We've mentioned the Brazilian aircraft manufacturer Embraer's KC-390 transport (shown below) in these pages before. It's scheduled to enter production next year.
It looks like the most recent test flight was a hair-raising experience. According to The Aviationist:
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.
There's more at the link.
Embraer SA has stopped flying the first prototype of its new military cargo jet after a stall test last month pushed the aircraft beyond its operating limits, the planemaker said on Wednesday, adding that the project is still on schedule.
The KC-390 prototype suffered no damage to its “primary aircraft structure,” but some of its access hatches and aerodynamic fairings must be repaired before the aircraft can resume flights, Embraer said in a statement.
. . .
Embraer said the incident would not affect the certification schedule of the KC-390, which enters service next year. The Brazilian Air Force has already ordered 28 of the aircraft for 7.2 billion reais ($2.3 billion), with two deliveries in 2018 and three in 2019 ... Executives have said the KC-390 could eventually account for $1.5 billion in annual exports, as Embraer aims to compete with Lockheed Martin Corp to replace more than 700 aging C-130 Hercules turboprops around the world.
Again, more at the link.
If a big, heavy piece of test equipment rolled to the rear of the plane, it could certainly upset the aircraft's center of gravity, causing a very dangerous situation. That sort of incident can be disastrous, as evidenced by the crash of a 747 freighter in Afghanistan in 2013. I'm sure many readers will remember watching video of the crash, which is horrific. This report summarizes the apparent cause of that accident.
I'm sure the KC-390 wasn't carrying a similar weight of test equipment; but it's a smaller aircraft, so even a lesser weight displacement might have had tragic consequences. Full marks to the test pilots for controlling their descent and bringing the plane back into controlled flight. I bet they had to change their underwear afterwards, though!