Originally Posted by
onetrack
Sounds like there's a possibility someone might have specified an incorrect steel specification for the replacement tie rods?
The only way threaded sections fracture in this manner, is if;
1. The steel used does not have adequate tensile strength ..
2. The steel used is too brittle and does not have adequate elongation abilities under load stress ..
3. The steel used is affected too readily by minute amounts of corrosive attack, thus creating a reduction in cross-section thickness (and surprisingly to many, high tensile steels corrode much more readily than low-grade steels).
H37869A - Fuselage Joint "H" Tie Rod
There's also a major difference between hot-rolled threads and threads cut on a lathe. Hot-rolled threads are inherently stronger, because the grain in the steel follows the thread form - whereas a lathe cuts the grain, and thereby weakens the grain structure.
It is interesting for me to come across this today in Stuart Mackay's Tiger Moth book:
One Cowley built Tiger Moth did come under scrutiny in the late summer of 1941 when operating with 17 EFTS Peterborough. In the rear seat, the pupil heard a loud crack and watched bemused as the starboard lower wing detached from its root end pickup on the lower longeron. The instructor only realized his pupil had bailed out when he saw the descending parachute, at which point the aircraft broke up and he too abandoned what had become a spinning fuselage, which devoid of all wings, landed on a house in Peterborough town alongside the engine which had already buried itself in the garden.
Flying at 17 EFTS was suspended for two days while the fuselage tie rods of the entire fleet were examined in detail partly by engineers wielding magnets. There was some suspicion that during assembly or later maintenance, tie rods manufactured from Dural instead of high tensile steel had been installed or substituted and the whiff of sabotage as much as carelessness in quality control was prevalent, although the engineering staff was never advised of the outcome.