By Bjorn Fehrm
October 1, 2019, ©. Leeham News: The FAA has issued an Air Worthiness Directive (AD) for high time Boeing 737 NGs, requiring immediate inspections for cracks in their wing attachments called pickle forks.
The cracks were discovered on high time aircraft which were torn down for conversion to freighters. The affected 737 types are NG only; the MAX and Classic have a different wing attachment design. The P-8 Poseidon, a derivative of the NG, also is unaffected.
The issue was first reported by Charlie Harger of KOMO radio station in Seattle. He reported cracks were found in the reinforced fuselage frames, which are the main attachments of the wing to the 737 NG fuselage. The frames are placed at the forward and rear wing spars where their lower parts are used to transfer most of the load from the wing into the aircraft’s fuselage.
These frames have reinforcements called “pickle forks,” as the reinforcements attached to the frames look like a cocktail pickle fork, Figure 1. In total the aircraft has four pickle forks, one on each side of the forward attachment frame and one on each side of the rear attachment frame.
The wing is also attached with the frames meeting the wings top surface between these attachment frames. These fuselage frames are not designed to carry much load, however, as the wing’s top surface is not a good attachment point for the fuselage.
The attachment frames with their pickle fork reinforcements are a so-called safe life design (see below for more on this) which means they shall last well above the life of the aircraft, in this case, 90,000 flights. The cracks were found on ex-Jet Airways of India passenger planes which were converted to freighters for Amazon with more than 33,500 flights behind them. When doing these conversions the airframes are stripped bare and the cracks were found.
The issued AD affects Boeing 737 NG aircraft with over 22,600 flight cycles (flights). These shall be inspected within one year. For aircraft with more than 30,000 flight cycles, the inspection shall be completed within one week from the effective date of the AD.
A typical 737 NG flies 2,000 flights per year so the AD affects aircraft aged 11 years or more. From 737 NGs of 15 years or more, the AD is critical, the inspections which are done with a boroscope and takes one hour shall be done within a week or the aircraft is no longer allowed to fly.
Cracks due to age are found all the time in our airliners. As they conduct the takeoff, flight and landing (a flight or Flight Cycle), their structures are subject to stresses which fatigue the metal over time. The airframes are designed to handle this metal fatigue either with redundant load paths, a so-called fail safe design, or a design with very low stress levels giving very long life, a so-called safe life design.
The prescribed maintenance of the aircraft has inspections of the structure at regular intervals to find any issues like cracks which develop in the airframe before these become dangerous. The major tear down for the 737 NG, the so-called heavy checks, are at six and 12 years after entry into service. These are specially designed to search for cracks and other issues affecting the aircraft’s structure.
The 737 NG pickle forks are a safe life design, according to our information. This is what makes the cracks worrying. They shouldn’t be there on an airframe with only 37% of its design life behind it.
What can be the cause of the cracks? Here some first thoughts before we have more information from Boeing and the FAA:
These are the kind of questions Boeing and the FAA are seeking the answer to. Important information in this search for the root cause is the results of the ordered inspections. So far three aircraft have had cracks. Fifteen aircraft had been inspected through yesterday.
The flight spectrum and production numbers of the aircraft with cracks will be an important lead to the root cause of the issue and what further actions shall be taken.