Striking mechanics and baggage handlers reached a tentative settlement with Pan American World Airways because the company planned to start firing them this week, the chief union negotiator said yesterday.

"The issue when we settled on Saturday was no longer the contract," said John Kerrigan, director of the Transport Workers Union's (TWU) airline division. "The issue was whether to prolong the strike at the cost of members' jobs."

The 5,800 ground workers will now be asked to ratify a contract that contains the same wage increase the company offered before the strike began Feb. 28, and smaller bonuses.

Pan Am has refused to comment on the settlement before the vote later this week.

The union's 15 negotiators said they had concentrated on obtaining a job-security pact. The job-security provisions include a choice of alternate employment or severance payments of up to $30,000 for 700 laid-off kitchen workers and a guarantee that TWU members on the payroll when the contract is ratified have a job for life, they said.

The new contract offers a 20 percent wage increase over three years -- but 14 percent of that is a raise that the union postponed in 1982 to help Pan Am avoid bankruptcy.

It grants mechanics and flight dispatchers an immediate bonus of $1,000, and a $600 bonus to the other workers, compared with $1,200 and $900 offered before the strike.