Originally Posted by hillrider

100% of the airlines use either the

• 2 x 3.5 mm mono
• 3.5mm stereo or
• 6.35 mm stereo (very rare)

If you travel with adapters for all you're covered 100%, but personally I encounter the last one only 0-5% a year.

The impedance is all over the map, and an audio transformer (impedance adaptor) may be required; thankfully my 2 x 3.5 mm mono to 1 3.55 mm stereo adaptor (which came with a pair of Sony NC in-ear headsets) also has a transformer, as with certain airlines' systems I would be blasted even at the lowest volume without it.