History of the Flight
The pilot of G-BRWO, a Piper PA-28-140 Cherokee, was conducting a private flight. After being identified on radar, the pilot was instructed to join a left base leg approach to Runway 26. The aircraft was informed it was number two in the approach behind a Sikorsky S76 helicopter. A Cessna 150 had landed on Runway 20 and was cleared to backtrack and exit at Taxiway Echo.
The S76 was cleared to land at 1542:44 hrs and subsequently descended into a low hover above Runway 26 before being given clearance to vacate. The pilot of G-BRWO was then given wind conditions (250° at 8 kt) and told to expect a late landing clearance. At 1543:33 hrs, the S76 reported it was clear of the runway, and at 1543:47 hrs, G-BRWO was cleared to land, which the pilot acknowledged.
One minute and ten seconds after the acknowledgment, ATC requested a position report from G-BRWO. After several unacknowledged transmissions and requests to other traffic, the Airport Fire and Rescue Service (AFRS) was placed on local standby. The pilot of a Robinson R22 helicopter, operating on the airfield, offered to check the approach and was cleared to fly towards the fire training facility. At 1547:32 hrs, the R22 pilot confirmed an aircraft inverted near the fire training facility. The AFRS was deployed and arrived at approximately 1549:30 hrs, where they made the aircraft safe.
The pilot reported that the final approach was normal, but after crossing the runway threshold during the flare, the aircraft rolled uncontrollably to the right. The right wing then contacted the runway surface, failed structurally, and the aircraft became inverted. The cockpit door had jammed due to the remains of the right wing, but the pilot was able to open it with considerable force and escape unaided before the AFRS arrived. The pilot received first aid and was transported to hospital. There was no fire.
Investigation
Investigation included examination of ATC transcripts and reports from the airport operator, ATC staff, and AFRS. After loss of communication, the aircraft was initially misidentified as a Cessna 172. During the emergency, several aircraft were in communication with ATC, including a commercial flight cleared to land on Runway 20 about one and a half minutes after AFRS deployment. The airfield's fire category remained unchanged until four minutes after AFRS deployment.
The fire training facility, positioned close to the threshold of Runway 26, obstructed the view from the ATC tower, preventing direct observation of the accident site. G-BRWO probably crossed the runway threshold approximately one minute after clearance, and the R22 pilot located it about three minutes after the accident. AFRS units arrived approximately two minutes after being deployed.
Wake Turbulence
Every aircraft, including helicopters, produces wake vortices, which are two counter-rotating air masses trailing aft from the aircraft. Vortices form when the weight is supported by wings or rotors. In stable airflow, they drift downwards and, near the ground, move outward from the generating aircraft's track. Vortex strength increases with aircraft weight and decreases with speed.
Research shows helicopter vortices are more powerful than those of fixed-wing aircraft of equivalent weight, especially during the final decelerating flare to a hover during landing. In July 1992, a Piper PA-28 lost control due to wake turbulence from a Sikorsky S61N, resulting in fatal injuries (AAIB Report AAR 1/93).
UK CAA documents (CAP 493, AIC P64/2009, and Safety Sense Leaflet 15c) describe wake turbulence hazards and separation minima. Aircraft are categorized by MTOW: Heavy, Upper Medium, Lower Medium, Small, and Light. G-BRWO (Piper PA28) and the S76 are both classified as 'Light,' so no increased separation was required. However, Safety Sense Leaflet 15c advises pilots of light aircraft following a helicopter to consider allowing greater spacing, treating the helicopter as one category higher. If applied, the S76 would be 'Small,' requiring 4 nm separation on approach.
Analysis
The fire training facility's position prevented direct observation of the runway threshold, delaying confirmation of the accident by about three minutes. The wind conditions (250° at 8 kt) resulted in a headwind component of approximately 7.9 kt and a crosswind of 1.4 kt, making it unlikely that the S76's wake vortex dissipated before G-BRWO arrived. While no increased separation was required by CAP 493 or AIC P64/2009, information indicates helicopter wake vortices are more powerful. Safety Sense Leaflet 15c suggests treating helicopters as one category higher, but this advice is not incorporated into CAP 493 or AIC P64/2009.
Safety Recommendation
Safety Recommendation 2010-026: It is recommended that the Civil Aviation Authority review CAP 493 Section 1, Chapter 3 and AIC P64/2009 and provide clear advice regarding the potential hazards to fixed-wing aircraft when following a helicopter in the same weight category.