An airplane whose CG is significantly far behind the aft limit becomes neutrally stable or even mildly unstable. It can still be flown by a sufficiently alert pilot, but to a pilot who is unprepared for it, the experience is disorienting and can easily lead to overcontrol. Overcontrol in the nose-up direction, in turn, can end in loss of control.
That, the National Transportation Safety Board concluded, was the cause of the crash of a 1999 Piper Saratoga at North Captiva Island, Florida, in June 2014. The 2,000-hour instrument-Ârated private pilot, 62, was bringing a load of ceramic tiles from Fort Myers, 20 miles away on the mainland, to be used in a remodel of his house. He had made a similar run earlier that day and was the only occupant of the airplane.
A number of witnesses glimpsed the airplane on the private 1,800-foot grass strip colorfully called “Salty ÂApproach” because it lies athwart a narrow spit of land and has open water at both ends. Most thought the Saratoga was taking off; at least one knew that it had been attempting to land and was going around. All agreed that the engine was running — one described it as “screaming” — and that the airplane appeared to be struggling to climb as it crossed the beach. It rose only a few feet before the left wing dropped, caught the water and separated from the fuselage. The airplane sank about 200 yards west of the departure end of the runway, coming to rest on the shallow bottom with its right wing projecting vertically above the surface. Several witnesses paddled to the scene in kayaks hoping to find survivors, but in vain. The NTSB listed the cause of the pilot’s death as drowning rather than the usual “blunt force trauma,” and indeed most of the recovered airplane was minimally damaged.
Accident investigators determined that the gear had been down and the flaps at 40 degrees. Throttle, prop and mixture were full forward. Nothing was found to indicate any trouble with the engine, which had been recently inspected, and the bending and curling of the propeller blades were consistent with ample power.
The ceramic tiles had been on two wooden pallets in the aft passenger/baggage area, from which the four club seats had been removed. The total weight of tiles and pallets was 666 pounds. The NTSB describes the tiles as having been “unsecured,” and notes that the only cargo tie-down strap in the aircraft was found folded and stowed. It is suggested, but not stated, that passenger seat belts were not used to secure the pallets.
In addition to the six-seat passenger cabin, the Saratoga has baggage areas both behind the rear seats and between the cabin and the firewall. Each is limited to 100 pounds. The fuel capacity is 102 gallons, the gross weight 3,600 pounds, and the empty weight of the accident airplane, with the seats removed, was nearly 2,500 pounds. The Saratoga is extremely versatile, but with a useful load of 1,100 pounds it obviously cannot carry six people, maximum baggage and full fuel all at once. The Pilot’s Operating Handbook devotes more than the usual space to describing various kinds of loadings and urging pilots to do their weight and balance calculations carefully.
A placard in the airplane warned that the “maximum allowable combined weight in aft seats [including the aft baggage area] is 609 pounds.” The author of the NTSB report somewhat naively observes that the weight of the cargo exceeded that limit by 57 pounds and “would have degraded the airplane’s climb performance and increased its stall speed.” Actually, by the NTSB’s own reckoning the airplane’s weight, notwithstanding the excess cargo — there was not much fuel aboard — was at or below the legal gross. For that matter, the four missing seats weighed more than 57 pounds. At any rate, even if it had been 57 pounds over gross, the Saratoga’s performance would not have been perceptibly affected. The increase in stalling speed would have been four-tenths of a knot.
The NTSB offered the following explanation of the crash:
“Based on the evidence, it is likely that, during the approach to land, the unsecured tiles began to slide forward, which would have made the airplane’s nose feel heavy and might have led to the pilot’s decision to go around. However, when the pilot applied power and began to pitch the airplane’s nose up during the go-around, it is likely that the unsecured tiles slid aft, which resulted in the CG exceeding its aft limit, the airplane’s nose pitching up further, and the pilot’s pitch control authority decreasing. These conditions resulted in the airplane exceeding its critical angle of attack, experiencing an aerodynamic stall, and colliding with water.”
Likely is perhaps a bit strong. For one thing, the heavy wooden pallets were presumably resting on carpeted floor and would not have slid readily. If the tiles were going to slide backward, they would more probably have done so during the takeoff from Fort Myers, when the acceleration and the initial climb angle could have been much greater.
It seems at least equally likely that the tiles stayed put and that the pilot, having decided at the last moment — perhaps he was already on the ground — to go around, got barely airborne at minimum speed, encountered a gust of wind, and caught a wingtip. In other words, this accident could have occurred without the cargo shifting at all.
Let us suppose, though, that the NTSB’s theory is correct and that the tiles actually did slide aft. The airplane would have pitched up before the pilot could react, and the wing could have partially stalled. Flow separation over part of the wing, together with extended landing gear and 40 degrees of flap, would have accounted for the failure to climb, and the pilot’s fear of settling back toward the water would have kept him from pushing the nose down. The mushing airplane would have been difficult to control, and even a slight change in angle of attack could have caused a tip to stall and a wing to drop.
That may or may not be what happened. But the mere fact that it can happen is reason enough to monitor weight and balance carefully and to secure cargo against any possibility of movement.
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