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The Data and Facts About VLOS Flight Distance for UAS

The purpose of this article is to highlight the actual limits of flying small unmanned aerial vehicles (sUAVs), specifically multi-rotors, under the Federal Aviation Administration’s (FAA) Visual Line of Sight (VLOS) regulations.

This time-consuming research began with a simple question: how far you can see and fly while still meeting the regulations under 14 CFR § 107.31 that defines legal VLOS operation.

In talking about this with others, many had assumptions about how far they could see their quad but there had been no real-world flight testing to verify the hypotheses.

The flight test results surprised even experienced pilots. The data has made us realize that either the FAA needs to change the VLOS requirements or we pilots must assume the liability, responsibility, and potential fines since many of our flights simply do not comply with the current regulations.

Testing Results Summary

Day VLOS Flight Data

Compliant VLOS flight distances during the day were 1,200 feet if eyes were never taken off the aircraft and as little as a maximum of only 500 feet from the pilot and/or Visual Observer (VO) if the responsible party looked away, even momentarily. If the aircraft was further away when contact was lost the aircraft had to be flown back towards the party watching the aircraft to be reacquired.

Night Flight VLOS

To maintain UAS VLOS compliance night flight needed to be within about 65 feet of the pilot and/or Visual Observer.

Testing data can be found here.

Testing Background

Although our testing is limited, subjective, and somewhat crude at times, I believe it serves as a starting point to understand and compile VLOS flight realities. Amendments will be made to this commentary as additional flights, experiences, and contributions from other pilots are obtained.

I will also point out some of the confusion that we sUAV pilots face as a prelude to encouraging more clarity from our friends at the FAA and those VLOS areas of manned aircraft flight that I believe apply to the unmanned world and those that do not. Along the way, I will also be discussing the under-appreciated yet highly valuable use of visual observers (VOs) to reduce pilot workload and increase safety.

And finally, I will also be sharing takeaways (many based on real-life experiences and research) that I believe will help both the FAA and unmanned flight crews foster our joint quest to developing an amicable, long-term, mutually beneficial relationship.

I extend my heartfelt thanks to my partner Bruce Christianson, whose been by my side for years and helped me make all this possible, and Steve Rhode who has been a tremendous source of information and a great sounding board.

My Pilot Point of View

In this article I will be combining many personal experiences and perspectives specifically, that of an FAA certified manned aircraft pilot (MAP) with SEI, MEI, Commercial ratings and a skydiver driver with 2,583+ total flight hours, a previous FAA 333 exemption, and current NPRM Part 107 remote certificated pilot (RCP) and as a limited recreational operator (LRO – me since 1995, 1,600+ hours) who fly radio-controlled aircraft, mostly fixed wings, for fun.

MAPs look at the entire drone operation subject thru a different lens since we’ve had special training and flight experiences that shape our thought process and interpretation of sUAS (small unmanned aerial systems) operations.

I believe that some of our LRO and many of our RCP brethren don’t understand why and it would be helpful to get everyone on the same page of facts. We MAPs have gone thru rigorous education, training, testing, sim work, and check rides (not to mention the $’s involved) to achieve our licenses and ratings far beyond memorizing the answers necessary to become a remote certificated (albeit accidental) pilot. Imagine how you would feel if, on your next trip from MSP to PHX, the 767-captain announced on the intercom and that he/she had just passed their written exam but had not been trained or certified to fly this aircraft. Are you still sitting in 17C?

Concerning LROs, it takes real skill, training, and practice to fly radio-controlled model aircraft to keep them from auguring in. I have yet to see one of these critters take off, hover, return to home, and land on their own. At Academy of Model Aeronautics (AMA) fields, like the one I fly at, you must demonstrate competency and proficiency before a flight instructor signs you off for solo flight.

UAS VLOS Flight Testing

Shall we talk about weather, weight & balance, the center of gravity (especially when its off), dual rates, expos, control reversals, or profuse sweating during maiden flights?

It’s All About What We Can See

We all at times fly under similar rules. MAPs can fly under Visual Flight Rules (VFR), RCPs and LROs fly under Visual Line of Sight (VLOS) directives. Concerning LROs, if we lost contact for even a moment, we’d either find our aircraft has departed the area never to be seen again or we’d bring home what’s left in a dustpan or what one of my fellow pilots calls a “pile of sticks”.

The FAA is responsible for managing our national airspace system (NAS). In developing rules for drone flights, they’ve applied manned aircraft regulations to our flying since those rules already exist. But I believe this creates several ambiguities and concerns that make drone flying problematic when it comes to satisfying some of those regulations. I will attempt to explain.

