Possible Clues:

• Effective obstacle that's quickly broken?
• Invisible impediment in the sky
• Reliable fence needs to be approached by high flyers looking for speed
• Hollow roar breaking boundaries when flying -- at this?
• Gait that's broken at high speed?
• Speed-related problem (for aircraft)
• Urban order is broken with a crash of noise!
• It is passed with a boom
• Crash at speed and it's broken
• Runs repeatedly coming in boundaries smashed � it takes a lot of pace to break through