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Mach Trim System
The Mach trim system on the Boeing 707 is designed to aid in providing longitudinal stability during manual flight at high speeds. This is accomplished by adjusting the position of the movable horizontal stabilizer to counteract the aircraft-nosedown tendency at speeds of Mach 0.84; or above. This nose down tendency at high speeds, or "stuck under" as it is sometimes called, is a flight condition pertinent to all high-speed sweptwing aircraft. Although the B—707 aircraft is designed to be controllable throughout the Mach trim range without the operation of this system, it is however, required by Civil Air Regulations. The Mach trim system was tested and found to be operating in a normal manner. The Mach trim switch is turned on as a part of the after-takeoff checklist; the crew testified that they believed this was done on departure from London.
The Mach warning system serves to alert the crew or the approach of limiting airspeeds. This is done by means of a warning bell. A test of this system indicated that the bell center bolt was loose, thereby producing a buzzing sound rather than the clear ringing sound intended.
Flight Recorder
N 712PA was equipped with a Lockheed Aircraft Service Flight Recorder, model C, which continuously measures and records the aircraft's compass heading, barometric altitude, vertical acceleration, and indicated airspeed. The recorder in this aircraft is mounted in the left main gear wheel well and is connected to the aircraft's electrical power system through a switch on the landing gear scissors; it operates continuously when the aircraft is airborne. The measured parameters are recorded on aluminum foil tape by means of stylii which move transversely across the foil as the foil moves past the stylii at a uniform rate. A time trace is also impressed on the foal to facilitate the analysis of the recorded data. The tape, which winds from one spool to another, is driven by an electric motor through an escapement and gearing. The foil tape on a loaded spool is fastened to the spool by a piece of masking tape in much the same manner as photographic film is attached to its spool. The recorder will continue to operate even after the foil supply has been exhausted but the resistance of the masking tape is generally sufficient to "stall" the spool drive mechanism. When this occurs the various stylii markings are superimposed (or nearly so) upon themelves and it is difficult, if not impossible, to obtain any intelligence from the recorded markings. A 100-foot roll of aluminum tape is used in the recorder and this is sufficient for approximately 150 hours of recorder operating time.
When the recorder on N 712PA. was examined after the accident, it was determined that the foil supply had been exhausted at the time of the accident. Records revealed that the recorder, SN #103, had been installed on N 712PA on January 19, 1959, by a PAWA mechanic who had obtained the recorder from the PAWA stockroom at Idlewild. The unit had been stored in the stockroom since its return from Lockheed Aircraft Service following a repair by that organization. The mechanic verified that the tape was in the recorder but he did not, nor would he ordinarily, measure the length of the installed tape. Measurement of the tape after the accident disclosed that instead of the usual 100-foot length the tape was only 51 feet long. Further investigation disclosed that 18 flight segments, covering 71:26 hours, were recorded on the tape from January 21 until the tape was exhausted on a westbound flight on January 27, 1959. From this latter date nine flights, covering 13:18 hours, were made during which intermittent record advance totaling .35 inches
