Going over a cliff

[dopey dingo]

Has anyone ever flown off a high cliff?
Lets say the maximum height is set for 400 feet and you fly from your home point over a cliff that is 1000 feet high.
Will the drone descend to 400 feet and stay down there or will it just issue a warning and let you continue to fly at that height?
How would this effect the RTH function?

 

[JoelM2P]

Your altitude is set at your takeoff point, so if you maintain 400’ across the drop off then your controller will read 400’. But your actual altitude will be much higher.

 

[Meta4]

Has anyone ever flown off a high cliff?

Lots of flyers have.

Lets say the maximum height is set for 400 feet and you fly from your home point over a cliff that is 1000 feet high.
Will the drone descend to 400 feet and stay down there

This is a common question from new flyers (but I'm not sure why).
Your drone would do the same as a bird or plane that flies past the edge of a cliff - it will keep flying.
The drone is a flying machine and has no magnetic attraction to the ground.

or will it just issue a warning and let you continue to fly at that height?

It won't give any warning, the drone can't tell if it's 40 feet or 4000 feet above the ground below it.
There won't be any warning.

How would this effect the RTH function?

Not at all.

 

[dopey dingo]

Lots of flyers have.

This is a common question from new flyers (but I'm not sure why).
Your drone would do the same as a bird or plane that flies past the edge of a cliff - it will keep flying.
The drone is a flying machine and has no magnetic attraction to the ground.

Maybe a lot of new flyers are as confused as I am.
I have a Mavic 2 Pro....I have the maximum altitude set to 400 feet. When the drone reaches 400 feet it stops climbing and issues a warning; "you have reached maximum altitude" and I happily fly around at 400'.
But what happens when I go over the edge, I am now at 1400 feet and, although there is no magnetic attraction, the drone is designed not to allow it to fly at that altitude, so what happens?

It won't give any warning, the drone can't tell if it's 40 feet or 4000 feet above the ground below it.
There won't be any warning.

Maybe we are talking about different drones or I am really confused. My drone knows exactly how high it is, it feeds me that information instantly in real time as it is ascending or descending - it is visible on my controller and when I get too high or too low it warns me, isn't that the purpose of the sensors?

Not at all.

My concern is, if the drone is "locked" at a maximum altitude of 400 feet, it flies over a cliff and is now 1400 feet, will it descend to its maximum height restriction of 400 feet and if RTH is activated will it "force" itself to climb above the maximum set altitude.

What am I not getting?

 

[MAvic_South_Oz]

Maybe a lot of new flyers are as confused as I am.
I have a Mavic 2 Pro....I have the maximum altitude set to 400 feet. When the drone reaches 400 feet it stops climbing and issues a warning; "you have reached maximum altitude" and I happily fly around at 400'.
But what happens when I go over the edge, I am now at 1400 feet and, although there is no magnetic attraction, the drone is designed not to allow it to fly at that altitude, so what happens?


Maybe we are talking about different drones or I am really confused. My drone knows exactly how high it is, it feeds me that information instantly in real time as it is ascending or descending - it is visible on my controller and when I get too high or too low it warns me, isn't that the purpose of the sensors?


My concern is, if the drone is "locked" at a maximum altitude of 400 feet, it flies over a cliff and is now 1400 feet, will it descend to its maximum height restriction of 400 feet and if RTH is activated will it "force" itself to climb above the maximum set altitude.

What am I not getting?

The altitude shown is above take off point, not above ground level (AGL).
So if you flew more than say 50’, maybe 100’ (there’s no defined distance laterally from a clif etc here in Oz), you would still see 400’ on your go4 app, but flying outside CASA rules at 1400’.

Edit,
Meant to add, if you know the cliff is 1000’, you can descend to -600’ if flying from ground level, or if at 400’ then down to -1000’, and be around the 400’ AGL to remain legal.
Just have to be reasonable signal LOS to ensure you maintain connection, or RTH will activate and bring you back up to whatever you have that set for.

 

[Meta4]

But what happens when I go over the edge, I am now at 1400 feet and, although there is no magnetic attraction, the drone is designed not to allow it to fly at that altitude, so what happens?

Answered in post #3

My drone knows exactly how high it is, it feeds me that information instantly in real time as it is ascending or descending - it is visible on my controller and when I get too high or too low it warns me, isn't that the purpose of the sensors?

The height you see on the screen is the height of the drone relative to where you launched.
This comes from a barometric sensor that measures changes in air pressure ... just like a real plane's altimeter.
The drone has no sensor to tell whether the ground below is 40 ft or 400 feet down below.

