3000 feet, although the altimeter still reads 3000. Therefore, the airplane will intercept this same glidepath farther out from the runway - it's just geometry. If the pilot follows this glidepath down, then when the airplane crosses the FAF it therefore MUST be lower than the 3000 ft indicated altitude it started at - maybe 2900 ft*. Again, the airplane is actually at a true altitude of 3000 ft at this point - but the indicated altitude is less - 2900. When it gets to the stepdown fix COTOX, it will be lower than 1800 - but not by as much since altimeter errors reduce the closer in elevation you get to the reporting station (the airport).

If the pilot is flying an LPV, LNAV/VNAV (using WAAS), or ILS, this won't be an issue (and COTOX would be ignored anyway). But flying an LNAV+V or LP+V, the aircraft would cross the FAF and the SDF indicating below the published altitude and would at the least be a bust on a checkride.

A similar situation was the cause of some problems at LAX some years ago. Aircraft were intercepting the ILS glideslope 15 miles or more from the runway, well outside the FAF and flying it down. There were lots of intermediate stepdown fixes before the FAF on these approaches and on hot days the aircraft's indicated altitude would be below the published altitude. This was a problem because of aircraft flying underneath them to other airports, and caused numerous losses of separation and pilot violations.

* 100 feet is not an unreasonable error for hot days and being 1500-2000 feet about the airport elevation.

It was causing problems during the turns to final while doing simultaneous approaches to the north and south runways also. It was during the LA Basin temperature inversions that it was worse. The low side being turned to final at 2500 were quite a bit higher than 2500 and were picking off the high side who had been turned on at 3500 but had started down the glideslope.

 

[mtuomi]

The cold weather correction is because you are lower than you think, and that poses the risk of running into the ground while your altimeter says you're still at a safe altitude. Correct, there is no similar risk in high temperature conditions. However, high temperature very much still affects altimetry. Recall the familiar figure from the Pilots Handbook of Aeronautical Knowledge:

View attachment 55957

When it's warm, our true altitude is higher than indicated. So that's usually good from a safety standpoint. Following a glideslope/glidepath, though, it creates some issues. Chiefly because the glideslope (whether ILS or GPS-based) is NOT affected by temperature. Here's a fancy picture:

View attachment 55958

On a standard day, the airplane intercepts glidepath right at the FAF at 3000. But on a warm day, the airplane is actually higher than 3000 feet, although the altimeter still reads 3000. Therefore, the airplane will intercept this same glidepath farther out from the runway - it's just geometry. If the pilot follows this glidepath down, then when the airplane crosses the FAF it therefore MUST be lower than the 3000 ft indicated altitude it started at - maybe 2900 ft*. Again, the airplane is actually at a true altitude of 3000 ft at this point - but the indicated altitude is less - 2900. When it gets to the stepdown fix COTOX, it will be lower than 1800 - but not by as much since altimeter errors reduce the closer in elevation you get to the reporting station (the airport).

If the pilot is flying an LPV, LNAV/VNAV (using WAAS), or ILS, this won't be an issue (and COTOX would be ignored anyway). But flying an LNAV+V or LP+V, the aircraft would cross the FAF and the SDF indicating below the published altitude and would at the least be a bust on a checkride.

A similar situation was the cause of some problems at LAX some years ago. Aircraft were intercepting the ILS glideslope 15 miles or more from the runway, well outside the FAF and flying it down. There were lots of intermediate stepdown fixes before the FAF on these approaches and on hot days the aircraft's indicated altitude would be below the published altitude. This was a problem because of aircraft flying underneath them to other airports, and caused numerous losses of separation and pilot violations.

* 100 feet is not an unreasonable error for hot days and being 1500-2000 feet about the airport elevation.

Isn't this why we get local weather before starting an approach? And why some procedures are NA if no local wx is available?

 

[luvflyin]

Keep in mind that if the glide path is high or low at the FAF due to ambient conditions, it's also going to be high or low crossing the threshold. Why not just fly half a dot above, or whatever it happens to be?

