The Warden's key is necessary to build the plane. It is in two possible locations:
The first part is in the showers. To get in the showers you must activate the door with the Afterlife. When you go down into the showers you have to open a door with the warden's key and activate the electric trap
of the washing machine with the Afterlife. When you activate the washing machine you cannot get out of the showers until you can pick up the plane part. It´s probable that the Warden and a few zombies will appear to kill you.
You can find the second part when you go down on the gondola and get off near the second dog (or wolf). You need the Warden´s key to open a door and then you use the Afterlife to open the second door where you get the part.
The third part is in the Warden´s Office behind an electrified door that will kill you. To stop this from happening you have to electrify three generators, which are in the closest room to the second dog, with the Afterlife. Now you must go to the Warden´s Office and open the door which was electrified and there you will find the third part.
The fourth part is in the third dog´s room in a cupboard. You must open it with the Warden´s key to get the part.
The fifth part is in the snail race. When you go down this race you can find a closet that you must open with the Warden´s Key. Then you have to get to the top of the snail race to use the Afterlife, then you go back down to electrify the closet so that the numbers from the columns appear. When you do this you have one minute to revive yourself and get the part.
Once you have flown the airplane and died on the Golden Gate you only have to get five cans of gas so that the plane works again. These cans are in the same places as the original parts.
You can see how to make it in this video: Airplane parts
