[bearoutwest]

Important to look at heavy landings with wheels down separately to force landings with wheels up (either crash landings on land or ditching into the sea).

Wheels down means the fuselage is supported on two points – 1st in line with the two main wheels, and 2nd on the tailwheel. The fuselage then bends between the 1st and 2nd. The weakest part of the structure suffering the most bending load is more likely to fail under this circumstance.

Wheels up landing means the fuselage is supported by many points of contact with the ground (or water), so the bending load is different and affects the fuselage less than with wheels down. If the fuselage breaks up, it may be from different reasons.

The weak point on the fuselage occurs not necessarily from improper weight distribution, but rather to the dimensions of the fuselage. The wider/taller sections of the fuselage will have more space to carry load and also have more spacing for more efficient structural design. The narrower sections of fuselage carry less load as they have less space, but will also be less efficient in structural design strength. At some stage the change in fuselage profile causes a weak point. With the Fw200 it was near the trailing edge of the wing.

I don’t know the actual design loads for the Fw200, but let’s make up some numbers to demonstrate what I mean.
- Say a Fw200 carrying 2000kg of bombs and fuel, is capable of managing a 4g pull-out from a glide attack (i.e. 4 times the weight in gravity), with a “proper and correct ” weight distribution. It should be able to do this 10,000 (perhaps 100,000) times without any fatigue damage to the structure. If it has suffered microscopic cracking during this time, then the weakened (cracked) structure might only be able of sustaining a 3g pull-out. The next 4g pull-out might weaken it more…….eventually it fails.
- Say another similarly loaded Fw200, has to pull 5g to pull-out of an attack. As the structure is designed for only 4g, it is now damaged (either cracked or stretched but differently to fatigue cracking). It may not necessarily fail immediately, but it may only now be able to sustain 3g as a maximum pull-out, etc. It will also eventually fail, if it continues to be used on normal operations.
- Say a third similarly loaded Fw200 has to sustain an 8g pull-out while avoiding a fighter attack (i.e. twice the design load). It may well fail immediately, even without any other damage from the fighter attack.

This pull-out load is based on centrifugal force, and bends the aircraft fuselage in the same manner as when it sits on the ground on its wheels. So heavy landings can also add to the damage.

Note that these are not the only forces acting on the fuselage, but are the only ones discussed in my “simplified” example.