The House Defense Appropriations Subcommittee opens an investigation today into whether the Air Force disobeyed a congressional order by going ahead with part of an $800 million computer system called "Project Max."

Leadoff witnesses are scheduled to be Gen. David C. Jones, Air Force chief of staff, and Everett T. Keech, assistant Air Force secretary for financial management.

Subcommittee Chairman George H. Mahon (D-Tex.) also has directed his investigators to probe Project Max, part of the Advanced Logistics System Congress ordered canceled in 1975. The Air Force had described the system as a way to keep better track of aircraft repairs.

"A classic exhibition of contempt," charged Reps. John E. Moss (D-Calif.) and Charles Ross (D-N.C.) in releasing a file of internal Air Force documents last week on Project Max.

The file released by Moss and Rose included the handwritten notes of Maj. Gen. Robert L. Edge, head of Air Force computer resources, who wrote in one that "I'm not overly concerned about unapproved work."

Edge also said that congressional committee chairmen probably would not read the Air Force justification for Project Max nor understand it if they did. He apparently was referring Clellan (D-Ark.) of the Senate Appropriations Committee.

McClellan, right after reading about Edge's remarks in the press last week, scheduled a Senate Defense Appropriations Subcommittee hearing for Wednesday to question the two-star general and Keech.

A. Ernest Fitzgerald, the civilian management executive whom the Air Force fired after he revealed the cost overruns on the Lockheed C-5 transport plane, said yesterday that Project Max would justify costs, not help control them.

Fitzgerald, who is back at his old Air Force management job by virtue of a Civil Service Commission order, said a small version of Project Max, called "Mini Max," was tried out in Newark, Ohio, and "bombed. It was too complicated - just a mess."