Investigation Summary
Flight 706 was a multi-stop service from Milwaukee to Miami. During the second leg's departure from Chicago, the aircraft performed a running takeoff from runway 14R. Witnesses noted the climb was slightly shallower than normal for an Electra. At the 8,000-foot runway marker, while at an altitude of approximately 100 feet, the aircraft began a coordinated right turn that quickly developed into a steep bank. The crew managed a brief, garbled radio transmission before the aircraft lost altitude. The right wing struck power lines near the Chicago Northwestern Railroad tracks at a 70-degree angle. The aircraft then impacted a railroad embankment while banked at 85 degrees, cartwheeling and disintegrating over a 1,200-foot path. Investigators focused on the aileron primary control system, specifically the aileron boost assembly. They discovered that the input linkage to the aileron boost unit had disconnected because a critical bolt and safety wire, which had been removed during maintenance two months prior, were not properly reinstalled. This disconnection caused the ailerons to move to a full starboard-wing-down position, which the pilots could not override.
Final Conclusions
The Civil Aeronautics Board (CAB) determined the probable cause was a mechanical failure in the aileron primary control system. This failure was traced to the improper replacement of the aileron boost assembly. The malfunction resulted in a total loss of lateral control at an altitude too low for the crew to recover the aircraft.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
First Officer
Systems & Failure Modes
Aileron Boost Assembly
A hydraulic unit designed to assist the pilot in moving the ailerons; its failure due to improper maintenance led to the loss of lateral control.
Aileron Primary Control System
The mechanical and hydraulic linkage responsible for controlling the aircraft's roll.
Interesting Facts
- 01The aircraft was delivered in May 1959, making it approximately 28 months old at the time of the accident.
- 02The crash occurred approximately 3,800 feet to the right of the end of runway 14R.
- 03The impact occurred at a bank angle of approximately 85 degrees and a nose-down attitude of 10 degrees.
- 04The wreckage was strewn over an area 200 feet wide and 1,200 feet long.
Safety Actions & Advisories
Maintenance Procedure Review
The accident highlighted critical risks in the replacement procedures for aileron boost assemblies, leading to stricter oversight of primary flight control maintenance.