What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GS intercept on GRT EFIS

L

lr172

Well Known Member
I have been vectored on to an RNAV final approach course inside of the IAF or IF where the LPV GS begins (always on an LNAV, as I had no LPV capability before, but now I do). In these cases, it means that I will likely become established well above the LPV GS for that point. If I arm the GPSV approach during the vectors, will the GRT wait until it is within 2 dots of horizontal deviation before it attempts to capture the GS? The manual says that it performs this way for the ILS, but I wasn't sure if that would be true for the GPSV.

Also, I have read that on an ILS, I do not want to press the execute button, after arming, until inbound on the approach course (i.e. after procedure turns, etc.). Does this apply to a GPSS/GPSV approach? I would assume that I would not need to worry, as it is just following roll commands and not capturing a course, but I suppose I could be betting headings to the IAF. Still, it would seem that I am not capturing a course, but following waypoint sequencing and getting roll steering commands.

Thanks for your help here. I have just installed a 430W and thinking ahead to using some of these features.

Larry
 
lr172 said:
I have been vectored on to an RNAV final approach course inside of the IAF or IF where the LPV GS begins (always on an LNAV, as I had no LPV capability before, but now I do). In these cases, it means that I will likely become established well above the LPV GS for that point. If I arm the GPSV approach during the vectors, will the GRT wait until it is within 2 dots of horizontal deviation before it attempts to capture the GS? The manual says that it performs this way for the ILS, but I wasn't sure if that would be true for the GPSV.

Also, I have read that on an ILS, I do not want to press the execute button, after arming, until inbound on the approach course (i.e. after procedure turns, etc.). Does this apply to a GPSS/GPSV approach? I would assume that I would not need to worry, as it is just following roll commands and not capturing a course, but I suppose I could be betting headings to the IAF. Still, it would seem that I am not capturing a course, but following waypoint sequencing and getting roll steering commands.

Thanks for your help here. I have just installed a 430W and thinking ahead to using some of these features.

Larry
Click to expand...

Your first paragraph is confusing. Usually GS intercept happens at or near the FAF, not the IAF. ATC should never vector you so that final course intercept happens where you are above the GS. Activating execute at that point may result in an extremely steep dive down to the GS.
Second paragraph: wrt LPV approaches: I usually don't engage 'execute' until I'm turning onto the final approach course. I think this is particularly important if you're doing a procedure turn. The EFIS will follow the GPS Procedure turn course as long as it is in GNAV. But it can't do that in ENAV. Once inbound, it will switch to ENAV and arm the GS after you push Execute.
 
Last edited:
EXECUTE

The EXECUTE option should not be selected until you hear " cleared for the approach" when flying IFR.
 
Agreed with both Bob and RV8iator. When you are going thru IAF, you are below the GS and ideally you will intercept the GS just as you reach the FAF if your altitude is corresponding to the approach plate. Mine captures GS right when the needle is at or very near center.

One other tip just in case you are not aware of, on an ILS approach I set mine A/P to the heading mode and arm ILS. I will follow the heading instructions from ATC until the last turn when I am given to capture the LOC and cleared for the approach. The EIFS will automatically changes from following Heading mode to ENAV once it sees an active needle so there is nothing to do just monitor and enjoy the ride.
 
Thanks for Input. This helps. Several rnav approaches near me have you entering the approach at 3000' and the LPV starts at the IAF, not the FAF, likely due to the high initial altitude. A few times i was vectored onto the approach course between the IAF and FAF. Maybe this is somewat atypical.

Larry
 
lr172 said:
Thanks for Input. This helps. Several rnav approaches near me have you entering the approach at 3000' and the LPV starts at the IAF, not the FAF, likely due to the high initial altitude. A few times i was vectored onto the approach course between the IAF and FAF. Maybe this is somewat atypical.

Larry
Click to expand...

Please post the name of the approach.
Maybe we're confusing terminology. The approach starts at the IAF. It is not unusual to be vectored onto final inside the IAF. But you should always be below the GS at course intercept.
 
Larry,
I think I can guess the confusion. Look at the Rnav GPS 2L KDPA approach. On the profile view it appears that you intercept the GS at 3000' at the IAP (BOMER). But that is not true, it's just a poor drawing. The diagonal path down is meant to show that the minimum altitude for this leg goes down to 2500'. The actual GS intercept at 3000' is not shown, but will be a bit less than 2 miles before the FAF (ZETKO). So if you are told "Fly heading 050, intercept the final approach course, Maintain 3000 until established, cleared for the approach." Then you have two options: (1) Intecept final, descend to 2500', intercept the GS at ZETKO; or (2) maintain 3000', intercept the GS 2 miles before ZETKO, track it down, passing thru 2500' at ZETKO. If your vector has you intercept the final course less than 2 miles from ZETKO (e.g., above the GS) then the controller screwed up. You should declare a miss and explain why, hopefully they vector you for intercept further out next time.
PS In the real world with a moving map you can see if intercept is going to be too close to the FAF. Just change your course a bit to intercept earlier.
PPS Obviously only descend to 2500' if inside the IAF.
 
