Frangible tower is a structure that is able to survive the environmental loads encountered within the airport’s operating area and at the same time, offer little or no resistance to a colliding aircraft. Unfortunately, the frangible mast design is not this simple. In the case of the frangible mast, the engineer must develop a structure that is still limited in its deflection to provide the most stable support for the landing system but offer little or no resistance to an aircraft during impact. The FAA has defined this type of structure as Low Impact Resistant (LIR) or frangible.

FRANGIBILITY

Low Impact Resistant Structures need to be designed to withstand the static and operational/survival wind / jet blast loads with a suitable factor of safety, but fail readily when subjected to the sudden collision forces of a lightweight aircraft traveling at 140 km/hr

For analysis purposes, the structure must “break-away” at a peak force of 45 KN applied for 0.008 s. In theory, the tower must fail at this load while limiting the damage to the plane after impact with the tower. It is the intent of the FAA & ICAO that the crew will be able to maintain control of the aircraft until safely stopped after such an impact. Ideally, the tower should separate in a segmented fashion, creating a ‘window’, or opening, for the aircraft to travel through.