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GPS Used As DME On Approaches  
User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Posted (10 years 4 months 4 days 19 hours ago) and read 6474 times:

Hey guys, something I'm curious about that I haven't be able to find the answer to yet. We have a couple airplanes with IFR GPS's and uncurrent databases. I know that due to this, the GPS cannot be used to fly GPS approaches, but it still can be used for enroute functions.

Now my question, these airplanes do not have DME. Can an IFR GPS with an uncurrent database be used satisfy the DME requirements on VOR/DME and ILS/DME approaches?

If you have any FAR, AIM, etc references to this type of stuff also, that'd be great! Thanks guys.

14 replies: All unread, jump to last
 
User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 1, posted (10 years 4 months 4 days 19 hours ago) and read 6450 times:

You can't legally fly GPS IFR with an out of date database even if its enroute only. The installation guidance material called up for GPS installations is AC 20-138A. Under this guidance, the following airworthiness limitations must be included in the GPS Flight Manual Supplement:

IFR enroute and terminal navigation predicated upon the GPS 500’s GPS Receiver is prohibited unless the pilot verifies the currency of the data base or verifies each selected waypoint for accuracy by reference to current approved data.

and....

Instrument approach navigation predicated upon the GPS 500’s GPS Receiver must be accomplished in accordance with approved instrument approach procedures that are retrieved from the GPS equipment data base. The GPS equipment database must incorporate the current update cycle

If your AFMS has no such limitation, then either your GPS isn't approved for IFR or not approved properly.

As far as DMEs go, its not only illegal to use a GPS but unwise. Some DME reference points have no corresponding GPS waypoint. If you manage to find it and start navigating, the integrity and accuracy of the GPS distance isn't guaranteed to be within the tolerance of non-precision approach constraints.

The approach scaling and corresponding RAIM limits needed for approach aren't invoked outside of a GPS approach retrieved out of the database. Therefore your "DME" fix generated by the GPS can be several miles out with no indication of the error.

So...."no" on the IFR GPS with the expired datacard and "no" on the GPS distance as a substitute for DME.





User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 4 months 4 days 18 hours ago) and read 6430 times:

Airplay, those must be Canadian specific regulations. In the US, for IFR enroute, we are only required to determine that the information is correct, the database does not need to be current (AIM Table 1-1-8(2)). But, after doing some digging, it does look like the database does need to be current for DME usage on non-GPS approaches (AIM 1-1-20(f)(6)(c)).

User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 4 months 4 days 17 hours ago) and read 6336 times:

Nope. AC 20-138A is FAA guidance material. The predecessor AC 20-138 just happens to be the accepted guidance material for both the US and Canada with respect to GPS approvals.

I should make one correction though. Systems approved previous to the re-issue of this AC to the "A" revision should have the limitations I stated revised to read:

A valid and compatible database must be installed and contain current data.

If you want to check it out yourself, check the FAA website and go to the regulatory and guidance library.

You wouldn't believe the amount of aircraft importations that I've been involved with where an American airplane with a GPS is being imported and the system doesn't meet the IFR requirements. I invariably am accused of applying "Canadian" rules when in fact they are the same rules...at least when it comes to GPS approvals.

 Smile

[Edited 2004-06-19 04:29:56]

User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 4, posted (10 years 4 months 4 days 17 hours ago) and read 6219 times:

Reading through the AC as I'm typing this...pretty interesting. The AIM states that IFR terminal and enroute use of GPS "requires verification of data for correctness if database is expired" and that IFR approach "requires current database."

When I get a chance I'll have to read through both the AC and the AIM more carefully.


User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1959 posts, RR: 33
Reply 5, posted (10 years 4 months 4 days 17 hours ago) and read 6058 times:

You can use a GPS with an expired database for situational awareness only, same as any non TSO approved GPS.

See FAA advisory circular 90-94 and chapter 1 of the AIM. You should not use a GPS with an expired database for area navigation unless you can verify your route using other means. This means you should not be using that GPS in lieu of DME on a VOR/DME, ILS/DME, NDB/DME or any other approach which requires DME unless you have an actual DME unit installed, operating, and actively being monitored in the airplane.

A GPS with a current database CAN be used in lieu of DME and ADF equipment.


User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4211 posts, RR: 37
Reply 6, posted (10 years 4 months 4 days 16 hours ago) and read 6052 times:

Hmmmm.... whoops!!!

(walks off with a "whodathunkit" look on his face).

I always thought that GPS could be used as a DME substitute as long as you verified the lat/long was correct for that distance fix.



Chicks dig winglets.
User currently offlineMeister808 From United States of America, joined Jan 2000, 973 posts, RR: 1
Reply 7, posted (10 years 4 months 4 days 16 hours ago) and read 5988 times:

GPS can be used as a substitute for DME if it is called out on the approach plate.

My rule of thumb: do what the plate says. If the plate don't say it, don't do it.

-Meister



Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1959 posts, RR: 33
Reply 8, posted (10 years 4 months 4 days 15 hours ago) and read 5816 times:

I would encourage everyone to read AIM 1-1-21 (f)(5-8)

It goes pretty in depth on exactly how and when GPS may be used in lieu of ADF and/or DME during enroute and terminal phases of flight.

1-1-21(f)(6(c))

Waypoints, fixes, intersections and facility locations to be used for these operations must be retrieved from the GPS airborne database. The database must be current. If the required positions cannot be retrieved from the airborne database, the susbstituation of GPS for ADF and/or DME is not authorized.



