SAO PAULO, Aug 9 (Reuters) - A regional turboprop plane

fell into what aviation experts called a flat spin before

crashing in a residential neighborhood near Sao Paulo in Brazil

on Friday, killing all 61 people on board.

Regional carrier Voepass said the plane, bound for Sao

Paulo's international airport, took off from Cascavel, in the

state of Parana, and crashed at around 1:30 p.m. (1630 GMT) in

the town of Vinhedo, some 80 km (50 miles) northwest of Sao

Paulo.

Video shared on social media showed the ATR-72 aircraft

spinning out of control as it plunged down behind a cluster of

trees near houses, followed by a large plume of black smoke.

Nearby resident Daniel de Lima said he heard a loud noise

before looking outside his condominium in Vinhedo and seeing the

plane in a horizontal spiral.

"It was rotating, but it wasn't moving forward," he told

Reuters. "Soon after it fell out of the sky and exploded."

City officials at Valinhos, near Vinhedo, said there were no

survivors and only one home in the local condominium complex had

been damaged while none of the residents were hurt.

"I almost believe the pilot tried to avoid a nearby

neighborhood, which is densely populated," de Lima said.

The plane's unusual final circling motion before hitting the

ground triggered widespread curiosity among aviation experts,

especially given the lack of obvious bad weather, but by the end

of the day investigators said it was too early to determine the

cause.

"Today ice was predicted (at the altitudes the plane was

flying at), but within the acceptable range," Voepass Chief

Operations Officer Marcel Moura told a press conference.

"But the plane is sensitive to ice, that could be a starting

point," Moura said, adding the plane's de-icing system, along

with the rest, had been deemed operational before takeoff.

The head of Brazilian aviation accident investigation center

Cenipa said that the plane's so-called "black box" containing

voice recordings and flight data had been recovered from the

site.

U.S. aviation safety expert Anthony Brickhouse said

investigators would look at things like weather and whether the

engines and controls were functioning properly as they examine

causes for the loss of control in flight.

"From what I've seen it was definitely what we would call

loss of control," he said.

Flightradar data showed significant gyrations in speed

before the crash, U.S. aviation safety consultant and former

commercial pilot John Cox said, cautioning that he would want to

verify the data but that something "really significant" happened

to cause the plane to spin when it came down.

"It appears that there may have been some catastrophic event

before that loss of control," he said.

Cenipa head Marcelo Moreno told a press conference that

initial reports indicated the aircraft had not reached out to

traffic control to report an emergency.

Voepass, Brazil's fourth-largest airline by market share,

had originally reported 62 people aboard the aircraft. Local

outlet Globo News interviewed two men who said they had missed

the flight.

In total, the plane was carrying 57 passengers and four

crew, Voepass said. All were carrying Brazilian-issued

documents, the carrier reported.

Some of the passengers were doctors from Parana heading to a

seminar, Governor Ratinho Junior told journalists.

"These were people who were used to saving lives, and now

they've lost theirs in such tragic circumstances," he said.

Franco-Italian ATR, jointly owned by Airbus and

Leonardo, is the dominant producer of regional

turboprop planes seating 40 to 70 people.

ATR told Reuters that its specialists were "fully engaged"

with the investigation into the crash and its customers.

The motor on the plane was a PW 127 produced by Pratt &

Whitney Canada, its parent company RTX Corp ( RTX ) confirmed to

Reuters. RTX said that it had offered assistance in the

investigation.

Both French and Canadian investigators will participate in

the investigation, Moreno said. Europe's safety regulator also

said it would offer technical assistance.

The crash is Brazil's deadliest since 199 people were killed

in 2007 on a flight operated by TAM, which later joined LAN to

become what is now LATAM Airlines.