The deadline for comments on the FAA’s proposed regulations for operations of powered-lift aircraft closed on Aug. 14, and industry is calling on the regulator to adopt performance-based energy reserve requirements rather than carry over the prescriptive limits for airplanes as laid out in its notice of proposed rulemaking (NPRM).

Citing a lack of operational data to support a less-restrictive fuel reserve, the proposed Special Federal Aviation Regulation (SFAR) for pilot certification and operations would require powered-lift aircraft to meet the same reserve requirements as fixed-wing airplanes: 30 min. for daytime visual flight rules (VFR) operations and 45 min. for nighttime VFR and day or night instrument flight rules (IFR) operations.

Developers of electric-vertical-takeoff-and-landing (eVTOL) aircraft point out the initial aircraft have typical flight durations of 20-30 min. and requiring prescriptive 30-min. or 45-min. energy reserves could double or even triple the size of the battery required. They are calling for a performance-based requirement.

A report submitted by eVTOL developer Supernal in response to the NPRM estimates the proposed reserve requirements would increase operating costs by disrupting the optimum charging cycles for limited-range vehicles, reducing battery life and requiring more frequent and costly replacements.

Industry has cooperated to produce a white paper on performance-based energy reserves for the General Aviation Manufacturers Association (GAMA). “If you can fly for 30 minutes, a 30-minute reserve does not make sense. But we can’t just say we can’t do it,” Valerie Manning, chief commercial officer of eVTOL developer Overair, told the FAA’s Advanced Air Mobility Summit in Baltimore on Aug. 2.

The reserve requirements for airplanes were set to account for inaccuracies in fuel gauging, Greg Bowles, head of government affairs for Joby Aviation, told the summit. But eVTOL developers can “very accurately” measure the energy remaining in batteries and set zero as the power required to meet the vehicle’s rated performance. “If we’re below zero, we’re below the performance required,” he said.

Despite this, Joby has designed its eVTOL to meet the 30-min. energy reserve requirement. “We have a big battery,” Bowles said. “We can have a very rational conversation on what is reasonable performance. But there is risk that will take time, so Joby will make sure we meet the existing requirement.”

In its response to the FAA’s NPRM, Archer Aviation says “the application of airplane fuel reserves to the powered-lift category is inappropriate and fails to consider the unique operational capabilities” of the aircraft, namely the ability to take off and landing vertically and inherently limited flight endurance.

While there are no operating powered-lift aircraft on which the FAA can collect data, Archer says “operational data does not inform this determination, aircraft certification does ... [The] operational range of the aircraft will be known at type certification and this information can be used to inform appropriate levels of fuel reserves.”

Archer recommends that the FAA follow the long-standing precedent of performance-based contingency fuel requirements for Part 121 scheduled airlines, which reduces reserve requirements based on the carriers’ fuel planning and tracking capabilities.

The OEM says the regulator should allow similar approvals for Part 135 air taxi operations and “looks forward to demonstrating its ability to develop a performance-based concept for safely managing planned fuel reserves.”

Other than its comments on the FAA’s NPRM, Archer notes that its eVTOL design has multiple system redundancies and none of the single-point failures of helicopters, and this should be taken into account when determining reserve requirements. 

In its comments, Beta Technologies points out the rate of energy consumption for an eVTOL is significantly higher in vertical flight than in cruise and “not generally proportional to duration of flight or distance flown. Electric-propulsion powered-lift aircraft will have variable energy reserve needs based on the planned flight profile.”

And then there is the environment within which eVTOL air taxis will operate. “These operations will have an information layer that will support performance-based decision-making, and routes over cities will be completely surveyed,” says Erick Corona, director of conops and airspace ecosystem for Wisk Aero.