Tweet
I thought it worthwhile recording my experience with the tailwheel spring retaining bracket:
Background:
Some months ago during a short landing contest, I noticed that the retaining U-bracket which holds the back of the tail spring to the fuselage had become loose, despite being correctly torqued when installed. I assumed that the springs had settled-in with use, and that the bracket had not been sitting correctly to begin with.
In response to that, the bolts were re-tightened in the field. This was not done with a torque wrench because none was available, just torqued "by feel" until the bracket was tight again (read mistake was made!).
Effect:
Last weekend, one of these bolts failed during a landing on a rough strip.
The tailwheel had unlocked and spun around after initial touchdown (tail touched as I entered a wheel landing). The wheel had been facing diagonally forward when it touched the ground again, as I lowered the tail. Marks on the runway showed what had happened. The tailwheel re-aligned itself very rapidly upon contacting the ground, and put a high side-loading on the bracket as it did so. This high load caused the bolt to shear off at the threads.
Losing this bolt allowed the spring leaf attached to the tailwheel to swing out to one side as I struggled to steer the aircraft down a very small runway. The spring bent the retaining bracket out of it's way, until the spring interfered with the tailwheel steering horn and stopped moving (this also locked the rudder). I was still able to steer with differential brakes.
Probable Cause:
The failure surface was primarily brittle fracture, over 95% of which appears to be one final failure, by microvoid coalescence. This is almost certainly due to one large overstress event. There was a small fatigue crack which the final failure initiated from, with several large striations visible - but the crack only represented less than 5% of the failure area (damage from previous landings I assume). The fracture's shape shows it was single-direction bending stress which caused the crack, and a combination of torsional stress and bending which caused final failure.
All this tells me that the bolt was under a lot of stress when it failed, and that the nut was certainly overtightened.
Action taken:
I replaced the bolts with more of the same AN4 bolts, and torqued them correctly.
Reviewed with maintenance engineer - replaced washers with ground-back washers to allow the bolts to sit straight, the bend in the U-bracket was causing them to bend. Used a washer as a spacer to prevent the U-bracket from bending.
I suspect this is a once-off due to over-torque. However, the initial cause of the bracket becoming loose could also be strain on of those bolts. Has anyone else had similar experiences with that bracket becoming loose?
It has opened my eyes to the high operational stresses which tailwheel steering places upon the spring retaining bracket.
Background:
Some months ago during a short landing contest, I noticed that the retaining U-bracket which holds the back of the tail spring to the fuselage had become loose, despite being correctly torqued when installed. I assumed that the springs had settled-in with use, and that the bracket had not been sitting correctly to begin with.
In response to that, the bolts were re-tightened in the field. This was not done with a torque wrench because none was available, just torqued "by feel" until the bracket was tight again (read mistake was made!).
Effect:
Last weekend, one of these bolts failed during a landing on a rough strip.
The tailwheel had unlocked and spun around after initial touchdown (tail touched as I entered a wheel landing). The wheel had been facing diagonally forward when it touched the ground again, as I lowered the tail. Marks on the runway showed what had happened. The tailwheel re-aligned itself very rapidly upon contacting the ground, and put a high side-loading on the bracket as it did so. This high load caused the bolt to shear off at the threads.
Losing this bolt allowed the spring leaf attached to the tailwheel to swing out to one side as I struggled to steer the aircraft down a very small runway. The spring bent the retaining bracket out of it's way, until the spring interfered with the tailwheel steering horn and stopped moving (this also locked the rudder). I was still able to steer with differential brakes.
Probable Cause:
The failure surface was primarily brittle fracture, over 95% of which appears to be one final failure, by microvoid coalescence. This is almost certainly due to one large overstress event. There was a small fatigue crack which the final failure initiated from, with several large striations visible - but the crack only represented less than 5% of the failure area (damage from previous landings I assume). The fracture's shape shows it was single-direction bending stress which caused the crack, and a combination of torsional stress and bending which caused final failure.
All this tells me that the bolt was under a lot of stress when it failed, and that the nut was certainly overtightened.
Action taken:
I replaced the bolts with more of the same AN4 bolts, and torqued them correctly.
Reviewed with maintenance engineer - replaced washers with ground-back washers to allow the bolts to sit straight, the bend in the U-bracket was causing them to bend. Used a washer as a spacer to prevent the U-bracket from bending.
I suspect this is a once-off due to over-torque. However, the initial cause of the bracket becoming loose could also be strain on of those bolts. Has anyone else had similar experiences with that bracket becoming loose?
It has opened my eyes to the high operational stresses which tailwheel steering places upon the spring retaining bracket.
Comment