Handheld GPS on an Instrument Approach

bluelake

Well-Known Member
Ok, I think this ought to be easy but....

(Supposed) I am considering flying an instrument approach that requires DME but am using an airplane that contains no DME. All that I have a is a basic handheld GPS in which I can tune in DME.

I know this cant be allowed, but I am in a debate over this and I am having mild difficulty finding exactly where this is prohibited in the regs, explicitly or otherwise.

Any help from thos with photographic memory of the FAR's is greatly appreciated...

DeanR
 

Minuteman

“Dongola”
Interesting question...

To start, § 91.205 requires "Two-way radio communications system and navigational equipment appropriate to the ground facilities to be used."

If an instrument approach requires "DME," it requires "distance measuring equipment," which is a specific kind of NAVAID, with operational specifications which are defined in §171.157, "DME Performance requirements" (I think you have to pay to get a copy of the ICAO specification referenced in this section), where accuracy, fail-safe schemes, periodic tests, and inspections to be performed are defined.

The incompatibility of GPS with this scheme is that its performance cannot be guaranteed by the great and powerful Administrator, and an un-augmented signal will vary with time and position of the receiver (although it will still probably be as accurate as DME 99.9% of the time when available). Also, the GPS constellation is not "federally" controlled. Its operated by the military, meaning they can adjust the precision and availability of the signal as they see fit their needs, not the NAS'.

Someday, an approved WAAS-capable GPS receiver may be a viable DME substitute for a TACAN-compatible DME receiver because the FAA would ultimately have control over the quality and availability of the signal and meet the requirements of 171.157, but until then you'll have "GPS" NAVAIDS, and "DME" NAVAIDS as separate animals with different op-specs (as it were).
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
Someday, an approved WAAS-capable GPS receiver may be a viable DME substitute for a TACAN-compatible DME receiver because the FAA would ultimately have control over the quality and availability of the signal and meet the requirements of 171.157, but until then you'll have "GPS" NAVAIDS, and "DME" NAVAIDS as separate animals with different op-specs (as it were).

[/ QUOTE ]"Someday" happened in 1998 for operations within the US. You can find the permission to substitute IFR certified GPS for ADF and DME so long as the database is current in AIM 1-2-21. Most of it is in (f)(5) and (f)(6)

AOPA members can read a good write-up on it at:

http://www.aopa.org/whatsnew/air_traffic/gps_in_lieu.html

This does =not= apply to handhelds. So far, there are no handheld GPS units that qualify as IFR certified units.
 

kellwolf

Piece of Trash
[ QUOTE ]
"Someday" happened in 1998 for operations within the US. You can find the permission to substitute IFR certified GPS for ADF and DME so long as the database is current in AIM 1-2-21. Most of it is in (f)(5) and (f)(6)

[/ QUOTE ]

Just make sure that it is set for the NAVAID not the AIRPORT. I discovered my friend had been doing that on a trip to Vero Beach. The VOR is about 4 miles west of the airport.
 

bluelake

Well-Known Member
ok, thanks all so let me twist the question just a bit. What if its a mounted VFR-only GPS in an otherwise IFR qualified airplane that has no DME on the av stack. Can one use the DME feature of the GPS for shooting the approach?? My guess is still NO but not clear where it says so in the regs... the reference earlier is the closest so far.
 

JHines

New Member
Can't find an exact reg (AIM is just advisory). Did some searching and it appears there's an open debate on what the exact reg is.

AIM 1-1-21(f)(6) says that terminal-IFR approved GPS equipment can substitute for DME on an approach. AC 20-138 is saying that equipment meeting TSO C-129 class A1 or A2 may be approved for terminal IFR operations. Handheld equipment probably can't meet TSO C-129 because it lacks (1) RAIM and (2) the required CDI.

Of course, AC 20-138 clearly states that compliance with it is not the only means of getting airworthiness approval, but you'd basically be on your own for any kind of one-off installation.
 

Minuteman

“Dongola”
[ QUOTE ]
"Someday" happened in 1998 for operations within the US

[/ QUOTE ]
Ehh, err, that's almost five years ago ... oops. Thanks

Now (and I kind of feel like I should have my instrument rating yanked for not knowing something like this), how do you tell if an approach has a GPS overlay or not?
 

aloft

New Member
[ QUOTE ]
ok, thanks all so let me twist the question just a bit. What if its a mounted VFR-only GPS in an otherwise IFR qualified airplane that has no DME on the av stack. Can one use the DME feature of the GPS for shooting the approach?? My guess is still NO but not clear where it says so in the regs... the reference earlier is the closest so far.

[/ QUOTE ]
If the approach requires DME, and you have only GPS, it must be an IFR-certified one in order to substitute it for a DME receiver. A non-IFR-certified GPS (or a certified one with an outdated database) is usable only for enroute navigation.

Just so we're clear, you're not using a "DME feature" of the GPS, you're simply navigating via the known lat/long of the DME transmitter so all the regular caveats of GPS use apply.
 

MidlifeFlyer

Well-Known Member
[ QUOTE ]
Now (and I kind of feel like I should have my instrument rating yanked for not knowing something like this), how do you tell if an approach has a GPS overlay or not?

[/ QUOTE ]Don't give up your instrument rating. GPS is still new and expensive enough that it's just making its way into instrument training. Your best bet is to read the portions of the AIM and the FAA's Instrument Flying Handbook for an overview.

(BTW, the August 7 AIM revision changed the paragraph numbers. The GPS information is now 1-1-20 instead of 1-1-21. The full text is here:

http://www2.faa.gov/atpubs/aim/Chap1/aim0101.html#1-1-20)

The answer to your question is also in the GPS section of the AIM:
==============================
h. GPS Approach Procedures
As the production of stand-alone GPS approaches has progressed, many of the original overlay approaches have been replaced with stand-alone procedures specifically designed for use by GPS systems. The title of the remaining GPS overlay procedures has been revised on the approach chart to "or GPS" (e.g., VOR or GPS RWY 24). Therefore, all the approaches that can be used by GPS now contain "GPS" in the title (e.g., "VOR or GPS RWY 24," "GPS RWY 24," or "RNAV (GPS) RWY 24").
==============================

From a pilot standpoint, it's less complicated than it seems. The complexities is more of an issue for Gamin, King and the other GPS manufacturers. (All?) approach-certified GPS units will tell you whether you can fly it or not. Pull up an ILS approach in the box, for example and you will get some kind of reminder that this is for monitoring purposes only and can't be flown GPS-only.
 
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