B747-200 aircraft in cruise when aircraft commenced vibrating. Flight crew observed airspeed and dropped and was bordering on stall. Apparently autothrottles had disconnected without warning. Flight crew regained airspeed but lost 2000 ft doing it.

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Attributes
ACN	368297
Time
Date	199705
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : naha
State Reference	FO
Altitude	msl bound lower : 37000 msl bound upper : 39000
Environment
Flight Conditions	Marginal
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B747-100
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	enroute airway : naha
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 101 flight time total : 31000 flight time type : 4949
ASRS Report	368297
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 100 flight time total : 12500 flight time type : 50
ASRS Report	368195
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

In cruise, autoplt coupled to INS and pms and autothrottle, we felt a vibration. At first saw engines ok. Flight controls and flaps ok, then noticed airspeed mach .66 with no warning, no flashing lights, no stick shaker. Everything normal otherwise. I immediately disconnected autoplt and autothrottle, pushed power up and started descent to regain mach .84. We were 2000 ft below our assigned altitude momentarily before getting back 'in front of the power curve.' we still haven't figured out how it happened without warning. System checked ok, but strongly suspect autothrottles disengaged with no warning, and wrote it up accordingly.

Original NASA ASRS Text

Title: B747-200 ACFT IN CRUISE WHEN ACFT COMMENCED VIBRATING. FLC OBSERVED AIRSPD AND DROPPED AND WAS BORDERING ON STALL. APPARENTLY AUTOTHROTTLES HAD DISCONNECTED WITHOUT WARNING. FLC REGAINED AIRSPD BUT LOST 2000 FT DOING IT.

Narrative: IN CRUISE, AUTOPLT COUPLED TO INS AND PMS AND AUTOTHROTTLE, WE FELT A VIBRATION. AT FIRST SAW ENGS OK. FLT CTLS AND FLAPS OK, THEN NOTICED AIRSPD MACH .66 WITH NO WARNING, NO FLASHING LIGHTS, NO STICK SHAKER. EVERYTHING NORMAL OTHERWISE. I IMMEDIATELY DISCONNECTED AUTOPLT AND AUTOTHROTTLE, PUSHED PWR UP AND STARTED DSCNT TO REGAIN MACH .84. WE WERE 2000 FT BELOW OUR ASSIGNED ALT MOMENTARILY BEFORE GETTING BACK 'IN FRONT OF THE PWR CURVE.' WE STILL HAVEN'T FIGURED OUT HOW IT HAPPENED WITHOUT WARNING. SYS CHKED OK, BUT STRONGLY SUSPECT AUTOTHROTTLES DISENGAGED WITH NO WARNING, AND WROTE IT UP ACCORDINGLY.

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.