on the ground
Both - you fly it in the air, but you drive it on the ground.
Any electrical system needs a voltage reference point. In other words, voltages in an electrical circuit are given (or measured) in reference to a point, which is by definition "zero" volts. This reference point can be called "ground". In other words, ground is by definition "zero volts". Perhaps you wanted to ask why we need ground in a "plane" form in a digital printed circuit board. That is usually done to improve the noise characteristics of the printed circuit in question. A ground plane disrupts the electromagnetic field created by the signals and if laid out carefully, can reduce the unwanted electromagnetic emissions.
Taxiing
The plane was on the ground, not flying in the air.
It is simply a plane surface making an angle with the horizontal (ground).
Into a container in the plane where it can be removed when the plane is back on the ground.
Yes, it does, unless it's part of a "no ground plane" kit. "No ground plane" is a misnomer, since the outer conductor on the coax is in fact the ground plane in these kits and is a specially made and tuned part that should not be altered. The fiberglass whip that you're asking about is actually 1/2 of a dipole, the other half is the ground plane. Both must work together for efficient signal propagation and be tuned in the mounted location.
No, you cannot be unctuous if a plane hits the ground. "Unctuous" means excessively flattering or ingratiating in a way that comes off as insincere or false. It does not apply to a plane crash situation.
When a plane nose-dives, the nose of the plane turns downward and the plane descends nose first toward the ground.
When a plane nose-dives, the nose of the plane turns downward and the plane descends nose first toward the ground.
The liquids and solids go into a holding tank and are pumped out when the plane is on the ground, usually while the plane is refueling.