Air Traffic Control
The aircraft in Episode 6 caused the security alert because no-one was able to contact it by radio. Air traffic control maintain order in the skies by dividing the airspace into areas, each of which is monitored by an air traffic control team. As the aircraft travels through these areas, responsibility for it transfers from one air-traffic controller to another. The controllers will maintain contact with the pilots with radar and radio communication. However, small aircraft such as microlights are not required to file flight plans they only really come into contact with the ‘local controllers’ who control the airspace over the airfields themselves.

Once they are five miles out of this zone, the only contact and control over them would be via radio. And if, as in this episode, the radio is faulty, the pilot would be completely out of touch.

Microlights
Microlights were first developed when people thought of adding engines to hang-gliders. The idea was not to create an air-craft that had a utilitarian function, but that was for the fun of flight. Microlights, as their name suggests, are small and light and only capable of carrying one or two people. At the smallest end, they still look very much like hang-gliders, but at the top end they look like a conventional aircraft. The advantage of these aircraft is that they do not need an airfield or a large amount of space to take off and land. Most of them can easily be flown if you have access to an area the size of a field. They are most often flown by hobbyists who want to experience the sheer thrill of flight. And this shows in that although microlights are easily available fully built, many enthusiasts still choose to build their own aircraft from kits.

Some pilots of small aircraft fly by vision only (visual flight rules, or VFR). These pilots are not required by the FAA to file flight plans and, except for FSS and local towers, are not serviced by the mainstream air traffic control system. Pilots of large commercial flights use instruments to fly (instrument flight rules, or IFR), so they can fly in all sorts of weather. They must file flight plans and are serviced by the mainstream air traffic control system.