Incident Overview
On February 18, 2012, about 1900 central standard time, a Boeing 737-832, registration N374DA, encountered turbulence while descending through 29,000 feet as it approached the Detroit Metropolitan Wayne County Airport (DTW), Detroit, Michigan. Delta Airlines operated flight 1806 under 14 Code of Federal Regulations Part 121 as a scheduled domestic passenger flight. The flight departed Los Angeles International Airport (LAX) and was en route to Detroit.
Injuries and Damage
Two flight attendants and one passenger sustained minor injuries, and one flight attendant sustained serious injuries. The captain, first officer, another flight attendant, and 129 passengers were not injured. The airplane was not damaged.
Flight Crew Reports
The flight crew reported receiving reports of turbulence near DTW and informed the cabin crew to complete cabin service before starting the descent. Flight attendants stated they had finished service and were performing final checks before the approach. The crew made an announcement about two minutes before entering the expected turbulence area, advising passengers and crew to remain seated and fasten seatbelts. Flight attendants reported that the turbulence hit before they expected it and before they could get seated.
The first officer reported that as they descended from flight level 290, the airplane encountered a significant jolt of moderate turbulence. The turbulence decreased as they descended, and the flight reached smooth air below flight level 190.
Flight Data Recorder Information
According to flight data recorder (FDR) data, during the event, the maximum vertical acceleration was +1.568 G and the minimum was -0.30 G over a period of 0.625 seconds. The FDR data indicated that vertical accelerations were severe as defined by International Civil Aviation Organization (ICAO) standards.
Weather Conditions
A weather study conducted by an NTSB meteorologist analyzed the conditions. The High Level Significant Weather Prognostic chart depicted the polar jet stream moving northeastward from Texas into Ohio and eastward, with maximum winds of 180 knots at 35,000 feet and winds of 70 knots or greater extending from 17,000 to 51,000 feet. A second subtropical jet stream was located south of the first. A large area of moderate turbulence was forecast with the polar jet stream over several states, including Michigan. Another area of moderate to severe turbulence was expected from 25,000 to 32,000 feet, covering northern Missouri, Illinois, Iowa, Wisconsin, and Michigan. The accident site was within that area.
The National Weather Service (NWS) issued multiple in-flight weather advisories for the area, including a significant meteorological (SIGMET) advisory for occasional severe turbulence between 22,000 and 30,000 feet, a Center Weather Advisory (CWA), and an airman meteorological (AIRMET) advisory for moderate turbulence between 18,000 and 41,000 feet.
Weather Information Provided to Crew
The Delta Meteorology Department provided the flight crew with weather documents for the intended route, including several turbulence alerts. The crew also received updates en route via ACARS (Aircraft Communications Addressing and Reporting System). The first update at 1547 CST warned of light to moderate turbulence. At 1559 CST, the advisory was updated to include parts of Ohio and Michigan, advising to avoid altitudes from flight level 200 to 250 for moderate to severe turbulence and moderate turbulence between flight level 260 to 300. Another ACARS message at 1929 CST indicated that the area of moderate to severe turbulence had moved south and east of Detroit.