249g

[Max Headroom]

Could i ask how the decision was made that its safe to fly a 249g drone over people (not crowds) and property etc but not for heavier drones to do so?

(genuine question not a wind up)

 

[Cafguy]

It is based on some calculation by some person in some office within the FAA I guess 249 wont hurt as much. BUT who knows really why the 250 number became a limit of some kind.

 

[Max Headroom]

I guess 249 wont hurt as much. BUT who knows really why the 250 number became a limit of some kind.

I guess they had to make a cut off point somewhere i just wondered how they arrived at 249g

 

[Exphantomflyer]

I guess they had to make a cut off point somewhere i just wondered how they arrived at 249g

which came first, the 249g limit or the Mini 1 (the first sub 250g drone)

 

[tstr14]

It's loosely based on terminal velocity of that weight and the potential damage it could cause. Several volunteers were hit objects of various weight and when they got to 250 grams, it stopped killing them, so they subtracted one more gram for safety and that's what they chose.

 

[AlanL]

The more technical answer comes from some calculations of the kinetic energy of impact for an errant drone. Bullet points copied from the Pilot Institute Pt. 107 lecture notes. In the case of a Category 1 drone:



Category 2 for drones over 249g (0.55 lbs):



There are more categories than just these two but this addresses the original question of why 249 grams.

 

[MARK (LI)]

I don't believe that threshold of 250 grams is just particular to the US or our FAA...it seems to be universally accepted around the world, so it would have to come from some widely accepted authority on how much it hurts when you get clobbered by an out of control flying projectile.

 

[Meta4]

Could i ask how the decision was made that its safe to fly a 249g drone over people (not crowds) and property etc but not for heavier drones to do so?

The calculation was made (not necessarily accurately), based on the kinetic energy (ability to cause damage) of a falling drone.
Since kinetic energy is ...


... a heavier drone would have a higher kinetic energy.

 

[123taff]

It is just as well that it is a universally accepted standard, can you imagine the difficulty for manufacturers if different countries (markets) had different standards??

 

[YVRGUY]

It's loosely based on terminal velocity of that weight and the potential damage it could cause. Several volunteers were hit objects of various weight and when they got to 250 grams, it stopped killing them, so they subtracted one more gram for safety and that's what they chose.

Impact tester…@ 249 grams. Sadly, their co-tester didn’t survive when impacted by a 250 gram drone. Why do these brave subjects risk their lives in the name of science and technology??? WHY???

 

[EricJT]

A modified dart board in some bureaucrat's office?

 

[Zbip57]

Could i ask how the decision was made that it's safe to fly a 249g drone over people (not crowds) and property etc but not for heavier drones to do so?
(genuine question not a wind up)

The 250g threshold was arrived at by the November 21, 2015, final report of the "Unmanned Aircraft Systems (UAS) Registration Task Force (RTF) Aviation Rulemaking Committee". A copy of the report can be found at this link: www.hsdl.org/?view&did=788722

It should be required reading for everyone, as it's a great illustration of how government regulations can be created based on ridiculous assumptions, using made up numbers pulled out of thin air, with no actual data or genuine risk analysis to back them.

Furthermore, the 250g limit was never intended to be used as distinguishing between what's considered "safe" vs "unsafe". The goal was only to decide upon a threshold upon which registration would be required.

Quoted from the report:
The stated objective of the Task Force was to develop recommendations for the creation of a registration process, which ultimately would contribute to an enforceable rule imposed by the FAA.
The FAA stated that the intent of establishing this registration framework was to promote a culture of accountability while achieving a maximum level of compliance.

For example, had they recommended a ridiculously small weight threshold (i.e. the weight of an unmanned paper airplane), everyone would have laughed at that and simply not bothered registering. By setting a "reasonable" threshold, more people would be encouraged to register. And the sole purpose of registration was, "to promote a culture of accountability".

Applying some math, while relying on a bunch of highly suspect assumptions, they calculated that the risk of a 250g object shaped like a brick, but with a coefficient of drag similar to a baseball, when dropped from 500ft into a "relatively densely packed urban environment", had a 4.7x10-8 probability of causing a fatal injury.

"or less than 1 ground fatality for every 20,000,000 flight hours of an sUAS".

[...] there were Task Force members who believed it was too conservative, as the weight could negatively impact the credibility of the sUAS registration program and thus lessen compliance levels because it would require registration of some sUAS generally considered to be in the “toy” category.

Was it too conservative?

In the 9+ years since this report was published, exactly how many fatalities have resulted from such a remotely piloted 250g object falling from the sky?

The report also mentioned that this calculated risk from a 250g sUAS is about 1,000 times less than the risk of fatality from General Aviation!

Considering that the acceptable risk levels for commercial air transport are on the order of 1x10-9, and general aviation actual risk levels are on the order of 5x10-5, this level of risk at 4.7x10-8 seems to present a reasonably acceptable risk level to the Task Force for sUAS that meet the aforementioned assumptions. Some members of the task force questioned why sUAS risk level would ever be required to exceed the current general aviation risk level of 5x10-5.

The report can be found at this link: www.hsdl.org/?view&did=788722

The FAA subsequently decided upon the 250g registration threshold, and most other countries soon copied the same number (except Japan which uses 100g!)

 

[Zbip57]

Category 2 for drones over 249g (0.55 lbs):
[...] must not cause injury to a human being equivalent to 11 foot-pounds of kinetic energy upon impact,

That (11 ft-lbs) is even far more conservative than the actual number used by the Registration Task Force. Here's quoting from the report,

Referencing information from a 2012 MITRE report (which further references a United Kingdom Ministry of Defense 2010 study), an object with a kinetic energy level of 80 Joules (or approximately 59 foot-pounds) has a 30% probability of being lethal when striking a person in the head.

Of course this equally means an object with kinetic energy of 59 ft-lbs has a 70% probability of being non-lethal!

Of course, using any number at all based solely on kinetic energy is plain silly. You also need to consider the shape of that object.

Whether it's carrying 11ft-lbs or 50ft-lbs of kinetic energy, being struck by a basketball will create an entirely different probability of injury versus an object with that same kinetic energy but shaped like a lawn dart!