Page:CAB Aircraft Accident Report, American Airlines Flight 320.pdf/11

 the approved minimum of 820 feet asl over the range station is borne out by the observations of the witnesses in the vicinity of the station. A lay or even a skilled aeronautical witness has great difficulty in estimating accurately the height of an aircraft above the ground. This is especially true when the kind and size of the aircraft is not known to the witness and familiar reference points for establishing relative size and height are neither known nor discernible. It is possible, however, to obtain a compelling impression which, while not accurate in every detail, establishes the most significant facts. In this instance the Board is of the opinion that the one known eyewitness to Flight 320 just prior to its passage over the range station actually saw it, that it was flying low, and was headed in the direction of the range station. The Board is of the opinion, however, that his estimate of 100 feet is too low. In fact, to have descended to an altitude below 300 feet and successfully traversed the area without colliding with buildings or other obstacles, many of which are about 200 feet in height along the approach path leading to runway 22, or without having attracted the attention of many other witnesses, is highly improbable. It is believed, therefore, that Flight 320 definitely approached the range station at a height greater than 300 feet and probably higher than 400 feet above the ground.

If that be the case, one must return to the testimony presented by the crew, physical evidence of the wreckage, and the limitations on the possible flight paths of Flight 320 from the range station to the point of impact. All available evidence shows that FLight 320 struck the water in a very shallow descent approximately one minute after passing the range station. Furthermore, the point at which impact occurred was only two miles from the range station. In order to have passed the range station at 900 feet and strike the water where it did, Flight 320 would have had to have experienced an average rate of at least 900 feet per minute throughout its descent. During the engineering flight test of the static pressure system simulating the final approach of Flight 320 to La Guardia, it was observed that to stabilize the airspeed at 135 knots, and the rate of descent at 250 feet per minute, 980-1,200 h.p. were required.

Operational flight tests observed by Board personnel were conducted on June 16, 1959, between the range station and the crash site using an American Airlines Lockheed Electra 188, N 6113A, which had a gross weight of 97,192 pounds at takeoff. Atmospheric conditions existing at the time were: surface temperature 63 degrees F; surface wind, northwest at 15 knots; winds at 1,000 feet were from 300 degrees at 16 knots; at 2,000 feet, from 300 degrees at 21 knots. In simulating a 900-foot descent with constant horsepower of 900 and 1,200, descent rates up to 2,000-2,500 feet per minute were reached, with airspeed indications of 175/195 knots observed.

Such indications should normally have been immediately perceptible to the crew. If Flight 320 crossed the range station at approximately 400 feet, a rate of descent of approximately 400 feet per minute would place the aircraft at the approximate impact point. Under conditions of still air such a rate of descent would be compatible with the evidence concerning the horsepower output of the engines during descent and at the time of impact since the brief period during which the aircraft was reportereported [sic] to have reached a rate of descent of 600-800 feet per minute would require a rate of descent in the order of 200-300 feet per minute during the remainder of the descent to the water.

As a result of this accident and in connection with its investigation, the Board arranged extensive tests of the altimeter systems and components of the Electra.