History of the Flight
On 19 July 2018 at 1138 UTC, a Flight Design CTSW, registration G-KUPP, departed from Runway 08R at Redhill Aerodrome, Surrey, on the second training flight of the day. The pilot in command occupied the right seat, with one passenger on board. The wind was from the north at 3 kt. As the aircraft climbed through 500 ft, the pilot detected an electrical burning smell and observed smoke emanating from the instrument console. He declared a MAYDAY, turned left, and felt heat from the console on his left leg. He continued a tight left-hand circuit to land back on Runway 08R, selecting 40° of flap. Smoke obscured the airspeed indicator on final approach, but the pilot landed about halfway along the runway. The airport Rescue and Fire Fighting Service met the aircraft and extinguished the fire using carbon dioxide extinguishers. Total flight time was under two minutes. After exiting, the pilot noted the flaps were still in the takeoff configuration (15°).
Aircraft Examination
The aircraft was recovered to a repair organisation for examination. Disassembly of the instrument console revealed that it had been previously modified to install new avionics equipment, and the charge indicator lamp had been relocated from the upper right to the lower central section. The repair organisation assessed the wiring standard as very poor: wires were over length, insufficiently supported, at risk of chafing or entrapment, and improper terminals were used. The charge indicator lamp wire (22 AWG, nominal 3 A rating) had burned through; adjacent wires showed heat damage. The composite instrument console, made of woven carbon fibre with resin, exhibited fire damage; in some places the resin was completely consumed, leaving only the carbon mat. The engine fuel supply pipe and fuel filter were located behind the console.
Analysis
The repair organisation determined that the modification to the charge warning lamp, performed without shortening the wires, was a causal factor in initiating the fire. The excess wire length was damaged, leading to a short circuit against the composite structure. The composite material is electrically resistive; the short circuit current of up to 50 A generated sufficient heat to ignite the resin, producing smoke. The cause of the insulation damage could not be determined, but chafing or pinching in a panel joint was possible. Following the accident, the UK type approval holder issued Service Bulletin 150, which recommends fitting a 1 A fuse to the charge warning lamp supply wire and ensuring wiring is tidy, well secured, and protected from chafing or short circuits. It also requires that any fuel filter inside the instrument console be of a fire-resistant type.
Conclusion
A modification to the charge warning lamp resulted in an over-length wire that was damaged and short circuited to the composite structure. The heat generated by the short circuit caused the resin to ignite shortly after takeoff. The pilot successfully landed the aircraft on the active runway despite reduced visibility and system failures.
Safety Actions
The UK type approval organisation issued Service Bulletin No 150 to modify Flight Design CTSL, CTSW, and CT2K aircraft, aiming to reduce the risk of electrical and fuel fires.