Forget acceleration, forget speed etc. You want a pressure sensor to detect a rate of climb of around 400 feet per minute or more to detect the "take-off", and an absolute pressure around 1500 to 2000 feet above sea level,
and a zero rate of descent, with say a twenty minute timer after those configurations for the landing. It would be better, in fact, not to bother detecting the landing - but to make this an "auto off - manual on" device as the cabin pressure on descent can level off quite some time before the actual landing in pressurised aircraft (depending on holding etc.)
The technology is small, easily packaged (many watches and phones already have barometric sensors), and if you want a separate device you can look at parachute automatic deployment safety sensors,
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