The latest in the Boeing 737 story has developed wider than the all-new 737 MAX model, at least in the public eye.
Further product problems are now surfacing with the former aircraft to the 737 MAX, the 737 NG – shortened for New Generation.
The Boeing 737 NG model was launched to airlines in 1993 and is the predecessor to the currently troubled MAX as the third-generation of the Boeing 737, and has been manufactured and delivered to customers since 1996. The model sold over 7,000 variants all over the world.
The NG series includes four models, the −600/-700/-800/-900; for Boeing Business Jet clients namely -700 BBJ, –800 BBJ (BBJ2) and -900ER (BBJ3) variants.
What’s the latest issue?
The aircraft manufacturer Boeing has discovered cracks in a vital part of its Boeing 737 NG aircraft that connects the body of the aircraft to its wings, referred to as the ‘Pickle Fork’.
The problem was initially thought to effect aircraft that had made 30,000 flights or more.
However, in recent days, airlines such as Qantas and Southwest have found cracks in aircraft that have flown 27,000 and 28,500 flights respectively.
The crack found by Qantas is reported to be “one inch long”.
What is a pickle fork?
The pickle fork is a major stress bearing structure interconnecting the fuselage and the wing, technology prior to a design and approval of an active suspension system.
The pickle fork is an assembly named so because of its resemblance to the kitchen utensil, which attaches firmly to each side of the fuselage and has two prongs that extend below it where they are attached to the wing spar.
There are 4 pickle fork assemblies on a Boeing 737 aircraft, positioned fore and aft of the wing respectively on each side of the aircraft.
Since they connect the fuselage (body) of the aircraft to the wing spar (wing) and take both lateral and vertical loading forces for every movement of the aircraft; from taxi, take-off, landing and in-flight.
Stress transmitted from aircraft wings into the fuselage via the wing-to-body joint is a significant concern in aircraft design, since it affects the strength, durability and other aspects of the aircraft.
Previous designs using rigid wing attachment points present limitations to the fuselage/wing construction and sizing due to deflections imposed upon the fuselage by wing bending.
According to a Boeing flied patent on the design currently, there are several common structural configurations for joining the main wing to the fuselage of a commercial airplane. Two common variants of fixed wing-to-body joints are shown in FIGS. 2 and 3. Shown in FIG. 2 is a cross-sectional view of an aircraft 200 having a main wing 202 and a fuselage 204. This aircraft 200 includes what is called a “pickle fork” fitting at the front and rear main spars of the main wing. In the cross-sectional view, a main spar 212 of the wing is visible, to which a pair of pickle fork fittings 210 are fixedly attached.
The pickle fork fittings 210 extend upward into the side wall of the fuselage 204, and thus transmit horizontal and vertical forces, represented by arrows 214, and bending moments, represented by arrows 216, from the wing 202 into the fuselage 204. Because of this configuration, when the wing 202 deflects, as shown in dashed lines at 220, the pickle fork fittings 210 also deflect, as shown in dashed lines at 218, causing corresponding deflection and deformation of the fuselage 204. It is to be understood that the magnitude of deflection of the wing 202 shown at 220 and of the pickle fork members 210 shown at 218 in FIG. 2 may be exaggerated for illustrative purposes.
Some aircraft employ a combination of a “pickle fork” fitting at the front spar, a trap panel at the rear spar, and a “flex-tee/Pi-fitting” over-wing attachment. The term “trap panel” is well known to those of skill in the field of aircraft structures and is short for trapezoidal panel. A trapezoidal panel is a panel that is attached to the rear spar of the main wing, in line with the fuselage skin. Its purpose is to transfer loads between the fuselage and the wing. Trap panels are one step closer to a wider spread load exchange between the wing and the fuselage.
Is Boeing concerned?
Pickle forks are designed to last the lifetime of a plane, so the discovery is alarming to both Boeing, the wider aviation industry; and ultimately owners, operators and customers.
It’s a big deal considering the fact it was discovered not at scheduled maintenance, but by chance as part of a selected aircrafts custom conversion program.
It is likely that heavy landings are the main cause of the problem, although official reports are yet to be made public as of publishing.
Since about 90,000 or more take-offs and landings is considered ‘lifetime’ of an aircraft, the 30,00 flights or less is more than concerning for Boeing.
Boeing reported the issue to the FAA after it discovered the cracks while modifying a “heavily used aircraft,” according to the statement. The company said it found similar cracks in a small number of other planes, as well.
What is Boeing doing about it?
The Federal Aviation Administration has ordered operators to inspect the planes for structural cracks.
Up to 50 737NG planes around the world have been grounded so far, inspections are taking place and reports being filed.
Its only when these investigations and route-cause analysis has taken place can Boeing and its customers begin to understands the cause(s), and then the potential and/or recommended solution(s) for the issue.
Experts previously said that the presence of a pickle fork crack “does not immediately compromise the safety of the aircraft,” Australia’s ABC News reported.
Should BBJ owners be worried?
With far fewer flights that airline variants, BBJ customers would on the face of it have less to worry about. However, concerns should be directed to your BBJ aircraft flight department and maintenance provider for professional advisory on the matter.
However, should your aircraft have been pre-airline owned, or have flown as considerable number of flights approaching the 30,000 flights reported by Boeing as a concern, then it’s of paramount importance to content your aircrafts flight department and maintenance provider for professional advisory.