The discovery of unexpected fatigue cracks in at least four American Airlines Boeing 767s in the last month has cast a shadow over the widely used twin aisle jet.

Qantas is understood to be ‘alert but not yet alarmed’ by the US reports, and is seeking clarification from Boeing. It operates 26 of the jets in configurations of around 250 seats, mainly on east coast CityFlyer services.

The Qantas 767s were due to be replaced by Boeing 787 Dreamliners from late 2008, but it may not receive that much-delayed plastic jet until 2015, and only after its low cost brand Jetstar receives its first 787s about a year earlier.

US sources claim the Federal Aviation Administration is drawing up a mandatory set of inspections for all 767s to be announced in the near future in relation to engine pylon cracks found in two of 54 American Airlines 767s.

One of the pylon cracks had grown to a stage where it could have broken apart, dropping the engine off the wing.

In April 2001, dangerous pylon cracks were found within a fleet of  five very old and improperly maintained Ansett 767s, setting the scene for their infamous Easter grounding after yet another of the jets was found to have made multiple inter city flights with defective emergency door slides. It isn’t known if the American Airlines pylon issue is the same as the problem at Ansett, but like Ansett, American is a serial offender in terms of maintenance errors and has been repeatedly fined millions of dollars by the US safety regulator.

However, different surprise cracks were also detected in the main wing spar of several of American’s 767s which have been fitted with large heavy winglets  or ‘horns’ to smooth the aerodynamic performance of the wing over medium to longer distances, causing a net saving in fuel burn.

These optional winglets have long been widely used on Boeing 737s, but the larger version now being fitted to some 767s are of more recent design. They have not been adopted by Qantas, which now uses its 767s for primarily short routes but are being fitted to some Air New Zealand 767s that are flown for longer distances.

The FAA and Boeing are understood to be investigating these wing cracks to determine if they are related to the normal processes of airframe aging or are an unintended consequence of additional stresses caused by winglet installations.

It is a situation that is being studied with intense interest at Qantas, and just possibly at Virgin Blue, which is rumoured to be considering acquiring 767s to lift its domestic and regional international capacity above the 180-189 seat limit of its Boeing 737-800 fleet.