Current Rules and Regulations

Let’s begin with the salient sections of 14 CFR (code of federal regulations) §107.

14 CFR §107.31 Visual line of sight aircraft operation.

(a) With vision that is unaided by any device other than corrective lenses, the remote pilot in command, the visual observer (if one is used), and the person manipulating the flight controls of the small unmanned aircraft system must be able to see the unmanned aircraft throughout the entire flight in order to:

(1) Know the unmanned aircraft’s location.

(2) Determine the unmanned aircraft’s attitude, altitude, and direction of flight.

(3) Observe the airspace for other air traffic or hazards; and

4) Determine that the unmanned aircraft does not endanger the life or property of another.

(b) Throughout the entire flight of the small unmanned aircraft, the ability described in paragraph (a) of this section must be exercised by either:

(1) The remote pilot in command and the person manipulating the flight controls of the small unmanned aircraft system; or

(2) A visual observer.

An acronym some people use to remember this is LAASDON:
L–location.
A–altitude.
A–attitude.
S–speed. (which is not currently included in 107.31)
D–direction of flight.
O–obstacle clearance.
N–not a hazard to other aircraft or persons or property on the ground.

As I review 107.31 here is how I interpret it:

Section 1) Location, we interpret that as: can I see my drone in terms of azimuth (distance) and elevation (altitude).

Section 2) Attitude, as a MAP I’m thinking of pitch and roll displayed on the attitude indicator (AI) of my aircraft. Side note: I recently had to describe how the DJI GO FLY app’s artificial horizon display differs from that of a manned aircraft but that’s another story. Multi-rotors (in non-aircraft mode) fly in a flat plane so what is it I’m supposed to attest to? We chose to change this to Orientation. In other words, do we know where the front of the drone is pointed at any given time? This is part of our emergency SOP (standard operating procedure) if we lose our screen display (as we’ve experienced during cold weather ops here in Minnesota) or return to home (RTH) failures. We also practice this by having the RCP / PMC (person manipulating the controls) look away while the drone is reoriented by another pilot to see if they can bring it back to the home point.

Altitude, without some sort of telemetry, our guesses turned out to be the most frustrating and discouraging part of our experiments.

The direction of flight, if flying perpendicular to our line of sight, we felt comfortable reporting that. If the flight path was at an angle (say <45°), less so. If the flight is directly along our line of sight we would not know if it’s hovering, moving toward, or away from us unless close in.

Section 3) Observe for other traffic or hazards, which we can do via a preflight site inspection and checking the area we’re to fly in with the FAA VFR Sectional Charts. If we’re an emergency responder, SAR pilot, or the like, we don’t have that luxury of time to assess the flight environment.

Section 4) No endangerment, the same answer as the section 3) response.

Noticeably absent (for observation) is the airspeed of our drones.

The acronym we used in testing was LAADON: L–location, A–attitude (Orientation), A–altitude, D–direction of flight, O–observe airspace for traffic or hazards, N–not endangering life or property.

14 CFR §107.33 Visual observers.

If a visual observer is used during the aircraft operation, all of the following requirements must be met:

(a) The remote pilot in command, the person manipulating the flight controls of the small unmanned aircraft system, and the visual observer must maintain effective communication with each other at all times.

(b) The remote pilot in command must ensure that the visual observer is able to see the unmanned aircraft in the manner specified in §107.31.

(c) The remote pilot in command, the person manipulating the flight controls of the small unmanned aircraft system, and the visual observer must coordinate to do the following:

(1) Scan the airspace where the small unmanned aircraft is operating for any potential collision hazard; and

(2) Maintain awareness of the position of the small unmanned aircraft through direct visual observation.

§ 107.37 Operation near aircraft; right-of-way rules.

(a) Each small unmanned aircraft must yield the right of way to all aircraft, airborne vehicles, and launch and reentry vehicles. Yielding the right of way means that the small unmanned aircraft must give way to the aircraft or vehicle and may not pass over, under, or ahead of it unless well clear.

(b) No person may operate a small unmanned aircraft so close to another aircraft as to create a collision hazard.

But there are circumstances where RCPs need to be aware and vigilant for several reasons. As you know, if you’re inspecting a tower you can fly your drone within a 400-foot radius of that structure but no higher than 400 feet above the structure’s immediate uppermost limit. However, what if that altitude is above the Maximum Elevation Figure (MEF – listed in the Sectional Charts) in the area you are flying? A MAP theoretically could run into your drone if they are flying at the MEF. Also, if the tower is 900 feet high and the weather forecast is KMSP 261137Z 2612/2718 04005KT P6SM OVC008, can you still fly?

