Light Sport Aircraft
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 20; Flight Crew Total 1100; Flight Crew Type 30
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR
Flight Crew Became Reoriented
Aircraft
Upon takeoff from an airport within the Philadelphia Mode C transponder ring; the aircraft's transponder was 'On' but was not reporting Mode C altitude; due to a potentially misleading indication on the panel. According to the 'Garmin G3X Touch Pilot's Guide;' Rev. H; page 117: 'ON Mode can be selected at any time. ON Mode generates Mode A and Mode S replies; but Mode C altitude reporting is inhibited. In ON Mode; a green ON indication and transponder code appear in the mode field of the Transponder Data Box.' This can be a misleading design. The pilot puts the transponder into 'Manual On Mode' and sees the green 'ON' indication with the transponder code. But behind the scenes; the selection of 'ON Mode' inhibits altitude reporting. The manual further says: 'Altitude Mode is automatically selected when the aircraft becomes airborne. Altitude Mode may also be selected manually.' There is no indication that having selected the 'ON Mode' requires the further selection of the 'Altitude Mode' to turn on the mode C altitude reporting; especially since it says; without qualification; that altitude mode 'is automatically selected when the aircraft becomes airborne.' Thus; with this setup; it is way too easy to take off in an area where Mode C altitude reporting is required; believing that the transponder is doing exactly that; when it isn't. The indication on the G3X panel; when the system is 'ON' but not in altitude reporting mode; should indicate something like 'NO Alt' or similar. Further; since the G3X is quite a capable navigation tool; it should be able to determine if it within a 'Mode C' veil; and if so; flash a warning if altitude reporting is not enabled.
Light sport aircraft pilot reported that the transponder did not report altitude as expected/required.