B733, en-route, Santa Barbara CA USA, 1999

Summary: 

On 2 September 1999, a United Airlines Boeing Boeing 737-300 in the cruise at FL240, experienced severe turbulence due to an encounter with the wake vortex from a preceding MD11 on a similar track which had climbed through the level of the B737 with minimum lateral separation, 1.5 minutes earlier.

Event Details
When: 
02/09/1999
Event Type: 
Day/Night: 
Day
Flight Conditions: 
VMC

18953

Flight Details
Aircraft: 
Operator: 
Type of Flight: 
Public Transport (Passenger)
Flight Origin: 
Take-off Commenced: 
Yes
Flight Airborne: 
Yes
Flight Completed: 
Yes
Phase of Flight: 
Cruise
Location
Approx.: 
near Santa Barbara, CA
WAKE
Tag(s): 
ICAO Standard Wake Separation prevailed, In trail event
CS
Tag(s): 
Turbulence Injury - Cabin Crew
Outcome
Damage or injury: 
Yes
Aircraft damage: 
Minor
Non-aircraft damage: 
Yes
Non-occupant Casualties: 
No
Occupant Injuries: 
Few occupants
Off Airport Landing: 
Yes
Ditching: 
Yes
Causal Factor Group(s)
Group(s): 
Aircraft Operation
Safety Recommendation(s)
Group(s): 
None Made
Investigation Type
Type: 
Independent

Description

On 2 September 1999, a United Airlines Boeing Boeing 737-300 in the cruise at FL240, experienced severe turbulence due to an encounter with the wake vortex from a preceding MCDONNELL DOUGLAS MD-11 on a similar track which had climbed through the level of the B737 with minimum lateral separation, 1.5 minutes earlier.

Synopsis

This is an extract from the official report into the accident published by the National Transportation Safety Board (NTSB).

"The flight had just leveled off at FL240 with the autopilot still engaged. The seat belt sign had been off for about 5 minutes, and the cabin crew was in the process of preparing a beverage service. The airplane was crossing a VORTAC when the flight encountered turbulence the crew categorized as 'severe,' with a rapid right roll and pitch excursion. The captain disconnected the autopilot and attempted to counter the excursions with control inputs. As a result of the aircraft motions, 1 flight attendant sustained serious injuries, and another flight attendant and 13 passengers sustained minor injuries. The airplane sustained minor damage, limited primarily to ceiling tiles and seats. A review of the meteorological data showed no evidence of any atmospheric phenomena in the area. A simulation study comparing the known airplane response to control inputs versus the motion of the airplane as recorded on the DFDR data showed that the flight controls alone did not cause the recorded motion of the airplane. Radar data established that the flight was in trail of a MD-11 by 11.5 nm and 97 seconds when the upset occurred. The MD-11 had climbed through the flight's cruise altitude and was 600 feet above the flight at the time of the encounter. Wake vortex studies by NASA have documented vortex descent rates between 270 and 440 feet per minute."

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Further Reading

For further information see the NTSB Report (LAX99LA291)

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