GPS that meets the above requirement can be used in lieu of ADF or when the approach plate says ADF REQUIRED or DME REQUIRED as long as adf or dme are not the principle approach navigation source.

To shoot an NDB approach (the adf is the principle approach navigation source in this situation) without an ADF installed in the airplane using only GPS, the approach must be included in Phase III of the approach overlay program. In other words, the approach's title must say "NDB or GPS rwy XX" instead of just "NDB rwy xx". If you just want to use the GPS in lieu of ADF to identify a bearing from an NDB or a compass locator on an ILS approach, GPS can be used without any mention on the approach plate.

[Edited 2004-06-19 06:44:37]

User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 9, posted (10 years 4 months 4 days 5 hours ago) and read 5303 times:

And...keep in mind the RAIM constraints. The RAIM alarm in the GPS enroute mode (which is what it will be in unless you have activated a GPS approach from the database) may not alert you of position errors of several miles.

Of course "most" of the time the error will be much less than this.


User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 10, posted (10 years 4 months 3 days 23 hours ago) and read 5238 times:

You don't need an approach activated, or even loaded, to be in other than enroute mode. When you are within 30 miles of your departure or destination airport (as specified in the GPS flight plan), it will switch to terminal mode (assuming you have it set to auto switch).


09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 11, posted (10 years 4 months 3 days 23 hours ago) and read 5236 times:

You can't legally fly GPS IFR with an out of date database even if its enroute only. The installation guidance material called up for GPS installations is AC 20-138A. Under this guidance, the following airworthiness limitations must be included in the GPS Flight Manual Supplement:

Um, you proved yourself wrong in your following quote:

IFR enroute and terminal navigation predicated upon the GPS 500’s GPS Receiver is prohibited unless the pilot verifies the currency of the data base or verifies each selected waypoint for accuracy by reference to current approved data.

Note the "or verifies each selected waypoint for accuracy by reference to current approved data." That is in agreement with what the AIM says, like Flyf15 has said. You do not need a current database for enroute navigation, provided that you have verified the accuracy of each waypoint you use.



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 12, posted (10 years 4 months 3 days 23 hours ago) and read 5232 times:

If the database is not current, but the unit is approved for enroute IFR, you can still use it for enroute navigation, provided the waypoint accuracy is verified.

The GPS can be substituted for both DME and ADF, provided that the station being substituted is not the primary approach aid. This means that you can not fly an NDB approach using only a GPS, and if your chosen approach has a DME arc as the final approach course, you can not fly it using only a GPS (some TACAN approaches are set up like this). If the NDB (or DME) approach has a GPS overlay, then you can fly it, however you are still not flying the NDB approach, you're flying the GPS approach.

There are other limitations on using the GPS as a substitute for NDB and DME, such as the chosen waypoint must be retrieved from a current database, and a few others that are usually no-brainers and taken care of by the unit without thinking.

Also, another note about RAIM, if the unit loses RAIM capability after passing the FAWP of a GPS approach, it will NOT notify the pilot for up to five minutes. This allows the pilot to complete the approach. The allowance goes under the knowledge that a loss of RAIM capability does not mean that accuracy has been lost, and operates under the assumption that accuracy will not degrade any appreciable amount in the next five minutes. However, as soon as a RAIM annunciation is provided, a the missed approach must be executed.

How to execute the missed approach without RAIM? Good question, since you are not allowed to use the GPS without RAIM unless you are monitoring alternate navigation systems (and not at all on an approach). In spite of this rule, there are some instances where you will have no other choice but to follow the GPS course until coming into range of an alternate navigation system (often VOR), or climbing about the controller's MVA for the area you are in. God help you if your GPS automatically suspends course guidance when a RAIM alert occurs. Always have some sort of no-nav escape plan in mind when flying a GPS approach in real weather. Declaring an emergency may help, if the controller can see you on radar, then they can vector you even below the MVA.



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 13, posted (10 years 4 months 3 days 23 hours ago) and read 5232 times:

Ha, one last thing.

When you're receiving radar vectors to a GPS approach, it's a good idea to have the approach loaded starting with the IAWP that's closest to where you're being vectored instead of in VTF (Vectors to Final) mode. This way, if the controller suddenly tells you to proceed direct to the IAWP and clears you for the approach, you don't have to do some fancy reprogramming while trying to intercept the course. Once the controller says "fly heading xxx until intercepting the final approach course, blah blah blah", you can simply activate VTF mode for the approach (how to do this varies from unit to unit, GNS 430, hit PROC, then enter, CNX-80, PROC, then VTF).

Controllers aren't always up on GPS approach procedures, so even if you ask for vectors to the approach, they will sometimes spring an IAWP on you at the last moment. If they would just treat them like any other approach it would be much easier.



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 14, posted (10 years 4 months 3 days 5 hours ago) and read 5175 times:

Note the "or verifies each selected waypoint for accuracy by reference to current approved data." That is in agreement with what the AIM says, like Flyf15 has said. You do not need a current database for enroute navigation, provided that you have verified the accuracy of each waypoint you use.

I agree. I should have said it prohibited unless you have approved data on board to verify the accuracy of the waypoint position. But also note what the current installation guidance says, as I later posted:

A valid and compatible database must be installed and contain current data.

I should have posted the latest information first. I instead posted an excerpt from an AFMS developed using the older guidance material.

Of course thats not to say that there aren't several GPS installations out there that are perfrectly legal that have the old limitation.

I think the FAA has caught on to the fact that the vast majority of pilots (mainly private guys) won't bother to verify the accuracy of the database when using out-of-date databases.




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