So is this it for Logan Airport?

Massport, the government entity responsible for running Logan, has proposed a new airstrip. Predictably, a number of Boston residents, particularly those in East Boston, are strongly opposed. The Boston City Council, US Representative Michael Capuano and now Boston Mayor Tom Menino, have piled on.

The new runway, once thought to be an easy-to-accomplish feat (although in Boston politics, "easy" is a relative term), now faces a challenge that could derail it.

The truth be told, the notion behind the runway is not all that bad.

Logan is not particularly well designed. Under normal conditions, it has three runways available at one time for landing and departing jets. But when the wind shifts, any of those runways can become unusable (planes can't land and take off in crosswinds). Planes then need to land and take off from a different direction.

With three runways operating, Logan can accommodate 120 operations (each takeoff or landing is an operation) per hour. But a strong wind from the northwest can force Logan to use fewer runways.  If two runways are used, the number of operations drops to 90. If one runway is available, the number of operations drops even lower, to 60.

So with one shift in the wind, the airport may suddenly find its capacity to handle air traffic cut virtually in half.

The new airstrip is designed to cure this problem. It would give the airport a second large, effective runway that runs in the northwest/southeast direction. In theory, the purpose of the new runway is to reduce air traffic delays, not to expand the airport's overall capacity. In theory, the new runway only exists to manage traffic that otherwise would be circling the skies, waiting for a runway to open. In theory, the planes would only fly in and out over the water, disturbing few.

In theory. The problem is that theory and practice sometimes don't match, and there's good reason to fear they won't this time either.

Even though the alleged purpose of the runway is to reduce traffic delays when the wind shifts, Massport talks relentlessly about how the airport will continue to expand. Massport officials project that the number of travelers will increase every year for the foreseeable future by about 3.5 percent. That means that the number of people flying in and out of Logan Airport will double in 20 years.

So what's the purpose of the new runway? To handle a particular meteorological problem or to handle new growth? It's the growth issue that has everyone's nose out of joint. The airport is almost at capacity right now. New growth will require more expansion. New growth will also put more aircraft over the streets of Boston. Make no mistake about it: aircraft noise is loud and aggravating; if you don't need sleep and don't need to talk or hear people talking, it may not bother you. Otherwise, it can wreck your life. It's the kind of quality of life issue that most of those flying in and out of Logan would never tolerate around their own homes.

Massport claims it has a regional transportation strategy. It claims that over the next few years it will expand the use of other airports in the area. But Massport won't commit to zero growth at Logan.

And so the suspicions remain: the new runway is about growth, not traffic delays. A few years from now, Massport will be back with new proposals for more capacity. Ten years from now Hanscom (or some other secondary airport) will be no closer to reality than it is today.

Those are the suspicions that are behind Boston's considerable opposition to the new runway. Those are the suspicions Massport must allay. So far, though, the suspicions remain unanswered.