Synopsis
On 23 August 2016 at 1200 UTC, a Vans RV-9A (registration G-RPRV) experienced a nose landing gear collapse while landing at Nympsfield Airfield, Gloucestershire. The aircraft subsequently inverted. The sole occupant, the pilot, sustained minor injuries. The aircraft sustained substantial damage to the nose landing gear, propeller, canopy, upper fin/rudder assembly, and right wingtip.
History of the Flight
The pilot flew from Kemble Airfield to Nympsfield. Prior to the flight, he reviewed a briefing document from the resident gliding club's website, which described the airfield as 'a less than smooth grass field with various slopes along its length and width.' The document included an aerial photograph marking an 'unlandable' area adjacent to a track used by glider winch vehicles, and recommended that landings be made midway between the track and the northern airfield boundary.
Weather conditions were good and the surface appeared dry. The pilot touched down normally on a north-easterly heading and reduced speed. There were no markings to indicate the extent of the 'unlandable' area, but the pilot assessed that he remained clear to the north of it. He noted going over a hump on the runway surface and believed it possible that he instinctively moved the stick forward at the top of the hump, rather than keeping it fully back, while moving on rough ground. He estimated his groundspeed had reduced to approximately 20 kt by this stage.
Without warning, the nose of the aircraft dropped, the propeller hit the ground, and the aircraft flipped upside down. The pilot was uninjured and turned off the electric master switch before reaching for the fire axe. The canopy had been damaged, and the pilot used the axe to cut a hole through which he crawled out of the aircraft.
A check of the landing surface did not reveal any specific holes or troughs. It was apparent that the nose landing gear leg had failed and bent back on itself, reducing the propeller's ground clearance.
Gliding Club Comment
The gliding club explained that along a portion of the airfield, the grass slopes steeply either side of the track used by glider winch vehicles, which is why that area is marked as 'unlandable.' This area extends approximately 10 metres to the north of the track but might appear more extensive on the photograph in the club's document. The club confirmed that the accident aircraft did not encroach the 'unlandable' area and updated its briefing information for visiting pilots following this accident.
Previous AAIB Investigations
The AAIB has published reports on 131 previous occasions where the nose landing gear of a Vans RV-6A, 7A, 8A, or 9A has failed. Seven of these occurred while landing on soft or undulating grass and one occurred during takeoff from a sloping grass runway. Of these, three aircraft also flipped inverted. The remaining five incidents occurred while landing on asphalt runways, with four from bounced or heavy landings.
Light Aircraft Association (LAA) Oversight
The aircraft was being flown on a Permit to Fly administered by the LAA, which is aware of historical nose landing gear issues on certain aircraft types, including the Vans RV series. The LAA is also aware of research suggesting that, under some circumstances, rough ground can cause the nose landing gear leg to resonate, inducing a harmonic resonance that can lead to failure, which can then cause the nosewheel fork assembly to impact the runway surface.
One modification aimed at overcoming this resonance is an 'Anti Splat' kit, designed to alter the harmonics of the nose landing gear leg and restrain it from tucking under. This modification is approved by the LAA and detailed in its Type Acceptance Data Sheet (TADS) for applicable Vans types, including the RV-9A. The LAA has also approved the fitment of a modified fork assembly to facilitate an increase in tyre size, which increases the distance from the fork to the runway surface. According to LAA records, the Vans RV-6A G-RVSA, which inverted on 30 August 2008, was fitted with both these modifications.
The TADS for the Vans RV-9A includes the following: 'Problems have been experienced with the RV-9A noseleg, especially when operating off grass, with instances of the nosewheel bending back and the strut digging into the ground, causing a rapid stop and further damage. In order to avoid this risk, it is important to maintain the correct nosewheel tyre pressure, and to trim the spat to ensure generous clearance between the tyre and the wheel aperture in the spat (circa half an inch). It is also important to maintain suitable preload on the nosewheel axle bearings, torquing up the axle nut gently as required in the absence of a conventional spacer between the bearings. It is also important to land the aircraft on the mainwheels first and hold the nosewheel off the ground during the initial part of the landing roll, rather than landing on all three wheels together which encourages wheelbarrowing and overloading the nosewheel.' The final sentence reflects a programme initiated by the LAA to educate pilots of susceptible types to delay nosewheel contact as long as possible when landing. The LAA believes this has led to a reduction in the number of nose landing gear failures on light aircraft in recent years.