Airplanes have been known to float for so long that they became a menace to navigation and had to be sunk by gunfire.

10 seconds sound clip from the The High and the Mighty (1954) movie soundboard.

You can hear this line at 01:15:19 in the DVD version of the movie.

Quote context

[...]

- Some of which can land on water.

- And surface vessels will be directed toward us.

- Airplanes have gone into the water before and no one received the slightest injury, except maybe a little dampening.

- Casualties come from panic. So you must not hurry.

- Miss Spalding and our second officer, Mr. Wheeler, will be back here with you. Do exactly as they tell you.

- Airplanes have been known to float for so long that they became a menace to navigation and had to be sunk by gunfire.

- They've been known to sink in a few minutes, too.

- This is an inflatable life raft.

- It's really a wonderful gadget. It's got everything in it but T-bone steaks.

- It will hold all of you very comfortably.

[...]