UPDATE 1-Embraer's backlog falls to six-year low

* Commercial plane deliveries surge 66 pct quarter-over-quarter

* Executive jet deliveries climb 54 pct in period

July 10 (Reuters) - Firm orders at Embraer, the world's largest maker of regional aircraft, fell to the lowest level in six years at the end of the second quarter, in a further indication that a global economic downturn is hampering the company's backlog.

The company's backlog of firm orders tumbled to $12.9 billion at the end of June from $14.7 billion at the end of the first quarter, the company said in a securities filing on Tuesday. Deliveries rose on a quarter-on-quarter basis, the company added.

Demand for aircraft may remain somewhat uncertain in coming years as airlines grapple with rising costs and scarcer aircraft financing globally. The European debt crisis was the cause behind a flurry of deferrals in orders that had not yet translated into cancellations, a company executive said last month.

North America makes up 35 percent of Embraer's market by number of customers, with Europe at 27 percent. Embraer makes regional jets for up to 120 passengers in competition with Canada's Bombardier.

In estimates released late on Monday, Embraer said demand for executive and regional aircraft will keep growing over the next 20 years, with about 6,800 new planes worth $315 billion to be delivered in the commercial segment over that period.

Embraer delivered 35 commercial planes and 20 executive jets in the quarter, up from 21 units and 13 units, respectively, in the first quarter, according to a securities filing.

Shares of the São José dos Campos, Brazil-based company plunged 4.4 percent on Tuesday to 12.80 reais.