Homebuilt Accidents: Comparing the Causes

Do certified aircraft have different types of accidents than Experimentals?

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In the first article of this series, we looked at the relative accident rate between Experimental/Amateur-Built (E/A-B) aircraft and the overall general aviation (GA) fleet. We got a lot of answers. The homebuilt accident rate could be as low as 12% higher than GA, or 45% higher, or even 200-300% higher, depending upon a whole series of assumptions.

There is one common factor in these estimates: the word “higher.” The homebuilt accident rate is definitely higher than the overall GA rate.

The obvious question here is “Why?” What types of accidents are more likely to happen with a homebuilt versus a garden-variety production airplane? Are the planes too hot for the pilots? Are homebuilts much less reliable mechanically? Does their sporty nature lead owners into hazardous flight?

Establishing a Baseline

The first step is to establish a standard against which homebuilt safety can be compared. Too many GA aircraft have more than one engine (rare in the homebuilt world), and too many appeal to the business traveler rather than the recreational pilot. It’s not a fair comparison between these planes and the typical homebuilt. We need one—or perhaps two—GA aircraft to set a baseline, or control group.

One major drawback of the Cessna 172/210 control group used in this analysis is that it includes few taildraggers. Cessna 170s like this one are rare enough that they wouldn’t have much effect on the results.

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This is rather tough, when you consider how wide ranging homebuilts are. Homebuilt seat counts span from a single seat to six or more, designs range from lightweight fun machines to turbine-powered rockets. There is, in fact, no real good solution.

Still, we need to pick some sort of baseline. What would most homebuilt pilots fly if they had to own a production airplane?

Let’s start with the most common GA aircraft: the Cessna 172. It is the most produced airplane ever—it even beats out many planes built during WW-II. There are about 22,300 of them on the U.S. registry, giving us a nice large sample size.

One problem with the 172: It’s a fine machine, but it’s not really a high-performance aircraft. It can’t really be compared to a Lancair or even a run-of-the-mill RV.

So let’s mix in its high-performance stablemate: the Cessna 210. This gives us a complex aircraft with retractable gear and a constant-speed propeller.

Remarkably, the mix of Cessna 172s and 210s produce a fleet size similar to that of homebuilts—as of January 1, 2017, there were 26,315 fixed-wing homebuilts vs. 26,445 Cessna 172s and 210s.

One drawback: The Cessna 172 is used a lot for basic flight training, which is quite uncommon for homebuilts. For our analysis, we’ll eliminate accidents that were due to pilot mistakes during dual instruction. We’ll retain student solo accidents, though—about 3% of homebuilt accidents have a student at the helm.

Figure 1: Registered examples/number of accidents for homebuilts and selected control group.

Figure 1 shows the size of the Cessna 172/210 control group is not only a close match as far as fleet sizes, but the number of accidents affecting the two groups are also roughly equivalent.

Perfect solution? Hardly. No taildraggers are in our control group, and in almost half (48%) of the fixed-wing homebuilt accidents, the prop was bolted to something other than a Lycoming or Continental. And most of those 172s and 210s are quite old compared to the homebuilt fleet.

Not an ideal set for comparison…but probably about as good as we’re going to see. We’ll also only address fixed-wing homebuilts in this analysis.

Pilot Miscontrol

The biggest factor for both homebuilts and the control group is pilot miscontrol. This category covers accidents due to mistakes made in the physical control of the aircraft—undershooting, overshooting, inadvertent stalls, gear-up landings, etc.

Homebuilts come across much better. As Figure 2 illustrates, about 52% of the control group accidents were pilot miscontrol, versus only 38% of the homebuilt accidents.

Figure 2: Pilot miscontrol versus common GA types.

Now, it’d be easy to jump to a hasty conclusion here. Does this say that homebuilts are easier to fly…that pilots are less likely to lose control? Should folks be talking about more restrictions on Cessnas, not amateur-built aircraft?

Alas, no. Pilot experience is key. The median for total flight hours of the pilots in the homebuilt accidents is about 1000 hours, but for the control group, it’s exactly a third of that (333 hours). The pilot miscontrol rate for homebuilts is less because the pilots are more experienced.

Only 2% of the homebuilt accidents have a student pilot at the controls, versus over 20% for the control group. That 20% is overwhelmingly in the Cessna 172 side, of course. Only 0.7% of the Cessna 210 group involved students. Almost a third of homebuilt accidents involve pilots with advanced certification (Commercial, ATP), versus about a quarter of the control group cases.

Over the years, I’ve looked at the accident causes for other common general aviation production aircraft. The pilot miscontrol results for the Beechcraft Bonanza 36, certain models of PA-28 Cherokee, the Cirrus, and individual results for the Cessna 172 and 210 are also included in Figure 2. Note that E/A-B aircraft are actually in the middle of the pack here: better than some simpler aircraft, but worse than complex planes like the Cessna 210 and the Beech Bonanza 36 series.

