Can Changing Landforms Cause Altitude Violation?

[MARK (LI)]

And attempting a strict literal interpretation of the rules always ends up in pointless about flying over a hypothetical cliff edge like this.
The rules are brief and can't spell out in fine detail what applies in a specific extreme situation like that.
Common sense is needed.
If you fly 10 ft, 20 ft or even more out over the edge of a 1000 ft cliff, you won't be getting in the way of any manned aircraft and no-one is going to be chasing you for reckless flying.

@Meta4 ..I know that you are a long time member here...you are obviously quite knowledgeable ...and I do not want to get on your bad side....but it is just an open discussion ...and not really pointless at all....if my interpretation is wrong...and the other guys are right...it opens up a lot of opportunities to get some challenging flying accomplished ...I don't know where we could go to get a definitive answer ...but it would be beneficial to know

 

[smashse]

That cliff is part of the topography...it is not a structure...I did see where you say a CFI confirmed that....but a CFI teaches how to fly manned aircraft...and I surmise that he is well versed in the rules that apply to them...this is not manned aircraft and has its own different set of rules

Well on top of being an FAA licensed instructor, he owns a drone training school. Not trying to be argumentative, just pointing out this is someone who would know and have access to find out, and reason to find out as well. (also not trying to advertise for the school).

A structure is any complex object by definition, and even the US Geographical society refers to rock formations, cliffs, buttes, plateaus, mesas, etc as "structures". I'll leave this, while not a legal website, it is generally understood definition:

What Are Some Examples of Natural Structures?

Naturally occurring structures include canyons such as the Grand Canyon in the United States, mountains such as Mount Everest, and even homes and nests that animals make for themselves, including beehives, ant hills, bird nests and beaver dams. Any form that stands on its own and has a...

www.reference.com

The FAA in it's rules do not specify natural or man-made structure.

I normally do not like to take an open interpretation of laws because I generally agree with you that too many people take too open of an interpretation, but this creates an impossible scenario on a 500' cliff. You would have to descend to -100' AGL before going over the cliff by the above theory, making it illegal to cross the cliff edge and doing nothing to improve safety, and that's the FAA's stated mission, is to provide safe airspace use.

I've gone ahead and emailed the FAA for clarification on this exact scenario. If I am wrong when they eventually reply, if at all, I'll put it up here.

 

[MARK (LI)]

Not taking your reply as argumentative at all....it's just a component of the discussion.....and thank you for keeping it civil..I actually hope that you are correct...as I said...it would really create some nice opportunities.. I hope FAA does respond and I look forward to hearing what thy tell you

 

[eEridani]

Snake River Canyon in Idaho, or in the Columbia Gorger through Oregon & Washington: real life cases where simply launching your drone can put you in violation of the regs.

Having flown these in a light plane, you can easily violate the letter of the law that says you must maintain 500' AGL minimums. If you had a radar altimeter and a terrain following autopilot maybe you can maintain the 500' minimum, but there are things not on the sectionals that will break your effort to be 100% compliant. Heck, even training and doing emergency landing practice gets you below 500' (if you have a good instructor). And then there's mountain flying, where terrain coming into a small strip is close enough to touch in order to drop into the field safely.

The the letter of the law implies that your drone can't be higher than 400' above the ground you are over. That maintains a 100' clearance between your drone and a plane that is maintaining 500' AGL rules. But, IMO and with regard for odd terrain (and terrain is the operating word here), it comes down to safely piloting the aircraft and not interfering with other aircraft. Now, if only helicopter pilots obeyed those same rules.

 

[pilotinstitute]

For part 107 (not positive about Hobby/rec/TRUST flights), it is also considered 400' from a structure, so it actually lags a touch from the hill. I don't have a nifty illustration, but if you are within 400' from a cliff, you can actually go 400' above the height of the top of the cliff. I know some may disagree, because the FAA says "within 400' of a structure" but they never say man-made or natural structure. A cliff or tower makes more sense in explaining what I am saying.

Say you have a 200' cliff to the ocean, and you are flying 80' from the cliff over the water, your max altitude would actually be 400' over the cliff, or 600' MSL, even though your drone is over the water. Your drone is technically at that point 600' AGL viewed directly down, but because of the proximity of the cliff, you are allowed that.

