(CNN) -- Boeing says it has notified all 767 aircraft operators about cracks found on two American Airlines 767-300 planes, and by mid-July will recommend more frequent maintenance checks.
American Airlines found the cracks in large pylons that hold engines onto the wings of two of its 767-300 aircraft, which Boeing currently recommends inspecting every 1,500 takeoff and landing cycles.
"Boeing will recommend decreasing the number of flight cycles between inspections of the pylon mid spar fitting from every 1,500 flight cycles to possibly as low as every 400 flight cycles," the company said late Wednesday.
About 260 planes constructed in the same manner as the damaged American Airlines aircraft could be affected by the recommendation, Boeing said.
American Airlines spokesman Tim Wagner said the problem was first discovered on one of its 767-300s about two weeks ago during a maintenance check targeting other parts of the aircraft. The airline then found cracking on one more 767-300 after inspecting a total of 56 aircraft -- 767-300 and 767-200s -- a process American Airlines completed Monday night.
"We found these issues when it's best to find them, which is early on," said Wagner. The airline has sent the first damaged pylon to a metallurgy lab to try to determine why it cracked.
"Until that metallurgy testing is completed no one knows what the cause is," said Wagner. The damaged planes were among American's older aircraft, Wagner added.
American flies Boeing 767-300s on international routes as well as long-haul domestic routes. The 767-200s fly primarily transcontinental routes.
Airlines have previously found similar cracks at the holes where the pylon attaches to the wing, which led Boeing and the Federal Aviation Administration five years ago to recommend checks every 1,500 cycles. At that time, when the FAA issued an airworthiness directive it said, "We are issuing this (directive) to prevent fatigue cracking in the primary strut structure and reduced structural integrity of the strut, which could result in separation of the strut and engine."