A B747-400 flight crew departing on a transpacific international flight noticed a thump and abnormal...

Narrative:

After a maximum weight takeoff; we were north of ZZZ climbing through 13;000 ft in smooth air accelerating through about 320 KTS when we felt a bump in the airframe; followed by a noticeable vibration. Flight attendants called to report they heard and felt the vibration as well. I asked to level off at 16;000 ft and decelerated to 270 KTS gear speed. Thinking it may be a gear door problem; I cycled the gear. Being heavy weight and just above cms; when the gear came down the speed decayed a little to where we got a momentary stick shaker and lost approximately 150 ft of altitude. The flying first officer quickly asked for and received a block from 15;000-16;000 ft. As soon as the throttles caught up with the speed; we leveled at 16;000 ft and raised the gear. This was no help on the vibration so we called via satcom to dispatch who patched in maintenance control. Since we could not determine the source of the vibration; we decided to return to ZZZ. We arranged with ATC for fuel jettison and returned to ZZZ for an uneventful landing. I didn't want to over dump fuel; so we ended up landing overweight by approximately 1;000 pounds. We entered item into the maintenance log for overweight landing. Upon landing; it was discovered that an air conditioning pack access panel approximately 5 ft square had separated from the aircraft. Supplemental information from acn 819719: I was the first officer that performed the preflight walkaround. During the normal course of the body gear wheel well inspection; you have a very good look at the access doors for the air conditioning packs and exhaust areas. It would be very obvious if fasteners weren't closed or doors left in an un-faired condition. I noticed no such anomalies during my walkaround. Upon further inspection; the left-hand access door to the air conditioning pack was completely removed from the aircraft except for a small amount of aluminum clad honeycombing on the inside forward hinge area.

Original NASA ASRS Text

Title: A B747-400 flight crew departing on a transpacific international flight noticed a thump and abnormal vibration in climb. They consulted with Dispatch and Maintenance; dumped fuel; and returned to departure airport.

Narrative: After a maximum weight takeoff; we were north of ZZZ climbing through 13;000 FT in smooth air accelerating through about 320 KTS when we felt a bump in the airframe; followed by a noticeable vibration. Flight Attendants called to report they heard and felt the vibration as well. I asked to level off at 16;000 FT and decelerated to 270 KTS gear speed. Thinking it may be a gear door problem; I cycled the gear. Being heavy weight and just above cms; when the gear came down the speed decayed a little to where we got a momentary stick shaker and lost approximately 150 FT of altitude. The flying First Officer quickly asked for and received a block from 15;000-16;000 FT. As soon as the throttles caught up with the speed; we leveled at 16;000 FT and raised the gear. This was no help on the vibration so we called via SATCOM to Dispatch who patched in Maintenance Control. Since we could not determine the source of the vibration; we decided to return to ZZZ. We arranged with ATC for fuel jettison and returned to ZZZ for an uneventful landing. I didn't want to over dump fuel; so we ended up landing overweight by approximately 1;000 LBS. We entered item into the Maintenance Log for overweight landing. Upon landing; it was discovered that an air conditioning pack access panel approximately 5 FT square had separated from the aircraft. Supplemental information from ACN 819719: I was the First Officer that performed the preflight walkaround. During the normal course of the body gear wheel well inspection; you have a very good look at the access doors for the air conditioning packs and exhaust areas. It would be very obvious if fasteners weren't closed or doors left in an un-faired condition. I noticed no such anomalies during my walkaround. Upon further inspection; the left-hand access door to the air conditioning pack was completely removed from the aircraft except for a small amount of aluminum clad honeycombing on the inside forward hinge area.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	819717
Time
Date	200901
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Flight Plan	IFR
Component
Aircraft Component	Fuselage Panel
Person 1
Function	Captain Check Pilot Pilot Flying
Experience	Flight Crew Last 90 Days 150 Flight Crew Total 16000 Flight Crew Type 1500
Person 2
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 112 Flight Crew Total 13100 Flight Crew Type 4706
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Speed All Types Inflight Event / Encounter Loss Of Aircraft Control

A B747-400 flight crew departing on a transpacific international flight noticed a thump and abnormal vibration in climb. They consulted with dispatch and maintenance; dumped fuel; and returned to departure airport.