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Cases › WPR12LA057 · Part 121 airline

BOEING 757-222United Airlines

Modesto, CA, USA · 2011-12-01·N547UA

0
fatal
1
serious
Serious injury
highest injury
—
damage
Replay the flight data248,434 samples · 23 parameters12:03:00.0 – 14:03:01.0AnalyseThe analystOne timeline · precedents · ask the docket · what the record saysGenerateInvestigation briefFacts, cause, recorder extremes, sequence, sourcesSourceNTSB docket12 documents on data.ntsb.gov

What happened

NTSB summary, verbatim

While en route at flight level (FL) 360, the captain of the commercial air carrier flight requested and was granted a lower altitude (FL 300) to minimize the effect that turbulence was having on the airplane. After the captain received pilot reports indicating that varying degrees of turbulence, mountain wave activity, and low-level windshear existed, he illuminated the seat belt sign and made an announcement for the passengers to be seated. The flight crew then advised the cabin crewmembers of possible turbulence and instructed them to prepare the cabin for landing, stay near their seats, and be seated immediately if turbulence was encountered. About 7 to 10 minutes later, while the airplane was at FL 300, the flight encountered moderate-to-severe turbulence. A flight attendant who was in the lavatory when the turbulence was encountered sustained a broken ankle after being thrown in the air twice.

Photographs

1the aircraft, and what the investigators found
The aircraft before the accident
Boeing 757-222ER 'N547UA' United Airlines (14007836357)
Alan Wilson · CC BY-SA 2.0 · Wikimedia Commons

Probable cause

the Board's determination
The flight's encounter with known severe turbulence associated with mountain wave activity, which resulted in a flight attendant sustaining a broken ankle.

Occurrence sequence

1 stepsNTSB coding · CICTT taxonomy
  1. 1 · Approach-IFR initial approach
    Turbulence encounter
    defining event

Findings

1 causal · 0 contributing
  • CAUSE
    Environmental issues › Conditions/weather/phenomena › Turbulence › Terrain induced turbulence — Effect on equipment

Sequence of events

4 timed eventsfrom the FDR factual report · claude-sonnet-5
  1. 1215 PSTDeparturep.1
    Flight departed Denver International Airport (DEN)
  2. 13:39:16 PST to 13:39:44 PSTCruise/approach at FL300p.3
    Vertical acceleration fluctuated between approximately 0 g's and 2 g's, peaking at a maximum of 1.91 g's and a minimum of 0.25 g's; lateral acceleration fluctuated between about -0.2 g's and 0.2 g's, peaking at a maximum of 0.21 g's and a minimum of -0.17 g's. Autopilot was engaged, pressure altitude was at approximately 30,000 feet, and computed airspeed was about 309 knots.
  3. After 13:39:44 PSTDescentp.3
    Aircraft began its descent from 30,000 feet following the acceleration excursions
  4. 14:01:01 PSTLandingp.3
    Aircraft landed

NTSB analysis

from the final report

While en route at flight level (FL) 360, the captain of the commercial air carrier flight requested and was granted a lower altitude (FL 300) to minimize the effect that turbulence was having on the airplane. After the captain received pilot reports indicating that varying degrees of turbulence, mountain wave activity, and low-level windshear existed, he illuminated the seat belt sign and made an announcement for the passengers to be seated. The flight crew then advised the cabin crewmembers of possible turbulence and instructed them to prepare the cabin for landing, stay near their seats, and be seated immediately if turbulence was encountered. About 7 to 10 minutes later, while the airplane was at FL 300, the flight encountered moderate-to-severe turbulence. A flight attendant who was in the lavatory when the turbulence was encountered sustained a broken ankle after being thrown in the air twice.