The other thought that comes to mind is that Garmin doesn't do the "+V" without WAAS...are those glidepaths barometric or do they use WAAS? If the latter, they're not subject to temperature.

Same question for the L/VNAV glidepaths, for that matter.

it shouldn't be off by more than a few feet crossing the threshold. The altimeter is going to read correctly at field elevation as long as it's fairly recent and the setting was taken correctly. The temperature effects on true vs indicated altitude is because the column of air shrinks when it's cold and expands when it's hot. The higher you go, the bigger the error

 

[MauleSkinner]

This is one reason why, while I understand the intent of the +V approaches, I do feel that they really complicate training. An IR student is already pretty swamped by material, then you introduce all the weirdness of the +V. There's a glidepath that looks just like the LPV, but you can't intercept it prior to the FAF; you can follow the glidepath if you want to, but you don't have to; watch out for final stepdown fixes (that you can ignore on an LPV); you're flying a glidepath, but remember you can't go below DA like on an LPV, etc.

Obviously I'm still struggling with how best to present them to instrument students.

There are, as I'm sure you're aware, a LOT of things that complicate training. Back when dual NAV/COMMS (but only one glide slope receiver) and an ADF were king (both literally and figuratively ), we didn't need transponders, and we looked at the whiskey compass every five or ten minutes to reset our DG, life was simple.

Now we have to know about ADS-B, glass, laser gyros, and dual backups to the backup system...there's a LOT to teach/learn. As you're indicating here, your job to distill it down to a "need to know" basis while still teaching everything necessary becomes more complicated yet.

I like @midlifeflyer 's explanation. Another perspective might be that Garmin provides three types of glidepaths as appropriate...one for LPV minimums, one for VNAV minimums, and one that's advisory only. In the Garmin system, they're all dependent on WAAS, which means none of them are affected by altimeter errors. Feel free to use that glidepath at the level anunciated or any lesser level.

Specifically regarding the advisory VNAV, stabilized approaches are great, but there may be reasons that a dive-and-drive is preferred. The altimeter, however, is controlling, and in no case can you allow an advisory (or any) glidepath to take you below the designated altitude for the approach segment.

 

[TommyG]

As a CFII I only teach LNAV and LPV for the GPS.

 

[Charlie H]

Currently an IFR student and I'm learning all instrument approaches. Mind boggling

 

[MauleSkinner]

Currently an IFR student and I'm learning all instrument approaches. Mind boggling

It simplifies it in my mind to think of it as only three types...

1. Glideslope/glidepath to a DA
2. Dive and drive to an MDA
3. Advisory glidepath to an MDA (this may also include CDFA guidance)

The navigation source may change, but these three are the only ways to get down to DA/MDA with on-board navigation.

 

[azure]

The cold weather correction is because you are lower than you think, and that poses the risk of running into the ground while your altimeter says you're still at a safe altitude. Correct, there is no similar risk in high temperature conditions. However, high temperature very much still affects altimetry.

I'm not a CFI or CFII but this is a good point, and an excellent, very clear explanation of it, and because of it I would probably (IF I were a CFII) teach my students to ignore advisory glideslopes while they were still learning the basics of shooting instrument approaches. Later on, when a student has flown umpteen straight LNAV approaches and is getting ready for the checkride, it might be nice to discuss advisory glideslopes, and maybe even bring up this very point as it's an issue we should all be aware of.

Personally I have not seen an LNAV+V glideslope in several years, ever since the FAA began issuing notes on many approaches because of obstacles and Jeppesen began removing them from those approaches in the database. That includes approaches that I flew regularly back in Michigan and at my current home base. (I know, there are still some out there, just not as many as there used to be.)

 

[Clip4]

I've been teaching instruments a lot lately.

