Yes, you could do M&A if there is a very small number of other large shareholders who are very firmly in your camp and can be relied on to vote accordingly. Typically institutional investors like pension funds, or private equity, but could in theory be a rich-guy individual shareholder.

But if you're buying shares on the open market to acquire a large minority position, you will still have to a pay a premium so it would be risky business to rely on other votes, unless you're very certain.

Elliot did not do M&A with SWA, but they did a big management shakeup from a minority position, by recruiting other shareholders (not too hard, they were already annoyed at management).

I think generally when acquiring another airline for purposes of integrating into your operation (that's the norm due to scope) you would absolutely want a controlling interest so that you have complete freedom of action to make any and all changes. Don't want to risk other investor groups possibly vetoing certain parts of your integration plan.

Also the controlling interest threshold in some cases can be higher than 50% for purposes of approving major muscle movements. Depends on state law and also the way the company is structured... just because you *own* 51% doesn't always give you the right to dictate to the other 49%.