This would be a merger of unprecedented scope, combining the second- and third-largest U.S. carriers. In 2001, the Justice Department rejected a proposed merger -- one that had little overlap -- between United and US Airways ( LCC), then the first- and sixth-biggest airlines.

The review, by the way, took 14 months. That may or may not be a precedent. As US Airways President Scott Kirby has said, "The companies chose to have the process take 14 months because United got cold feet."

Perhaps it would take just seven months. It is not unlikely that the hedge funds and strategic investors would move on well before the process ended.

One thing about such investors: They are often in a hurry. In its letter to Delta, Pardus -- which holds 2.5% of Delta and 4.8% of United -- noted: "Time is of the essence as now is the right regulatory environment."

Similarly, in a September letter calling for a frequent-flier program spinoff by American Airlines parent AMR ( AMR), the Icelandic company FL Group, who owns about 9.1% of the company's shares, said "there is no time to lose."

By one important measure, the letters had the desired impact, because share prices rose after they were made public. Fittingly, the gains were largely short-term. Now, shares in all three carriers have largely receded to their levels of before the activists got involved.

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