MD80 CREW HAD A NOSE DOWN PITCH TRANSIENT WHILE IN ZOA CLASS A.

Narrative

AT FL350 MACH .76. DURING CRUISE; FELT A SUDDEN THUMP WITH A NOSE DOWN PITCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT WAS MET BY MAINT. THEY EXTENSIVELY DEBRIEFED THE CREW. THE ACFT WAS REMOVED FROM SVC FOR MAINT ACTIVITY. THE JACK SCREW ASSEMBLY HAD BEEN RECENTLY REPLACED. ALL TOLERANCES WERE AT NEW LIMITS AND LUBRICATION WAS CORRECT. AN ELEVATOR CTL CABLE PULLEY HAD A SLIGHT AMOUNT OF CORROSION AND WAS REPLACED. ONE CABLE WAS FOUND SLIGHTLY BELOW TENSION AND ADJUSTED. AT THIS POINT ACR MAINT WAS RUNNING OUT OF IDEAS. AN OLD TIME MECH INDICATED THAT WHEN THE DC9 FIRST CAME OUT HE REMEMBERED A SIMILAR INCIDENT THAT WAS CAUSED BY THE AFT AIRSTAIRS IN THE ACFT. THIS WAS CHKED. IT WAS FOUND THAT THE STAIR UPLOCK HAD FAILED AND ALLOWED THE STAIRS TO DROP TO THE MECHANICAL LOCK. THIS CAUSED THE SUDDEN THUMP FELT IN THE AIRFRAME. THE DISPLACEMENT OF THE STAIRS ALSO CAUSED THE PITCH DOWN THAT WAS OBSERVED. THE UPLOCKS WERE REPAIRED AND NO FURTHER PROBS HAVE BEEN NOTED.