No problem you say, the MAP can avoid my drone. Not so bucko!

Let’s say I’m cruising along in my Bonanza at 174 knots (200 mph), or 293 fps. If you are in my flight path and our best slant range visibility (see attached test results document) is 900 feet, that gives me a total of 3.1 seconds to not only find your drone but take evasive action to avoid hitting it.

There is no doubt in my mind that at the same altitude your drone will end up impacting my aircraft. Thus, it is the RCP’s responsibility to see and avoid this situation since they are in a far better position to avert a potential mid-air collision.

We MAPs have challenges spotting airborne traffic even when ATC gives us their location relative to our position, the direction of flight, and altitude. TCAS (traffic collision avoidance system) helps. Here are a few photos that help prove my point.

Testing

We conducted many tests (see attached test results document) to determine the extent to which we experienced drone loss contact under varying weather conditions and flight scenarios during both day, civil twilight, and nighttime.

Concerning LAADON, we made some concessions. Location we ignored since without it none of the rest would happen. We were curious to see (no pun intended) if we could do a good job (we didn’t) at estimating Altitude and identify Orientation when we spotted the quads. Observe airspace for traffic or hazards, and Not endangering life or property is part and parcel of our flying field safety rules and pilot in command (PIC) responsibilities so no measurements were taken on either of them.

The good news is that the FAA has established some rules that help us maintain airspace safety. We RCP and LRO pilots have a hard ceiling of 400′ AGL (above ground level), while we MAP – VFR pilots have a hard deck of 500′ AGL (over unpopulated areas), thus a 100′ of separation is there to ensure we don’t run into each other.

Helicopters and ultralight may operate below 400 feet AGL and must be given the right of way all drones.

Moreover, our RC club’s field provides a safe and unobstructed environment to conduct our experiments and since we’ve had numerous fix wings auger in, our neighbor farmer has yet to suffer any crop or soil damage. Wish I could say the same for the aircraft involved. Recently one ducted fan (jet) attempted to fly thru a 70’ tall oak tree; guess which one won?

For civil twilight and night flights, our UAS was equipped with additional anti-collision lights (ACLs) as part of satisfying our daylight waiver (more on this later) requirements. We also ran tests using the indicator/status lights that are an integral part of our drones. We had tried several ACL models previously with no success, so we were skeptical if the new lights we purchased would be any different; we were pleasantly surprised.

A question might be raised why 3-mile visibility for the ACLs? I believe the FAA is following already established MAP VFR weather minimums for night flights. Here is the relevant table:

Our preparation begins the day before our planned flights. The acronym we use is WEPAC (Weather, Environment, Personnel, Aircraft, Communications). This provides us time to deal with any problems that surface including aircraft, controller, and IMU updates, heading display functionality, telemetry, and compass calibration issues. Terminal area forecasts (TAF) are reviewed to determine if the anticipated flight day winds, visibility, and ceilings meet or exceed our criteria and run again in the morning or day of to ensure it’s still a go.

Testing Procedure

Our first day’s objective was to two-fold. 1) to determine the maximum slant range limits where we experienced loss contact. That way we knew that flights beyond those limits would be meaningless and 2) to incorporate previous flight experiences to confirm our suspicions or learn something we didn’t already know.

We then set about flying at various altitudes, directions (headings), and slant range distances to record our best guesses. We attached reflective tape and strobe lights (the old ones) during the day to determine if we felt they made any difference; they didn’t. Our aircraft of choice were a Mavic Air 2 and a Phantom 3 Pro.

Our second day was discouraging; the Phantom 3 Pro was our primary aircraft. On our first flight, due to cold conditions, the battery depleted faster than expected giving us an early, unanticipated low battery warning and, wouldn’t you know it, an RTH failure as well. Luckily with our LRO skills and a freshly plowed field, we were able to land it without damage. We then raised our low battery warning limits since we learned that once lipos (lithium polymer batteries) run down to 20% charge, depending on age, they tend to fall off a cliff. We tested both outbound (flight following) and inbound (VO tries to spot the ship) flight paths to confirm LC and our Zone of Uncertainty box. Lastly, we flew at several different altitudes and distances (up to 550′ feet out) to determine our skill at estimating the altitude of our aircraft. Frankly, we gave up on the estimate game since our guesses never got close to what telemetry reported, the latter obviously the more reliable source but sometimes was off by 8’ of 10’; not sure why but suspect cold weather may have played a part.

