The major tear down for the 737 NG, the so-called heavy checks, are at six and 12 years after entry into service. 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 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. 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. How dangerous are cracks in our airliners?Ĭracks due to age are found all the time in our airliners. 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. For aircraft with more than 30,000 flight cycles, the inspection shall be completed within one week from the effective date of the AD.Ī typical 737 NG flies 2,000 flights per year so the AD affects aircraft aged 11 years or more.
These shall be inspected within one year. The issued AD affects Boeing 737 NG aircraft with over 22,600 flight cycles (flights).
When doing these conversions the airframes are stripped bare and the cracks were found. 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. 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. 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 wing is also attached with the frames meeting the wings top surface between these attachment frames. Source: Leeham Co based on a patent drawing. The influence of a flexing wing can be seen in the picture. 737 NG pickle forks reinforce the frames attaching to the forward and rear wing spars. 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.įigure 1. These frames have reinforcements called “pickle forks,” as the reinforcements attached to the frames look like a cocktail pickle fork, Figure 1.
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. He reported cracks were found in the reinforced fuselage frames, which are the main attachments of the wing to the 737 NG fuselage. The issue was first reported by Charlie Harger of KOMO radio station in Seattle. The P-8 Poseidon, a derivative of the NG, also is unaffected.Ĭracks develop at one third expected lifetime. The affected 737 types are NG only the MAX and Classic have a different wing attachment design. The cracks were discovered on high time aircraft which were torn down for conversion to freighters. 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.
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