Accident Summary - continued

At the time of the No. 3 fan failure, the pilot and flight engineer were "experimenting" with the relationship between the engine's fan speed (N1) signals and the automatic flight control systems. The pilot and flight engineer appeared to be curious as to where the autopilot/autothrottle received its engine speed signal (i.e., N1 gauge or N1 tachometer generator). The crew reported that their "experiment" was conducted as follows:

1. Autopilot selected in airspeed mode - 257 KIAS.

2. Flight engineer pulled circuit breakers for N1 tachometers on engine numbers 1, 2, and 3.

3. Target airspeed reduced by 5 knots.

4. When throttles retarded, pilot disengaged auto throttle.

5. Crew then heard explosion and cabin decompression.

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