GPS Approach Problems

[SadisticRabbit]

Hey Guys,

I've been flying sims for a while now, but only really now getting into navigation properly. I can do ILS landings no problem and also know roughly on how to use the GPS. My usual method of flying is to set the GPS to a direct destination, then select the ILS approach for the runway I want to land on. At about 100nm out I activate the GPS approach.

Now this is where my problems start. My problem isn't with every landing.. sometimes it works perfectly, other times it does not. Please bear with me as I explain an example, I may be missing something and that's where you lot come in :) I have read here and other places how the GPS approach should line you up with the runway enough so that you can then land visually or via ILS. This is what I want to achieve and in some cases it works perfectly, but in other cases it end up putting me in the middle of the ILS glideslope without enough time to line up with the runway.

So for an example, flying from EGLL to EGNX. I put EGNX in the GPS then select my approach, for this example use ILS09 (if memory serves me correctly, I'm not near my SIM PC). I load the vectors. So I take off and fly to EGNX. Since it's a short hop I activate the approach fairly quickly. Meanwhile I set NAV1 to the ILS frequency.

Now on some flights to other airports the GPS takes a course that bring me in line with the runway with plenty of time to line up correctly and get on the ILS, but not at EGNX (and a fair few others). What happens is it takes me to a marker in the middle of the ILS path and there is not enough time to turn left and line up with the runway. The result is a terrible landing!

So what am I missing? It seems that some airports have GPS approaches that are just too close to the runway to then line up correctly and land. Now I can fly the last part manually, but I hear so many people saying that the GPS can line you up automatically and I see many people doing that on airports I have problems with.

The return flight from EGNX to EGLL is perfect. It takes me on a radial to the EGLL runway with plenty of time to line up.

What should be done in these circumstances? Should I load a different approach? Should I not load the vectors?

I feel that I must be missing something. Say you leave from a runway heading WEST but want to land on a runway heading EAST.. this also seems to make the GPS approaches useless!!?

Any tips appreciated.

[xxmikexx]

SadisticRabbit,

I've never used GPS but it seems to me that a GPS approach would want to transition to an ILS approach if one is available ...

That being the case I would think that the point to navigate to using the GPS would be the final approach fix (FAF). This is often an intersection but definitely is the point where the glide slope should be intercepted at a published altitude above the ground, maybe 1000 feet AGL and maybe 5-7 miles from the touchdown point depending on the angle of the glideslope.

If I've gotten any of this wrong somebody will be along to correct me,
I hope this helps,

[NikeHerk67]

SadisticRabbit

That's a good question. After reading your post I went to East Midlands EGNX just to have a look at it. Being that the airport in located in the UK, I can't get find a GPS approach plate. Do you have access to them where you are? At East Midlands however both RW 91 and RW 271 both have an ILS CAT III approach. In this case the GPS approach exists for the simple reason that it is intened to be used by small GA aircraft or experimentals that do not have instrument landing capibilities. I'd imagine, without flying the GPS approach is an approach set up to guide those small aircraft, keeping them out of harms way, to the OM where they can then turn and make the landing.

For that reason you would never use the GPS approach to land a jet or an instrument rated aircraft. There wouldn't be any reason for it. There are no full GPS instrument landing RW's that exist today, however they're in the process of creating them.

[Tim_A]

There are no GPS approaches in the UK. Some were trialled a few years ago (Shoreham, Gloucester, Blackpool come to mind; I don't know if East Mids was part of the trial), but there are none right now.

The charts are here for reference: http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=60&Itemid=109.html

Looking at the ILS procedure for 09, you're being vectored to the IAF (EMW), but you are then trying to turn onto final, whereas the procedure is to turn beacon outbound, descending to not below 2000ft by I-EMW DME 8.2 , and then turning right to intercept the localiser. But in no sense is this a GPS approach.

(I'm in no way instrument qualified, so I might easily have got it all wrong!)

[scott967]

There are approaches that are designed to be flown (either by hand or coupled to the autopilot) by a "computer" typically referred to as RNAV or GPS approaches (outside the US they call GPS GNSS). The approaches are designed to provide the best possible performance using the "computer" which can be the Garmin GPS unit or an FMS as found on the big iron.

Separately, the hardware and database suppliers for these units include the conventional radio navaid procedures within the database. IRL, the conditions under which you may legally use the GPS as your main source of navigation is pretty complex (see the FAA AIM for US rules). In the sim, it is easy to simply load the procedure using the proc key and follow it, using the gps/nav switch in gps position. These procedures are not designed to be computer flown (this is an oversimplification, but I think the concept is OK). MS bought a database from Jepps and then incorporated it into the FS sim. If you select for example an ILS approach in the GPS, and try to follow the magenta track display line either manually or with the AP, you can often see that the aircraft won't fly "correctly" (what I see most often is that the magenta track will fly to intercept the final approach course / ILS localizer and not turn to the localizer course until passing the intercept point. It works pretty good though when there is a course reversal / procedure turn to get on the final approach course). I believe this is a limitation of FS. Note that in the vertical profile, the GPS will give you a vertical speed required to achieve an altitude limitation at the active waypoint, which may not be the best if you are trying to intercept an ILS GS.

So what to do? If you really want to use the GPS in a coupled approach, use/request a proper GPS/RNAV procedure. If you want to follow the conventional approach in the GPS, be prepared to switch to heading hold mode and intercept the final approach course/ localizer without coupling the AP to the magenta track line.

scott s.
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