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approach position. The landing gear was extended while passing over New Rocelle at 1,500 feet and the pilot established a rate of descent which he believed to be in the order of 350 feet per minute. Because of the different calibration of the vertical speed indicator as compared with the instruments used by the captain during almost all of his previous 29,000 flying hours, the actual rate of descent was between 900 to 1,000 feet per minute until checked by Captain DeWitt. The captain's altimeter indicated an altitude approximately 125 feet higher than the actual altitude. Since the captain was utilizing the autopilot, his corrections of altitude and direction were somewhat slower than would normally be expected in a manual approach. Because of the successive rate of descent, the aircraft descended below the minim altitude prescribed for station passage. When crossing the La Guardia Range Station, the captain's altimeter indicated approximately 600 feet, which was slightly less than 500 feet above sea level.
When passing the range station, the pilot lowered the nose of the aircraft to establish a rate of descent of approximately 250 feet per minute, inasmuch as this rate of descent, if held for 60 seconds, would bring him to an indicated altitude from which a visual landing may easily have been made to runway 22. On passing the range station, he established a descent of from 600–800 feet per minute. Approximately 20 seconds after passing the La Guardia Range Station the aircraft passed through an indicated 400-foot altitude which was slightly less than 300 feet above the water. At this time the flight engineer and first officer observed lights below the overcast. In accordance with American Airlines' Operations Specifications, Flight 320 could descend to a minimum of 350 feet indicated altitude. Approximately 125 feet of calibration and setting error in the captain's drum altimeter would mean the aircraft actually could have descended to 225 feet. Brief visual check on the instrument panel indicated only a very slight descent and it is possible that, expecting to find the 100-foot pointer somewhere in the vicinity of 250 to 300 feet, he mistakenly accepted the indices on either side of the drum when, in fact, the 100-foot painter was already approaching approximately an indicated 125 feet. The illusion of a safe flight altitude with the limited visual reference available over sparsely lighted areas such as the Rikers Island Channel at night, is not an unknown phenomenon.[1] Furthermore, because of a dike located between the end of runway 22 and the water of the channel, the threshold lights which are slanted at between three and five degrees would not be observed by the crew of Flight 320 unless the aircraft had been at or above the following elevations when at designated distances from runway 22:
| Distance | Three-degree-Slant | Five-degree-Slant |
| 1/4 mile | 79 feet | 132 feet |
| 1/2 mile | 159 feet | 265 feet |
| 3/4 mile | 240 feet | 400 feet |
| 1 mile | 338 feet | 530 feet |
| 1-1/4 miles | 1400 feet | 665 feet |
| 1-1/2 miles | 557 feet | 798 feet |
| 1-3/4 miles | 557 feet | 929 feet |
| 2 miles | 637 feet | 1,062 feet |
Under these circumstances the descent continued for the few seconds remaining until impact was made by the landing gear and right wing with the water.
- ↑ See footnote 5, page 14
