El Segundo-based DeCrane Aircraft Holdings Inc. agreed to buy closely held Avtech Corp. for $83 million in cash, strengthening its line of aircraft control systems, DeCrane said. Seattle-based Avtech makes on-board aircraft communications systems primarily for Boeing Co., the world's largest aircraft maker. The company, which has 350 employees, makes the intercom systems that allow communications between pilots, flight attendants and passengers. DeCrane had 1,233 employees at the end of the year, and had 1997 sales of $108.9 million and makes aerospace electronics, including cockpit displays, cabin and temperature and lighting equipment and passenger entertainment systems. The acquisition will strengthen DeCrane's position in the growing market for high-end regional, commuter and business jets. DeCrane shares rose 13 cents to close at $17.13 on Nasdaq.