Excerpts from
NTSB Report on N611FE accident in 1997, which includes a Submission of Proposed Findings by Boeing’s Long Beach Douglas Products Division:
Because a fuse [pin] in the vertical plane may not prevent substantial loads from
entering the wing structure once the fuse has released, and because the review of
historical data indicated that failure due to overload was most likely to occur as a
result of high drag loads, a different approach was taken to assure fuel tank
integrity for the high vertical load (above 2.0 g’s) condition. For vertical loads
above 2.0 g’s, the [MLG] is not designed to separate from the wing. Instead, the
landing gear and its back-up structure are designed to be very robust, i.e., they are
designed to withstand significantly greater descent rates than the 12 fps (ultimate)
required per Part 25.723 (b). Analysis has indicated that for a maximum landing
weight, typical-landing-configuration landing, the MD-11 [MLG] can withstand
up to a 16.9 fps descent rate without bottoming the shock struts or failing its
backup structure including the wing rear spar. Similarly, for a rolled landing (8
degrees one-wing-low attitude, with lift equal to aircraft weight), the landing gear
can withstand up to 15 fps descent rate without bottoming the shock strut or
failing its back-up structure including the wing rear spar. 57
For ‘fused’ aircraft the (remaining) energy of vertical descent would then be
absorbed by flexing the low-side wing, or by some combination of exercising the
high-side landing gear, and flexing the low-side wing. For some combinations of
sink and roll rates the low-side gear may fuse (followed by the wing
engine/nacelle) and the aircraft may ‘settle in’ on the remaining gear and the lowside
wing without compromising fuel tank integrity. For higher sink and roll rates
(or lower amounts of wing lift) the low-side wing may fail nonetheless, as a result
of exceeding its flexure (bending) limits.
57 Boeing further stated in its submission that it had “begun an evaluation into the net safety benefit of installing a fuse for vertical overload in the DC-10 and the MD-11 [MLG]…that could take a year or more to complete.” Boeing also stated that it would include the Newark accident scenario in its study of the potential safety benefits of vertical fusing.
...it is most probable that, as a result of loads applied to the right [MLG] that were
substantially beyond design limits, the right wing structure failed. The failure
most probably initiated at the rear spar/bulkhead (trunnion) rib interface and
progressed through the primary wing box structure. As a result of this failure, the
right main gear trunnion moved substantially upward and aft with respect to the
trap [trapezoidal] panel fitting. This motion was sufficient to cause the fixed side
brace to bind against the pillow block footing, tearing the pillow block loose from
the trap panel. 63
63 The Boeing submission also noted that, according to simulations, “subsequent to the failure [of the spar web structure], the right wing twists substantially nose-down under the imposed loads. This twisting causes the right wing to ‘dump’ most of its lift and results in a sudden and substantial outboard motion of the right main gear bogie, caused by the fixed and folding landing gear side braces pivoting about their (common) attachment at the trap panel fitting attachment point.”