Search and rescue teams have still not traced any locator signal from Air France flight AF447, more than 24 hours since the Airbus A330 departed Rio de Janeiro on its ill-fated voyage.
The Brazilian defence ministry's air command says there has been "no capture" of any locator transmission from the aircraft, such as the sonic emission from a flight recorder.
It adds that no other aircraft on the same route received a request for assistance on the dedicated radio distress frequency 121.5MHz.
The main focus of the search is the region beneath a section of the UN873 airway, between the INTOL and TASIL waypoints, overseen by Brazil's Cindacta-3 area control centre in Recife.
These waypoints, about 360nm apart, mark the transit point into Cindacta-3's Atlantico oceanic flight information region, and the exit point into the Dakar oceanic flight information region.
"The search will focus [on the area] where flight AF447 sent an automated message about technical problems," says the ministry. These transmissions identified a pressurisation problem and electrical system fault.
As part of the search effort, the ministry adds, a Lockheed C-130 aircraft, stationed at Las Palmas in the Canary Islands, has been assigned to fly along the reverse direction of the A330's route.
Brazilian military authorities are also using a second C-130, an EADS Casa C-105, and two Embraer Bandeirante aircraft - as well as Eurocopter Super Puma and Sikorsky Black Hawk helicopters.
Aircraft for the airborne search are being based at Natal and the island of Fernando de Noronha. Brazil's naval forces are also participating with the deployment of a frigate, corvette and patrol vessel. Learmount: Missing Air France A330 Map: Search and rescue area Profile: Air France Profile: Airbus A330 AirSpace discussion: Air France aircraft from Brazil to France disappears