What happened
Shortly after takeoff using standard power, at approximately 100 feet above ground level (AGL), the #3 engine fire warning light illuminated. Despite normal engine indications, the crew began executing the engine fire/severe damage checklist. During this process, the #3 oil low pressure light activated and the #3 generator tripped off.
The fire warning light remained illuminated until the second fire bottle was discharged. The crew declared an emergency with the tower and performed a normal landing. After taxiing clear of the runway, emergency crews inspected the aircraft but found no evidence of fire. The aircraft subsequently taxied back to the gate and underwent a normal shutdown.
The investigation
Mechanical examination of the #3 engine revealed that the starter turbine was missing from the starter assembly, though the turbine was recovered from within the engine cowling. Inspection of the full-length airfoils showed rub marks on all tips, with material displaced away from the direction of rotation.
Further findings indicated that the starter shaft had separated 2 1/4 inches from the impeller. The bearing was found to be broken and rubbed flush against the shaft. Additionally, the ignition exciter box was burned open, with its brackets burned and the unit left hanging by its cable.