LOSS of CONTROL OVER WATER **EXPLAINED**

[Drone-Retriever]

First,
We see an elevation value on our remote control indicating feet above takeoff point.
We need to understand how a drone knows it elevation above ground.
Simply put it is barometric pressure
What is barometric pressure?
To simplify the discussion the following is important to know.

• The measurement of atmospheric air pressure is done with a device called a barometer
• Sea level pressure is expressed in millibars.
• Standard sea level pressure is 1013.2 millibars, 29.92 inches of mercury, 760 millimeters of mercury, or about 14.7 pounds per square inch.
• Pressure increases about 1 inch per 1,000 feet
• For a drone this equates to 10 feet for every 0.01 change in atmospheric pressure change.

EXAMPLE:
When I turn on my drone the electronics records and SETS atmospheric pressure in memory. This establishes my 0 foot elevation height.
As the drone increases in elevation the atmospheric air pressure changes and a calculation is performed against the set point to report the elevation in feet above ground (take off point).
This works great and as long as the atmospheric pressure that I am flying in does not change. The elevation reported and calculated is accurate.
What happens if the weather is changing which it is always doing? If after locking in the atmospheric pressure for 0 feet the drone flies and the actual ground air pressure changes by .01 inches (100th of an inch).
The drone will be flying with an elevation error of 10 feet.
So if flying over water you may think you are at 15 feet but in reality you may be at 5 feet above water assuming a change in pressure of only 100th of an inch in air pressure change.

Now let’s talk about temperature.
As temperature changes during the day the rate of heating causes air movement as air near the ground is warmed it rises. This rise in temp and movement of air may also contribute to the change in atmospheric pressure as air movement is what causes atmospheric pressure to change.

Now let us talk about drones and downward ultrasonic flight sensors.
When a drone finds itself above a solid surface the sensors provide a reflection in signal and a trigger to land and then a calculation is performed to control rate of landing.
Over land this is a simple safe landing.
Over water ultrasonic waves from these sensors PASS through water and are not reflected. (This assumes a flat water surface below the sensors.)
If the water has movement then some reflection may occur. This fluctuation may or may not be recognized by the sensors.
Since the drone cannot sense a surface it simply flies into the water.

Finally a combination in the above variables should be considered when flying over water.

 

[Yogi053]

Your explanation has dented my confidence somewhat for flying over water!

 

[CanadaDrone]

Your explanation has dented my confidence somewhat for flying over water!

Always take extra care but don't let it ruin your experience. I've flown over water hundreds of times with zero issues, including in sport mode at low altitudes. Would I try and skim the water though? Definitely not. Just use the same common sense as you would over land and give yourself enough room for a wide margin of error. The drone does not use the ultrasonic sensors to keep you above the water, it uses the barometer - and if it responded that way to miniscule atmospheric pressure changes, crashes would be far more common even on land. Unless you are trying to land it on the water, you don't have much to worry about.

I've also had my drone in a hover while stormy weather passed nearby, and altitude did not change in the slightest despite what I assume were wide swings in pressure.

 

[Deleted member 103366]

Your explanation has dented my confidence somewhat for flying over water!

Most of my proper flights are over salt water, normally it would not be low, more likely at 60ft + but I have occasionaly brought it down low in a vertical descent, got the shot I wanted and them climbed back to 'height'. I have maybe even had it creep forward when low but my thumb was always ready to throttle up if there was any unintended descent and I seem to remember doing that on a couple of occasions. Better to lose the shot than the drone. I am not going to gamble the safety of the drone on it being just a slight drift etc., any descent and up it goes.
I would not fly low over water at speed.

 

[Yogi053]

Thank you for your comments lads (and lasses?). I actually have flown over water especially with my Mini 2 but right now I am waiting the return of my M2P from the Dji repair centre in the Netherlands. It fell to the ground close to home onto a concrete driveway and caused problems with some menus not operating correctly. I found this out after changing all props and although minor test flights proved good, I sent it for repair. It has been repaired under warranty so no cost to me. I have Dji refresh on both drones but if the M2P happened to have been flying over water where I normally fly, there would have been no way for me to recover it. I suppose I could afford to replace my Mini 2 but my finances would be stretched to replace M2P.

 

[eEridani]

I've flown over water at low altitudes several times. No serious issues at all. That said, there were some unusual responses from the drone to control movements; and elevation in the OSD was clearly off. But nothing in any of this felt like is was dangerous to the flight: just indicators to be a bit more careful when dropping altitude for a camera move.

One example, over relatively still water provides no visual reference, so even pointing the camera down doesn't help much. I was trying to set my altitude over the water to 8' so I'd have plenty of clearance passing under a bridge. The drone somehow thought I wanted to land. Not really sure the water mattered, but maybe. I was still pretty high when the gimbal zeroed (as is normal for landings). A quick up on the controls stopped the nonsense, but this could have just as easily happened over a parking lot, too.

