The FAA's EB-105 update seems to beg more questions than it answers...

What?

The FAA recently released an update to Engineering Brief (EB) 105 , which covers vertiport design (it is now officially EB-105A).

Who?

The FAA.

Comments

While I won't go into detail on the entire EB, I will point out some concerns that I have with specific parts of the document.

Keep it simple, right?

One change that perplexed me is how the FAA took a standardized, already known and used concept and made it more complicated than it needs to be. What I am referring to is the Controlling Dimension "D" which has been around for quite some time in the helipad design world. The FAA decided it needed to muddy the water with an additional dimension for vertiports designated "D-p." Here are the definitions of D and D-p next to one another.

Controlling dimension (D): The diameter of the smallest circle enclosing the VTOL aircraft projection on a horizontal plane, while the aircraft is in the takeoff or landing configuration, with rotors/propellers turning, if applicable.

Propulsion dimension (D-p): A subset of the controlling dimension, defined as the diameter of the smallest circle enclosing all the propulsion units, including their propellers, rotors, fans, etc., on a horizontal plane, while the aircraft is in the vertical takeoff or landing configuration, with rotors/propellers/fans turning, if applicable.

Here is how the two compare on something that looks like a BETA eVTOL:

For a Joby-esque eVTOL it might look like this:

To me, the addition of D-p seems unnecessary and potentially confusing. Theoretically, the existing D should enclose the entire aircraft, rotor/propellers included. If the definition is the "smallest circle enclosing the VTOLaircraft projection on a horizontal plane" this would go out to the prop/rotor tip, as applicable to a specific aircraft (e.g. Volocopter or Joby). Having a distance D-p does not include all components of the aircraft, such as the outer wing/tail shown in the example resembling a BETA aircraft, seems counterintuitive. If the goal of D is to segregate the aircraft from obstacles and surfaces that could negatively impact operations, then it should always use the "worst case" dimension, i.e. what is the imaginary circle around the aircraft that should overlap with objects/locations incompatible with safe operations.

More mud in the water...

For whatever reason, the FAA further complicated things by subdividing the TLOF, FATO, and Safety Area dimension requirements into non-powered lift and powered lift categories. I am all for the distinction between these two types of VTOL finally being properly defined, but it is unclear why they need their own dimensions. See below for the FAA's new table on the topic:

It also does not make sense to have a distinction between non-powered vs powered lift in terms of the size of the TLOF. The document states the non-powered lift a/c have "the ability to land at facilities with smaller FATOs" yet this document shows a TLOF of 1.88 x D-p vs. simply 1 x D-p for powered lift a/c. Then the two a/c types differ again for the FATO size, with the non-powered lift still being 1.88 x D-p and the powered lift changing to 2 x D-p. The safety areas for both aircraft types are 2.5 D (not D-p).

These different numbers and distance measures (D vs D-p) make the whole planning process burdensome and confusing. Also, these do not appear to have (properly) considered the DWOW from VTOLs. It is almost assured that DWOW will be an issue beyond 1 x D-p and likely the issue for 1.88 x D-p. This will also pose a challenge to vertiport stakeholders as they try to detemine what dimension(s) to use. If there is a range of VTOL types that will operate at the site, it makes the whole TLOF and FATO size issue increasingly complex.

Referring back again to the D vs D-p issue. This section talks about the FATO being clear of obstructions except for frangible navaids. If the goal is having an area clear of potential collision hazards, D makes more sense than D-p if D includes all parts of an aircraft and D-p does not.

There she blows...

The maximum outwash/downwash (DWOW) velocity value reported in the EB (34.5 mph) were nowhere to be found within the document the FAA references. Instead, in the reference (pp 150-152), the wind speeds noted to be of concern were 51 knots to unsteady a person and 75 knots to knock someone over. Even 34.5 mph could be too high for some populations to withstand. Per the article linked HERE , it is recommended that persons be protected for wash speeds over 18 mph which uses the recommendations of the Lawson Comfort Criteria.

Considering the concerns mentioned in lines 610-616, the recommendations of the CARI article are even more relevant. Evidence also shows that some VTOLs have stronger "jet" flows that travel further and are more unstable than traditional DWOW. The question is at what distance will DWOW influence the surface or objects? This distance is the one that should be used to determine the size of safety areas and protection requirements for persons and/or objects.

Another point about DWOW... There should be protected or caution areas to separate persons and property from DWOW. A warning sign for DWOW should also be provided and recommended for use similar to the vertiport warning sign example in the EB.

As Beavis once said...

Considering what IS known about LiPo battery fires, especially in EVs, there should be recommended standards for the vertiport surface/environment to be able to withstand long-term (2+ hours) of temperatures associated with these types of fires, which can reach 5,000 F. Rooftop facilities that fail to meet such a standard could result in catastrophic damage (or destruction) of the host building. Also, vertiports co-located with an airport building could have the same issues in case of a fire.

That's all folks...

Hopefully the FAA will get some good comments and will revise their guidance accordingly. Now isn't the time to reinvent the wheel - keep it simple and safe.

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