At the cessna, you can either pry the door open with a crowbar or break the glass window to get inside.

If you break the window, you find a random item. If you knock on the door afterwards, you get the option to fight a zed climbing onto the cessna (optional, doesn't change anything) or open the door. Opening the door from inside has the same result as prying it open with the crowbar earlier.

Once the zed in the plane is dead, you can search the plane. If you broke the window to get in, you have the option to check the area--if you do this, you will miss a great deal of items and skip directly to the fighting.

If you search instead (and keep searching), you gather two random items, three random items, and finally a single random item.

Afterwards, you have the option in taking on several back to back fights: two single zed fights, a fight with a runner, then a small horde of 6 zed, then a small horde of 3 zed, then a large horde of 16-29 zed, and finally three hordes at the same time; a small horde of 3, a medium horde of 8, and a large horde of 18-25.

If you manage to destroy all of the zombies, you can do a final search of the plane to gather four random items. If you didn't search the plane earlier, you don't get any more than four items in this last search.

To find Manton, you will need to these things, try the door then take the window off, then just open the door without calling out, then kill the single zed, then search and you will find a random item and/or Manton. Hope i helped!