and slats were extended varied, but most responded that they were extended, although they could not tell how far. The CVR provided evidence of the flight crew's omission of the taxi checklist. Although the stall warning was annunciated, investigators determined from the CVR that the aural takeoff warning was not annunciated by that warning system. The NTSB was unable to determine a cause for the electrical-power failure in the central aural warning system (CAWS): The investigators had spoken with other MD-80 pilots, and learned that many pilots found it a nuisance to hear a take-off configuration warning ("Slats ... Slats... Slats....") while they were simply taxiing. It was so common for pilots to pull the P-40 circuit breaker that the area around the circuit breaker was smudged from routinely being manipulated. This circuit breaker also controls some of the stall warning sounds. This coincided with the missing sounds from the CVR of the incident flight. While the investigators felt that the P-40 circuit breaker probably had been pulled by the pilot on the incident flight, they could not definitely confirm if the circuit breaker had been tripped, intentionally opened, or if electric current failed to flow through the breaker to the CAWS while the breaker was closed: NTSB conclusions The NTSB published its final report on May 10, 1988, in which it concluded that the accident was caused by pilot error: Aftermath After the crash, Northwest retired the flight number 255 along with its counterpart flight number 254, which was the outbound flight from Phoenix to Detroit. They were changed to Flights 260 and 261 beginning in September 1987 until the company merged with Delta Air Lines in early 2010. It was still operated by MD-82 alongside DC-9 and Boeing 727 but were replaced by Boeing 757 and Airbus A320 in the 1990s. Delta continues the retirement of 255 by Northwest; , Delta has no flight 255. Memorials In memory of the victims, a black granite memorial was erected in 1994; it stands (surrounded by blue spruce trees) at the top of the hill at Middlebelt Road and I-94, the site of the crash. The memorial has a dove with a ribbon in its beak reading, "Their spirit still lives on ..."; below it are the names of those who perished in the crash. Another monument to the victims (many of whom were from the Phoenix area) stands next to