The more I thought of it on reading the account from the British source, the more I could visualize a very valid situation.
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It starts out being some technical issue or disagreement, which escalates from professional to personal (given their romantic status, he a married man, she a mistress) and they get into a lover's spat in the cockpit. It gets loud...she begins screaming and shrieking with increasing volume as the man intentionally or unintentionally eggs her on, and soon the sounds of screaming and yelling are clearly audible to the crew in the galley, and even some of the people in first class who are close enough.
The flight attendants glance at each other with wide eyes and open mouths as they listen, and the passengers are nervously looking on since they don't have any idea, it could be terrorists for all they know, people begin texting to their families...passengers start shouting "What's going on?".
Inside the cockpit, the woman is now unhinged in her tirade, and the pilot cannot concentrate on flying the plane. He tries to get her to be quiet because he understands the ramifications (In the middle of the exchange, he realizes this is a very bad thing) but he isn't being nice or soothing about it, and she is determined to have her say (at extra high volume) pilot duties or not.
Soon, he cannot fly the plane between his not so polite requests to settle down and shut up, and the course corrections unanswered or delayed dialogue with the various ground stations, and realizes it is getting out of control. This is a high stress situation, and he slaps her.
She gets up and runs out of the cockpit sobbing, to the horror and astonishment of the flight attendants and passengers. The flight attendants gather around her, and seeing how her appearance is unsettling and disturbing the passengers (and themselves) implore her to go back onto the flight deck. At some point the pilot gets out of his seat and goes to the galley, and she goes back to the flight deck.
But like a gasoline fire that has been put out with CO2, but is still far above the flash point, the argument reignites, and she exits the cockpit a second time.
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I can easily visualize this. Given what we know of these kinds of things, she might just as well have smacked him on the top of the head with a stainless steel coffee pot.