Pilots of the KC-390 were only able to recover the aircraft 300 meters from the ground.
Embraer has grounded the first prototype of the new KC-390 military cargo jet after a stall test incident on Oct. 12. According to the Brazilian planemaker, the scheduled test pushed the aircraft beyond its operating limits, however Brazilian Aero Magazine media outlet, that has talked to an engineer involved in the project who asked to remain anonymous, something else happened in the skies near Embraer Unidade Gavião Peixoto Airport, the private airport located near Gavião Peixoto, Brazil, owned and operated by Embraer: an incident that almost ended in a tragedy.
As reported by Aero Magazine, the KC-390 registered PT-ZNF was performing critical pre-stall tests, that involved high-AOA (Angle Of Attack) and ice formation on wings. During the maneuver, an equipment used for the tests, detached from its place and rolled to the back of the cargo compartment causing a sudden change in the center of gravity (CG) of the aircraft. As a consequence of the rapid displacement of the CG the pilots lost control of the airlifter, that stalled and started to spin towards the ground. Reportedly, the pilots were able to recover the aircraft as it was only 1,000 feet (about 300 m) above the ground, and landed the KC-390 safely in Gavião Peixoto airfield.
Analysis of the track based on the KC-390 ADS-B transponder using the popular Flightradar24.com plane-tracking website suggests that the cargo prototype plunged from about 20,000 feet to around 3,000 feet, between 13.25UTC and 13.28UTC, with a peak vertical speed of -30,976 fpm. However, based on the ADS-B raw data, the area where the test flight was taking place is not covered by receivers at altitudes below 2,800 feet, therefore it is possible that the aircraft was recovered well below 3,000 ft and that the transponder signal was detected once the aircraft had climbed again to a safe altitude after recovering from the spin.
Two KC-390 prototypes have been built, the second one being example PT-ZNJ that made its first flight on Apr. 28, 2016. PT-ZNF made its maiden flight on Feb. 3, 2015.
H/T Jaime Maia for the heads-up