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Our tail spring snapped just after landing in the weekend. We were on an especially rough airstrip, but the tail just broke under a moderate rolling load, nothing special.
IMG_20190329_174606.jpg
If we examine the failure surface, we can quickly see the "beach marks" (fatigue striations) which originates from cracks on the lower surface. These are a tell-tale sign of a fast growing crack, experiencing high stresses, which is not surprising. This is the bottom of the lower-most spring, so it's likely that rocks flicked up by the wheels have impacted the spring, created micro-cracks in the spring which have grown quickly. This was after 668 hours service. The cracks were probably not that old.
What is surprising is the size of the large grey area, perhaps 80% of the spring's cross-sectional area. This is the "final failure area", which tears apart when the metal is under high stress. It appears dull instead of shiny because of the tearing metal (called micro-void coalescence). The point is, this spring leaf is regularly stressed to 80% of it's breaking load (ultimate tensile strength) during an off airport landing. That is interesting.
Clearly the design is fit for purpose, as no damage was done and we flew home.
We were in a remote location with a considerable load aboard, so after finishing our hunting trip we had to delicately get the plane into the air and gingerly land back at home base. We jury-rigged the tail with a 600lb strap we had aboard, which after 20 wraps was more than strong enough to take some weight. It was a non-event though, as I was able to lift the tail in less than 10 metres under full power / full flaps for take-off. We landed and I kept the tail up until we had taxied off the runway, then placed the tail down carefully whilst at a standstill.
If you land tail-first or off-airport a lot, then you may wish to consider changing your tail spring every 500 or 600 hours.
Or perhaps inspecting it for cracks with dye penetrant. This is probably what I will do.
Or just live with the knowledge, the two remaining spring elements are strong enough to get you home if treated carefully.
Go Bearhawk!
(Edited for typos)
IMG_20190329_174606.jpg
If we examine the failure surface, we can quickly see the "beach marks" (fatigue striations) which originates from cracks on the lower surface. These are a tell-tale sign of a fast growing crack, experiencing high stresses, which is not surprising. This is the bottom of the lower-most spring, so it's likely that rocks flicked up by the wheels have impacted the spring, created micro-cracks in the spring which have grown quickly. This was after 668 hours service. The cracks were probably not that old.
What is surprising is the size of the large grey area, perhaps 80% of the spring's cross-sectional area. This is the "final failure area", which tears apart when the metal is under high stress. It appears dull instead of shiny because of the tearing metal (called micro-void coalescence). The point is, this spring leaf is regularly stressed to 80% of it's breaking load (ultimate tensile strength) during an off airport landing. That is interesting.
Clearly the design is fit for purpose, as no damage was done and we flew home.
We were in a remote location with a considerable load aboard, so after finishing our hunting trip we had to delicately get the plane into the air and gingerly land back at home base. We jury-rigged the tail with a 600lb strap we had aboard, which after 20 wraps was more than strong enough to take some weight. It was a non-event though, as I was able to lift the tail in less than 10 metres under full power / full flaps for take-off. We landed and I kept the tail up until we had taxied off the runway, then placed the tail down carefully whilst at a standstill.
If you land tail-first or off-airport a lot, then you may wish to consider changing your tail spring every 500 or 600 hours.
Or perhaps inspecting it for cracks with dye penetrant. This is probably what I will do.
Or just live with the knowledge, the two remaining spring elements are strong enough to get you home if treated carefully.
Go Bearhawk!
(Edited for typos)
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