SFO's long-problematic main runway, 28L, which is used for a majority of landings at the airport, is going to see the closure of its most heavily-trafficked section starting September 7, and the airport is starting the delay-warnings now.

The 20-day closure, which affects all flights taking off after 9 a.m. and ends on September 27, was announced back in March. The portion of the runway getting reconstructed — all the way down to the base layer of rock and concrete — is in the area where two other heavily trafficked takeoff runways intersect it, 1L and 1R. As airport officials explained, 68 percent of flights last year crossed through these intersections. The whole project is expected to cost $17.2 million.

This is the same runway, by the way, that saw a pothole open up on it in April that caused widespread delays for a good part of a weekday while it was getting repaired. And work to upgrade this runway dates back two years.

United Airlines sent out a "hard hat heads-up" via text to customers on Monday, suggesting that the construction "might lead to delays" and offering to switch flights for people for free if they want to avoid the construction period. As the Chronicle reports today, "Delays of 30 to 45 minutes on average are expected for all flights after 9 a.m., with some flights delayed by up to 2 hours."

In a statement, a United spokesperson said, "We know this could be a challenging time for our customers traveling through San Francisco, and we want to provide them the flexibility they need to adjust their plans and get to their final destinations."

The second biggest carrier at the airport, Alaska Airlines, hasn't made the same offer about switching flights.

SFO's aging infrastructure may be to blame for its delay issues, which are worse than average compared to other U.S. airports. About 22 percent of departing flights and 27 percent of arriving flights see some sort of delay at SFO, compared to 19.2 percent and 19.8 percent respectively, nationwide.