Here's the Fermi Estimate version, in case the math in Aeroalias's answer is difficult to follow:

It's been said that the ISS experiences 0.9 G (90% of standard gravity at sea level). Airplanes are said to fly a mile high, and the ISS is over 100 miles high--these are not particularly accurate numbers, but they're good enough for order-of-magnitude estimations. Therefore, without any complicated math, we would expect an airplane to experience 99.9% of standard gravity.

As Aeroalias's answer works out to 99.63%, this is a pretty good estimate.

Here's the Fermi Estimate version, in case the math in Aeroalias's answer is difficult to follow:

It's been said that the ISS experiences 0.9 G (90% of standard gravity at sea level), which is of course canceled out by their orbital velocity so the astronauts inside feel like they're in 0G.

Airplanes are said to fly a mile high, and the ISS is over 100 miles high--these are not particularly accurate numbers, but they're good enough for order-of-magnitude estimations.

Therefore, without any complicated math, we would expect an airplane to experience 99.9% of standard gravity. As Aeroalias's answer works out to 99.63%, this is a pretty good estimate.