Page:CAB Accident Report, Pennsylvania Central Airlines Flight 19.pdf/52

 accident. Other witnesses made varying statements as to their estimate between the time of the lightning strike and the roar of the engines. Mrs. Jacobs estimated 10 seconds. The noise during descent sounded to her like a "siren" or "scream". Mr. Harry E. Everhart, who lives 1¾ miles southeast of the scene of the accident, estimated 15 seconds. Mrs. Everhart, who lives 2¾ miles east of the scene and who heard an "awful roar," reported 7 seconds. Mrs. Hickman, who lives about 2½ miles southeast of the scene of the accident, estimated 16 seconds. Mr. McGaha, 4¼ miles southeast, also heard a "roar." Mr. Ridgeway, 3¾ miles southeast, and Mrs. Hickman heard a noise that sounded like an old truck.

Information received from the N.A.C.A. indicates that a large increase in sound emission from a propeller would result only from overspeeding and that the engines must have been temporarily overspeeding to create the high pitch noise which could be referred to as a "shriek" or "siren" effect. This would explain the "siren" or "scream" which Mrs. Jacobs heard and would distinguish the high tip speed of the propeller from the "roar" of the engines. The testimony of the witnesses therefore corroborates the evidence on over-revving since the noise issuing from an airplane would not ordinarily be described as a "scream."

Over-revving might occur if the airplane were to accelerate faster than the propeller pitch could keep pace with it if in addition there had occurred a momentary interruption of the oil by which the pitch changing mechanism operates. This would create a tendency to rotate the propeller blades toward the low pitch position or at least retard them from moving to the high pitch position which they would normally tend to assume while the airplane was accelerating as in a dive.

Not only would the interruption of oil thus tend to invite overspeeding but the interruption itself could be caused by the same maneuver which created the acceleration. If the airplane went into a sudden dive a