We will climb to 15,000 feet, perform manoeuvres, then descend to around 1500ft, where the aircraft's low speed search and rescue capabilities will be demonstrated.
"In case of emergency, please follow the instructions of our flight test engineer. Of course it will not happen, but we need to be prepared for any situation."
Embraer says its two prototype planes have passed all tests so far in more than 620 hours aloft - including a trouble-free long range mission to Farnborough in Britain. But the aircraft has not yet been certified for military or civilian use.
The plane is labelled "EXPERIMENTAL" on its fuselage near the cockpit entry door we climb through before being strapped into benches in the belly of the plane.
Joining we three aviation scribes are executives and engineers who have worked on the KC-390 project for many of the past seven years - most of them getting their first ride in the plane.
With engines only slightly modified from the standard units used on an Airbus A320, the sound is familiar - and in the plane's cargo bay it is only a bit noisier than a commercial jet.
The bench seats are comfortable enough and the aircraft has other creature comforts: a small galley near the loadmaster's station, a pressurised cargo cabin, oxygen masks for more than 80, a heated floor and an airline-style vacuum toilet - a big step up from a tube tank setup in the fuselage wall of the RNZAF Hercules.
The plane often carries water tanks on test flights to replicate future loads of up to seven pallets of supplies, but this day it is empty, so takeoff comes quickly, with each engine generating 29,000lb of thrust.
Flight engineer Leandro Bigarella, co-pilot William Souza and pilot Airton Manuoel Rodrigues with Grant Bradley.
We leave the ground at 120 knots, and then comes an exhilarating 50 minutes hooning around in a 64-tonne beast high over the centre of São Paulo state, an area chosen for its relatively empty skies.
We are stationed behind Rodrigues and co-pilot William Souza for most of the flight in the flight deck - which apart from side sticks rather than yokes, looks much like a Boeing 787 Dreamliner.
It is near-silent and for much of the flight (at less than half the KC-390's maximum altitude of 36,000 feet) it is almost as though we're suspended in one spot.
I had timidly "flown" the plane briefly in a simulator the day before, so had some rudimentary sense of how it was done.
We have one of the best views in all of Sao Paulo, above the orange groves and sugar cane plantations, in a flight so smooth it feels as if we are floating in a balloon.
That's until Rodrigues decelerates sharply, as though he's yanked on a handbrake, to demonstrate how difficult it would be to stall the plane, then applies power and accelerates - which is when the G forces really kick in.
He then banks to 45 degrees, smiling at the looks on our faces.
Next comes a rapid descent. The plane is designed to dive into hostile airfields or respond to emergencies at up to 12,000 feet a minute, and while we are descending at a relatively sedate 8000 feet per minute, to me, the rapid approach of the earth and the Tiete River resembles a recreation in an episode of Air Crash Investigation - and one that doesn't end well.
However, everything is under control and as we approach 2000ft Rodrigues applies the airbrakes on the plane's massive wing - again a bit like pulling on a handbrake in a car - and brings the nose up to level before banking at 45 degrees so we can get an extra good look out one side of the cockpit at what is growing in the fields below. Easy.
Landing is glass smooth.
Afterwards, there is a debrief in a crew room, where the pilots and flight engineer Leandro Bigarella review the flight. Rodrigues says at low level, the KC-390 handles like a fighter jet and Souza says pilots with a little less than average skills will be able to handle basic flying.
