Most frequent airline travelers have been just barely on one side or another of the “ten minute rule.”

While flights have a scheduled departure time, the gate agent will close the door to the plane ten minutes before departure. Which, depending on what side of the door you’re on, can either mean a huge sigh of relief, or a delayed or even ruined trip.

Sometimes it’s the passenger’s fault, either for checking in late or dawdling in the airport. On other occasions, it’s because of a late connection. But, the most maddening thing is that the rule isn’t consistently applied.

Last week, two very frequent travelers ended up on the wrong side of the door.

The first involved a United Global Services passenger, the airline’s highest frequent flier rank. The frequent flier got to the airport seven hours in advance of his scheduled red-eye and barely in time for stand-by on a earlier flight.

The first agent he talked with told him there was space, “Run!” But when he got to the gate, the door was closed and despite empty seats, the agent there told him he couldn’t board, although the plane didn’t leave for another 15 minutes.

In the second case, a United 1k (100,000 miles a year) traveler landed late into Denver on a connection from Washington, D.C. to Bozeman. He still had about 15 minutes to spare, was upgraded, and was only three gates from his connecting flight.

So, while he felt confident, the traveler still raced off the plane and to his connecting gate. While the plane had not departed, he was told he had just missed the cut-off. Again, they had closed the door to the ramp. It was the last flight of the night, too. So, he ended up spending the night in Denver. (United did at least pay for his hotel.)

I understand that schedules matter and there are a number of factors involved in holding or not holding a plane. What I don’t understand is why airlines don’t stick to one rule instead of closing the door early when it is convenient for them and keeping passengers waiting on board in other cases.

While these clients both missed flights, I’ve had situations where the planes were held and have been on flights delayed for connecting passengers. (In more than one case, just for one or two people.) There doesn’t seem to be a consistent pattern.

Moreover, often even airport reservations agents’ departure boards aren’t updated with actual real flight times. Sometimes a flight that shows boarding can be closed. More than once I’ve gone to a gate where it’s less than 10 minutes prior to departure and the door is still open.

(The last time, when I thought I’d missed a connection in Denver, I ran at top speed, only to have the gate agent laugh and ask, “What’s your hurry?” Adding, “We wouldn’t leave without you.”)

In these recent cases, I can understand why United wanted to close the door on the Dulles flight. It was already over an hour late, although I have to wonder — with empty seats and a valued customer, would it have made that much difference? As an added inducement to United, the traveler had an upgraded first class seat on the sold-out redeye flight, which no doubt they could have used.

One thought is that the crew could have been reaching their maximum hours and United either needed to close the door or get a new crew; though, if so, presumably the gate agent could have easily told the traveler.

The Denver to Bozeman flight problem makes less sense. It was the last flight of the day. It was United’s delay that made the connection tight. And, the connecting plane left on time, so it was less likely to be a crew issue.

Travelers understand that “stuff happens.” But, the apparent capriciousness of how airlines decide and when they will hold a plane, or even just keep the door open a few more minutes, is maddening, especially when gate agents give no explanation.

In a perfect world, I’d love to see some approximate standards for holding flights — especially the last flight of the night to anywhere — or at least more communication.

At this point, however, even a travel agent or reservations agent with access to up-to-date departure information still can’t tell exactly when an airline door is going to be closed or even always when it is already closed.

My rule: Ten minutes before departure time is when getting on a plane is no longer within your control. It’s better to be sitting or standing around the gate area waiting to board than looking for another flight or a hotel room.

So while it is within your control, run, don’t walk, to the gate.