My concern is, if the drone is "locked" at a maximum altitude of 400 feet, it flies over a cliff and is now 1400 feet, will it descend to its maximum height restriction of 400 feet and if RTH is activated will it "force" itself to climb above the maximum set altitude.

What am I not getting?

Just as the height you see on screen is relative to the launch point, so is the max altitude limit.

 

[dopey dingo]

The altitude shown is above take off point, not above ground level (AGL).
So if you flew more than say 50’, maybe 100’ (there’s no defined distance laterally from a clif etc here in Oz), you would still see 400’ on your go4 app, but flying outside CASA rules at 1400’.

Edit,
Meant to add, if you know the cliff is 1000’, you can descend to -600’ if flying from ground level, or if at 400’ then down to -1000’, and be around the 400’ AGL to remain legal.
Just have to be reasonable signal LOS to ensure you maintain connection, or RTH will activate and bring you back up to whatever you have that set for.

Great, I think I understand
The altitude is set at the home point, thats what confused me.
If I descend lower than the home point the controller will show I am (- feet) lower than the home point
If RTH altitude is set at 50 feet then drone will ascend back to home point altitude plus 50 feet.
Only danger is signal loss but if that occurred the drone would automatically RTH and climb back up.
Have I got that right?
Thank you so much for clearing that up

 

[Yorkshire_Pud]

In essence the drone knows only its height relative to the take off point and all height limits set by you in the app are relative to the take off point.
The height above the ground, known as AGL, is unknown to the drone ........ unless the drone is low enough for the 'ground' to be within the range of the downwards looking sensors on the bottom of the drone, let's call this detected height "VPS height".
Most, if not all, DJI 'Mavic' drones have such sensors but the drone only acts upon them when 'very' low. They are what causes the reduction in descent speed as the drone nears the ground, they also are the source of "landing protection". Be aware they are not infallible and actually detect the nearest object beneath the drone that is within their field of view ... so stick your hand under a drone and the drone may climb away from it slightly.
From memory the M2P reports the VPS height in either the App or on the screen of the RC1A/B controllers, I do not know about the smart controllers.

With regards to your question, in mid air flights the drone makes NO ATTEMPT * to correct for changes in the height of ground. So, with regards to your flying beyond the drop off, the drone will stay at the height it was relative to the take off point and you would be breaking the 400ft AGL rule as soon as the drone was beyond the drop off.

* there is an exception to that when you fly the drone SLOWLY up a slope, landing protection can force the drone to climb to avoid the rising ground but it does not respond to descending ground. Fly too fast and landing protection does not act quickly enough to prevent a crash.

The drone will happily fly to heights below the take off point and will show negative height values when below the level of the take off point.
An RTH would entail a climb to the RTH height relative to the take off point, so if you set the GoHome height at 257FT and started the RTH when the drone was at -361ft an RTH would entail a climb of 618ft.
Note HOWEVER that there are exceptions to that rule that are governed by the drone's distance from the homeport. You will need to read the manual to find out the distances, the M2P's RTH behaviour can be quite complicated depending on which settings are on. From memory the drone will RTH at its current height if the drone is within 'x' ft of the home point, check "x" in the manual.
Also note that obstacle avoidance can complicate behaviours including RTH behaviour.


Beware of flying to close to cliffs etc. just incase they cause loss of GPS, that can complicate matters and has caused some drones to get 'stuck' below the lip of canyons etc., I can never remember the exact circumstances in which that has happened, I just remember that it can happen.

My lowest point so far is somewhere around -400?ft at a sea cliff.

 

[dopey dingo]

In essence the drone knows only its height relative to the take off point and all height limits set by you in the app are relative to the take off point.
The height above the ground, known as AGL, is unknown to the drone ........ unless the drone is low enough for the 'ground' to be within the range of the downwards looking sensors on the bottom of the drone, let's call this detected height "VPS height".
Most, if not all, DJI 'Mavic' drones have such sensors but the drone only acts upon them when 'very' low. They are what causes the reduction in descent speed as the drone nears the ground, they also are the source of "landing protection". Be aware they are not infallible and actually detect the nearest object beneath the drone that is within their field of view ... so stick your hand under a drone and the drone may climb away from it slightly.
From memory the M2P reports the VPS height in either the App or on the screen of the RC1A/B controllers, I do not know about the smart controllers.

With regards to your question, in mid air flights the drone makes NO ATTEMPT * to correct for changes in the height of ground. So, with regards to your flying beyond the drop off, the drone will stay at the height it was relative to the take off point and you would be breaking the 400ft AGL rule as soon as the drone was beyond the drop off.

* there is an exception to that when you fly the drone SLOWLY up a slope, landing protection can force the drone to climb to avoid the rising ground but it does not respond to descending ground.