Last edited:
BobTurner said:
Larry,
I think I can guess the confusion. Look at the Rnav GPS 2L KDPA approach. On the profile view it appears that you intercept the GS at 3000' at the IAP (BOMER). But that is not true, it's just a poor drawing. The diagonal path down is meant to show that the minimum altitude for this leg goes down to 2500'. The actual GS intercept at 3000' is not shown, but will be a bit less than 2 miles before the FAF (ZETKO). So if you are told "Fly heading 050, intercept the final approach course, Maintain 3000 until established, cleared for the approach." Then you have two options: (1) Intecept final, descend to 2500', intercept the GS at ZETKO; or (2) maintain 3000', intercept the GS 2 miles before ZETKO, track it down, passing thru 2500' at ZETKO. If your vector has you intercept the final course less than 2 miles from ZETKO (e.g., above the GS) then the controller screwed up. You should declare a miss and explain why, hopefully they vector you for intercept further out next time.
PS In the real world with a moving map you can see if intercept is going to be too close to the FAF. Just change your course a bit to intercept earlier.
PPS Obviously only descend to 2500' if inside the IAF.
Click to expand...

Thanks for the clarification and detail here. You're pretty clever; It was the 2L RNAV at DPA where this happened, as you described. After your explanation, it makes sense that it is not 3000' at the IAF, but much closer to the FAF. As I mentioned, I have only done these approaches as non-precision in the past.

Thanks again for taking the time to help me better understand this.

Larry
 
BobTurner said:
Larry,
I think I can guess the confusion. Look at the Rnav GPS 2L KDPA approach. On the profile view it appears that you intercept the GS at 3000' at the IAP (BOMER). But that is not true, it's just a poor drawing. The diagonal path down is meant to show that the minimum altitude for this leg goes down to 2500'. The actual GS intercept at 3000' is not shown, but will be a bit less than 2 miles before the FAF (ZETKO). So if you are told "Fly heading 050, intercept the final approach course, Maintain 3000 until established, cleared for the approach." Then you have two options: (1) Intecept final, descend to 2500', intercept the GS at ZETKO; or (2) maintain 3000', intercept the GS 2 miles before ZETKO, track it down, passing thru 2500' at ZETKO. If your vector has you intercept the final course less than 2 miles from ZETKO (e.g., above the GS) then the controller screwed up. You should declare a miss and explain why, hopefully they vector you for intercept further out next time.
PS In the real world with a moving map you can see if intercept is going to be too close to the FAF. Just change your course a bit to intercept earlier.
PPS Obviously only descend to 2500' if inside the IAF.
Click to expand...
Bob,
How do you get the "2 mile before ZETKO" from this approach chart. I would have interpret this the same as Larry, fly at 3000 till BOMER, then start my decent to pass ZETKO at or above 2500.
 
Bavafa said:
Bob,
How do you get the "2 mile before ZETKO" from this approach chart. I would have interpret this the same as Larry, fly at 3000 till BOMER, then start my decent to pass ZETKO at or above 2500.
Click to expand...

A standard 3 deg GS is about 300 vertical feet lost for every nm horizontally. (From the FAF in this works on the chart). That also means that (going backwards on the GS) going from 2500' to 3000' takes less than 2 nm if you track the GS. The chart is drawn as if the slope of the line from 3000' to 2500' is the same as the slope of the line from 2500' to the runway. But it isn't, the slope of the line from 3000' to 2500' is (or should be) much shallower. The actual GS intercept at 3000' is not shown on the chart.

The approach is designed to be flown as (1) 3000' to the IAF; (2) step down to 2500' at the IAF, getting there prior to the FAF; (3) intercept the GS at 2500' and at the FAF and track inbound. However, since 2500 is underlined and not overlined it is a minimum, but not mandatory, altitude. So it is okay to maintain 3000' until GS intercept about 2 miles out from the FAF, and track inbound from there. It is also okay to begin a slow decent at the IAF (but with no vertical guidance) until you either intercept the GS or 2500, whichever happens first. If you're good you'll hit both at the same time.
 
BobTurner said:
A standard 3 deg GS is about 300 vertical feet lost for every nm horizontally. (From the FAF in this works on the chart). That also means that (going backwards on the GS) going from 2500' to 3000' takes less than 2 nm if you track the GS. The chart is drawn as if the slope of the line from 3000' to 2500' is the same as the slope of the line from 2500' to the runway. But it isn't, the slope of the line from 3000' to 2500' is (or should be) much shallower. The actual GS intercept at 3000' is not shown on the chart.

The approach is designed to be flown as (1) 3000' to the IAF; (2) step down to 2500' at the IAF, getting there prior to the FAF; (3) intercept the GS at 2500' and at the FAF and track inbound. However, since 2500 is underlined and not overlined it is a minimum, but not mandatory, altitude. So it is okay to maintain 3000' until GS intercept about 2 miles out from the FAF, and track inbound from there. It is also okay to begin a slow decent at the IAF (but with no vertical guidance) until you either intercept the GS or 2500, whichever happens first. If you're good you'll hit both at the same time.
Click to expand...
Awesome explanation, thanks Bob.
 
You must log in or register to reply here.
Back
Top