Usually the cause of a general aviation accident is apparent, or at least can be determined through investigation. But recently the NTSB posted probable causes in two accidents involving piston twins where the Board essentially says we can see what happened but we don’t have any idea why it happened.
The first accident involved a Piper Aztec. The sturdy Piper twin was maintained to the appropriate standards and had about 6,000 hours total time. Not a lot of time for an Aztec.
The weather was a typical Texas summer day with temperature at 90 degrees, scattered clouds at 5,000 feet, wind about 10 knots and visibility of 10 or more miles.
The commercial pilot also held a CFI and had more than 900 hours of total experience. He had recently earned his multi rating and had logged just over 39 hours in twins, but since both engines operated normally during the accident multi experience doesn’t seem to be an issue.
There was absolutely nothing remarkable about the conditions for the VFR flight from Houston Hooks to Tyler. But as the Aztec flew over Lake Palestine near Bullard people on the ground saw the piston twin roll into “several turns” while flying thousands of feet above the surface. The report doesn’t mention how steeply banked the turns were, but the witnesses then saw the Aztec pitch nose up and then head straight down.
The really odd aspect of the accident is that the Aztec was heading straight down without its wings. Investigators found that both wings failed symmetrically in positive overload. There was no evidence of metal fatigue, cracks or corrosion that could have caused the failure. Somehow the pilot had literally pulled the wings off the Aztec in good VFR weather with no reason to believe there was significant turbulence.
The Aztec is certified in the normal category so its structural limit load is 3.8 Gs positive. During certification the Aztec was tested to that loading and showed no deformation of the structure. It was then tested to ultimate load of 5.7 Gs and probably suffered some skin wrinkling or other stress but did not break.
That’s a lot of Gs. A level 60 degree bank turn only loads an airframe to about 2 Gs. Unless the Aztec had accelerated well beyond its design maneuvering airspeed (Va) it would probably be impossible to load more than 5.7 Gs without stalling it. But witness did not report seeing the airplane in a dive before it pitched up and the wings failed.
Control flutter was a possibility but the elevator, stabilizer, fin and rudder all reached the ground in one piece and showed no evidence of any flutter or other failure. The NTSB’s probable cause finding is that the pilot exceeded the structural limits of the airplane which is as obvious and meaningful as also pointing out that the crash was caused by the airplane hitting the ground.
The other piston twin accident mystery involved a Beech Duke departing Sedona in the Arizona Red Rock country. Sedona’s runway is on a plateau with an elevation of 4,830 feet. It was a warm day creating a density altitude of 7,100 feet.
Anyone who has ever flown the sleek turbocharged and pressurized Duke knows that it looks fast just sitting still, but is pretty much of a slug on takeoff with modest at best initial acceleration. But when the NTSB worked the performance numbers it found that the Duke should have lifted off after using 2,805 feet of the Sedona runway, despite the high density altitude. The POH also showed that the Duke could have accelerated to rotation speed, and then aborted the takeoff and stopped in a total distance of 4,900 feet. The Sedona runway is 5,132 feet long.
For some reason the Duke never rotated, and the pilot made no apparent attempt to abort the takeoff. The Duke, with engines sounding at full power, went off the end of the runway dropping off into a deep gully where it caught fire and killed the three people onboard.
Several witnesses, mostly pilots, watched the accident unfold. All agreed the engines sounded normal and like they were making full power. Some said the airplane was moving fast, others said acceleration slowed about half way down the runway. None saw any attempt by the pilot to raise the nose.
The private pilot was not highly experienced but had more than 600 hours total, more than 100 hours of multi time, and more than 60 hours in the Duke. He had attended initial and recurrent simulator training for the Duke and his instructors told investigators he had been very attentive, knew the airplane and its systems well, and was a good student.
What comes immediately to my mind is that the gust lock may have been in the controls. But the lock was found in the cabin, not the cockpit, and the holes in the control column where the lock pins the column were not deformed.
Examination and testing of the engines, propellers and other components didn’t reveal any reason the Duke wasn’t making full power confirming witness reports of the sound of the engines.
At least the NTSB is forthright in its probable cause finding on this one. It reported that it just doesn’t know why the pilot didn’t raise the nose of the Duke or abort the takeoff. Both accident causes remain mysteries.