(Adds details of earnings, conference call with executives)
By Marcelo Rochabrun
SAO PAULO, March 14 (Reuters) - Embraer SA reported an unexpected loss on Thursday due to a writedown in its underperfoming executive jets division, adding to concerns over its potential future earnings after it sells its profitable commercial jet unit to Boeing Co.
The Brazilian planemaker lost a net $18.1 million in the fourth quarter due to a $61.3-million writedown for research and development spending on its "Legacy" line of business jets, which have not yielded expected returns.
Embraer's fourth-quarter results missed a consensus estimate of an $8.4 million profit in a Refinitiv poll of six analysts.
The loss comes amid concerns over Embraer's future earnings potential once it closes a transaction to sell 80 percent of its profitable commercial plane division to Boeing, narrowing its focus to its money-losing executive jet and defense segments.
The defense unit posted an operational loss of 700 million reais ($183 million) in 2018 and the executive jets division had an operational loss of 150 million reais.
Shares fell more than 2 percent in Sao Paulo trading, the biggest loss on the Bovespa stock index.
Embraer had told investors in January that it had missed its revenue estimates for 2018 and the company would see little no profits in 2019 and 2020. Embraer restated those projections on Thursday.
Embraer's stock has fallen 15 percent year, with almost all of the drop following its revised earnings projections.
The results show Embraer has become more financially dependent on its passenger jets even as it prepares to transfer that business into a joint venture with Boeing in return for $4.2 billion.
The commercial jet operation, which will allow Boeing to compete directly with Airbus in a smaller passenger jet segment, accounted for 50 percent of Embraer's revenue in the fourth quarter, up from 38 percent during the same period of 2017.
The joint venture with Boeing, which Embraer now expects will close in the fourth quarter of 2019, will leave the Brazilian planemaker with a pile of cash but will not pay any potential dividends until five years after Boeing takes control.
Embraer lost $178 million in all of 2018, compared to a profit of $264 million in 2017.
On an earnings call, executives said they were not concerned about the loss last year and the company plans to deliver more executive jets in 2019.
Deliveries of Embraer's turboprop defense plane, the Super Tucano, should also edge up to 10 in 2019, from nine last year, management said.
That plane could be included eventually in another joint venture with Boeing, they said. The companies are already collaborating on global sales and support of a military cargo jet developed by Embraer for the Brazilian Air Force.
($1 = 3.8339 reais) (Reporting by Marcelo Rochabrun and Ana Mano Editing by Brad Haynes and Bernadette Baum)