Since Dan Cooper lowered the aft stairs near Tacoma, Washington, the cabin suction from the stairs lowering obviously did not pull the placard free. What then caused the placard to rip free north of Kelso? There is one handy explanation. The first time Flight 305 had leveled off was at 7,000 feet. This was a little north of Kelso. Air suction changes in the open bay may have pulled the placard off.

     The next time the airliner leveled off was near Ariel, when it came to its cruising altitude of 10,000 feet. This may have changed air suction again, causing the aft stairs to bounce up, enough to cause the internal air pressure oscillations as the gangway closed and then lowered again.

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