• 18 Jan 2025 05:56 AM
• 0

Is Autorotation Necessary for Powered-Lift Aircraft?

By Marty Shubert
Vertiflite, Jan/Feb 2025

In October, the US Federal Aviation Administration (FAA) published RIN 2120-AL72, “Integration of Powered-Lift: Pilot Certification and Operations; Miscellaneous Amendments Related to Rotorcraft and Airplanes,” which included the Special Federal Aviation Regulation (SFAR) for powered-lift aircraft, applicable to both tiltrotors and winged electric vertical takeoff and landing (eVTOL) aircraft. Several associations worked collaboratively to study the document (see “Washington Report,” Vertiflite, Jan/Feb 2025) and identified areas needing further clarification.

One of these areas was regarding autorotation. The new rule for Title 14 of the US Code of Federal Regulations (CFR) Federal Aviation Regulation (FAR) Part 194.302(d) states: “The final rule provides that powered-lift operating in vertical-lift flight mode that have demonstrated a capability to autorotate or conduct an approved equivalent maneuver are allowed the same minimum safe altitudes as those afforded to helicopters” (emphasis added).

This begs the question of what is an “equivalent maneuver” and whether it can be at least as safe as an autorotation in order to meet the two goals of airworthiness and operations: protection of the passengers and crew and not providing undue hazard to other persons and property. While the industry associations have been engaging directly with the FAA for written determination on how powered-lift aircraft will proceed, this article describes what this equivalent maneuver might look like to meet the safety requirement.

The following details should also be useful in understanding some of the hurdles that the eVTOL industry faces in certifying their aircraft and how they might be negotiated. It also provides an opportunity to address one of the prevailing questions of how eVTOL aircraft using distributed electric propulsion (DEP) can be safe while not being able to autorotate.

Understanding the Old

The FAA uses the minimum safe altitude (MSA) to denote an altitude below which is unsafe to fly — due to the presence of terrain or obstacles — for different categories and classes of aircraft. Legacy FAA documents appear to be predicated on the need for an immediate landing and the ability to find and use appropriately safe landing areas that do not risk damage to other persons or property.

For this reason, helicopters are granted a lower MSA due to their ability to perform an autorotation in an emergency. The autorotation would be considered the extreme case for emergency failure states, while less extreme precautionary landings are assumed to provide more control and aircraft performance that can utilize the same or smaller landing areas.

The emergency case of autorotation in a helicopter could be required for one of the following reasons: engine failure in a single-engine helicopter, dual-engine failure in a dual-engine helicopter (e.g. due to engine fratricide), a tail-rotor failure or fuel exhaustion.

The New Powered-Lift Requirement for MSA

While tiltrotors can provide some limited autorotation capability, most other powered-lift aircraft, characterized as DEP aircraft, utilize RPM-controlled propellors or low-inertia, variablepitch propellers that do not provide for autorotation capability. The FAA has, therefore, introduced a path for certification of these aircraft, but one that sets a high hurdle for safety and will require substantial justification for the eVTOL aircraft in performance-based safety objectives.

To better understand what is meant by an autorotation maneuver equivalent, we must first look at wording of the requirements for the new poweredlift aircraft that would indicate the need for an equivalent maneuver to autorotation in an emergency. To summarize the requirements from the FAA’s Draft Advisory Circular AC 21.17-4 “Type Certification—Powered- Lift” released in June 2024: the FAA requires that an aircraft that has experienced the most adverse effect on performance or handling qualities from failures of the flight control or propulsive systems — which are not shown to be extremely improbable — must be able to continue safe flight and landing (CSFL). When unable to meet CSFL requirements due to other, more improbable failures or conditions that may occur, the aircraft should be able to achieve a controlled emergency landing (CEL) that provides for pilot control of the direction and area for landing while reasonably protecting occupants from serious injury. It is the CEL that addresses the emergency conditions that are inclusive of autorotation or an “equivalent maneuver.” This requirement also recognizes that in DEP aircraft the effectors that provide aircraft performance also likely provide for control of the aircraft.

How do we justify what is considered an equivalent maneuver? The Powered-Lift 21.17-4 provides a path that may be considered through FAA Orders 8110.4 and 8110.112: “Equivalent level of safety (ELOS) findings are granted when literal compliance with a certification regulation cannot be shown and compensating factors exist which can provide an ELOS (see 14 CFR § 21.21(b)(1)). Compensating factors are normally any design changes, limitations, or equipment imposed that will facilitate granting the equivalency. An issue paper documents the evolution and conclusion of the request for an ELOS finding.”

Equivalent Level of Safety for Powered-Lift

DEP powered-lift aircraft (and similarly tiltrotors) can provide for an equivalent level of safety to that of autorotation for the following reasons. It should be emphasized that the following is not meant to be proscriptive but offered as one of many ways that this ELOS might be offered.

