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.