Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/7

 The maintenance records on N724US indicate that it had been involved in a landing accident at Fort Lauderdale, Florida, on January 26, 1962. An investigation by the Board, at the time, revealed that the aircraft landed short of the runway. Structural failure occurred when the right main landing gear separated, with resultant damage to the adjacent wing, flap, and fuselage areas, and the No. 3 and No. 4 engine nacelles. Following the return to service the aircraft sustained a bird strike on the right wing leading edge which was also repaired. These were the only occurrences of significant structural damage to the aircraft, prior to this accident. The maintenance records reflect compliance with FAA Standards of airworthiness.

In order to more fully develop certain areas of its investigation, the Board convened a public hearing during which experts from the aviation industry were called to testify. Three basic areas of concern were the weather and its potential, the pilot and his ability to control the aircraft, and the aircraft and its characteristics throughout a maneuver such as indicated on the flight recorder readout.

The Director of the National Severe Storm Project (NSSP) testified that the turbulence encountered in a thunderstorm varies directly with the amount of rainfall and the diameter of the storm during its building or mature stage. During the deteriorating stage, the diameter of the storm is no longer indicative of the turbulence. The large updrafts occurring within thunderstorms are frequently 15 miles wide, and invariably contain smaller gusts which produce the turbulence. The strength of these smaller gusts generally varies directly with that of the draft in which they occur. The report submitted by NSSP in June, 1963, concluded in part that it is not unreasonable to assume that severe turbulence exists at some point in any storm, and in a growing, or large mature thunderstorm one may expect extreme turbulence.

Thunderstorm data of a more specific nature were developed by meteorologists of the USWB, who evaluated the nine indicators of turbulence which might have been present in the crash area at the time of the accident. They reported the most reliable of these indicators seems to be the rainfall rate, which indicates gusts values in the severe range, other fairly reliable indications such as buoyancy, hail, and surface gusts indicated somewhat higher gust values.

A representative of the Naval Medical Research Institute, and a pilot who performed as his subject during a series of tests on negative G maneuvers conducted by the U. S. Navy at their Johnsville, Pennsylvania facility, were called as witnesses at the hearing. They advised that from a physiological standpoint the acceleration evidenced by the flight recorder readout should not have physically incapacitated the crew members, assuming they were restrained in their seats. The Navy tests subjected the pilot to repeated loads of -3G for