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Low wing may equal lower risk on landingLow wing may equal lower risk on landing

The last issue of Flight School Business noted a surprising detail that emerged from the accident records of the Diamond DA40, Cessna 172, and fixed-gear Piper Cherokee. After restricting all three makes to airplanes built in the past decade (to ensure comparability with the DA40, which only entered the market in 2002), it appeared that landing accidents accounted for more than half of all those in the Skyhawks and Diamonds, but just more than a third of those in PA-28s.

At the time, we didn’t see an obvious explanation. Some students do find low-wing airplanes easier to land, presumably due to the increased cushion provided by greater ground effect, but that should benefit the Diamond at least as much as the Cherokee. The most relevant design differences would seem to be the Diamond’s longer wingspan, which might provide crosswinds with greater leverage, and its use of differential braking rather than a steerable nosewheel for directional control on the ground. Could one or both of these have increased susceptibility to ground loops and runway excursions enough to offset any advantage provided by the low wing?

As usual, the truth turns out to be more complicated, and finding any pattern is made more difficult by the small numbers of accidents in both late-model Cherokees and Diamonds. Even so, there are some strong similarities between the two low-wing models and some notable differences with the Skyhawk.

The first surprise was that four of the 19 accidents in Cherokees (21 percent) occurred while taxiing. Two involved collisions with other airplanes and two with stationary objects (a light pole and a runway hold-short sign). This compares to 5 percent of the relevant accidents in Cessna 172s and none in Diamonds. Again, there’s no particularly obvious reason for this; and since the number of accidents in late-model PA-28s was small to begin with, a few fluke events exert a disproportionate influence. Exclude those taxi smacks, and landing accidents made up almost exactly the same share of what’s left in both DA40s and PA-28s.

Looking at the details of the landing accidents, we find that five of seven in Cherokees and six of eight in DA40s were losses of directional control. In each model, three were on student solos. Both of the hard landings in DA40s and the only one in a PA-28 happened during dual instruction of primary students. Two accidents in each model were on single-pilot flights by private pilots.

The Skyhawk’s record looks very different. Not only did it have the highest percentage of landing accidents of the three (133 of 227, almost 60 percent), but less than a quarter of them (31) were losses of directional control. Stalls and other hard landings were more than three times as common with 47 of each; together, they accounted for more than 70 percent of all landing accidents in newer Cessna 172s. And just about exactly 65 percent of all three types of accidents—hard landings, stalls, and loss of directional control—were on student solos. So while the record doesn’t prove they’re harder to learn to land, it doesn’t contradict that interpretation, either.

Of course, comparing percentages can be a slippery business, especially when they come from observational data instead of controlled experiments. A higher share of landing accidents could represent a greater risk during landing, or a reduced risk of accidents of other kinds. As we saw last week, a consistent measure of exposure provides a benchmark to support more meaningful comparisons.

Unfortunately, we don’t have good data on the number of landings attempted in each model. Neither do we have reliable estimates of the number of hours flown, even without distinguishing between older and newer examples. What we do have is the same measure we used last week: the number of aircraft-years in service for airframes built since 2001. Using this to compare the rates of landing accidents rests on the assumption that all three models average about the same number of landings per year, which is certainly arguable. It’s still the best we have.

By this standard, the eight DA40 landing accidents in about 9,600 aircraft-years work out to about 0.8 per thousand. Seven Cherokee landings in 3,300 aircraft-years translate to 2.1 per thousand, about two and a half times as high. And 133 in 18,600 aircraft-years in Cessna 172s comes out to about 7.2, more than three times higher still—most of which can be attributed to their much higher frequency of hard landings and stalls. Had the rate of those been about the same in Skyhawks as in the other two models, their rate of landing accidents would have been about 2.4 per thousand aircraft-years—almost identical to that of the Cherokees.

“High-wing vs. low-wing?” A recent article in AOPA’s Flight Training magazine nominated that as one of four questions “not worth a debate.” Well, maybe. Then again, maybe not.

ASI Staff

David Jack Kenny

Manager, Safety Analysis
David Jack Kenny analyzes GA accident data to target ASI’s safety education programs while also supporting AOPA’s ongoing initiatives and assisting other departments in responding to breaking developments. David maintains ASI’s accident database and regularly writes articles for ePilot, Flight School Business, Flight Training, CFI-to-CFI, and other publications.

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