VTOL stands for vertical takeoff and Landing. Seems simple enough. So how does this description fit into the regulatory universe? VTOL designs today are possible through the use of distributed propulsion systems. But there is a bit of a regulatory quandary with these systems. Distributed propulsion provides airframe designers with a vast number of configuration options. Some of these designs look more like helicopters than airplanes, but some look more like airplanes than helicopters. The use of multiple rotors or “thrust units” opens up a lot of possibilities. So, when writing regulations, what do we call them?
On August 11, 1995, the FAA proposed a new aircraft category for pilot certification called Powered-Lift. While the NPRM attempted to be generic, the motivation was really the AW609. The AW609 is a twin engine turbine tilt rotor which is what FAA was envisioning for Powered-Lift. The definition for Powered-Lift can be found in 14 CFR § 1.1.
Powered-lift means a heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.14 CFR § 1.1
Electric distributed propulsion wasn’t really even on the horizon in the mid-1990s. I’m not sure anyone at that time was envisioning flying machines with small multiple rotors. If so, the designs likely included a wing for horizontal flight to increase efficiency. The AW609 used a wing for horizontal flight and twin turbines and rotors for both vertical and horizontal flight. This provided it speed and efficiency. The definition of Powered-Lift ended up including the words “nonrotating airfoil(s) for lift during horizontal flight” based on what was envisioned at the time.
Electric Distributed Propulsion
Regulatory language written around a vision for the future always becomes outdated. Consequently, the definition of Powered-Lift appears to be one of the next victims of technological change. Over the last couple of decades there has been rapid innovation in battery, microprocessor and accelerometer technology. So, this has opened the possibility of VTOL designs which use multiple rotors that do not rely on wings for horizontal flight.
Airplanes, Rotorcraft, and VTOLs
If a vertical takeoff and landing design doesn’t use wings, is it a rotorcraft? Or, if a VTOL uses wings, is it an airplane that can takeoff and land vertically? What’s important is understanding how the definition fits within the regulations. Today the regulations cover normal and transport category airplanes and rotorcraft (14 CFR Parts 23, 25, 27 and 29). However, certain features of VTOL configurations are not covered by the existing regulations.
Continued safe flight and landing is one of the most significant considerations. VTOLs that use several smaller rotors can’t use autorotation as a viable emergency landing technique like a traditional single or dual rotor helicopter. Therefore, system safety requirements are necessary to account for potential failures and meet the intended level of safety. VTOLs that use a wing for horizontal flight will require different emergency landing requirements since they takeoff and land vertically. These are just a couple of examples.
EASA and FAA VTOL Definitions
So, if VTOLs need to be defined so they fit into a “regulatory bucket”, how will they be defined? Powered-Lift no longer seems viable since it doesn’t fit all distributed propulsion configurations.
EASA defines VTOL as “applicable to aircraft with lift/thrust units used to generate powered lift and control and with more than two lift/thrust units used to provide lift during vertical take-off or landing” in SC-VTOL-01. EASA determined that any vehicle with one or two rotors is most likely a helicopter. However, more than two lift/thrust units brings in distributed propulsion requirements to address those features. EASA discusses applicability in this video from the 2019 Rotorcraft & VTOL Symposium.
The FAA hasn’t officially defined VTOL yet. However, there is an indication that FAA will soon need to adopt the EASA definition due to current rulemaking. On March 1, 2021, S.516 Advanced Air Mobility Coordination and Leadership Act was introduced into the US Senate. The Bill defines “The terms ‘‘vertical take-off and landing’’ and ‘‘VTOL’’ mean an aircraft with lift/thrust units used to generate powered lift and control and with more than two lift/thrust units used to provide lift during vertical take-off or landing.” If this bill passes and the definition becomes law, then FAA will have to address the current Powered-Lift definition in 14 CFR § 1.1.
A Note on Addressing Requirements Gaps
An ASTM working group called AC433 is working through all of the gaps between fixed wing and VTOL means of compliance. ASTM published an article in Standardization News which describes the process. AC433 is a working group under ASTM F44. Contact the ASTM F44 staff manager for more information. An overview of ASTM F44 can be found here.
Joel Heck has been with Cessna, now Textron Aviation, for 24 years in engineering. His entire career has been focused on the design approval aspects of FAA certification, and his technical work experience in certification spans the Cessna, Beechcraft, and McCauley product lines.