A Chilean national airline plane, ignoring a court order, took off from New York last night after a federal judge froze the assets of the airline to satisfy a $2.9 million judgment growing out of the murder of former Chilean ambassador Orlando Letelier here in 1976.

Letelier and his co-worker, Ronni Karpen Moffitt, were assassinated by Chilean secret police agents when a bomb destroyed the car they were riding in on Massachusetts Avenue NW.

Judge Charles B. Brieant's order prohibiting the airline, LAN-Chile, from removing any assets from the United States was served on the airline and the New York Port Authority at Kennedy International Airport yesterday afternoon. But the operations office at the airport said last night that it had not been aware of the court order, and thus permitted the plane to take off at 6:15 p.m.

Brieant also ordered the Chilean government to appear March 29 and show cause why Michael Moffitt, Ronni Moffitt's husband and Letelier's colleague at the Institute for Policy Studies at the time of the murders, should not be appointed receiver to run the airline's U.S. operations until the $2.9 million judgment and $800,000 in interest are paid.

A spokesman for the LAN-Chile in New York said that he had no comment on Brieant's order.

The order came 2 1/2 years after a U.S. District Court here awarded a total of $4.9 million in damages to the survivors of Letelier and Moffitt. In November, 1980, Judge Joyce Hens Green said that the Chilean government should pay $2.9 million of the damage award because agents of DINA, the Chilean secret police, had carried out the assassinations on the orders of the military junta and the country's dictator, Augusto Pinochet.

John J. Privitera and Samuel J. Buffone, partners in the firm of Tigar, Buffone and Doyle, which represented the survivors without charge, accompanied a process server to Kennedy airport and informed the Federal Aviation Administration of Brieant's order.

Green's original judgment said the remaining $2 million in damages should be paid by Juan Manuel Contreras, the former head of DINA, two other DINA agents, two Cuban exiles involved and Michael V. Townley, an American citizen who admitted planting the bomb while working as a DINA agent.

As the key government witness in the murder case against the Cuban exiles, Townley testified that he used LAN-Chile employes working for DINA as couriers for explosives and remote-control detenators of the type used in the assassination.

During the civil suit brought by Moffitt and Letelier's widow, Isabel, the Chilean government denied the allegations of involvement, but contended that, if it had been involved, it was protected from liability for official governmental actions such as political assassination under the Foreign Sovereign Immunities Act of 1976.

Green, however, rejected the Chilean argument that Congress intended to limit the liability of foreign governments to such private acts as automobile accidents.

Chilean government representatives refused to appear in court, and responded to the lawsuit through diplomatic notes relayed by the State Department. Green eventually ruled that the Chilean government and its agents were responsible for the murders of Letelier and Moffitt.

"We intend to leave the Chileans no place to hide," the attorney who represented the families, Michael E. Tigar, had said at the time of the judgment. After the Chilean government ignored efforts to collect the damages, Tigar and his associates decided to pursue LAN-Chile in federal court in New York.

The attorneys presented documents to Brieant demonstrating that LAN-Chile is wholly owned and controlled by the Chilean government and that it was directly involved in the assassination plot.

The documents were largely court records showing that DINA officials provided Townley with a passport in a false name and LAN-Chile tickets; that they instructed LAN-Chile pilots to carry Townley's explosive devices through customs, and that LAN-Chile personnel helped provide Townley with an alibi by placing a U.S. immigration exit slip made out in Townley's false name in a batch of exit slips for another airline.

The Chilean government and LAN-Chile will have another opportunity to answer the charges at the hearing to show cause why Moffitt, a consultant to a New York financial institution, should not be appointed receiver to administer LAN-Chile while the airline tries to raise the $3.7 million due the survivors.

Judges have wide discretion in the appointment of receivers in such cases. The Chilean government or LAN-Chile may post a bond of $7.4 million, twice the value of the damage judgment plus interest, and continue to operate the airline.