This was confirmed by an FAA licensed flight instructor... Twice just to make sure he knew what I was asking. I have not yet found anything official that contradicts that.

For your river canyons, if they aren't 400' wide, you can use the elevations of the cliffs/banks for ground 0.

As others have said, a cliff is not a structure, it's considered part of the ground (as in the ground in AGL) so it wouldn't qualify for the Part 107 "400 within 400" rule.

Here's an excerpt from the Part 107 Preamble that explains what the FAA had in mind when they created the rule:
"To address the concerns expressed by commenters requesting higher operating altitudes in proximity to buildings, towers, power lines, and other tall structures for the purposes of inspections and repair, the FAA is establishing new provisions in the final rule that will enable those operations in a way that does not compromise aviation safety. Specifically, the FAA notes that 14 CFR 91.119 generally prohibits manned aircraft from operating in close proximity to structures. Section 91.119 requires manned aircraft to stay 500 to 1,000 feet away from the structure, depending on whether the area is congested. Because manned aircraft are not permitted to operate in close proximity to structures, this rule will allow a small unmanned aircraft to fly higher than 400 feet AGL as long as that aircraft remains within a 400-foot radius of a structure up to an altitude of 400 feet above the structure’s immediate uppermost limit. Allowing higher-altitude small UAS operations within a 400-foot lateral limit of a structure will enable additional operations (such as tower inspection and repair) while maintaining separation between small unmanned aircraft and most manned aircraft operations. "

 

[Heavydpj]

Keeping it a friendly discussion....the 400' ceiling is AGL....there are places....probably most of the country where you can be over 400"MSL at launch .......with a mountain...you can follow the topography and maintain the 400' AGL.....with a cliff..if you were to ascend ..go higher than the cliff and go forward.....now you are BVLOS...
I think loose interpretation of the rules will just get someone in trouble

I completely agree with you about the BVLOS concern, and would certainly never advocate that one should do anything so reckless and unlawful.

However, I can think of several scenarios where the rising/descending terrain issue would come into play without violating the VLOS rule.

- Fly to the top of a 450 hill to see what's on top.
- I am on the top of a 450 foot hill, and I want to see what's on the other side of a hilltop that is more than 400' away, but within VLOS. In this case, IMO, noting that the rule applies the other way as well, I would need to descend to maintain 400'AGL while transiting to the other hilltop.
- Fly only partway up a very large mountain, halfway up a cliff.
- In a clearing or meadow and I want to see what's beyond the 150' trees.

 

[Heavydpj]

As others have said, a cliff is not a structure, it's considered part of the ground (as in the ground in AGL) so it wouldn't qualify for the Part 107 "400 within 400" rule.

Good point on the interpretation.

So here's the very nerdy and hypothetical question.

Say I was flying one day and there happens to be an FAA examiner having lunch right next to me. and he has his handy dandy altitude detector with him (just go with me here).

I fly at balls-on 400' AGL, following the absolutely flat terrain. As I stay within VLOS range, I fly over a 500' deep chasm that's exactly 10' wide.

Can the examiner violate me? Must I descend into the chasm to maintain my altitude limit? (and probably lose VLOS)?

Question is for drunken debate and mental masturbation purposes only.

 

[smashse]

Good point on the interpretation.

So here's the very nerdy and hypothetical question.

Say I was flying one day and there happens to be an FAA examiner having lunch right next to me. and he has his handy dandy altitude detector with him (just go with me here).

I fly at balls-on 400' AGL, following the absolutely flat terrain. As I stay within VLOS range, I fly over a 500' deep chasm that's exactly 10' wide.

Can the examiner violate me? Must I descend into the chasm to maintain my altitude limit? (and probably lose VLOS)?

Question is for drunken debate and mental masturbation purposes only.

By the other logic above, you would be cited for being at 900' AGL. By my understanding you would not be in violation of altitude, because you are <400' of the edge at all times. I don't know if the examiner would violate you, but if he did, I'd report it to the FAA.

 

[dronerdave]

There is no lateral distance from terrain in the US only AGL to the terrain directly below the UAV. This happened to me once. I was flying from a hilltop over a lake. I descended along the terrain at about 200 AGL to the water and began to video. A plane coming up the canyon was on me before I could react. If that plane happened to hit me at that altitude I'd still be at fault because I didn't see and avoid. Manned aircraft are allowed to fly below 500' AGL feet in some circumstances such as at this lake. But if I had ignored the rules and flew above [edit +400' to] 500' AGL because I didn't want to drop to the legal height and did hit a manned aircraft, I'd expect the book to be thrown at me.

