Frangible Tower Explained
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.
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.