Air pressure should be the same as when on the ground.

Don't think that is correct Geoff.

My understanding is that cabin pressure is maintained at a pressure equivalent to around 10,000 feet ASL - hence your ears "popping" during ascent & descent. You can see this with things like crisp packets and PET drinks bottles - crisp packets expand at cruising altitude & a drinks bottle opened & drained but re-sealed at cruising altitude will be pulled nearly flat on landing. 10,000 feet is the limit for flying without oxygen.

Too much stress on the airframe to maintain a high pressure differential, hence running the cabin at a lower pressure than that experienced at sea level but at a level known not to induce hypoxia.

Nigel B.