The proposed tiltrotors and DEP aircraft:

• Do not have tail rotors, which require autorotation when they fail.
• Provide for separation of engines/ motors to reduce the likelihood of fratricide. An assumption for tiltrotors in this regard is that the redundant interconnect drivetrain of the tiltrotor is sufficiently isolated from the failing engine or that a failure of the interconnect drivetrain will not cause engine failure.
• May still employ autorotation — as is the case in tiltrotors — as one of many ways of handling CEL.
• Utilize redundancy in drivetrain or motors that provide for partial-power operations for engine/motor failures. These partial-power operations may still provide for CSFL, which provides for greater landing-site opportunities than the autorotative state. The variable configuration of these aircraft will also provide for various landing alternatives. In the event that the partial-power does not allow for CSFL and requires a CEL, one can expect that performance will be demonstrated to still allow for transition from wingborne flight to the vertical takeoff and landing (VTOL) configuration for controlled landing to a site where minimum-power landings can be made at low speed with minimum ground distance required.
• One can expect that there will be supporting capabilities in this regard, such as continuous real-time performance calculation and realtime predictive algorithms for both nominal and failure conditions that support safe, partial-power landings, as well as pilot assistance algorithms for continuous real-time calculation and display of landing sites that enhance the pilot decision process.

Finally, energy exhaustion must be addressed. This is a standing question with eVTOL because energy depletion occurs differently than in aircraft employing fossil fuels, and current battery technology allows for only limited energy supply and flight time. This, then, is included here as a reason for CEL. This also recognizes that the effectors that DEP uses for lift are also used for aircraft control in making emergency landings in small areas in the VTOL configuration. For this reason, energy reserves should be above some threshold of partial-power performance and controllability for landing. The idea here is to never fully deplete energy reserves.

This could be accomplished in some of the following ways, to be considered during each aircraft’s type-certification process. Ideas include:

• Continuous, real-time electrical power monitoring and real-time predictive algorithms for both nominal and failure conditions (to include thermal runaway) that support energy thresholds for safe landings.
• Pilot assistance algorithms for continuous, real-time calculation and display of landing sites that enhance the traditional pilot decision process. Landing site size could also figure into required landing ground-roll distances that may be required.
• An auto-descent or descent cueing mechanism that is triggered when a CEL threshold is breached, where controlled, partial-power descent will be commanded while preserving power for aircraft flight controls that support safe landing. This might look a lot like autorotation in the VTOL configuration. As opposed to actual autorotation, this mechanism would be predicated on height above ground and calculated landing site availability. In this regard, one can acknowledge that lower operating altitudes would be beneficial. This would also be considered advantageous to traditional autorotation, because descent rates will likely be manageable to some extent and provide for timely decision making.

Conclusions

Folding the argument back to the MSA discussion, we can see that the lower MSA would arguably provide for higher safety margins in powered-lift aircraft regarding risks of energy exhaustion, fire or thermal runaway, as long as the “equivalent maneuver” can be demonstrated.

Additionally, by employing the methods described above, one can anticipate that the DEP pilot is more likely to anticipate the CEL more rapidly than in traditional immediate failures that result in autorotation. This timeliness will likely provide for enhanced safelanding opportunities when the variable configuration of the aircraft is considered. In this regard, wing-borne flight will provide for less energy draw than VTOL flight and therefore increase safe landing opportunities. The challenge comes in being able to rapidly understand those alternatives and decide on the best option. With the rapid advance of software and avionics, this may be quite addressable.

To accomplish the ELOS described previously will require designs in propulsive and flight control systems that are robust to failure, provide sufficient redundancy and employ supporting subsystem innovations as described above. These supporting subsystems must also achieve a level of robustness and redundancy that might be equivalent to that of flight controls.

This discussion provides an example of the nuanced considerations in certification and operations of these new revolutionary aircraft. The CEL definition as provided in AC 21.17-4 is inclusive of airplane-like glide, helicopter-like autorotation, and an equivalent means that may be similar to glide or autorotation. This would indicate that the CEL definition is based solely on the mitigation of risk to the occupants of the aircraft and not any other persons or property. The operational MSA discussions, however, would then represent a subset of CEL, or an inferred higher restriction that not only minimizes risk to the occupants of the aircraft but also other persons and property. What has been offered here is one view of how to achieve this.

About the Author

Marty Shubert is a consultant test pilot at Tiltrotor Flight Test Consulting, LLC (www.tiltrotorflighttest.com), in Patuxent River, Maryland. He has performed as an experimental test pilot for 27 years. He served for four years as an experimental test pilot with the U.S. Army Airworthiness Qualification Test Directorate and then as a Bell test pilot for the next 23 years, where he conducted experimental flight tests of the MV-22 and CV-22 Tiltrotors. He retired as Bell’s Associate Technical Fellow and Experimental Test Pilot in 2020 and has since consulted with NASA, the FAA, and others on design and certification of evolving eVTOL concepts.