Airplane GPS Failure

Recently I have had a few run-ins with a cranky GPS receiver in our aircraft and have learned (re-learned) some lessons along the way.

Part of a good pre-flight, especially if IFR operations are part of the plan, is to verify that all of our navigation equipment is operational.  Regular VOR checks either using ground points or a dual VOR check is required.

Other navigation systems may be a little more obscure as far as pre-flight checks or may require a few more steps of system investigation to determine if everything is ready for flight.

The Garmin® 1000 system on many installations requires a check of databases, onboard equipment (LRU’s) and individual GPS receivers prior to takeoff.  Typically the number one GPS receiver is displayed and the number two receiver is ‘hidden’ so you need to make the effort to find out if it is fully functional.

The GPS receiver system that is on the aircraft I fly most regularly is tied into an FMS display / control head.  The FMS system on this aircraft is capable of using any or all of the sensors available to it.  GPS, DME, and VOR information is blended together to provide position and navigation information.  When one or more of the sensors is not available or fails, a message is generated, but usually position information is not degraded greatly.

So on to the rest of the story…during the pre-flight setup and check of the FMS (second leg of the day), a message warning about the FMS position is generated and I assume that because of our location near the hangar the GPS is not getting enough satellites to generated good position information.  The weather is good, the aircraft is ready to go and nothing else seems amiss.

Just after takeoff another message from the FMS indicates that we need to tune a local VOR/DME station.  This is the clue that the system is now ‘degraded’ to using VOR and DME in place of the missing GPS sensor information.  Because we failed to tune the specific DME frequency or tuned it too late, the FMS decides to drop to Dead Reckoning (DR) mode, which basically provides no useful information on the Multi-Function display (MFD) for the remainder of the flight.

After a bit of scrambling and cross-checking of charts (those paper things that we keep folded up in the flight bag), we return to VOR to VOR navigation (tune-identify-turn) and let ATC know we have lost some of our navigation capability.

One more event tied to this problem happens just prior to landing.  The database for the Ground Proximity Warning System (GPWS) is using the location information from the GPS to determine local terrain elevations.  On final to our home airport the GPWS starts warning of terrain and continues to complain all the way to landing.  Seems the GPWS (via the FMS) thinks we are somewhere else where the terrain is a factor.

Lessons learned…when a blended sensor system complains about position errors find out why (check all the sensors-GPS/VOR/DME/IRS, whatever).  During a pre-flight check know how to quickly determine the accuracy and efficacy of the equipment on board.  Have your paper chart out and marked with the planned route…sounds simple but in the ‘modern electronic cockpit’ the old-fashioned paper charts are usually buried in the bottom of the flight bag.  As an alternative, a good flight log with intersections, VOR frequencies and airway courses will provide much of the needed information.

Our maintenance found a bad antenna connection, that lead to the intermittent failure. I will prepare  better the next time by being  more suspicious of any sensor error.

Tagged as: g1000, garmin, gps