Page:CAB Accident Report, Pan American Flight 161.pdf/14

 Docket No. S-99

The V-G recorder was recovered and after a cursory inspection of the recording glass the complete instrument was shipped to the National Advisory Committee for Aeronautics for study and interpretation.

Although it had been subjected to the action of salt water for a period of about 24 hours immediately after the crash, relatively little of the smoked surface had been washed off the recording glass. Special tests conducted by the NACA revealed that newly applied smoked surface is immediately washed away simply by immersion in water. However, the effect of immersion is appreciably reduced if the smoked surface is old. This recording glass had been smoked prior to December 5, 1944, and therefore the smoked surface was apparently old enough to resist, to a great extent, the action of salt water. Thus it was possible to obtain, from the record some significant information.

On the record were three areas of high "G's"; a negative area in a velocity range within and up to about 130 m.p.h., a positive area in the range of 100 to 170 m.p.h., and another negative area above 170 m.p.h. Except for a portion of the last mentioned the high "G" scratches in all three areas are typical of marks made by a recorder during a crash. Such marks are usually in the form of irregular scribbling, with the stylus skipping while the various loops are being formed. Furthermore, the high speed negative area and the positive area appear to be interrelated inasmuch as the two loops have the same slope and match each other; also the maximum absolute "G" values are about the same; i.e., 4G.

Greatest significance was attached to a loop within the high speed negative area. This loop intersected the +1G line at approximately 170 and 200 m.p.h., and the net acceleration at the crest was noted as -3G. That this loop was not a part of the irregular "crash" scratches within the same area is evident by the following characteristics. First, the loop has a smooth and regular appearance devoid of stylus skip or chatter. Secondly, the higher speed segment of the loop is not in line with, but somewhat separated from, the irregular scribbles previously noted. This regular loop evidently was caused prior to the breaking up of the airplane. The characteristics of the loop, as expressed by its slope and the negative "G" value, are not consistent with the supposition that the loop had occurred in free flight. Therefore, the only remaining possibility is that the loop was the result of the airplane's contact with water.

In order to reconcile the latter supposition with the negative acceleration present and with the relatively high velocity registered, the NACA conducted a series of hydrodynamic tests. These tests were run at Langley