NTSB Factual Report

HISTORY OF FLIGHT

On April 22, 2003, at 1252 mountain daylight time, a McDonnell Douglas MD-88, N974DL, owned by Wilmington Trust Company Trustee, Wilmington, Delaware, and operated by Delta Air Lines, Atlanta, Georgia, as Delta flight 964, was substantially damaged when it struck a tug during pushback at Denver International Airport (DEN), Denver, Colorado. The airline transport certificated captain, airline transport certificated first officer, two flight attendants, and 56 passengers were not injured; however, one flight attendant received minor injuries. Visual meteorological conditions prevailed. The scheduled domestic passenger flight was being conducted on an instrument flight rules flight plan under the provisions of Title 14 CFR Part 121. The flight, to Cincinnati, Ohio, was originating at the time of the accident.

According to Delta Air Lines, the airplane arrived in Denver at 0951 as Delta flight 1185. Delta Flight 964 was scheduled to depart at 1100, but was delayed due to the previous flight crews' report of a take off slat warning light indication problem. A maintenance check was completed and pushback for flight 964 occurred at 1206.

According to data obtained by the airplane's cockpit voice recorder (CVR), at 1224, while the crew was completing the pre-take off checks during the taxi, the captain reported another take off slat warning light indication problem. Delta maintenance requested that the captain return the airplane to the gate so maintenance personnel could re-examine the fault. If possible, maintenance could "placard" the minimum equipment list (MEL) item so the flight could continue. The airplane arrived back at the gate at 1239.

At 1241, a Delta maintenance technician boarded the airplane to assess the problem. He stated to the captain that "were gonna [sic] have to mess up with your takeoff condition here and you know after we do this, you're not supposed to move anything." The captain stated "right." During the maintenance check for the slat indication problem, the procedures, as identified in MD-88 MEL 27-00-37, state that "either or both" engine throttles must be advanced to verify the operation of the slat takeoff warning horn. The operation of the system was verified and the slats were set in the takeoff configuration. During the entire maintenance check, the captain and first officer remained seated.

At 1251, following the completion of the second maintenance check, the first officer called for and received pushback clearance. The engines were started during pushback, and the captain stated that "prior to the engine start sequence, I did not direct re-accomplishment of the before start checklist." The captain said "the pushback and initial stages of the engine start sequence appeared normal." However, shortly after engine start and with the tug still attached, the airplane began to move forward under its own power. At that time, the captain noticed that the throttles were still advanced. The captain and first officer both reached to pull the engine throttles back to idle, but the airplane had moved forward enough to cause the tug and tow bar to jackknife to the left. The captain shut down the engines, secured the cockpit, and assessed the situation. Although one flight attendant received minor injuries to her shoulder, there were no other injuries to any ground handling personnel, crew, the remaining two flight attendants, or the 56 passengers.

PERSONNEL INFORMATION

According to FAA records, the captain held an airline transport certificate with an airplane multiengine land rating. He held a first class medical certificate dated November 14, 2002, with no waivers or limitations noted. According to the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), submitted by Delta Air Lines, the captain had a total flight time of 2,941 hours in all aircraft, 2,112 hours in this make and model, of which, 190 hours were in the last 90 days and 4 hours in the previous 24 hours.

According to FAA records, the first officer held an airline transport certificate with an airplane multiengine land rating. He held a first class medical certificate dated July 5, 2002, with no waivers or limitations noted. According to the 6120.1/2 submitted by Delta Air Lines, the first officer had a total flight time of 3,345 hours in all aircraft, 2,234 hours in this make and model, of which, 173 hours were in the last 90 days and 4 hours in the previous 24 hours.

AIRCRAFT INFORMATION

The airplane was a transport category, fixed wing, multiengine, 149 seat McDonnell Douglas MD-88. The airplane was manufactured by the McDonnell Douglas Aircraft Company in 1991 as serial number 53242. The airplane was equipped with two 19,000 lbs. Thrust, Pratt & Whitney (P&W) JT8D-219 turbofan engines. At the time of the accident, the airplane had accumulated a total of 33,331.5 flight hours, which included 26,221 cycles.

WRECKAGE AND IMPACT INFORMATION

The nose landing gear was rotated approximately 120 degrees left of center. The right front corner of the tug struck the left side of the airplane's fuselage approximately 6 feet aft of the main cabin door and approximately 4 feet below the cabin floor. The impact with the tug tore a hole approximately 4 feet by 2 feet in length along the left side of the fuselage, substantially damaging several station bulkheads.

TESTS AND RESEARCH

According to an NTSB Vehicle Recorders Division Engineer, on April 25, 2003, the airplane's flight data recorder (FDR), a Lockheed Model 209, s/n 4387, manufactured by Lockheed Aircraft Services, was examined. The recorder was in good condition, and the data was extracted normally from the recorder. This model 209 FDR records configuration data using an analog signal. The FDR records 64 words of digital information every second of relative time. Each second of recorded information is called a sub frame and is identified by a Sub frame Reference Number (SRN).

The data indicated that electrical power was restored to the FDR at SRN 46176. At SRN 46178, the data indicated that the left engine fuel flow increased from 328 pounds per hour (PPH) to 626 PPH. At SRN 46182, engine pressure ratio (EPR) for the left engine began increasing and N1 for left engine was at 10 percent. During this time, the right engine indicated holding with N1 at 0 percent, EPR at 1 and engine fuel flow unchanged at 109 PPH. Between SRN 46212 and 46216, longitudinal acceleration increased from -0.1 to 0.12 g, and then decreased to -0.31 g by SRN 46220. At SRN 46221, the left engine parameters continued increasing, the EPR was at its maximum of 2.09, and N1 had increased to 98 percent. The right engine EPR continued unchanged at 1, and N1 was at 10 percent. During these times, vertical acceleration reached a minimum of 0.95 g and a maximum of 1.06 g. At SRN 46226, the left engine parameters began decreasing and EPR was at 1.13, and N1 was 51 percent. Electrical power was then removed from the FDR.

ADDITIONAL INFORMATION

As outlined in MD-88 MEL 27-00-37, the procedures in step 11 identify that "either or both" engine throttles must be advanced to verify the operation of the slat takeoff warning horn. However, MD-88 MEL 27-00-37 does not include a step to retard the throttles.

Following this investigation, the maintenance procedures, as identified in MD-88 MEL 27-00-32, MD-88 MEL 27-00-36, MD-88 MEL 27-00-37, and similar procedures, as identified in MD-90 MEL 27-84-01, and MD-90 MEL 27-84-02, were revised with approval from Boeing. The MEL's now include a final step that states, "Return both throttles to idle and system controls to normal position."

NTSB Probable Cause

the flight crew's failure to maintain aircraft control, which resulted in engine start with the throttles advanced and the subsequent impact with the tug. Contributing factors include, the flight crew's improper procedures/directives and failure to re-accomplish the before start checklist, the captain's diverted attention, the manufacturer's inadequate MEL procedures, the manufacturer's insufficiently defined conditions/steps, and the tug.