Following a serious incident where an aircraft hit a donut lighting structure at San Francisco International airport , the FAA instigated frangible design rules for such structures . A frangible object was defined as " an object of low mass , designed to break , distort or yield on impact , so as to present the minimum hazard to aircraft " . This characteristic is seemingly contradictory to the operational requirements for stiffness and rigidity imposed on this type of equipment
serious incident
Pan Am Flight 845
Frangibility