Moments after takeoff aboard the gleaming new McDonnell Douglas DC9 Super 80, a flight attendant for the fledging Muse Air gave a different sort of on-board message over the public address system.

"We need your help," she said, launching into a plea to the passengers to support the airline's lobbying campaign for permission to expand its routes. Without the new routes, Muse Air says it can't survive.

"Muse Air has bought four more of these beautiful airplanes for a total of $88.5 million," she said, but it can't use them unless it gets additional landing rights from the Federal Aviation Administration.

If the passengers want to see Muse survive, she added, they must write Transportation Secretary Drew Lewis and tell him they support free enterprise and feel Muse Air should be allowed to stay in business.

Today is the day Muse intended to start service to Tulsa and Midland/Odessa, the first expansion of its business since starting up last July. Instead, two new planes delivered last week for the new service are being used on Muse's Dallas-Houston route, and its two other planes have been parked at Dallas Love Field.

Failure to get an adequate number of landing rights--called slots--to expand services will bankrupt this well-financed airline, according to M. Lamar Muse, the airline's chairman, and his son, Michael Muse, its president.

The father-son team that started the airline won't say how long that would take, but they noted that, with payments of $25,000 a day on two parked aircraft, it won't take "very long."

The two planes, which together cost $41 million, were bought Jan. 15 with the help of a $37 million loan guaranteed by the FAA. When Muse ordered the planes, the FAA was allocating slots on a first-come, first-served basis, and they were assured they would be able to fly them, Lamar Muse says. The next month, the FAA altered the method by which it had been allocating slots since the air traffic controllers strike in a way that favors the established airlines and discriminates against the new entrants, he contends.

Unless Muse gets relief from the FAA, the airline's problems will get significantly worse. Late next month, Muse is to take delivery of its four other DC9-80s, purchased for a total of $88.5 million. Muse put down a $13 million nonrefundable deposit on the planes, but won't be able to accept them unless it has enough slots to convince lenders that the planes can be used. The going rate for slots in this area is about $25,000 each.