History of Flight
On July 7, 2008, at approximately 0845 central daylight time, a McDonnell Douglas MD-81, registration N804ME, operating as Midwest Airlines Flight 8663, experienced an inflation of the tailcone evacuation slide shortly after departing Chicago Midway International Airport (KMDW), Chicago, Illinois. Visual meteorological conditions prevailed. The charter flight, operating under 14 CFR Part 121 on an instrument flight plan, carried 2 flight crew, 4 flight attendants, 2 airline representatives, and 43 passengers. No injuries were reported. The flight, en route to Charlotte-Douglas International Airport (KCLT), diverted to Lambert-St. Louis International Airport (KSTL), landing uneventfully at 0930.
Flight crew statements indicated they did not hear the slide inflate; an airline mechanic seated in the rear also did not notice. After liftoff, the captain reported the airplane's pitch increased without corresponding control input, reaching 20–25 degrees nose-up before regaining control via column and stabilizer trim. The crew noted higher-than-normal pitch control pressure required to level the aircraft. After the incident, the crew consulted maintenance and completed abnormal checklists, but corrective actions were ineffective in reducing pitch control restriction. They elected to divert to KSTL. Normal control pressures returned during cruise descent as the airplane passed through 15,000 feet mean sea level; no further anomalies occurred.
Personnel Information
The captain, age 50, held an airline transport pilot certificate with single and multiengine land ratings, a flight instructor certificate, and type ratings in Cessna Citation, Learjet, and DC-9. His last FAA medical was June 18, 2008, with a first-class certificate requiring corrective lenses for near vision. He reported 15,014 flight hours. Hired January 3, 1990, by the airline, he flew DC-9, MD-80, and Boeing 717 variants, logging 850 hours in the past 12 months, with 0.4 hours in the 24 hours before the incident. His last regulatory checkride was June 9, 2008, in the same make/model.
The first officer, age 42, held an airline transport pilot certificate with single and multiengine land ratings, a flight instructor certificate, and type ratings in DC-9, Gulfstream G-IV, Learjet 60, Saab 340, and Boeing 737, 757, and 767. His last FAA medical was September 28, 2007, with a first-class certificate and no restrictions, reporting 6,400 hours. Hired August 21, 2000, he flew DC-9, MD-80, and Boeing 717 variants, logging 603 hours in the past 12 months, with 0.4 hours in the 24 hours prior. His last regulatory checkride was August 13, 2007, in the same make/model. Neither pilot had prior accidents, incidents, or enforcement actions.
Aircraft Information
The MD-81 is a low-wing, twin-engine transport-category airplane, FAA-certified in 1981, derived from earlier DC-9 models. The incident airplane, N804ME, serial number 48030, manufactured in 1981, was registered to Midwest Airlines on September 23, 1998, and added to the operating certificate on April 23, 1999. It had accumulated about 46,844 total hours. Configured for charter operations, it seated up to 82 passengers, plus 2 flight crew and 5 cabin crew. It was equipped with two Pratt & Whitney JT8D-217C turbofan engines producing 20,000 pounds thrust each.
Meteorological Information
At 0851, the KMDW automated surface observing system reported: wind 230 degrees true at 10 knots, gusting 15 knots; visibility 10 miles; scattered clouds at 8,500 feet, broken ceilings at 15,000 and 20,000 feet, overcast at 25,000 feet above ground level; temperature 26°C; dew point 21°C; altimeter 29.95 inches of mercury. Moderate rain showers were noted in the distant west.
Flight Recorders
The airplane carried a Fairchild Model A100S cockpit voice recorder and an L-3 Communications Fairchild F1000 flight data recorder. The CVR contained no relevant data and was not transcribed. FDR data showed the maximum pitch angle during initial rotation was 16.3 degrees, with a maximum pitch rate of 3.5 degrees per second. The maximum pitch during takeoff and initial climb reached 26.8 degrees. Boeing states typical initial climb pitch angles are 16–20 degrees, occasionally reaching 25 degrees; typical rotation rates are about 3 degrees per second. Inertial calculations indicated that during takeoff rotation and initial climb, the airplane experienced inertial loads of sufficient magnitude and duration to allow an unsecured slide container to rotate open, initiating slide inflation. Prior flights had similar loads but of insufficient duration.
Wreckage and Impact Information
Post-flight inspection confirmed in-flight inflation of the evacuation slide within the tailcone. The deflated slide lay in and around its cover, which was overturned aft of its normal location. The inflation cylinder was empty inside the cover. The slide cover and base, including hinges, forward tie-down straps, deployment lanyard, and floor hardware, were undamaged. A bracket securing one walkway railing to overhead structure had fractured. The unsecured railing and an inflated slide may have impinged on elevator control cables running vertically nearby.
Tests and Research
The slide and hardware were examined under NTSB supervision. The slide had one BB-sized hole in the left support tube with a 6-inch scrape near it. Dark linear grease-like markings on the right tube were consistent with contact to elevator control cables. Inflation system components were within design specifications; the inflation valve trigger was rotated about 30 degrees from nominal, but the manufacturer stated this would not affect initiation. Two deployment tests were conducted using an exemplar slide and cover. In the first, with properly installed tie-down straps, the slide expanded internally, deforming the cover base, but the cover did not rotate open. In the second, with forward tie-down straps unsecured, the slide cover immediately rotated on its aft hinge when the lanyard was released, allowing the slide to fully extend. Examination showed the slide must rotate about 90 degrees from its resting position to activate the inflation cylinder; a properly secured cover would prevent sufficient rotation. A pull test on the floor retention brackets required 22.8 pounds to release them; Boeing requires functional testing at manufacture but no specific force requirement.
Additional Information
The incident flight was a charter for a political candidate, staff, reporters, and US Secret Service personnel. Since the last evacuation slide service check, the airplane had flown 3 legs with the candidate aboard. An internal USSS investigation determined no USSS personnel or support personnel interfered with or altered aircraft hardware or systems related to the slide. After the incident, the airline inspected its 11 MD-80 fleet between July 11 and 25, 2008, finding no anomalies with tailcone evacuation slides. A query of the FAA's Service Difficulty Reporting system found one similar incident on an MD-81 in September 1991, where an inadvertent slide inflation occurred during preflight; the cause was not determined but the operator suspected an inadequately restrained slide cover. Boeing reported a similar incident in July 2002 involving an MD-82 that experienced in-flight slide inflation after severe turbulence; one forward tie-down strap was torn, and the slide cover hinge sustained minor damage. On October 8, 2008, the airline released Maintenance Alert Bulletin #10-08, adding a step to the MD-80 Service Check to ensure security of slide cover tie-down straps. On October 14, 2008, a revised work card added specific language calling for examination of tie-down straps for proper installation and security.