I’m not against UAVs. What I am against is fencing off parts of the sky from the flyers who were here first in the interest of UAV testing. I’m also not for rolling the dice with the lives of fellow aviators and their passengers.
A central challenge for integrating UAVs into the National Airspace System (NAS) safely is their current inability to see/sense-and-avoid. However, I look for solutions to situations given the limitations of existing science and technology. I have a solution to propose given what exists today.
Much of the see-and-avoid concept required of us under 14 CFR 91.113 depends upon both recognition of the potential for the collision with another aircraft and our rate of closure. That means we have an idea of the distance and speed of another aircraft based upon our sense of scale: A Boeing 747 that fills a particular space in our windscreen is certainly further away than the Piper Cub filling that same space.
So when aircraft of a smaller scale fill that same space ahead of us and we don’t recognize that difference in scale, we don’t recognize that the smaller aircraft is closer — requiring a faster response to avoid a mid-air collision.
To date there has been no systematic training to educate civil aviators in the United States about UAVs. Up to now we really haven’t had to worry much about them. But that’s about to change. There are six sites that are to begin UAV testing later this year. So where’s that safety training? You can only mitigate the risks you know about.
I’m absolutely against the use of TFRs in support of UAV testing. If the same principles of risk mitigation fence us out of UAV testing airspace, then why shouldn’t those same principles result in UAV operators not launching their UAVs, too? What’s good for the goose is good for the gander. It’s national airspace, not corporate airspace.
Here’s a potential solution for sharing the friendly skies with our remotely piloting colleagues that I wish to share.
I’m going to use as a basis for this solution a base altitude of 400 feet AGL. That’s because there is a long established practice for remotely controlled aircraft to be flown below 400 feet in accordance with FAA AC 91-57, Model Aircraft Operating Standards. That means any remotely piloted aircraft flying above 400 feet must be a UAV.
According to FAA Policy 8900.227, Unmanned Aircraft Systems (UAS) Operational Approval, a UAV that flies under IFR must have an operating Mode C transponder (a Mode S transponder is preferred). Go only half a step further and require that — as a minimum — a UAV always have a transponder with both Modes A and C. This means that the pilotless aircraft could be seen by ATC’s secondary radar system. Regardless of the size of the UAV and its radar cross section, the squawking UAV will be seen on the ATC controller’s radar scope, if the UAV is within line of sight of the radar antenna.
Next, don’t make the direct line of communication between the UAS controller and ATC preferred; make it a requirement in the Certificate of Authority or Special Airworthiness Certificate. If ATC detects conditions that could lead to a possible mid-air collision, they shouldn’t have to waste time establishing contact with the UAS controller. We’re talking safety here. Why wouldn’t you stack the odds in your favor?
Finally, make sure ATC can see a primary return on their radar scope from 400 feet AGL and up. That is, make sure they can detect a manned aircraft that is not squawking its transponder — referred to by the FAA as a non-cooperating aircraft.
Being able to see those two targets on their scopes affords ATC the opportunity to issue avoidance vectors to the UAV operator — a workaround for the see-and-avoid requirement in the NAS. There are, however, limits to all of this being possible.
The foundation to this solution is that the UAV must be able to be detected by ATC. This detection is easy to predict using the Radar Horizon Equation. For example, using a radar antenna height of 100 feet AGL and the RC-to-UAV transition altitude of 400 feet AGL, the maximum detection range at 400 feet AGL is approximately 37 nautical miles from the antenna. (For an explanation of why this works, click here. If you just want to enter numbers and let the webpage do the math, try Foruno U.S.A.’s Radar Horizon Calculator.)
Beyond 37 nm the radar controller can’t track aircraft down to 400 feet AGL. Yes, there can be obstacles and obscurants that could make the maximum distance less than 37 nm. Each UAV range would have to be considered individually based on existing conditions and obstructions, just like they are supposed to now.
The other reason why 37 nm works well is that distance is less than the 60 nm maximum detection distance for the current ASR-11 Air Traffic Control Radar System to pick up the primary (non-transponder) radar return of a typical small general aviation aircraft. With its transponder off or in standby, an aircraft can still be “seen” by ATC from only the reflected radar energy.
This approach does place a small manpower burden on the FAA’s ATC resources at the supporting radar facility. But it’s also the FAA that’s moving the UAV agenda forward — at the direction of Congress — without a fair sharing of the see-and-avoid responsibility between the pilots in manned aircraft and the UAV operators on the ground.
This approach of flying UAVs in such a way that ATC can issue avoidance vectors to UAS operators in lieu of see-and-avoid capability appears a practical way forward using existing capabilities within their limitations.
I’m sure there are alternate solutions that I haven’t considered. In fact, I believe that none of us is as smart as all of us. I hope that if there are other and better ideas out there among my readers that you will comment below and contribute your solution.
What I hope we can avoid is injury or loss of life during the upcoming UAV testing period and beyond.
Cheers from The Alamo.