 

[Heavydpj]

There is no lateral distance from terrain in the US ...

So by this interpretation, using the example above, the sUAS pilot would be in violation of altitude restrictions because the instant the chasm opened up to its 500 foot depth, the pilot should have descended 100' into it.

Interesting.

By this rationale, a pilot flying an aircraft over a the an 800' deep chasm that is 50' wide should descend into the chasm by 100' to maintain proper airspace clearance?

 

[MARK (LI)]

So by this interpretation, using the example above, the sUAS pilot would be in violation of altitude restrictions because the instant the chasm opened up to its 500 foot depth, the pilot should have descended 100' into it.

Interesting.

By this rationale, a pilot flying an aircraft over a the an 800' deep chasm that is 50' wide should descend into the chasm by 100' to maintain proper airspace clearance?

No...the lateral distance has no bearing ..you would have to follow the land contour at max 400 feet AGL ...so you would have to descend at least 400 feet from the original ground level

 

[smashse]

OK, so I have to concede, the FAA defines structures as man-made, landforms do not count. However, they do consider the sidewall to be land, so as long as you are within 400' of the cliff, you are still 400' from the "land" and therefore still legal. This is literally the image sent to me from the FAA, showing 625' AGL straight down, yet still legal, with no "structures" in sight. With only requiring you to be <400' from the closest point of land. Yes, this image is not an FAA Drawing, and they admitted to such in the email, but it is what they sent to explain.



They also made clear that this is for Part 107, not TRUST, and that you still have to maintain all other provisions, I.E. VLOS, etc. Notice the 2 blue dotted lines showing 625' and 450' AGL, while maintaining 400' from the closest point of land.

So I was correct, you can traverse a Cliff wall, by staying less than 400' away from it, just my reasoning was incorrect.

As for the @dronerdave 's example, yes, you would have been at fault because of the see and avoid provision, that you always yield right of way to manned aircraft, even if you are flying legally and they are not, you still have to avoid them.

Would you have been cited for an altitude violation? I do not think so, but you still would have failed to yield right of way.

As for the question from @Heavydpj yes you would have been in violation, but not by 500', but more like .03125 ft, because that is the distance from the closest point of "land" when you make a 5'x400' right triangle flying over it. Lose a ft of altitude and the email from the FAA says you would be fine.

 

[MARK (LI)]

Good to have a discussion like this where there were different ideas and point of view...and we all remained under control...the vague ( confusing ) manner the FAA presents it is what led to this lively discussion...I hope you other guys did get as much out of it as I did...and that we remain friends ...that we most likely will never meet

 

[smashse]

Good to have a discussion like this where there were different ideas and point of view...and we all remained under control...the vague ( confusing ) manner the FAA presents it is what led to this lively discussion...I hope you other guys did get as much out of it as I did...and that we remain friends ...that we most likely will never meet

Completely agree Mark. I struggle with exactly this, and really tried not to be too argumentative. We have an answer from the FAA, which sort of clears/at the same time muddies the water. However I did learn a lot, and especially the distinction that the FAA does not consider any land to be a "structure". And that somehow land doesn't have to be directly below you.

I still do not have an answer on perfectly vertical or inverted cliffs, like could you fly up under a ledge with the "ground" being above you by <400'. But that's rare enough I can discuss that later.

 

[MARK (LI)]

I think a different concern with flying close to a cliff...or a building is that drafts occur that could be dangerous to the drone

 

[MAvic_South_Oz]

Cliff edges too, real cliff edges near vertical, will possibly also present connection issues if descending too far.
Being very near the edge is required to keep signal LOS in some cases.
The hilltop diagram example above is not so much a problem.

 

[smashse]

I think a different concern with flying close to a cliff...or a building is that drafts occur that could be dangerous to the drone

Yeah, I thought I had mentioned that we were talking legality, not actual ability because of wind shear. But there are some plateaus and buttes that certain time of the day have little shear.

But this is useful information just in case you are 301' over a 100' cliff, where the wind shear would be minimal, you could cross the edge and not have to immediately descend.