The drone will happily fly to heights below the take off point and will show negative height values when below the level of the take off point.
An RTH would entail a climb to the RTH height relative to the take off point, so if you set the GoHome height at 257FT and started the RTH when the drone was at -361ft an RTH would entail a climb of 618ft.
Note HOWEVER that there are exceptions to that rule that are governed by the drone's distance from the homeport. You will need to read the manual to find out the distances, the M2P's RTH behaviour can be quite complicated depending on which settings are on. From memory the drone will RTH at its current height if the drone is within 'x' ft of the home point, check "x" in the manual.

Beware of flying to close to cliffs etc. just incase they cause loss of GPS, that can complicate matters and has caused some drones to get 'stuck' below the lip of canyons etc., I can never remember the exact circumstances in which that has happened, I just remember that it can happen.

My lowest point so far is somewhere around -400?ft at a sea cliff.

Mate, thanks for that.
When explained like that it is very clear to understand
Obviously you know what you're talking about.
Clears it up for me nicely, thanks again

 

[Yorkshire_Pud]

Mate, thanks for that.
When explained like that it is very clear to understand
Obviously you know what you're talking about.
Clears it up for me nicely, thanks again

Cheers, best re read my post, I added a few bits that I missed first time through.

 

[MAvic_South_Oz]

Have I got that right?

Yep.

 

[Zbip57]

Beware of flying to close to cliffs etc. just in case they cause loss of GPS, that can complicate matters and has caused some drones to get 'stuck' below the lip of canyons etc., I can never remember the exact circumstances in which that has happened, I just remember that it can happen.

People have reported "getting stuck" like this when flying down into narrow canyons. It's a quirk of the VPS system.

When flying in areas without sufficient GPS reception, i.e. indoors or in spots without a sufficient view of the sky (deep canyons, or between tall buildings), the drone may not see enough GPS satellites to fix its position. In that case it will rely on its VPS sensors, but there are limitations. Check your specific User Manual for details.

The manual for my Mini says the VPS sensors work best at heights up to 10m (~30ft), assuming there's enough light and detectable ground pattern for the optical sensor to get a good positional fix. That's 10m actual height above any obstacle detected by the infrared sensors of the VPS system.

If you have no GPS, and the VPS sensors are out of range, all that's left is Atti mode in which the drone will not be able to brake to a halt or maintain a fixed position when the control sticks are centred, which really isn't that big of a deal as you still have stick control.

The potential bigger problem is if you lose GPS reception at the bottom of a steep canyon, but do have solid VPS control. In the footnotes describing the VPS system, the user manual for my Mini says, "The max hover altitude of the aircraft is 5m if there is no GPS."

That's one of those "useful" safety features that DJI have assumed necessary to design into their systems. They figure if you have insufficient GPS signal and are relying on the VPS sensors to hold your drone in position whenever sticks are centred, you probably wouldn't want to risk ascending any higher than 5m above the ground for fear of climbing out of reception range of the VPS sensors, thereby leaving you in ATTI mode (which many people seem terrified of).

The result of that limitation is, if you fly down into a narrow canyon and lose GPS reception, but your drone is close enough to the ground with enough light to get a solid VPS lock, your drone will then be unable to climb higher than 5m from the floor of the canyon. And, without GPS reception, RTH won't work either. So the drone is left unable to climb back out of the canyon.

It's never happened to me, but that's how it's been described to me. Nice, eh?

 

[scott135]

This comes from a barometric sensor that measures changes in air pressure ... just like a real plane's altimeter.

This statement surprises me. I assumed the altitude of the aircraft was calculated from GPS data, not an onboard altimeter. If an onboard altimeter was used, wouldn't the specific barometer reading for the location have to be entered each time flying, similar to a real plane?

 

[mobilehomer]

This statement surprises me. I assumed the altitude of the aircraft was calculated from GPS data, not an onboard altimeter. If an onboard altimeter was used, wouldn't the specific barometer reading for the location have to be entered each time flying, similar to a real plane?

It self zeros at start up each time. Location has nothing to do with it. Regardless of actual BP, the measured BP is set at zero for altitude calculation.

 

[scott135]

It self zeros at start up each time. Location has nothing to do with it. Regardless of actual BP, the measured BP is set at zero for altitude calculation.

Thank you for the explanation. It makes sense since we are only concerned about AGL vs. MSL, for altitude, right?

 

[Robert Prior]

But what happens when I go over the edge, I am now at 1400 feet and, although there is no magnetic attraction, the drone is designed not to allow it to fly at that altitude, so what happens?

