Aren't you always below the glide path before you fly level into it to intercept it?
What is frustrating then, why does the autopilot arm GP! The indication should simply remain ALT.
Thank you for the detail but the question remains, why does the autopilot arm GP mode? There is a Glide Path 3.50 depicted on the chart.
The flight director (not autopilot, there is a difference) arms GP mode because you told it to when you pressed the "APR" key on the GMC.
Thank you for the detail! But if GP mode is armed why doesn't the autopilot fly it?
As mentioned above, this is an LNAV-only approach; there is no published glide path. Some LNAV-only approaches include advisory vertical guidance, which will be annunciated on your HSI as "LNAV+V", but this one does not appear to. So although your flight director might have had GP mode armed, there probably was no vertical guidance available for it to fly.
Technically not always but yes you are correct. I guess I should have been more clear with my question, if the GP was intercepted but thanks for pointing this out.
My problem the ambiguity can get you killed! I've reached out to Garmin to see if they can explain.
Logic fallacy on your part. How is the armed GP going to capture something that‘s not there? You should recognize that and select a different vertical mode that will accomplish the altitude changes as depicted at an appropriate non-precision decent rate. Bottom line is the system worked as intended.
Because the GP was ARMED! And there is a glide path described on the chart.
The glide slope angle is there solely for you to use to calculate a decent rate. This approach does not have a vertical path, it has an MDA. There is no vertical path guidance that can be generated for the system to capture and fly.
Respectfully you are missing the point. If there no vertical guidance DON'T ARM GP!
I kind of agree with you. Imho this is one of the very few (minor) ergonomic screw ups Garmin has made. “Approach” is the wrong label for this button. It should say ‘Precision Approach’ or ‘approach w/ GS’. As explained above, for no glide slope approaches you shouldn’t push the approach button,
This is a pet peeve of mine. On the government approach charts what looks like a GS are actually a step down fixes. The symbology is subtly different, but you can tell by looking at the numbers that the first and second segments have different slopes. Get ahold of a Jepp chart for this approach, much better symbology, it’s obvious there that these are step-downs.
Yes, and this will be explicitly annunciated as "LNAV" instead of "LNAV+V" on the HSI.
Sir. I have both plates opened, NOS and Jeppesen. That 3.5 degrees is a "descent path" only (TERPS AKA as a VDA -Vertical Descent Angle). It is not a "glide path" as one would see on an LNAV/VNAV, LPV or standard ILS. That particular approach has circling minimums probably because the descent gradient exceeds 400 ft/NM from the FAF (JATGA) to the threshold crossing height of 40' at RW01. (TERPS Criteria) But by using the GS/KTS chart and the 3.5 degree descent angle the "rate" of descent can be calculated (example 100 kt GS = 619 fpm descent). That ROD should get your to the VDP at the MDA to capture the 3.5 degree PAPI lights on the right side of the runway without a dramatic power change The auto-pilot buttons do not know what approach has been loaded in the FMS and what is operational, it is up the the crew to determine what is the correct selection. i.e if you don't push any buttons, she'll just sit in the last mode she was assigned, For your instance, an example would be an simple ILS approach that we know has both lateral and vertical guidance. But the new ATIS NOTAMS say the GS is OTS. So we would need to know that "APPR" mode is not going to work today (she'll do what happend to you on the GPS NPA and just hold last selected ALT of 2000') So we need to resort to a LOC only (LNAV type) programming. We would select NAV only and use VS as the descent criteria. In your scenario it was the fact that the ALT selector will always be honored unless a valid GS signal is received. (This is also why when ALT is showing in the PFD on an ILS and the GS is captured you can then change the ALT selection to the MAP altitude without the AP climbing back up the hill) I can provide some "lite" reading to assist in learning Continuous Descent Final Approaches which can also help in all NP approaches you may encounter. - cappy
Why would you expect a GP to be followed on an approach that does not have one?
I have both charts up at the moment. Sorta agree as the NOS chart depicts the newer technique of a CDFA profile, (Continuous Descent Final Approach), while Jepp still shows the old step-down “dive and drive” from years ago. The fact that the plate(s) both show only an MDA, not a “Decision Height” is the clue that no GS is available. See my previous post.
Craig,
). After retiring from the 737 world (Luv'd it ), I've jumped into a G200, and went from flying a lot of ILS and RNAV (RNP) approaches, to a fairly different AP/FD system, and a wider variety of RNAV GPS approaches, both precision and non-precision. The G200 has the same APCH button as you describe, that arms both lateral and vertical flight director and autopilot modes (the 737s I flew did not). Took a little getting used to.
