-warning device-takeoff warning system may have been disabled without the crew realising-
Or, even worse, in the NW255 crash, it is thought that the crew may have pulled the takeoff warning breaker to avoid the nuisance horn while revving an engine up during single engine taxi inbound to DTW. Pulling cb's was common practice a couple of decades ago from my observation. In one of Delta's DFW crashes the takeoff warning horn was found to be inop and the flaps were up although the checklist response for flaps fifteen was given.
Modern Boeings will not engage the autothrottles for takeoff with flaps up (so I'm told
), perhaps this came from the NW255 crash.