Unsecured load results in flat spin

Aircraft: Mooney 20R. Injuries: 1 Fatal. Location: Hollister, Calif. Aircraft damage: Destroyed.

What reportedly happened: The pilot departed for an 11-hour ferry flight across the Pacific Ocean. The aircraft had been outfitted with a ferry fuel system in the form of a 238-gallon collapsible bladder tank located in the cabin behind the pilot’s seat, above the rear seat pans.

About 84 seconds after takeoff, the pilot initiated a left turn. During the next 47 seconds the airplane continued to climb to its maximum altitude of about 1,500 feet AGL. The airplane began to descend, and about 24 seconds later, it reached a groundspeed of 144 mph and entered a second left turn.

Over the remaining 79 seconds, the left turn continued with an accompanying series of three diverging groundspeed and altitude oscillations, ending at a groundspeed of 69 mph and an altitude of about 300 feet AGL. Witnesses observed the oscillations, which were followed by a spin to the ground.

The post-accident examination revealed no evidence of any airframe or engine failures or malfunctions that would have precluded normal operation. The throttle control was found in the idle position, and it was determined that the engine was operating at low power at the time of impact. These findings are consistent with the spin recovery procedure listed in the Pilot’s Operating Handbook that requires the engine throttle be set to the idle position.

At the time of the accident, the bladder tank contained about 121 gallons of additional fuel, which supplemented the 89 gallons carried in the two wing tanks. The ferry system design required that the bladder tank be attached to the fuselage utilizing ratcheting straps. The pilot installed the system two days before the accident, and a mechanic inspected the installation. The mechanic reported that he observed yellow tie-down straps installed over the bladder tank, however, no straps were found at the accident site, and the tank appeared to be unrestrained.

The ferry system operating instructions required that the fuel selector valve be set to the right tank during takeoff. During the post-accident examination, the valve was found in the left tank position. However, fuel was noted in the engine-driven fuel pump, flow divider, and fuel lines forward of the firewall, indicating that the incorrect position of the valve did not result in an interruption to the engine’s fuel supply.

In addition to the extra weight of the fuel in the bladder tank, 187 pounds of unsecured baggage was located behind the tank in the aft baggage area, which had a weight limit of 120 pounds. Although the ferry system design allowed for a one-time 15% increase in the Maximum Takeoff Weight, the weight of the airplane at the time of the accident was estimated to be about 23% beyond the Maximum Take Off Weight. Additionally, an estimate of the airplane’s center of gravity position at the time of the accident revealed that it was about 0.8 inches beyond the aft center-of-gravity limit. No weight and balance sheet referring to the airplane in the ferry flight configuration was located.

Investigators determined that the aft loading resulted in the airplane encountering longitudinal instability during the initial left turn. The airplane subsequently entered a series of altitude and pitch oscillations, which would have been extremely difficult for the pilot to control. Also, the unsecured fuel tank and baggage could have moved during takeoff or after the oscillations began, shifting the center-of-gravity farther aft and exacerbating the instability.

The previous owner of the airplane reported that he had experienced autopilot anomalies, with symptoms similar to those observed during the airplane’s divergence from controlled flight. However, the autopilot had been repaired about eight months before the accident, and post-accident examination of the autopilot components revealed no evidence of any anomalies that would have precluded normal operation.

Probable cause: The pilot did not ensure the airplane was loaded within its weight and balance envelope, which resulted in longitudinal instability and a loss of aircraft control during the initial climb.

NTSB Identification: WPR11FA059

This November 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.

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