On our third day, we attempted to determine where we felt we would lose orientation (where the front of the quad was pointed). It is a subjective line where we felt we had trouble determining aircraft orientation; above the line (viewing the bottom of the ship) being better than below the line (aircraft profile view). We then tested our new ACLs during civil twilight and actual nighttime flights to verify the 3-mile FAA minimums. Concerning civil twilight, we flew our aircraft into the setting sun for a worst-case scenario.

Civil Twilight

UAS VLOS Flight Testing Civil Twilight

On our fourth, day we tested the indicator/status lights our ships came with. We aren’t suggesting that they be used to satisfy the 3-mile visibility requirement (not to mention that they are located on the bottom of the aircraft) but were curious about their effectiveness during civil twilight and nighttime hours.

With both drones powered off, we set them on the ground to determine if we could see them at 75, 150, 225, and 300 feet away again during civil twilight and nighttime hours.

On our fifth, day we re-tested our new ACLs under civil twilight conditions. We learned that as our flights approached nighttime hours, the ACLs became much more visible to see. We used strobe/flash mode for all tests.

UAS Light Descriptions for DJI

Takeaways

Day Time

If you are a single RCP, realize that the minute you focus your attention on your display, you have violated the VLOS mandate.

The use of a VO(s) will greatly reduce the RCP’s or PMC’s workload not to mention adding an additional set of eyes to promote safety. Based on the slant range maximums we identified; the use of VO’s may be mandatory for engagement VLOS compliance. It is most reassuring to have another pair of eyes on your quad, much like us MAPs having someone sitting next to us, looking for ground contact, when we’re flying an ILS (instrument landing system) down to RVR (runway visual range) minimums.

As slant range distances increase, accuracy falls off as does proximity reference to obstacles. On cloudy, hazy, smog, fog, or smoke-filled days, LC distances decrease. How about these conditions?

UAS VLOS Flight Testing Day Visability

As the direction of flight changes from perpendicular to parallel viewing, the direction becomes questionable (as does airspeed if it were to be measured) since as stated previously you don’t know if your ship is hovering, moving toward, or away from you until its close in. Another phenomenon we observed is that as our ship flew down range it appeared to lose altitude even though it didn’t.

Depending on the background, like the sun, buildings, city and various landscapes, or other low contrast environments, a drone can hide in plain sight.

Civil Twilight

We found that most of the same daytime flight experiences applied to civil twilight except when we tested our new ACLs. They reduced/eliminated the Zone of Uncertainty and provided contact well beyond what we experienced during daylight hours. It was interesting to note that as ours tests progressed from civil twilight to the nighttime threshold, our drone became easier to spot. Too bad we can’t use them to satisfy VLOS mandates.
Nighttime

Altitude accuracy falls off quickly since reference ques (much like daytime) are non-existent. Attitude (Orientation) and Direction improves since the indicator/status lights are far more visible and our new ACLs provided an additional level of brightness. The latter also helped us when we had display lighting issues and used the green ACL light (mounted to the right side of our ship) to help guide it safely back home.

We found the white and green ACLs to be the brightest and most easily spotted; the red, although still visible, less so. We’re not sure how other colors may compare to our white and green experience. The test included one VO observing the ship’s ACLs as they rode away (passenger) with the other VO (driver) who attempted to spot the ship when they arrived at our 3-mile destination–and they found it. Impressive! But again, ACLs can’t be used to meet VLOS requirements.

Given city, building and related nighttime backgrounds (even the moon), the drone can easily disappear among those environments.

The Results

Our test results are included in the attached document.

What We Learned from Testing

The tests revealed that the distances from the home point we thought we could see our drones was greater than what we observed and recorded. I thought we could see them ¼ mile away. We did expect that actual and estimated altitudes would be off based on our radio-controlled model aircraft flying experiences and they were.

The inherent indicator/status lights of the ships were not that effective during civil twilight but surprised us at night. The VO reported seeing them from their position approximately 2,600’ away although they were insufficient for meeting 107.31 VLOS requirements.

UAS VLOS Civil Twilight Flight Testing

Lastly, we were curious if we could see our ships at night without any lights activated on the drones with them on the ground. We really could not as our pictures show. During nighttime hours, the aircraft disappeared at 150 feet out. VLOS compliance was lost at a shorter distance.

