eVTOL landing down/side wash a GPWS???
Below is an informative article titled “FAA study reveals hurricane-force levels of eVTOL downwash and outwash.” This appears to be the first report questioning the compatibility of these aircraft in certain areas.
Is the Aviation Policy GPWS announcing “Whop, Whop, PULL UP, PULL UP”?
Global aviation—manufacturers, potential operators, regulating authorities and local governments—has been actively considering and planning for eVTOL flights since 2021. Much of the discussions were centered around these innovative AIRCRAFT WILL BE GREEN MACHINES : how quiet they would be; how they were likely use environmentally friendly power sources; how these aerial taxis would relieve ground congestion; etc.
The FAA study and the 2023 UK CAA’s Understanding the downwash/outwash characteristics of eVTOL aircraft warn that the landing of these new aircrafts will likely ”surpass most of the air velocity safety thresholds” for people in proximity to landings. The Beaufort scale, a standard for measuring wind impacts, put the speeds measured in these studies as ranging from a strong gale to hurricane threshold levels and beyond.
The authors were pessimistic about the suitability of existing buildings and parking lots as landing areas. It flagged the need to reconsider the safety dimensions of vertiports and creation of areas to protect ground crew and passengers.
This PULL UP aural warning does not mean that aviation should pass on eVTOLs, but as the GPWS signals, an altitude or attitude change may be timely. Not stop, but reexamine some of the operating parameters given this new information.
FAA study reveals hurricane-force levels of eVTOL downwash and outwash
By Dominic Perry10 January 2025
Electric air taxis could generate downwash and outwash (DWOW) equivalent to a hurricane-force wind which ”surpass most of the air velocity safety thresholds” in published guidance, according to research caried out by the US Federal Aviation Administration (FAA).
Released in late December, the report – Electric Vertical Takeoff and Landing (eVTOL) Downwash and Outwash Surveys – is intended to inform vertiport design as the regulator prepares for potentially widespread adoption of the novel aircraft in the coming years.
Rotorcraft downwash and outwash poses hazard for people, aircraft and objects in the landing area
“eVTOL aircraft DWOW can pose significant risks to people and property and must be accounted for in vertiport design,” says the FAA. There have been multiple cases of BYSTANDERS BEING KILLED OR INJURED BY ROTORWASH from helicopters coming in to land, it notes.
Using three unidentified eVTOL aircraft prototypes provided by their manufacturers, the FAA’s study found that the maximum instantaneous DWOW velocity recorded was almost 100mph (86kt/160km/h) at a distance of 41ft (12m) from the centre of the take-off and landing area.
All three aircraft tested generated a PEAK DWOW VELOCITY IN THE 100MPH RANGE, the FAA study shows.
Even 100ft from the centre of the take-off area, speeds of 60mph were still recorded, the FAA adds, and the highest moving three-second 95th percentile velocity was 84mph at 23ft from the centre. The moving 95th percentile data for one of the three eVTOLs – a remotely piloted example – shows a figure of almost 64mph 126ft from the centre of the take-off area.
The Beaufort scale – a means of estimating wind speed based on visual cues – says a speed of 32-38mph, or a strong gale, would set whole trees moving, and offer resistance when walking against it, while ANYTHING OVER 74MPH IS CATEGORISED AS A HURRICANE-FORCE WIND.
FAA guidance says “most rotorwash-related mishaps can be avoided if separation distances are maintained so that impacting rotorwash-generated velocities do not exceed 30-40kt” or 34.5-46mph.
“eVTOL aircraft DWOW at the [safety area] and beyond surpass most of the air velocity safety thresholds” found in FAA guidance, the report states.
“The eVTOL aircraft surveyed produced high-velocity DWOW flow fields that could easily go beyond the safety area of a vertiport,” it adds, noting that it could “create safety risks to people, aircraft, equipment and infrastructure”, both on and off site.
“eVTOL OEMs propose high-volume, high-tempo eVTOL operations in urban areas, which have even greater potential of impacting bystanders with DWOW than traditional helicopters at heliports.”
The findings suggest that the eVTOL industry’s hope that landing areas could be established atop existing structures, parking garages for instance, are unlikely to be easily realised.
To mitigate the risk, the FAA recommends that vertiports create “downwash caution areas” which should be operational “when and wherever DWOW velocities exceed 34.5mph” which “moves with the aircraft as it maneuvers”.
The FAA study vindicates earlier research carried out by the UK Civil Aviation Authority, in which advanced computational techniques were used to predict very similar airspeeds in the outwash to those that were measured.
The three aircraft studied, all pre-production prototypes, “varied in configuration, the number of propulsion systems, blades per propulsion unit, and maximum take-off weight, all of which were less than 6,500lb [2,950kg]”. Only one of the trio was flown by a pilot on board, with the other two operated remotely.
Performed in 2023 and 2024, the test flights were carried out in daylight, visual meteorological conditions. Each pilot was to conduct “several preset maneuvers based on their and the OEM’s capability and risk tolerances”.
However, the FAA notes: “At the time of testing, aircraft flight profiles were limited due to the experimental nature of the aircraft and their early stage of development.”
The novel multi-rotor designs and complexities of the wake-to-wake, wake-to-fuselage and wake-to-ground interactions – which also varied with speed, heading and altitude – resulted in “non-uniform and high-velocity DWOW flow fields that can easily go beyond the safety area of a vertiport”, it says, meriting further research.
In a real-world environment those flow fields could also be impacted by ground structures that were not present in the study.
This article has been edited to correct a mistaken comparison to downwash figures for certain rotorcraft types.