Thanks Bob, you did a better explanation than I did at 00:20 with a nice Homebrew in my belly!Craig,
I understand your frustration, and also feel this is an opportunity to learn (or in my case, reinforce a lesson
On an ILS or LNAV/VNAV (precision) approach, pushing that button arms both FD modes and if the AP is on, it captures both. If, instead, we're flying a NP approach (LOC only or LNAV only...like the one you are referring to), we capture course guidance with the NAV button, and fly the vertical path with VS. Selecting the APCH button would be a programming error for this type of approach. The recommended rate of decent and GP angle from the chart, in this case, is a reference that can be used in selecting a VS to fly the vertical path smoothly. If we were to try to fly a LOC only or an LNAV (only) NP approach (like your approach) by hitting the APCH button, what happened to you, would happen to us. There is no GP to capture, so it would fly happily along the lateral path and not begin descent. That GP "armed" annunciation would lie to us the whole time, and it would not capture a glide path, because there isn't one to capture. It's there because we chose the wrong mode on the FD/AP, not because it has an inherent flaw. Our bad, not it's bad. That would have been an approach planning error on our part, that should have been caught in the approach brief (noting and briefing the NP nature of the approach, and briefing the appropriate modes to be used...in this case, NAV and VS).
Another way of saying what you said above is, if there is no vertical guidance, don't select APCH..., i.o.w., don't try to arm GP (meaning us, the pilots...because it's not going to work). Understanding the approach we are flying, and using the correct modes, is the key to safety in this situation. With two pilots to vet it, it may perhaps be easier to catch and trap those errors. In single pilot IFR ops, its even more important to study the plates, and self-brief all the nuances (and limitations) of a specific approach.
Good reminder for me
Cheers,
Bob
Dave,
.
There is no GP. You don’t seem to understand the different types of approaches or how to differentiate between them. Perhaps more study of instrument procedures would answer your question.
Exactly. The OP is incorrect in his interpretation that a Glide Path is depicted on the chart. Rather, as Dr/_Dave indicates it's a descent path that can be used along with winds, airspeed, etc... as a basis for mental calculations of a descent VS in order to smooth out the descent profile (the pilot still being responsible for not descending below crossing altitudes at step-down fixes). Another poster's use of a VOR or LOC approach, or an ILS where the GS fails, is a good analogy as well.
Yes I remember this confusing me when I first got my G5/307 combination. I wrongly assumed that for every approach you would press the APR button.
It’s definitely choosing the wrong mode, and that’s the real problem, but I think there’s still room for some blame all around. A button labeled “APR” that you press for some approaches, but not others, is certainly not optimal from a human factors standpoint.
The answer to your op question is not immediately clear, but with all the posts it is pretty well done.
If you use a pseudo GP on a NPA remember to set a DDA (Derived Decision Altitude) rather than the MDA, I.E. +50’ of the MDA.
By flying GPS NAV mode for this approach and doing the step downs either manually or with VS, not selecting Approach mode may be fine for a successful approach, but if you don't arm approach, will you get the missed sequencing? I guess it's part of the loaded procedure.. so the navigation to the missed hold is loaded and just have to hit TOGA? In the climbing left turn back to TENUY will you get vertical or are you still on your own?Craig,
I understand your frustration, and also feel this is an opportunity to learn (or in my case, reinforce a lesson
On an ILS or LNAV/VNAV (precision) approach, pushing that button arms both FD modes and if the AP is on, it captures both. If, instead, we're flying a NP approach (LOC only or LNAV only...like the one you are referring to), we capture course guidance with the NAV button, and fly the vertical path with VS. Selecting the APCH button would be a programming error for this type of approach. The recommended rate of decent and GP angle from the chart, in this case, is a reference that can be used in selecting a VS to fly the vertical path smoothly. If we were to try to fly a LOC only or an LNAV (only) NP approach (like your approach) by hitting the APCH button, what happened to you, would happen to us. There is no GP to capture, so it would fly happily along the lateral path and not begin descent. That GP "armed" annunciation would lie to us the whole time, and it would not capture a glide path, because there isn't one to capture. It's there because we chose the wrong mode on the FD/AP, not because it has an inherent flaw. Our bad, not it's bad. That would have been an approach planning error on our part, that should have been caught in the approach brief (noting and briefing the NP nature of the approach, and briefing the appropriate modes to be used...in this case, NAV and VS).
Another way of saying what you said above is, if there is no vertical guidance, don't select APCH..., i.o.w., don't try to arm GP (meaning us, the pilots...because it's not going to work). Understanding the approach we are flying, and using the correct modes, is the key to safety in this situation. With two pilots to vet it, it may perhaps be easier to catch and trap those errors. In single pilot IFR ops, its even more important to study the plates, and self-brief all the nuances (and limitations) of a specific approach.
Good reminder for me
Cheers,
Bob
The GFC500 with a valid VNAV profile in the flight plan will leave selected altitude on descent without APR (GP/GS) armed.
