United Airlines is sending 50-minute videocassettes to the homes of its 5,000 striking pilots across the nation containing televised messages from company executives urging pilots to end their 20-day-old walkout or risk being permanently replaced.

The management video parcels mailed this week represent an escalation in a high-tech, video-centered battle for the loyalty of the members of the Air Line Pilots Association. The union has been using videocassettes, a television production crew and 15-city satellite "teleconferences" to maintain solidarity within its ranks.

"This is a public relations war as much as anything else," said United spokesman Chuck Novak. "The union's got badges. We've got badges. The union's got videos. We've got videos."

"We think their sending out the video shows that they are getting desperate," ALPA spokesman David Jewell said. "They are having a hard time getting pilots to cross the line, and they are not having an easy time getting replacements."

The use of television and video production has become increasingly common among labor unions and companies seeking to communicate with the work force, but the United-ALPA confrontation represents the "most coordinated" use to date of video technology during a labor dispute, said Larry Kirkman, executive director of the Labor Institute for Public Affairs, the AFL-CIO's $3 million-a-year video arm.

The proliferation of video recorders and the ease of satellite hookups have created an "electronic union hall," Kirkman said, adding that it offers management new ways of reaching workers.

ALPA, among the wealthiest of unions, has spent more than $500,000 on three slickly produced national satellite conferences beamed to up to 15 cities that are part of a "family awareness network" designed to maintain support for the strike among union families. ALPA said 6 percent of its pilots, who earn $86,000 on average, have crossed picket lines.

The union hired a full-time film crew at its Chicago strike headquarters, where it produces weekly videocassettes that are mailed to more than 500 pilot "group leaders" who each invite about 10 pilots to their homes for discussions, Jewell said. The union videos include "news and propaganda," Jewell said, as well as financial advice, information on health insurance and footage of strike-related events, including shots taken from a helicopter filming grounded airplanes.

United's video includes a May 28 news conference outlining the company position on the strike, which has grounded 85 percent of the flights of the nation's largest airline. Executives outlined plans to hire 300 new pilots per month to "rebuild" United, but ALPA contends that the airline will never reach that goal and will continue losing about $10 million a day. United would not comment on losses.

The dispute now centers on "back-to-work" issues. Economic matters were resolved in federally mediated bargaining. The strike began May 17 over United's attempt to inaugurate a two-tier wage system that would have paid new pilots lower wages than veterans. Other airlines have similar pay scales.

That impasse was resolved in a compromise that would create a lower pay scale for new pilots for five years, followed by arbitration over whether it would continue.

But the talks broke down over United's refusal to hire 566 newly trained pilots who refused to cross picket lines when the strike started. United is also seeking to reward the estimated 250 ALPA members who crossed picket lines by giving them higher seniority status than pilots who struck.

"We want our pilots to return to the cockpit," United President James J. Hartigan said in the video. " . . . But we will not forsake those employes who have chosen to continue working."

The National Mediation Board said yesterday that it has called both sides to resume negotiations here Thursday. ALPA said it would participate. United spokesmen said the airline will announce its response today.