On June 4, 2019, about 1700 eastern daylight time, an experimental Alisport SRL Silent 2 Electro glider, N66911, was substantially damaged when it was involved in an accident near Danbury Municipal Airport (DXR), Danbury, Connecticut. The pilot sustained minor injuries. The glider was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
According to the pilot, he performed a preflight inspection of the glider and the self-launching electric motor. He stated the glider’s battery was fully charged before the flight. He utilized the glider’s electric motor to take off from DXR and proceeded northbound using thermals and ridge lift. He flew for about 2 hours before turning southbound near the Massachusetts/Vermont border to return to DXR. He stated high overcast conditions pushed him west toward the New York/Connecticut border and that he needed to use the electric motor to maintain a safe altitude due to low thermal activity. His last calculation of remaining battery energy was about 20 minutes from DXR, when he noted about 20% battery life remaining.
About 8 miles from DXR, the pilot contacted the tower controller and was cleared to land, but the glider encountered sink and the pilot turned on the electric motor. The pilot reported that the motor produced “no thrust,” and the glider continued to lose altitude until it impacted trees and a house 2.4 miles northeast of DXR. The impact with the house resulted in substantial damage to the glider’s foldable fixed-pitch propeller, nose cowling, cockpit canopy, both wings, and the tail boom. After the accident, the pilot reported that there were no mechanical malfunctions or failures of the glider that would have prevented normal operation prior to the accident.
According to the glider manufacturer’s website, a typical self-launch can be expected to consume about 20% of the available battery capacity, leaving about 40 minutes of power available for use as a sustainer.
The glider was equipped with a front electric-self launch (FES) control unit that contained nonvolatile memory on an SD card. The glider was also equipped with an LX9050 GPS navigation system, from which data for the flight were extracted and correlated with data from the FES. The contents of the FES SD card contained about 6 1/2 hours of data associated with the accident flight and indicated that the FES motor was used five times during the accident flight for a total duration of 1:09:16.
The FES control unit also recorded battery annunciations and provided system warnings. A yellow warning message was provided when the battery power dropped below 95 volts and a red warning message and red light were provided when the battery dropped below 90 volts. The yellow warning required a reduction in power, while the red warning was a critical voltage warning and required the pilot to stop using the FES motor.
The data from the accident flight showed that pilot received three yellow warning messages and two red warning messages during the flight. The yellow warning messages were displayed at 05:06:32 (elapsed flight time), about 41 nautical miles (nm) from DXR; at 06:08:46 about 12 nm from DXR; and 06:21:51, just 1 minute before impact and at an approximate distance of 2.4 nm from DXR. The red warning messages were displayed at 06:18:12 and 06:18:31.
The pilot reported that the accident might have been prevented with “better management of power and understanding of battery power source.”