is using phone's GPS to track plane speed allowed during flight? The phone
is still in airplane mode, of course, but the GPS Test app can still
show the plane's speed and GPS coordinates.

as long as I'm asking... what about bluetooth headphones?(not headset)
I have a large pair of bluetooth headphones but I sometimes get asked
by the FAs to turn them off during fligth. I comply, of course. However,
on other flights, the FAs don't seem to care at all. The headphones
are paired to either my phone or my Macbook Air for listening to music
or movies.

thanks!