Using a GPS glide slope when no ILS available

[Perco88]

Hey Folks,

 

First, I'd like to thank ED for this great sim. I am learning everyday with this sim and that is a great feeling.

 

I generally don't like to point the obvious but I'll pass on my principles on that one : english is not my native language (stupid french here :music_whistling:) so please feel free to point out any typos, bad spelling, bad grammar and so on, in this post or in the attached document.

 

Two weeks ago I was practicing ILS landing in bad weather, and I told myself that training on the same airfield was not that great as it was beginning to feel almost mechanical (turn here, drop gear now, ...)

And I wish there were ILS available on more airfields not to train on the same airfield everyday.

 

And my answer to this problem came to me while reading the manual.

 

You can simulate glide path using the 3D Nav Mode of a waypoint. Maybe this is known to all the hardcore simmers out there, but I have not seen any thread concerning this on this forum for the moment.

It revealed to be a great tool when there is little to no visibility like in the flight used in exemple in this tutorial (visibility inferior to 150m)

 

Basically, this technique lies in using the waypoint created in the DIVERT page of the CDU to drive the glide bars of the ADI to provide a glide path. As this waypoint is placed in the middle of the runway, good control of speed in final is required as you will not have the full length of the runway to speed your aircraft down.

 

I hope this tutorial can serve to other people.

I may not be doing eveything by the book, so any critics, comments, suggestions are welcome.

Again, typos and bad grammar comments are also welcome.

 

I am working on a Youtube video to illustrate this technique, but anyone whishing to post one is welcome.

 

Hope you'll enjoy this and practicing bad weather landing with this technique :pilotfly:

GPS Glide Path 3D Nav.pdf

Edited May 3, 2011 by Perco88

[Spetz]

That's a great little .pdf you made. Thanks ! I'll try this out tonight !!

[Nate--IRL--]

That is very very cool, I really must read up on the CDU again. Please add your guide to the Wiki.

 

Nate

[CharlieMike24]

Formidable!

[bartleby]

Good work. I was just thinking about trying to figure this out. I was going to manually create a glideslope intercept waypoint using offset, but this is way better. Had no idea about VANGLE setting. Going to give this a try in a bit.

 

Are the DIVERT waypoints centered on the runway? If they are, ought to be easy to use offset to make a threshold waypoint to use with this method. A little fiddly, but might make for an easier landing.

[skooby]

Thank you for the guide, it's very helpful to a newbie like me. :)

[GGTharos]

I'm going to take a guess here, but since your ADI is not operating in ILS mode, this is not a realistically acceptable means of IFR precision approach. There are huge differences in how ILS and STP modes operate in terms of needle deflections (albeit I think the ILS mode might not be working quite right in the sim) plus the fact that you're guiding to the middle of the runway = doom.

 

As long as your HUD is functioning, you might be better off eyeballing the glidepath by using the divert to airfield STP and sticking it at -3deg, then keeping your TVV on it.

[Kaiza]

plus the fact that you're guiding to the middle of the runway

 

Was just about to ask whether it did that. Interesting non the less.

[Furia]

This is somehow similar how we do FMS Non precission approaches in real life (VOR, NDB), The FMS/MCDU calculates an standar glideslope for a non precission approach using the aproach published data on the FMS. Actually the aircraft flies itr nicely and it is approved for approach.

However in real life the sistem provides and does all calculations including following the published approach lateral navigation route.

You can even reproduce the same published non precission approach on the CDU creating a flight plan with the approach plate and doing as it is detailled on this document about the 3D VNAV for the approach points.

This way you could fly safely full published approach plates even those non precission approach ones.

Anyway this is a great implementation here for the A-10C. Nice document.

;) ;) ;)

[Perco88]

Thanks for the comments.

 

To Bartleby :

Yes, the waypoint is indeed centered on the runway. I do not know how to offset a waypoint with the CDU so any help is appreciated.

For the moment, everytime I tried landing with this method, I could manage easily to stop the aircraft way before the end of the runway. The only obligation is to arrive not too fast.

 

To GGTharos :

I don't really know if the precision is equivalent to an ILS approach. The fact that the touchdown point is in the middle of the runway is indeed not as safe as an ILS approach.

Concerning the sensitivity of the glide slope indicators on the ADI, if I read the manual correctly, the sensitivity option located under the NAV page does not include a separate ILS sensitivity.

So I assumed that it is the same in STEER PT and ILS mode (according to the manual : 1 dot on the ADI Glide indicator = 0.35 deg by default)

 

 

Bear in mind that this is more like a bonus guidance in final, as the vertical bar only appears like 3nm from the waypoint (I a not sure if there is a way to make it appear sooner, I'll have to dig this a little more).

 

I'll be trying tonight to use this method wiht the HUD off and only use the HSI and ADI indications to validate the precision of the whole thing.

I'll post a track for those interested.

[Frederf]

I'm going to take a guess here, but since your ADI is not operating in ILS mode, this is not a realistically acceptable means of IFR precision approach. There are huge differences in how ILS and STP modes operate in terms of needle deflections (albeit I think the ILS mode might not be working quite right in the sim) plus the fact that you're guiding to the middle of the runway = doom.

 

As long as your HUD is functioning, you might be better off eyeballing the glidepath by using the divert to airfield STP and sticking it at -3deg, then keeping your TVV on it.

 

The primary instruments for an ILS or RNAV approach would be the CDI for alignment and the GSI on the side of the ADI for the vertical. The ADI steer bars are secondary and it's just as good to fly an ILS with the steer bars stowed but perhaps not as easy or smooth.

