- - Invisible limit of a sense
- - Effective obstacle that's quickly broken?
- - Invisible impediment in the sky
- - What 69 Across broke in October 1947 ... and what you must break nine times to reveal this puzzle's hidden phrase
- - Reliable fence needs to be approached by high-flyers looking for speed
- - It's responsible for a boom in flying
- - Chuck Yeager broke it
- - flawless defence, yet speedily broken
- - sonic wall
- - solid obstruction very quickly passed through
- - Aviation term.
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