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Overweight and off balanceOverweight and off balance

(WPR11FA059)(WPR11FA059)

By David Jack Kenny

Pilots who do a lot of cross-country flying can become somewhat casual about loading the aircraft, particularly when traveling solo. In a four-place single, the back seat and the baggage compartment are about your only options. Putting the heavier stuff on the seat and whatever’s left in the cargo hatch usually works out just fine. But the more challenging the flight, the more crucial it becomes to attend to every detail. That includes making sure things won’t start moving around the cabin at inconvenient times.

Early in the morning of Nov. 25, 2010, two Mooney M20s took off from Hollister Municipal Airport in northern California. Both airplanes had recently been sold, their U.S. registrations canceled, and re-registered in Australia. Ferry tanks had been installed for a planned 11-hour flight to Honolulu, the first leg of the trans-Pacific transport. Two witnesses saw the first airplane climbing westbound until it disappeared over the hills. Shortly afterward they saw the second Mooney coming back from the same direction at a slower speed and lower altitude, apparently returning to the airport. They saw it “start to spiral downward and come back up, like a trick … then did another spiral downward and … slammed into the ground.” Neither saw any sign of smoke or vapor trailing from the airplane.

The first pilot heard his friend announce his departure over Hollister’s common traffic advisory frequency, and then switched to NorCal Approach to open his flight plan. After the other pilot failed to check in, however, he went back to look for him. He found the wreckage in a farm field about four miles west of the airport. The 28-year-old airline transport pilot had been killed on impact.

Track data retrieved from a handheld GPS showed the Mooney climbing through 685 feet agl on runway heading, then turning left on course. It reached its maximum altitude of 1,270 feet agl 47 seconds into the turn, and then descended 222 feet before recovering. Over the next 89 seconds, another left turn was accompanied by “three diverging groundspeed and altitude oscillations varying between 67 and 144 mph” before the last data point was recorded at 300 agl.

No preimpact anomalies were found in the engine, which had accumulated only 612 hours of operation in 11 years of service. Both main tanks were full. The 238-gallon ferry tank had been installed on a sheet of half-inch plywood in the space normally occupied by the rear seats. It was plumbed into the left tank’s quick-drain port and used to replenish that tank via transfer pumps, so no modification of the Mooney’s original fuel system was required. However, the system’s design did require that the tank (a collapsible bladder type) be restrained by two ratcheting tie-down straps rated for 5,000 pounds apiece. No such straps were found in the wreckage, even though the A&P who had inspected the system two days before the accident and signed it off as airworthy insisted they’d been in place at that time. He had not performed the actual installation, which had been done by the accident pilot himself. The mechanic also recalled that before the flight, the rear baggage compartment appeared to be “loaded to the roof” with suitcases and other baggage. The two rear seats had been stowed back there as well.

The tiedown straps should have been fastened to the two front seat rails and the rear seat belt attach points. Investigators found the rear seat belts still in place beneath the plywood, which covered them completely. Tiedowns built into the ferry tank had not been secured to anything, either. The lower set of tiedown straps for the baggage compartment also were under the plywood, preventing them from being used to restrain that pile of gear. The surviving pilot told investigators that the ferry tank was “held in place by the airplane's sidewalls and luggage in the aft baggage area.”

For the ferry flight, Mooney had authorized a one-time increase of 15 percent in the airplane’s maximum gross weight, up to 3,873 pounds. The aft CG limit remained constant at 51 inches at all aircraft weights. Its actual weight at takeoff was estimated at 4,128 pounds, 255 above the ferry limit. The baggage compartment was found to contain 187 pounds, 50 percent more than its placarded maximum of 120, helping push the Mooney’s center of gravity back to 51.79 inches, almost 0.8 aft of limits. The NTSB noted in its finding of probable cause that

“It is likely that the aft loading resulted in the airplane encountering longitudinal instability during the initial left turn and entering 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 longitudinal instability.”

In addition to his multiengine ATP, the pilot held commercial ratings for single-engine airplanes, seaplanes, and gliders. His 2,540 hours of flight time included seven international ferry flights and a number of aerobatic competitions. The fact that he could not regain control suggests that the airplane’s loading left it uncontrollable—if not on takeoff, then after its contents shifted in flight.

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