US-based operators of next-generation Boeing 737 aircraft could face costs of up to $10.1 million to comply with a new airworthiness directive (AD) issued by the US Federal Aviation Administration today.

The administration is issuing the AD for 737-600, -700, -700C, -800, -900 and -900ER aircraft after the FAA found that incorrect procedures were used when applying protective coatings for wear and corrosion to the attach pins of the horizontal stabiliser rear spar.

The AD will require operators to undergo an inspection on the aircraft to check the part numbers of the pins on the horizontal stabiliser rear spar and to replace certain pins with "new, improved" attach pins.

"We are issuing this AD to prevent premature failure of the attach pins, which could cause reduced structural integrity of the horizontal stabilizer to fuselage attachment, resulting in loss of control of the airplane," the directive states.

The FAA estimates that inspection and replacement of the attach pins would cost $9,627 per aircraft, with a total of cost of up to $10.1 million for the entire US-based fleet of the affected 737s.

The FAA estimates the directive will affect 1,050 aircraft registered in the USA. A substantial portion of these 737s are operated by Southwest Airlines, which had 458 737-700 and -800 aircraft at the end of 2012 according to regulatory filings. United Airlines flies 237 next-generation 737s, data from Flightglobal's Ascend Online database shows. Alaska Airlines has 97 737-700s, -800s, -900s and -900ERs and Delta Air Lines operates 83 737-700s and -800s according to fleet data provided by the airlines.

The AD is effective from 20 May. The AD follows a notice of proposed rulemaking issued by the FAA on 12 September, 2012.

Source: Air Transport Intelligence news