He urged anyone with information, CCTV, dash cam or other footage to get in touch.
The public has been asked to avoid the area.
An AAIB spokesperson said: “An accident at Hanningford Reservoir involving an aircraft has been notified to the AAIB.
“An investigation has been launched, and a multidisciplinary team of inspectors have deployed to the accident site.”
A spokesperson for the Essex Wildlife Trust, which manages a nature reserve at the reservoir, confirmed its visitor centre would remain closed on Monday.
Hanningfield Reservoir, which is between Chelmsford, Billericay and Wickford, is owned and operated by Essex and Suffolk Water. It has a surface area of 870 acres (352 hectares).
