MD80 crew had a nose down pitch transient while in ZOA class a.

Attributes
ACN	484674
Time
Date	200009
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : sac.vortac
State Reference	CA
Altitude	msl single value : 35000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zoa.artcc
Operator	common carrier : air carrier
Make Model Name	MD-88
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 20000 flight time type : 5000
ASRS Report	484674
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	none taken : anomaly accepted
Consequence	other other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

At FL350 mach .76. During cruise, felt a sudden thump with a nose down pitch. Callback conversation with reporter revealed the following information: the aircraft was met by maintenance. They extensively debriefed the crew. The aircraft was removed from service for maintenance activity. The jack screw assembly had been recently replaced. All tolerances were at new limits and lubrication was correct. An elevator control cable pulley had a slight amount of corrosion and was replaced. One cable was found slightly below tension and adjusted. At this point air carrier maintenance was running out of ideas. An old time mechanic indicated that when the DC9 first came out he remembered a similar incident that was caused by the aft airstairs in the aircraft. This was checked. 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 problems have been noted.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.