Again, though, there’s that correlation with median flight time. The pilots in accidents involving the 210 and Bonanza had more flying time, and the percentage of pilot miscontrol accidents is less.

Other Causes

Pilot miscontrol dominates the accident causes so much that it skews analysis of the other factors. For the control group, for example, the second-leading cause (fuel exhaustion) happens one-tenth as often. Since homebuilts have such a lower rate of miscontrol, cause comparison isn’t quite the same.

So for the other causes, we’ll look at them as a percentage of the non-pilot miscontrol accidents. Figure 3 shows the results, grouped in several sub-categories.

Figure 3: Accident causes other than pilot miscontrol.

Homebuilts are better than the control group in several pilot judgment categories—continued VFR into IFR conditions and fuel exhaustion. Like pilot miscontrol, this may be attributable to more experienced pilots. Homebuilts have a higher rate in the maneuvering-at-low-altitude category. This includes traditional buzzing, as well as low-altitude aerobatics and other events involving ground proximity.

While 12% of the fixed-wing homebuilts in the accident database have converted auto engines, auto-engined Cessnas are an extreme rarity. This example was operated in the Experimental/Research and Development category with a Chevrolet engine.

Homebuilts also suffer in comparison in most of the categories related to mechanical factors. About a third of homebuilt accidents (32.3%) begin with a loss of engine power (versus 19% for the control group), and this is reflected in categories related to engine and fuel system mechanical issues. Builder error is also a major factor, with about eight accidents a year directly attributable to mistakes made during construction.

The NTSB cannot find the cause of engine failure in a significant number of cases for the control group, and it’s even more prevalent for homebuilt accidents. This is probably due to NTSB resource limitations and the fact that almost half of homebuilt accidents occur to aircraft with non-traditional engine types.

Finally, considering some homebuilders’ enthusiasm for formation flying, it’s surprising that the control group has a higher rate of midair collisions. This isn’t just a statistical anomaly; homebuilts have less than half the number of midairs as the control group. The vast majority (106 out of 112) of these involved Cessna 172s, so it may, again, be related to pilots with less experience.

It’s hard to find a production general aviation aircraft to compare to the Turbine Legend, especially when studying accidents.

The Problem with Percentages

Comparing percentages is the traditional way of looking at data like this, but it tends to isolate one from the tragedy. What do these results actually translate into, as far as broken airplanes?

As mentioned earlier, the fleet sizes of the fixed-wing homebuilts and the control group were almost the same size. What happens when we compare the numbers themselves, rather than percentages?

Figure 4 shows the tale. It depicts the difference in the average number of accidents for each year in a number of major categories.

Figure 4: Average accidents per year for main accident causes.

The effect of the control group’s greater incidence of pilot miscontrol is obvious—in a typical year, the control group sees 16 more accidents due to problems with stick-and-rudder skills. Similarly, the control group sees more accidents in most of the pilot-judgment categories.

However, despite the near-parity in fleet sizes, the fixed-wing homebuilt group does suffer more accidents than the control group. And as the lower half of Figure 4 shows, the primary reason for this is mechanical issues.

The percentage of accidents due to maintenance error in Figure 3 were about the same for both categories. Figure 4, though, shows that the fixed-wing homebuilts still suffer more occurrences. It’s not bad—in an average year, the fixed-wing homebuilts see between two and three more accidents than the Cessna control group.

Small, light, low-powered homebuilts like this Nieuport replica don’t really have a counterpart in the production aircraft fleet.

All told, the homebuilt group sees about fifty more accidents per year due to mechanical issues, including builder and maintenance error. On the other hand, the control group suffers about 30 more accidents a year in major pilot error categories.

The net is that fixed-wing homebuilts suffer about 20 more accidents than their control group counterparts. Over the 1998-2015 period covered by the data, the control group saw about 165 accidents per year. Twenty additional accidents would mean the homebuilt group had about a 12% higher accident rate—which is right on the lower bounds of the fleet accident rate mentioned earlier.

Almost half (48%) of fixed-wing homebuilts involved in accidents mount something other than a traditional certified aircraft engine.

Conclusions

What can we conclude from all this?

One concern brought up for homebuilt aircraft is that their handling qualities excessively tax the capabilities of their pilots. Results tend to indicate that this is not the case. No doubt about it, E/A-B pilots need to be prepared for the non-standard handling characteristics their aircraft occasionally exhibit. But pilot miscontrol statistics show that, for the most part, we are meeting the challenge.

Unfortunately, where we suffer in comparison to production aircraft strikes right at the core of the homebuilt aircraft movement: our ability to safely build the aircraft and keep it operating. About 29% of homebuilt accidents are due to mechanical issues, versus about 13% of the Cessna 172/210 control group.

We need to continue preparing our piloting skills for flying our aircraft, of course. But if we wish to improve our safety record, we need to up our mechanical skills as well.

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