As others have said, a cliff is not a structure, it's considered part of the ground (as in the ground in AGL) so it wouldn't qualify for the Part 107 "400 within 400" rule.

Here's an excerpt from the Part 107 Preamble that explains what the FAA had in mind when they created the rule:
"To address the concerns expressed by commenters requesting higher operating altitudes in proximity to buildings, towers, power lines, and other tall structures for the purposes of inspections and repair, the FAA is establishing new provisions in the final rule that will enable those operations in a way that does not compromise aviation safety. Specifically, the FAA notes that 14 CFR 91.119 generally prohibits manned aircraft from operating in close proximity to structures. Section 91.119 requires manned aircraft to stay 500 to 1,000 feet away from the structure, depending on whether the area is congested. Because manned aircraft are not permitted to operate in close proximity to structures, this rule will allow a small unmanned aircraft to fly higher than 400 feet AGL as long as that aircraft remains within a 400-foot radius of a structure up to an altitude of 400 feet above the structure’s immediate uppermost limit. Allowing higher-altitude small UAS operations within a 400-foot lateral limit of a structure will enable additional operations (such as tower inspection and repair) while maintaining separation between small unmanned aircraft and most manned aircraft operations. "

You actually have it exactly as the FAA explained. The cliff is part of the ground, and you have to be within 400' of the ground. This ground happens to be vertically oriented.

Cliff edges too, real cliff edges near vertical, will possibly also present connection issues if descending too far.
Being very near the edge is required to keep signal LOS in some cases.
The hilltop diagram example above is not so much a problem.

Being able to go out 100' or so makes RC LOS much more plausible especially if you aren't trying to go all the way to the bottom of said cliff or rock face.

 

[dronerdave]

Could be wrong because I thought the same a few years ago and was corrected (perhaps wrongly) and really would think that you can say 400' away but as I understand it the measurement is directly below the aircraft. So no matter how close you are to the cliff edge you can't have more than 400' AGL directly below. I doubt anyone from FAA would nitpick about it but this question always comes up and so far it has always came back AGL below the aircraft. I would love to hear I can be within 400' of any land surface...right now I've avoided that type terrain but would like to know what the FAA thinks.

 

[smashse]

Could be wrong because I thought the same a few years ago and was corrected (perhaps wrongly) and really would think that you can say 400' away but as I understand it the measurement is directly below the aircraft. So no matter how close you are to the cliff edge you can't have more than 400' AGL directly below. I doubt anyone from FAA would nitpick about it but this question always comes up and so far it has always came back AGL below the aircraft. I would love to hear I can be within 400' of any land surface...right now I've avoided that type terrain but would like to know what the FAA thinks.

If you look at post #32, Can Changing Landforms Cause Altitude Violation?

This is the image the FAA themselves gave me. I did not make it nor did I pull it from some random website, the FAA pulled it and sent it to me, and if you need a screen shot of the email to believe it I can definitely provide that with some redactions. While not showing a specifically 90 degree vertical, it does show that the FAA considers it the closest land, not what's directly below as they are saying this is legal with a 625' directly below altitude. This is also what my FAA licensed instructor and Drone pilot school owner confirmed for me.

If you look at the FAA rules, it actually does not specify directly below, it says you have to be no more than 400' from ground. and a cliff face is "ground" according to the FAA.

I do not know how this works with an inverted cliff/overhang, but logic would say it would still adhere since there is nothing to contradict it.

 

[MAvic_South_Oz]

Being able to go out 100' or so makes RC LOS much more plausible especially if you aren't trying to go all the way to the bottom of said cliff or rock face.

Totally agree, and if pilots really ARE allowed to fly within 400' of that cliff face, as mentioned in the post above and post #32 diagram, then all good . . . it means that such a flight out over a cliff edge (for that dramatic fall away shot) is ok to do.

It is hard to imagine 100' would be a problem, 400' ?
Hard to tell, but if in good VLOS and you can hear a plane or heli, then it's no different to flying where aircraft can and do fly below 500' from time to time.

I think pilots in the US should REALLY want to see that diagram actually shown on an FAA website page, just to confirm.
I can't see why FAA wouldn't have it on their site somewhere.

I saved that diagram, and intend asking CASA here about this too.
We have the same 400' / 120m rule in Australia.