I just passed my instrument ride yesterday so I am looking forward to shooting some approaches if I ever need to.
My club just got a 2002 C-172SP with a GPS. The aircraft only has two VOR's (no DME). We are purchasing the IFR GPS so we can shoot GPS approaches. My question is can we use the DME on the GPS to shoot a VOR/DME or LOC/DME approach?? Can we do that with the VFR GPS or do we need the IFR GPS insert to use the DME?
My last question is about GPS approaches. What is the difference between a GPS, GPS-A, GPS-B, RNAV (y), and RNAV (z) approach? I will be learning these soon, but I just wanted a general overview.
Thank you,
Aerospacepilot
I'm goinjg to answer your second question first, as it's the easier of the two. The letters at the end of the approach mean the same thing regardless of whether it's a GPS or any other type of approach. The -A, -B, etc. means that it's an approach that's designed for circling minima only. You'll notice on those approach plates that there is no straight in minima posted in the profile view, and that the title doesn't include a runway. So, for example you may see GPS-A, or VOR/DME-B, but you'll never see GPS
RWY 13-A. The circling approach letters come from the beginning of the alphabet. As for the sequence, the first circling only approach at the field gets "A", the second gets "B", the third "C", etc.
As for the ones with the letters at the end of the alphabet, it's a whole different story. Those do include particular runways and straight in minima. All the letter means in this instance is that there are two different approaches that both use the same equipment for final to the same runway. For example, at Eglin I there are 2 different ILS approaches to runway 19. One of them is ILS Y RWY 19 and the other is ILS Z Rwy 19. The letters start at the back of the alphabet and work forward, so if they ever put in a THIRD ILS approach to that same runway it would be ILS X RWY 19. If you are wondering why they might design two different ILS approaches to the same runway, at Eglin it's because one of them requires an aircraft to be TACAN equiped to identify the IAF's and to fly the missed approach, but the other one would be compatible for an aircraft that had a VOR/DME, but not a TACAN receiver. Lot's of military aircraft have BOTH VOR and TACAN, but there are still quite a few out there that are VOR/DME but not TACAN (T-37, T-6, for example), and I think the T-38's are still TACAN only, but not VOR.
Next, the difference between GPS and RNAV on the approach plate title. This one I'm only guessing at, but I've seen two variants. This first on something called a GPS-overlay approach, where the approach could be flown using the underlaying navaids or the GPS. These are typically listed as VOR/DME
or GPS Rwy 13. The key to identifying these is the "or GPS". The approaches that are flown using ONLY the GPS are the ones that have the RNAV (GPS) in the title.
As for using the GPS on approaches, I think the restrictions are:
1. The GPS MUST be certified in accordance with TSO-129A (basically IFR qualified) to use it for ANY part of an approach, legally, including identifing a DME fix.
2. The point must be retrieved from the database (and the database must be up to date), so you can't "fat-finger" in the Lat/Long of the field. If the point in NOT in the database, but the fix is defined by DME from a NAVAID that IS in the database you can use that instead.
3. If the database is expired, BUT YOU CAN verify the location using an approach plate that IS CURRENT, that's ok. (So if the Lat/Long from the database matches the Lat/Long from the current plate, you're good to go.)
4. You CAN use the GPS to identify the location of a fix defined by NDB or an LOM, but you CAN'T use the GPS to track an NDB bearing on an ADF approach unless it's labelled "OR GPS". Basically if the approach is "ADF RWY 13" you need to have ADF, but if the approach is "ADF or GPS RWY 13" you can use the GPS. However, if it's an ILS or VOR approach, (for example) that has a crossing bearing from an NDB, you can use a crossing bearing off of the GPS instead.
5. I think you need to have at LEAST RAIM (receiver autonomous integrity monitoring) or an equivalent fault monitoring feature. I'm not sure about the specifics on this, but the military makes us do a predictive RAIM check before shooting a GPS, and we also need to have operative FDE (fault detection exclusion). I don't think the civilians are required those second two parts, but you definitely need RAIM or equivalent.
It get's pretty in depth, and unfortunately I couldn't find the source regulation, but here's a guide from AOPA that sums up quite a bit:
http://www.aopa.org/whatsnew/air_traffic/gps_in_lieu.html
I think they are missing some information, though. For example, I'm SURE that having GPS satisfies the DME requirement for flight in Class A airspace above FL240, but they don't mention it. (I think it needs to be -129 TSO'd, though, but I'm not sure if it needs TSO-129a or if TSO-129 is enough).
