Mysteries Of Flight: Why Were Tomahawks Crashing?

When a type of aircraft suffers a pattern of what could be design-related mishaps, the NTSB might recommend to the FAA that the regulatory agency do something about it. The NTSB, as you might know, doesn't have regulatory powers, so if it sees a problem, which it so often does, it has to recommend to the FAA that it do something about it. Due to cost concerns or philosophical differences between the organizations, those recommendations don't always get acted upon, but in the case of some aircraft, including the Mitsubishi Mu-2 turboprop twin and the Robinson R22 two-seat utility helicopter, for example, they result in the FAA looking anew at the issues surrounding the aircraft in question. And in the end, they invariably come up with some kind of remedy based on the investigation, which often comes in the form of increased training requirements or operating recommendations.

Early on in the Piper Tomahawk's production life, it seemed likely that there was some problem. The planes were experiencing stall-spin accidents, fatal ones, at an alarming rate. For those of you not familiar with it, the Tomahawk was a two-seat, side-by-side low-wing, Tee-tail trainer that Piper introduced in the mid-'70s to compete against Cessna's ubiquitous C-150/C-152 high-wing trainers. The Tomahawk was, like the Cessna 152, a side-by-side two-seat trainer. It was even powered by the same Lycoming four-cylinder IO-235 engine. But apart from that, it was a whole new ballgame. The bubble canopy-like enclosure and the T-tail were cool and very modern, and the plane handled very snappily (no pun intended) as well.

According to an Air Safety Foundation story, that was the whole idea. The author of that story, Bruce Landsberg, who's now an NTSB board member, wrote that Piper designed the plane in part through input from instructors, which makes sense, right? And a lot of them wanted a trainer that spun more easily than the 152. Well, in a classic case of be careful what you ask for, that's what they got.

It tended to be like this. The plane, if flown just so---that is, just the wrong way---would enter a spin quickly, and, once in the spin, it would rotate very quickly---a big departure from the 152. When this happened at low altitude, as it did with nearly all such accidents, there wasn't enough altitude to recover, and the results were fatal for those aboard, which was usually a student pilot and an instructor.

In the early '80s, the NTSB and FAA stepped in and recommended a fix, adding stall strips to change how the stall progressed along the chord line, from root to tip, and the problem was!solved?

There were two big mysteries. The first was, why was the Piper Tomahawk experiencing more stall spins than other trainers? The NTSB compared it with the Cessna 150/152, the Beech Skipper and the Grumman American AA1, though the Cessna two-seat trainers represented an enormous share of the training market at the time, and found that the Tomahawk really was more prone to fatal stall spins by a factor of greater than five. Was it an inherently poor design? The Tomahawk was using a different airfoil than previous, famously docile Pipers. Or was there some problem with the way Piper was building the planes?

Was it design or manufacture or pilot/instructor ineptitude causing the stall-spin mishaps?

As it turned out, the answer was yes. The manufacturing part had to do with changes that Piper had made, not to the design but to the structure, reducing the number of main ribs and lightening the spar. These changes were apparently done only after the FAA had flown the spin series that resulted, investigators found, in the wing being quite flexible, so much so that it stalled in unpredictable ways. Others went so far as to say that each Tomahawk flew quite differently from the next one and differently from the one before that.

In addressing the problem, the FAA came up with a very low-tech, low-cost solution: stall strips. By adding these strategically to the leading edge of the Tomahawk wing, the snappy stall characteristics were tamed. The Tomahawk now enjoys one of the best safety records in its class. PP