Mention “all-metal homebuilt kit” to most folks, and thoughts immediately go to a certain Oregon-based company. But there’s another major line of all-aluminum kits: Zenith Aircraft.
The CH 701 suffers a larger percentage of accidents with low-time pilots. This applies to builders as well as the purchasers of completed, flying aircraft.
The comparison goes pretty deep. Chris Heintz, Zenith’s designer, is a contemporary of Richard VanGrunsven (Van’s Aircraft founder). They both built their first homebuilts around 1970, and both use their own names in their product lines (Zenith is an anagram of Heintz). Their designs have evolved over the years into the sophisticated aircraft the companies sell today.
Both companies have seen controversy over wing problems. The early Van’s RV-3 suffered some failures, as did the prototype RV-8. Zenith’s problems with the CH 601 are more recent, and the company responded with a comprehensive set of modifications. We covered the Van’s fleet in the July 2010 issue of KITPLANES. Let’s now turn our attention to the Zenith line.
Zenith has had a variety of models, but two predominate: The CH 601 and the CH 701. Most of the other models aren’t common enough to produce meaningful accident statistics. These models are included, however, when we discuss “All Zenith.”
The CH 601 has a low rate of pilot-error accidents compared to other homebuilt types.
The CH 601 accidents in this analysis do not include those aircraft registered as either Experimental Light Sport (ELSA) or Special (factory-built, SLSA) Light Sport Aircraft. Note that the 10-year period covered by my database (1998 to 2007) mostly predates the spate of wing-failure accidents. In any case, many of those aircraft were registered LSAs or occurred to non-U.S. aircraft and wouldn’t have been in my database anyway.
Figure 1 shows some pleasant news for Zenith fans. The rate of Pilot Error accidents is quite a bit lower than average. However, there are some troubling results when we look at the total hours in type at the time of the accident. As Figure 2 illustrates, when accidents occur, the Zenith pilots tend to have quite a bit less time in their aircraft than pilots involved in accidents in other homebuilt types.
Figure 1: Zenith Pilot Accident Rate.
Figure 2: Time in Type.
About 22% of all homebuilt accidents occur during the pilot’s first 10 hours, versus about 36% of all Zenith accidents. Half of the CH 701 pilots involved in accidents had 10 hours or fewer in CH 701s. In the next 30 hours, the percentages are much closer. Figure 3 shows the relative percentage of accidents for several homebuilt types, both during the first 10 hours, and from 10 hours to 40.
Figure 3: Pilot Time in Type (Overall).
The above figures are for pilot time in type rather than aircraft total time, so they include a mix of both new aircraft and completed homebuilts owned by a subsequent purchaser. The numbers scale down a bit when considered by aircraft (versus pilot) time. Looking at accidents that occurred during the overall test period, about a third of the Zeniths in my database had 40 hours or less at the time of their accidents, as compared to 20% of the overall fleet.
CH 701 builders often enhance their airplanes’ bush capabilities by using large wheels and tires. These high-drag items can result in a steeper descent rate.
One important point: These numbers are the percentage of accidents, not the percentage of total aircraft. The fact that 20% of all homebuilt accidents occur during the nominal test period does not mean that 20% of all homebuilts crash during that time period. The vast majority of all homebuilts get through their test period without an accident—about 97.5% of them, in fact.
Other Accident Causes
Figure 4 illustrates the distribution of the accident causes when Pilot Miscontrol is not included. The individual statistics for the CH 601 and CH 701 aren’t included, as their small sample size can produce deceptive results. They scored close to each other, so the “Zenith (All)” category can reasonably represent both.
Figure 4: Accident Causes.
Of key interest are the “Undetermined Loss of Power” and “Engine Mechanical” results. The Zenith series shows a higher percentage of accidents in these categories. One factor could be the wider use of Experimental and non-certified engines in these aircraft (See Figure 5).
Figure 5: Installed Engines.
The CH 750 has all of the features of the CH 701 but in a larger package.
As Figure 6 shows, the Zenith CH 701 has the lowest percentage of fatal accidents of any of the types examined. Generally speaking, when an accident occurs, lower-speed aircraft hit the ground slower, and hence there’s less energy to be transferred to the occupants. But there are a number of aircraft with about the same speed range that have a higher fatal percentage than the CH 701. The 701’s robust construction must receive some credit here.
The CH 601 scores a touch above average, but right in the ballpark with several other aircraft in the same general class.
Figure 6: Fatal Accident Rate Comparison.
The CH 601 series has used a variety of engines. This example mounts a Corvair auto-engine conversion.
Overall Accident Rate
Figure 7 shows the overall accident rate for each type, compared to the results from previous articles in this series. The accident rate is computed by taking the total number of accidents over the 10-year analysis period, dividing it by the average number of registered aircraft of that type, and then dividing by 10 years to get an average annual fleet accident rate. The Zenith series scores a bit high. This could be due, to some extent, to a higher number of accidents during the test phase.
Figure 7: Overall Accident Rate Comparison.
With great (elevator) power comes great responsibility. Trained pilots can accomplish more, but less experienced pilots may run into trouble.
As mentioned, most of the wing-failure cases with the Zenith CH 601 involved either the Light Sport variants or they occurred after the 1998 to 2007 time span of my U.S. accident database. As a result, my study showed the CH 601’s safety record is about average.
The CH 601 seems to appeal to lower-time pilots. The pilots involved in accidents had a median total time (versus time-in-type discussed earlier) of less than 40% of the homebuilt fleet in general (366 hours versus 950 hours). Yet the plane has a pilot-error rate that is less than half of the overall fleet! Obviously, the CH 601 presents few handling surprises to new owners.
The CH 601 XL series’ reputation was tarnished somewhat by a number of wing-failure accidents recently. The company has responded with a set of modifications.
The median total time for Zenith CH 701 pilots is almost identical (360 hours), but the pilot-error rate is closer to the norm. The CH 701 has a deserved reputation as an excellent low-cost STOL airplane. However, designs that support such capabilities often develop a higher sink rate when the airplane gets slow. This isn’t a problem if there has been proper training, but it can be a surprise to some pilots.
There’s no way to tell if this is the cause of the low time in type for CH 701 pilots involved in accidents. The sample size is low enough (only 20 accidents over 10 years) that this may just be a statistical glitch. When seven additional CH 701 accidents that occurred in the time period after my database are included, the median rises from 11 to 14 hours. Still, getting a proper checkout is a vital aspect of the first flight of any homebuilt aircraft, no less for the CH 701 than for hundreds of other designs.
Ron Wanttaja is a systems engineer, engaged in satellite orbit/constellation design and analysis, launch vehicle and onboard propulsion system trades, and operations concepts for space systems. He worked on the early design studies for the International Space Station.