Page:CAB Accident Report, Pan American Flight 161.pdf/15

 Field in the impact basin and employed a hull model with bottom lines comparable to those on the M-130. The results indicate that when the hull strikes the water under certain conditions, its bottom is subjected not only to local negative pressures (suction) but also to a not negative acceleration. The conditions during which this occurred were found to be a -3° trim of the hull keel, a relatively high speed, and a flight path angle slightly less than zero. This seems to correspond to an airplane in a shallow descent with nose down and at a high speed, probably with some power being applied. The tests further revealed that under the above conditions extremely high drag deceleration is developed. Although the V-G recorder is not designed to register directly the acceleration (or deceleration) in the drag direction, nevertheless the presence of decelerating forces acting in the drag direction is apt to affect the recorded velocity values. With the instrument orientated as it was on the subject aircraft (velocity axis in the fore direction) the inertia of the stylus in the presence of a drag deceleration will result in higher than actual speeds being recorded

Due to the complex nature of the problem and with the testing facilities available at the time, it was not possible to conduct tests which would give accurate quantitative results. However, the qualitative results were such as to be consistent with the negative acceleration within the high velocity range indicated on the V-G record. The speeds registered (170-200 m.p.h.) appear be extremely high, and are not easily reconciled with known facts surrounding this accident. A study of both the airspeed indicating system and the system supplying the V-G recorder offers no explanation as so why the latter registered a speed greatly exceeding that which Captain Goyette last noted on the airspeed indicator. There was no evidence found that water had accumulated in any portion of the two systems, nor that these systems were otherwise not operating within reasonable accuracy. Consequently, it is not possible to establish, even approximately, the speed with which the aircraft hit the water except to state that was probably higher than would be expected during a normal landing.

It was therefore concluded by NACA that the regular negative loop within the speed range of the V-G record was caused most probably by the impact of the aircraft with water at a relatively high speed. The resultant negative value of acceleration seems in line (at least insofar as direction is concerned) with the results of the hydrodynamic tests. The high values of impact speed (170-200 m.p.h.) are not readily explainable and should be tempered in view of other known facts. Further it appears that the impact took place while the aircraft was descending at a very shallow flight path angle with the nose slightly depressed from the normal horizontal trim attitude.