Fluor Earnings Higher Due to One-Time Gain : Construction: The company’s operating income falls 20.5% in the second quarter.

Fluor Corp., the Irvine-based engineering and construction giant, Tuesday reported net second-quarter earnings of $38 million, but half the gain came from a one-time adjustment of tax reserves that it is no longer required to maintain.

The company said net earnings increased 59% for the three months ended April 30 from a year ago when it reported net income of $23.9 million. But without the non-recurring $19-million tax adjustment, its net operating income declined 20.5%.

The earnings report disappointed Wall Street. As a result, Fluor’s stock, which has been a strong performer for the last 18 months, closed down $1.875 at $46.88 in heavy trading Tuesday on the New York Stock Exchange.

“Earnings from Fluor’s operations were only 24 cents a share, compared to everyone’s expectations of 35 cents plus, and last year for the same quarter the earnings were 30 cents,” said Herbert E. Hart, an analyst for S. G. Warburg & Co. in San Francisco.

Fluor said second-quarter revenue was $1.83 billion, compared to $1.63 billion a year ago. The tax adjustment related to reserves it had to maintain during a federal tax audit of its A. T. Massey Coal Co. The audit, for tax years 1975 to 1980, is now complete.

For the first six months of fiscal 1990, Fluor’s net earnings were $66.2 million on revenue of $3.71 billion, compared to net income of $45.5 million on revenue of $3.05 billion for the same period last year.

The company said that for the first half of fiscal 1990, Fluor Daniel, its primary engineering and construction unit, achieved a 25% increase in operating profits compared to the prior year.

But the profitability of the company’s natural resource investments declined slightly.