When flying an LPV, LNAV/VNAV, or ILS approach, you start on down at glideslope intercept - GS intercept is by definition the FAF for the approach, whether or not you have reached (or passed) the charted FAF.

Due to altimeter errors caused by non-standard temperatures, the actual point of intercept can vary widely from the charted FAF. Especially during the summer, glideslope intercept can occur 1/4 to 1/2 mile before the charted FAF (in the winter, of course, it would be past the FAF). No problem, on an ILS/LPV/LNAV-VNAV, you start down at glideslope intercept and follow glideslope.

But then we've got those oddballs, the LNAV+V and LP+V, where Jeppesen/Garmin publish an advisory glideslope that is not evaluated by the FAA. Since these are still non-vertically-guided approaches, the FAF is still where we are to start descending, just like a VOR or LOC-only approach (or LNAV without the +V).

However, in warm weather, inbound to the FAF, you will first pass through advisory-glideslope intercept, but can't start descending yet. Then you reach the FAF and can start descending, however the advisory glideslope is now below you.

You have basically two choices - do you intercept it from above (which we usually specifically teach not to do with glideslopes), or ignore it?

For CFIs - how do you teach how to use the advisory glideslope on "+V" approaches?

§91.175 Takeoff and landing under IFR.
Link to an amendment published at 81 FR 90172, Dec. 13, 2016.

(a) Instrument approaches to civil airports. Unless otherwise authorized by the FAA, when it is necessary to use an instrument approach to a civil airport, each person operating an aircraft must use a standard instrument approach procedure prescribed in part 97 of this chapter for that airport.

LNAV is a published procedure - LNAV+V is not a published procedure, thus advisory. Do not start down until FAF.

 

[midlifeflyer]

Currently an IFR student and I'm learning all instrument approaches. Mind boggling

Don't worry.

It simplifies it in my mind to think of it as only three types...

1. Glideslope/glidepath to a DA
2. Dive and drive to an MDA
3. Advisory glidepath to an MDA (this may also include CDFA guidance)

The navigation source may change, but these three are the only ways to get down to DA/MDA with on-board navigation.

That's good!

Unfortunately, so much training is rote. We take a bunch of concepts as treat them individually rather than as part of a cohesive whole. Instrument training is worse because of the sheer volume of procedural stuff, but it's there even in our earliest training. I remember a student having so much difficulty finding applicable FARs during a pre-checkride session with his instructor. I asked if I could suggest something. When I pointed out the FAR has a table of contents and is organized by subject like any other reference book he's looked at, the student's eyes lit up. No one had ever shown him that.

 

[luvflyin]

Currently an IFR student and I'm learning all instrument approaches. Mind boggling

Hang in there. It can be pretty "boggling." The whole system has been in a state of change for many years now. Give it time for the "picture" to come together.

 

[aterpster]

A similar situation was the cause of some problems at LAX some years ago. Aircraft were intercepting the ILS glideslope 15 miles or more from the runway, well outside the FAF and flying it down. There were lots of intermediate stepdown fixes before the FAF on these approaches and on hot days the aircraft's indicated altitude would be below the published altitude. This was a problem because of aircraft flying underneath them to other airports, and caused numerous losses of separation and pilot violations.

It took several of those pilot violations to wake up the airlines. The interim solution was to use Baro VNAV prior to the PFAF. The permanent solution was a redesign of the procedures to provide more vertical separation over underlying airspace in the KONT area.

 

[luvflyin]

It took several of those pilot violations to wake up the airlines. The interim solution was to use Baro VNAV prior to the PFAF. The permanent solution was a redesign of the procedures to provide more vertical separation over underlying airspace in the KONT area.

PFAF??

 

[midlifeflyer]

PFAF??

Precision Final Approach Fix. You know it as glidepath or glideslope intercept, but we don't have enough acronyms

 

[MauleSkinner]

Precision Final Approach Fix. You know it as glidepath or glideslope intercept, but we don't have enough acronyms

We ran out of TLAs, so they had to go to FLAs?