Amateur/Home Built/Experimental
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Flight Instructor
Flight Crew Last 90 Days 87; Flight Crew Total 2400; Flight Crew Type 461
Troubleshooting; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft
This report is for the dangerous situation of inadvertently approaching an airport in mountainous terrain. at night at too low of a height above the airport.This issue happened because I did not crosscheck information provided by the Garmin GTN 625 Navigator for VNAV profile with airport information.  The button sequencing on the GTN 'zeroed out' the 1;000ft AGL offset I programed into the VNAV feature.  This appears to occur with all GTN 6xx/7xx navigators.I was flying a personal flight to SAF.  I knew that I would be arriving at night; and planned to approach from the Albuquerque area to avoid terrain.  In order to have a safe approach to the airport; I setup VNAV in my GPS Navigator; a Garmin GTN 625.  I used the VNAV feature to give a 3 degree glide path to a point 1 mile from the airport at 1;000ft AGL.In setting up the VNAV; for altitude I entered a value of 1;000ft; then hit the MSL/AGL soft key to enable AGL.When I reviewed the added point VNAV waypoint in the flight plan on the GTN; it autofilled a crossing altitude of 6;349ft MSL.  I had understood this to be the MSL altitude of 1;000 AGL.  When approaching the airport; I descended through 7;000ft and noted that the terrain illuminated by city lights appeared closer than the approximately 1;600ft I was expecting.  I arrested my descent; and climbed visually to an appropriate height visually with the airport in sight.  I finished my traffic pattern and landed.  I noted that the elevation of the field was 6;349 MSL.Later I tested my keystrokes in the GTN625.  I tried to enter a VNAV waypoint with the following keystrokes:Enter 1;000ftHit the MSL/AGL buttonUpon doing this; I noted that when you hit the MSL/AGL button; the altitude/height reverts to 0.  I learned that you must enter the AGL value AFTER hitting the MSL/AGL value.  I also learned to always check your VNAV crossing altitudes with the chart.
Pilot reported problems with setting up the Garmin Navigator correctly and almost having a CFIT encounter.