 

The scales for the CDI and GSI are variable depending on the waypoint attributes with the "approach" setting being genuinely angular in display and similar if not exactly a match for the ILS deflections-per-error indications.

 

Of course legally the EGI isn't an RNAV-approach certified system and using it for the published RNAV equivalent of a Cat I ILS approach would be prohibited. However, functionally it's absolutely as good as the ILS if set correctly.

 

With a waypoint at the MAP for the approach set with a -3.0° 3D nav, attribute APPROACH, and TOFROM set to the right course it will guide you down to 200' minimums with aplomb. With non-precision approach or circle to land altitude standards even aiming for the middle of the runway is usually fine. Popping out of the clouds at 1,000' on course for a midfield touchdown is pretty smooth to transition for a normal landing.

[Smokin Hole]

This is a real world concern. I was once based in the Pacific with a major airline. Some of our destinations were small islands with distant alternates. Many of us discussed (as an ABSOLUTE last resort) extending the RWY centerline which automatically provides via GPS a PDI glide path. The actual RNAV approaches to these runways were offset due to terrain so this would have been the only way to get close enough to the runway and be in a position to land.

 

This is also a perfectly acceptable method to assist with a visual approach. Of course GG is right that you need the coordinates of the approach end of the runway.

 

Nice job, OP!

[Le0kong]

Thanks! From another CDU chapter drop-out. :D

 

 

To correct the problem of the waypoint being in the middle of the runway, what about using the OFFSET page from the CDU, using half the runway length (as offset distance) and runway heading (as bearing) to generate a second waypoint at the beginning of the runway.

[warthogmadman987]

From my real world experience, I can tell you that on GPS approaches you only use the needles for lateral navigation. Your "glidepath" would come from following specific step down procedures from an approach plate that is specific to each airports runway. The only way I know of a gps approach getting a glideslope like an ILS is if the aircraft is equipped for WAAS and the approach supports it. Pretty sure this is not built into the sim though.

[bartleby]

Thanks for the comments.

 

To Bartleby :

Yes, the waypoint is indeed centered on the runway. I do not know how to offset a waypoint with the CDU so any help is appreciated.

For the moment, everytime I tried landing with this method, I could manage easily to stop the aircraft way before the end of the runway. The only obligation is to arrive not too fast.

There were a couple good threads and some discussion on here recently about how to use the OSET page to generate offset waypoints. WarriorX made a good tutorial here, too: http://forums.eagle.ru/showthread.php?t=72101

 

The only thing is you'd need to make sure the waypoint was centered right on the runway, otherwise you might end up with a calculated threshold that's in the grass. Which might not be good for your aircraft.

[Tacbat]

Or you could extract the coordinates for the buttons of the runway, and make a new waypoint from that.

[Baggy MacDouche]

Just thinking aloud here, but I can imagine using the TGP to designate a mark point at the runway threshold and then converting that into a 3D waypoint for a more accurate landing point.

 

I also like to turn on the radar tape on the HUD to use as a reference to ground height when on final. Heck, if you are really good at math you should be able to adjust your slope by radar alt, time to waypoint, and vertical velocity. :p

[Zag]

:music_whistling: You have to be REALLY good at math to do that

 

but if you're established on the glide slope, a descent rate of about 700 ft/m should work

[GGTharos]

You barely need any math at all. 300' altitude/nm. is your glideslope.

 

Just thinking aloud here, but I can imagine using the TGP to designate a mark point at the runway threshold and then converting that into a 3D waypoint for a more accurate landing point.

 

I also like to turn on the radar tape on the HUD to use as a reference to ground height when on final. Heck, if you are really good at math you should be able to adjust your slope by radar alt, time to waypoint, and vertical velocity. :p

[sungsam]

I'm going to take a guess here, but since your ADI is not operating in ILS mode, this is not a realistically acceptable means of IFR precision approach. There are huge differences in how ILS and STP modes operate in terms of needle deflections (albeit I think the ILS mode might not be working quite right in the sim) plus the fact that you're guiding to the middle of the runway = doom.

 

As long as your HUD is functioning, you might be better off eyeballing the glidepath by using the divert to airfield STP and sticking it at -3deg, then keeping your TVV on it.

 

Eventhou I agree with GGTharos, if anyone want to test Perco88 method, additionally change precision scale to terminal instead of route mode. This is at the same page you change VNAV Mode to 3D and defines indication precision on HSI indexers.

 

Personally I would use Perco88 method at the early final (far away) as an indication of putting the plane on glide scope, but then on late final, GGTharos recommendation is the acceptable way for me.

 

Reason: Check CDU>GPS>EVE (Expected Vertical Error) values, it usually averages to 30 meters (realistic) even with GPS FOM 1 and ST 5 (five satellites in use), while EHE (Expected Horizontal Error) is near to zero. This 30 meters vertical error value is unacceptable for late Final and just before touchdown, that is doom IRL and assume also on DCS A10C. Height measurements is not a "strong" aspect of GPS receivers generally.

 

Anyway, VNAV is a means of reaching Desired Altitude assigned in a given Steer point by indications of HSI glide slope indexer.

Edited June 6, 2011 by sungsam

[effte]

Or you could extract the coordinates for the buttons of the runway, and make a new waypoint from that.

 

Or find the published information for the runway concerned, in which the threshold coordinates are given... :music_whistling:

[Rdiehl3]

Yes above...

 

Could you use your MAP view F10 to get these?

 

I think you can.

Edited June 29, 2011 by Rdiehl3