Gross profit for the Engineering Services segment was $3.2 million, or 16.5 percent of sales, for the quarter ended December 31, 2010, compared to $3.7 million, or 20.7 percent of net sales, for the prior year quarter. During the fourth quarter of 2010, the company recognized a charge of $0.5 million on its MJET program due to unplanned efforts to meet critical design review requirements.
Selling, general and administrative expenses (SG&A) were $8.4 million, or 15.4 percent of net sales, for the fourth quarter of 2010, up from $7.8 million, or 13.9 percent of net sales, excluding the impact of goodwill impairments, for the year ago quarter. SG&A for the Aerostructures segment was $6.5 million in the fourth quarter of 2010 compared to $6.0 million in the fourth quarter of 2009. The increase in Aerostructures SG&A relates to hiring to support the growth that we expect in 2011 and higher professional fees. The Engineering Services segment had SG&A of $1.9 million in the fourth quarter of 2010, up from $1.7 million in the prior year quarter, largely related to increases in payroll and fringe benefit costs.
Net interest expense was $142,000 in the fourth quarter of 2010, down from $345,000 in the prior year quarter because there were no draws on the credit facility in the fourth quarter of 2010. The effective income tax rate for the last quarter of 2010 was 10.5 percent compared to 36.5 percent in the year-ago quarter. The fourth quarter tax rate was impacted by a $0.5 million tax benefit associated with recent tax legislation extending the research and development tax credit as well as $0.7 million of additional tax benefits from an available manufacturing deduction and additional tax credits.
Free cash flow in the fourth quarter was $2.3 million. The company generated free cash flow for the full-year 2010 of $19.7 million and had no debt outstanding on its revolving line of credit at the end of the year. Cash flow has continued to be strong in early 2011.