The drone calculates altitude relative to the altitude of its launch point. This means if you start on a hill you can fly greater than 400' AGL; if you launch from a valley you might hit the altitude limit at actual ground level.

The legal limits in your country are another matter, but the software limits are based on where you launch from.

 

[Zbip57]

If an onboard altimeter was used, wouldn't the specific barometer reading for the location have to be entered each time flying, similar to a real plane?

For real aircraft it's important that everyone is talking about the same actual altitudes measured from sea level. For example, the airport I used to fly out of was situated at 321 feet above sea level. The altimeter setting for the day varied with local atmospheric conditions. The tower tells you what the correct altimeter setting is. When properly dialed in with the altimeter calibration knob, the altimeter would then read correctly as 321 ft while the plane was sitting on the ground.

The drone ignores all that and simply sets its barometric altimeter to zero when the motors are started. Everything after that is measured relative to that recorded zero height.

The problem with this system is, unless you only ever launch from and fly over a perfectly flat landscape, you never really know how high above ground the drone actually is.

Back to the original question, if you launch your drone from the ground at the edge of a cliff and climb 10 ft, then fly out over the edge of the cliff, the drone continues to report its height unchanged at 10ft. It's the pilot's responsibility to know how tall the cliff is and whether you're now still within the 400' AGL legal limit.

The drone doesn't know or care how high it actually is above the actual ground.

So if you configure your settings for a max 400' altitude limit so as not to ever exceed the legal limit, well that's only ever going to be accurately limited or displayed on your telemetry when directly above your takeoff location. You could take off and climb straight up until hitting that set limit. But if you're taking off on a hillside, and then fly away from the hill at that same 400' indicated height limit, you're immediately breaking the actual 400 AGL legal limit. Or, if you're at the indicated 400' limit and fly toward the hillside, the telemetry will continue to indicate the same 400' height right up until it smacks into the ground somewhere uphill from you.

 

[dopey dingo]

For real aircraft it's important that everyone is talking about the same actual altitudes measured from sea level. For example, the airport I used to fly out of was situated at 321 feet above sea level. The altimeter setting for the day varied with local atmospheric conditions. The tower tells you what the correct altimeter setting is. When properly dialed in with the altimeter calibration knob, the altimeter would then read correctly as 321 ft while the plane was sitting on the ground.

The drone ignores all that and simply sets its barometric altimeter to zero when the motors are started. Everything after that is measured relative to that recorded zero height.

The problem with this system is, unless you only ever launch from and fly over a perfectly flat landscape, you never really know how high above ground the drone actually is.

Back to the original question, if you launch your drone from the ground at the edge of a cliff and climb 10 ft, then fly out over the edge of the cliff, the drone continues to report its height unchanged at 10ft. It's the pilot's responsibility to know how tall the cliff is and whether you're now still within the 400' AGL legal limit.

The drone doesn't know or care how high it actually is above the actual ground.

So if you configure your settings for a max 400' altitude limit so as not to ever exceed the legal limit, well that's only ever going to be accurately limited or displayed on your telemetry when directly above your takeoff location. You could take off and climb straight up until hitting that set limit. But if you're taking off on a hillside, and then fly away from the hill at that same 400' indicated height limit, you're immediately breaking the actual 400 AGL legal limit. Or, if you're at the indicated 400' limit and fly toward the hillside, the telemetry will continue to indicate the same 400' height right up until it smacks into the ground somewhere uphill from you.

I hadn't thought of the reverse scenario, flying into a hill!
Are you saying that the drone will not sense land rising, assuming a gentle slope up, on the controller?
Whilst I understand that the drone will not automatically change its altitude, is there some way to see (other than LOS) that the terrain is rising?
This could be scarier than dropping off a cliff

 

[Meta4]

I hadn't thought of the reverse scenario, flying into a hill!
Are you saying that the drone will not sense land rising, assuming a gentle slope up, on the controller?
Whilst I understand that the drone will not automatically change its altitude, is there some way to see (other than LOS) that the terrain is rising?
This could be scarier than dropping off a cliff

That's why you need to look where you are flying.
Whether it's a rising slope, a cliff or a tree you don't want to run into obstacles.

 

[Robert Prior]

Are you saying that the drone will not sense land rising, assuming a gentle slope up, on the controller?

It will eventually sense the ground, assuming it has downward-facing proximity sensors. It may stop, or it may auto-land, depending on flight mode and settings.

This is definitely a case where reading the manual is a good idea.

I have landed both higher and lower than launch altitude when hiking. It is a bit strange looking at an altitude of "-40 m" and still being airborne, or "35 m" and landing. One quickly gets used to taking the displayed altitude with a grain of sodium chloride.