EXPERT REACTION: Cracks in Qantas planes

What is not clearly understood is that all mechanical devices, cars, bridges, ships, aeroplanes etc. all have structural cracks. The real question is how design engineers manage these phenomena.

In the case of aircraft, the engineer designs the structure to ensure a crack is detected before it becomes critical. To ensure this occurs an inspection procedure is designed to make sure the crack is found before it becomes critical.

The significant factor is the rate at which the crack grows, usually this is easy to calculate. If the crack might grow such that it could lead to a catastrophic failure then it has to be remediated before continued operation. 

The size of the allowed crack is clearly stated in the maintenance manual and signed off by the design authority and the regulator, CASA, FAA etc. Thus, the detection of not-critical cracks means the safety system is working. What we do not want to see is a failure with no warning.

The cracks that have been found in the B737 NG’s are in an area called the 'pickle fork'. The pickle fork connects the centre section of the fuselage to the rear spar in the wing. The rear spar helps the wing react to lift loads.

There is an airworthiness directive issued by the FAA to inspect this area for cracks on all B737 aircraft if the aircraft has undergone 30,000 flights or more. There are two pickle forks on the rear spar. The inspection should take about an hour for each aircraft.

The design is what is called 'fail safe' which means the structure can tolerate some level of damage or degradation for example due to cracking. The inspection period is set to ensure the cracks do not continue to grow to a dangerous length and then significantly compromise the strength of the airframe.

Modern Jet transport aircraft are all designed to be damage tolerant and this ensures integrity of the airframe provided the directed inspections are carried out when specified and repairs are made if cracks are found.