Last month I examined the effects of aft CG loading on stall recovery, ending with the conclusion that the aft limit must be exceeded by a fair margin before pilots experience truly dangerous consequences in stall recovery. This fortunate result proceeds from the considerable conservatism with which the FAA oversees the design and flight test requirements of Normal and Utility category airplanes.
Every now and then one runs into an incident that proves that margins of safety are not infinite. Here are a couple:
When I first started to flight instruct, my employer provided me and my partner CFI with two brand new Cherokee 140s. This was the introduction of the “Hershey bar” series, the first new single-engine Piper since the Comanches. It was also the first plane I had encountered with a one piece horizontal tail, the “stabilator.”
Like Cessna, Piper was then producing light singles approved for intentional spins when Utility category restrictions were observed. In the 140, this means partial fuel, limited weight and no rear seat passengers. A buddy and I spun the innards out of those two planes; it was fun, and we did not then appreciate the harm to the gyros.
Over the ensuing years, I came across a disturbing number of reports of spin accidents in the 140, all with fatal consequences. It was puzzling: the planes had seemed completely reliable.
Then came this NTSB analysis: the spin accidents occurred when pilots exceeded the aft CG limit for the Utility category, in some cases by less than an inch. This “little” error caused spins to flatten just a “little” bit more than usual, robbing tail surfaces of the authority necessary for spin recovery.
Two deaths per accident: a high price to pay for careless inattention to operating limits. In retrospect, all my spins in the 140 had been uncomfortably close to the limit, far closer than I would have ever guessed.
Here’s another example:
I knew a charter pilot who boasted of his ability to barrel roll his company’s Aztec. Result: two broken motor mounts (that’s all they have) and a very near miss with disaster somewhere over the blue Caribbean.
And another, particularly gruesome:
A 1980s pilot was convinced that his Normal category Partenavia twin could handle aerobatics. The result: widely published film of him snapping both wings off in front of an airshow crowd.
Although all three examples involve extremes beyond “normal” flight, they serve notice that safety margins are not infinite. When you push too far, something breaks.
OPERATING LIMITATIONS
When pilots decide to explore the territory beyond published limits, they are moving into the unknown, betting the farm that safety margins are sufficient to keep them alive. This is nothing more or less than an act of faith and is ultimately very silly: when you abandon flight-tested limits, all that remain are trial and error. In aircraft, as often as not, the consequence of error can be fatal.
CONCLUSION
That wraps up the series on Weight and Balance, a subject commonly dismissed by CFIs after a half hour and the introduction of a couple of rote calculations. I was fortunate to have the help of a couple of engineering-type CFIs to answer questions that had long troubled me, and I learned a fair amount as the series progressed. I hope it has helped readers to move beyond the “basics” to a better appreciation of how the airplane and its load really interact with the relative wind to achieve balance, stability and safe structural loads.
