History of the Flight
The aircraft was returning to Nottingham after a local flight. The pilot set up a visual approach to Runway 09 with full flap selected. Reported wind was from 090° at 15 knots. The pilot described the landing as a "hardish landing," touching down almost on the runway identification numbers with the stall warning horn activated. He applied brakes, then released them as he initiated a right turn to exit the runway onto the first taxiway. The pilot estimated groundspeed at approximately 10 knots when he began the turn. As the aircraft started to turn, the left main landing gear collapsed, causing the left propeller and left wing to strike the ground. The aircraft came to rest on the runway. The pilot reported the collapse to the control tower, completed shutdown procedures, and evacuated with his two passengers. An eyewitness described the landing as "firm but nothing serious" and estimated groundspeed at the start of the turn between 20 and 40 mph (17 to 35 knots). The distance from the estimated touchdown point to where the gear collapsed was approximately 240 metres (790 feet).
Maintenance History
On 18 December 1995, the aircraft (then registered F-GGGG) suffered a wheels-up landing at East Midlands Airport (see AAIB Bulletin 2/96). Re-registered as N234SA in March 2002, it was eventually repaired and issued an FAA Standard Airworthiness Certificate on 27 November 2002. Repairs included removing wings, replacing the fuselage front spar, belly skins, and damaged ribs. Main landing gear doors were repaired, but no repair work was carried out on the main landing gear itself. Worksheets noted that the main landing gear was checked and rigged according to the service manual, and several gear retraction tests deemed the system satisfactory. The collapse occurred on the aircraft's fourth flight and fourth landing after repair.
Landing Gear Mechanism
The main landing gear on the Cessna 310 is held down and locked by a side brace rigged to an over-centre position. A downlock link applies pressure to keep the side brace in that position. The rigging procedure is complex, covering nine pages in the service manual. The manufacturer states that small deviations from the procedure can cause the landing gear to collapse when the side brace comes out of its over-centre position.
Aircraft Examination
After recovery, the AAIB inspected the aircraft. Damage included left wing tip and left horizontal tail, and bent left propeller tips. The downlock link supporting the left gear lower side brace had failed at the threaded portion. The pivot bolt securing the downlock link to the side brace was bent, and the bush through which it passed was plastically deformed downward. Minor additional damage was noted inside the left main landing gear wheel well.
Metallurgical examination showed the downlock link failed from bending overload. The overload was applied through the pivot bolt, which itself plastically bent. Vickers hardness tests on undeformed regions of the threaded stud and pivot bolt indicated tensile strength consistent with normal values (980 N/mm²). The length adjustment of the threaded stud was almost at its minimum, providing minimal movement of the side brace into the over-centre position.
Landing Performance
Landing performance calculations assessed whether the pilot's reported 10-knot turn speed was achievable given the short distance to the taxiway. Aircraft weight at touchdown was approximately 4,700 lb. Runway elevation 138 feet amsl, temperature 9°C, wind 15 knots from 090°. Predicted landing ground roll using full flap and maximum braking was 420 feet (from Cessna 310R Flight Manual). The distance from runway numbers to collapse point was about 790 feet, allowing sufficient distance to slow to 10 knots with heavy braking.
Landing Gear Side Load Limits
No published ground turning speed limitation exists for the Cessna 310R. The manufacturer stated side loads during turns are very small compared to landing side loads during a crab or side-slip. They could not state a turning speed limit as loads depend on turn sharpness, not just speed. They did not think a turn at 20 knots would cause collapse from excessive side load.
Regulations (BCAR and FAA 23.485) specify a limit side inertia factor of 0.83 g. This corresponds to speed limits of 30 knots for a 100-foot radius turn or 37 knots for a 150-foot radius turn. Exiting the angled taxiway (120° from runway) could be done with a 150-foot radius, requiring at most a 100-foot radius if turn initiation was late. Therefore, the aircraft should sustain at least 30 knots before exceeding certification loads, and likely would skid before reaching 0.83 g.
Analysis
The overload failure of the downlock link was consistent with collapse following the side brace coming out of its over-centre position. Under load, an unlocked side brace would push against the downlock link, bending the pivot bolt and eventually failing the link.
The Cessna 310R main gear rigging procedure is involved and must be followed completely. The manufacturer warns that small adjustments can result in gear collapse. The threaded stud adjusted to near-minimum length provided minimal movement against the side brace, meaning any slack could cause unlocking and collapse.
Landing performance allowed the aircraft to slow to 10 knots before the taxiway after touching down near the numbers. The aircraft may have been travelling faster (as witness reported), but extra speed should not have collapsed a properly rigged gear. It is likely that side load during the turn, combined with a slightly out-of-rig left main landing gear system, caused the side brace to unlock and the gear to collapse.