Sheer pins
Newt,
As ShyTorque and NutLoose have referred, the sheer pin forms the weak link when towing, but only really effective when pulling. You can sheer the sheer pin when pushing, but you may not notice until the prime mover stops and the A/C continues. The sheer pins were manufactured from a fairly hard aluminium alloy and had a hole drilled through the centre. One end had a 'head' like a clevis pin, and the other end was cross-drilled for a split pin or similar to prevent the sheer pin from jumping out. They are manufactured in different ratings to reflect the weight of the aircraft being towed, and the ratings are identified by stamps on the head.
So, if the correct pin is fitted, can you damage the aircraft? Well, yes. In the incident Wensleydale referred to earlier, Shackleton AEW II WL 756 had its tail wheel assembly, including surrounding structure, ripped out. The correct rating sheer pin was installed in the tow bar at the time.
The aircraft were being put away into K17 hangar, a building that could accommodate three Shacks...if done correctly. 8 Sqn line had a Douglas Medium tractor (max tow weight 100,000 lbs) and the SOP was to tow the first one into the hangar, and at the last minute, swing the tractor to one side, just missing the far-end doors. This would ensure the tail of the aircraft would be as close as possible to the doors thereby leaving enough room to get another two Shacks into the hangar. If the first one wasn't in far enough, the third one would have its nose sticking out the other end.
On this particular occasion, the Douglas was U/S, so '56 was being towed in by the station Tug, a vehicle that due to its size was unable to do the 'Douglas swing'. The towing party stopped and opened the two centre doors to allow the Tug to drive straight through. Doors open and tug moves forward. Because the NCO I/C hadn't requested "brakes ON", he didn't request "brakes OFF" either. The awfully helpful chap in the cockpit had decided to play safe when the aircraft came to a halt and put the brakes on, just to be safe. Due to the noise of the Tug running in the hangar, none of the external towing team heard the air brakes operate (loud hissing like a lorry) and therefore were none the wiser. Because the Tug applied an even and smooth pull, and because the Shack towing arm had a very big spring, the forces wound up until the tail wheel structure overloaded. Fortunately, the bomb doors were fully open, and the aircraft settled onto the rear of the bomb doors. Because the bomb doors were hinged off the strongest longitudinal part of the aircraft, the longerons, the weight of the aircraft was supported by this strong path, thereby preventing major crush and ripple damage to the rear fuselage.
A sheer pin will fail quickly if subject to a 'snatch' load, and more slowly if the overload is gradual. I think most of the contributors in these pages have seen sheer pins in an almost 'S' shape when removed.
Camlobe