Paul Richter said the company didn’t expect to see wear in the middle section of the fuselage until the plane involved was much older.

When the company designed the 737-300, it expected the skin joint that failed to be robust enough not to require inspections for at least 60,000 pressurization cycles.

The cycles are the number of times a plane takes off and lands.

The Southwest jet that needed to make an emergency landing Friday had less than 40,000 cycles.

Southwest pulled nearly 80 planes for inspections and found five with similar cracks.

Richter says that it had given repair instructions on three of those planes.