Northwest Airlines Flight 853

The accident flight was the first one for the airplane after having maintenance performed on the number two (right) engine during the previous evening. The first officer reported the engine cowls were flush and he did not see any "hanging" cowl latches when he looked underneath the engine cowls during the preflight. The captain reported the preflight, taxi, and takeoff were normal. During climb out the lead flight attendant informed the pilots that a passenger reported that the number two engine cowling was flapping after takeoff. The captain then began to monitor the N1 vibration indications on the number two engine. He reported that the vibration was approximately double the indication from the number one engine, but the indicator was not flashing, nor had it turned amber. The captain reported that during cruise flight the number two engine vibration decreased and about 20 minutes after they leveled off, the airplane shuddered. He reported the remainder of the flight was normal until they were taxiing after landing and the lead flight attendant called the cockpit stating that "part of the right engine had come off." Half of the engine cowl departed the airplane when it was on a one mile final. The other half departed the airplane as it touched down. Another airplane subsequently contacted this part of the cowling when it landed after the accident airplane. Contract maintenance personnel reported they changed the N1 sensor on the number two engine. They reported they shut the fan cowling, but did not latch it as they still needed to perform an engine run and check for leaks. They performed the engine run and were in the cockpit when another mechanic asked for help on another airplane. The mechanics left the accident airplane to assist the other mechanic. Neither mechanic returned to the accident airplane to either latch or verify that the fan cowling had been latched. The aircraft maintenance manual (AMM) instructions for replacing the N1 sensor states, "Close the fan cowl doors (Ref. AMM TASK 71-13-00-410-040)." This task was not completed prior to the airplane being returned to service. The separated cowling contacted the right side of the horizontal stabilizer which resulted in substantial damage to the stabilizer. In addition, the fan cowl doors, number two engine pylon, the number two engine reverser, and the right wing number one slat were damaged.

Verbatim NTSB analysis and probable cause from the NTSB dataset