What was found
ON CLIMB OUT THE MCP ALT WINDOW FROZE AT 5000. TRIED SPINNING THE KNOB PUSHING THE KNOB WITH NO RESULT. DISCONNECTED THE A/P MANUALLY FLEW THE AIRPLANE TO FL230. SHORTLY AFTER LEVEL OFF THE ALT WINDOW BEGAN WORKING. THIS ALT WINDOW SITUATION IS A SAFETY OF FLIGHT ISSUE.THE AIRPLANE CANNOT FLY AGAIN UNTIL FIXED. PERFORMED T/S IAW FIM 22-12-77-00 TASK 22-02 - 801 AFTER SEVERAL TEST OFF MCP PANEL FOUND ALT SWICH FAULT REPLACED MODE CONTROL PANEL I.A.W. AMM 22-11-02-2, ALL OPS CHECK SATIS. COMPLIED WITH LLM M-3574 AND RVSM M-3586. A/C OK FOR CONTINUE IN SERVICE PANEL.