Yes, airplanes are solid.
An aeroplane is a solid: constructed mainly from light-weight aluminium.
hardest as in most complicated? or as in most solid?
because the ground is solid
If an airplane passes through a mountain it will be tunnelling underground, probably as a result of having crashed. Anything passing through solid rock is going to be subject to an element of turbulence.
The orbiter, the airplane looking part, and the solid rocket boosters are reused. However, the SRBs (solid rocket boosters) have to be completely dismantled and almost totally rebuilt.
The airplane in the movie Airplane is a Boeing 707
3 oz. or less can be brought on as a carry on an airplane as long as it is in solid form. Gel deodorant cannot be brought in carry on luggage, but any size/type of deodorant can be placed in checked baggage.
The front of an airplane is called nose of the airplane.
Well, it all depends on what you are considering. You may be talking about the shell/body of the aeroplane which is solid, the fuel for instance which may be considered as liquid/gas, or the gas (air) inside the aircraft as gas