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On Feb. 16, 2022, Joby Aviation’s first pre-production prototype was destroyed during flight testing near Fort Hunter Liggett, California. In a preliminary report published a few weeks after the crash, the National Transportation Safety Board said only that the uncrewed electric vertical take-off and landing aircraft had experienced an unspecified “component failure”. Last week, the NTSB issued its long-awaited final report on the accident, concluding that the test vehicle had broken apart in flight following the separation of a propeller blade at higher speeds than the aircraft is designed to fly.
Because the accident involved an early prototype operating beyond its design limits, nothing in the NTSB report or lightly redacted supporting documents significantly alters Joby’s certification prospects. As a Joby spokesperson put it, “Experimental flight test programs are intentionally designed to determine the limits of aircraft performance and, in doing so, provide critical insight and learnings that support the safe operation of aircraft as well as inform final design elements.”
Nevertheless, some industry observers expressed consternation that the broken propeller blade had triggered a cascading failure by impacting the adjacent prop assembly. Advocates of eVTOLs have been so vocal and insistent about the safety benefits of redundant propulsion systems that the risks associated with closely spaced propellers have been largely overlooked. Promoters of the new technology may have assumed that designing a small aircraft to be both aerodynamically efficient and incapable of this type of cascading failure is easier than it actually is.
Related: Special Report: The number at the center of an eVTOL safety debate
Regulators have engaged in no such magical thinking. Both the Federal Aviation Administration and European Union Aviation Safety Agency have addressed propeller safety in their proposed criteria for certifying eVTOLs, although their approaches vary significantly in stringency, reflecting their larger divide over the appropriate level of safety for this new generation of aircraft. Much as it does for conventional small airplanes and helicopters, the FAA is prepared to accept some critical parts on eVTOLs; that is, components designed, manufactured and maintained in such a way that they generally do not fail. EASA, by contrast, wants to eliminate these single points of failure, potentially constraining the design space for weight-sensitive electric aircraft.
As with its insistence on a 10-9 numerical level of safety, EASA’s approach is grounded in the assumption that eVTOLs will eventually achieve such a high frequency of operations over densely populated urban areas that any lower standard would be unacceptable. Yet, the market for urban air mobility is still largely theoretical, and while critical parts do sometimes fail, they are not a leading cause of aircraft accidents. There is consequently room for disagreement over how much emphasis should be placed on eliminating critical parts versus guaranteeing safety by other means — a point of contention that, if not resolved, could create very different markets for UAM on either side of the pond.
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