Civil Twilight UAS VLOS Flight Testing

Questions to Ponder

VLOS Aircraft Operation (from AC 107-2A, 2/1/21)

The remote PIC and person manipulating the controls must be able to see the small unmanned aircraft at all times during flight (§ 107.31). “The small unmanned aircraft must be operated closely enough to ensure visibility requirements are met during small UAS operations.”

This requirement also applies to the VO, if used, during the aircraft operation. The person maintaining VLOS may have brief moments in which he or she is not looking directly at or cannot see the small unmanned aircraft but still retains the capability to see the small unmanned aircraft or quickly maneuver it back to VLOS.

These moments may be necessary for the remote PIC to look at the controller to determine the remaining battery life or for operational awareness. Should the remote PIC or person manipulating the controls lose VLOS of the small unmanned aircraft, he or she must regain VLOS as soon as practicable.

Even though the remote PIC may briefly lose sight of the small unmanned aircraft, the remote PIC always has the see-and-avoid responsibilities set out in § 107.31 and § 107.37.

The circumstances that may prevent a remote PIC from fulfilling those responsibilities will vary, depending on factors such as the type of small UAS, the operational environment, and the distance between the remote PIC and the small unmanned aircraft.

For this reason, no specific time interval exists in which interruption of VLOS is permissible, as it would have the effect of potentially allowing a hazardous interruption of the operation.

If the remote PIC cannot regain VLOS, the remote PIC or person manipulating the controls should follow pre-determined procedures for the loss of VLOS. The capabilities of the small UAS will govern the remote PIC’s determination as to the appropriate course of action. For example, the remote PIC may need to land the small unmanned aircraft immediately, enter hover mode, or employ a return-to-home sequence. The VLOS requirement does not prohibit actions such as scanning the airspace or briefly looking down at the small, unmanned aircraft CS.

Who determines what is meant by “as soon as practicable” or “no specific time interval exists in which interruption of VLOS is permissible”?

Concerning Night Flights

If I equip my aircraft with anti-collision lights (ACLs) that the manufacturer claims can be seen from three (3) miles away, am I in compliance with FAA night flight requirements? Nope. I know of only two ways to be compliant. One is if the ACLs are FAA TSO-C96c Class II / TSO-C30c Type II & III certified (Technical Standard Orders – minimum performance standard for specified materials, parts, and appliances used on civil aircraft plus a separate FAA approval is required to install the article on an aircraft) or two, you find an unobstructed 3 mile stretch of road (like we did), launch the drone and ask your partner if they can see it (don’t cheat on this, it can and will come back to haunt you); logging the test date and results in your aircraft logbook. Also, remember the ACLs are for the benefit of MAPs, not us RCPs, although as explained previously some provide questionable value while others work quite well. As such, you need to consider mounting them in a way that helps our airborne friends.

I’ve also wondered why the multi-rotor manufacturers implemented the lighting configurations they have. For example, on a DJI Phantom 3 Pro, the front indicator lights glow steady red, while the rear status indicator lights can change from green to red. Standard nautical and aviation lighting standards are red light (left or port side), green light (right or starboard side), and a white light at the tail or aft section. That way we MAPs know which way to fly to avoid a midair. It most likely is the fact that multis can fly in all different directions thus navigation lighting configurations would not provide the safety their original design intended.

Final Thoughts

Remember, as pilot in command, you are 100% responsible and accountable for complying with the existing regulations. You must determine ahead of time how you will navigate and handle the inherent ambiguities just in case your position is ever challenged.

About Robert Zarracina

Robert helps companies determine if using drones will improve their business operations by utilizing his tested APL (Applicability, Profitability, Liability) program to ensure that all decision aspects are covered. His goal is to eliminate the misinformation, market hype, and costly mistakes that can result from being led down the wrong path. He is President of Flight Ventures Ltd and is an FAA-certified pilot, with single, multi-engine instrument and commercial ratings. He has acquired over 2,500 pilot in command flight hours with 30+ years of flying experience including being a skydiver driver. He has an extensive background in sUAV / sUAS business use and flight operations covering FAA Part 107 Remote Pilot Certification, previous FAA 333 exemption, daylight (night) operations waiver, and continues to add to his 1,600+ remote pilot in command flight hours. Previously he was hired as the remote pilot in command for Raytheon / BBN’s hostile drone interception program. He has helped establish sUAV flight operation departments for several local clientele, developed and presented numerous introductions to and history of drones, and Beyond Part 107 test preparation courses. He is a member of FAAST (FAA Safety team), possesses a BA and MBA in finance, marketing, computer automation and has taught senior-level marketing and business development classes at local colleges and universities.

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