Another example, again over water, but this time 430 feet away. I was flying off a 450 foot sheer cliff down to the river below. Seeing the drone wasn't an issue - if I ignored the vertigo - but altitude over the water was impossible to judge looking almost straight down. I was finally able to align with a nearby shelf - and got the shot I was looking for - flying between a couple of pillars - but again, the drone was not responding to controls as expected as I neared the water surface. Hesitant is the best description for how the drone was acting. But then, I was flying at -439' OSD, not sure I'd expect normal behavior at that point.

 

[sar104]

I've flown over water at low altitudes several times. No serious issues at all. That said, there were some unusual responses from the drone to control movements; and elevation in the OSD was clearly off. But nothing in any of this felt like is was dangerous to the flight: just indicators to be a bit more careful when dropping altitude for a camera move.

One example, over relatively still water provides no visual reference, so even pointing the camera down doesn't help much. I was trying to set my altitude over the water to 8' so I'd have plenty of clearance passing under a bridge. The drone somehow thought I wanted to land. Not really sure the water mattered, but maybe. I was still pretty high when the gimbal zeroed (as is normal for landings). A quick up on the controls stopped the nonsense, but this could have just as easily happened over a parking lot, too.

Another example, again over water, but this time 430 feet away. I was flying off a 450 foot sheer cliff down to the river below. Seeing the drone wasn't an issue - if I ignored the vertigo - but altitude over the water was impossible to judge looking almost straight down. I was finally able to align with a nearby shelf - and got the shot I was looking for - flying between a couple of pillars - but again, the drone was not responding to controls as expected as I neared the water surface. Hesitant is the best description for how the drone was acting. But then, I was flying at -439' OSD, not sure I'd expect normal behavior at that point.

The FC doesn't care about the OSD height unless it exceeds the maximum specified in settings. Negative values are perfectly fine, and should not lead to any strange behavior. Erroneous VPS heights on the other hand, which can occur over water, may cause problems.

 

[eEridani]

@sar104 : in your experience have you discovered differences between the ultrasonic altitude versus infrared altitude sensors?

 

[sar104]

@sar104 : in your experience have you discovered differences between the ultrasonic altitude versus infrared altitude sensors?

Not that I have noticed. Of the DJI aircraft that I have, the Phantom 4 and the Mavic Pro have ultrasonic while the Mini, M2P and M2EA are infrared. But I don't fly over water that much. I would expect the ultrasonic sensors to perform a bit better in that situation.

 

[Meta4]

This works great and as long as the atmospheric pressure that I am flying in does not change.

It's common and normal to see the indicated height drift over the duration of a flight and errors of 10 ft or more are common.

... The drone will be flying with an elevation error of 10 feet.
So if flying over water you may think you are at 15 feet but in reality you may be at 5 feet above water assuming a change in pressure of only 100th of an inch in air pressure change.

Minor errors in the indicated height don't cause the drone to fly into the ground or water.
If a pilot flies their drone into the ground or water, that's a pilot error.

When a drone finds itself above a solid surface the sensors provide a reflection in signal and a trigger to land and then a calculation is performed to control rate of landing. Over water ultrasonic waves from these sensors PASS through water and are not reflected. (This assumes a flat water surface below the sensors.)
If the water has movement then some reflection may occur. This fluctuation may or may not be recognized by the sensors.
Since the drone cannot sense a surface it simply flies into the water.

Drones don't lose control over water.
Drones don't simply "fly into the water" without the pilot pulling the left stick down to fly the drone into the water.
This thread is just contributing to the misinformation around flying over water.
If you are flying close to any obstacle, whether horizontally or vertically, whether the obstacle is dry or wet, don't fly into it.

 

[Meta4]

Hesitant is the best description for how the drone was acting. But then, I was flying at -439' OSD, not sure I'd expect normal behavior at that point.

Control response should be completely "normal" regardless of height above or below the launch point.

 

[sar104]

It's common and normal to see the indicated height drift over the duration of a flight and errors of 10 ft or more are common.

Minor errors in the indicated height don't cause the drone to fly into the ground or water.
If a pilot flies their drone into the ground or water, that's a pilot error.

Drones don't lose control over water.
Drones don't simply "fly into the water" without the pilot pulling the left stick down to fly the drone into the water.
This thread is just contributing to the misinformation around flying over water.
If you are flying close to any obstacle, whether horizontally or vertically, whether the obstacle is dry or wet, don't fly into it.

I suspect he is referring to the situation that we have seen quite a few times here, where a pilot decides to fly very low over the water surface (within the margin of drift of the barometric sensor) relying on the OSD height to stay dry. That's exacerbated by the lack of depth perception in the camera view of the water surface and the difficulty of estimating height above water for a distant object.

 

[Meta4]

I suspect he is referring to the situation that we have seen quite a few times here, where a pilot decides to fly very low over the water surface (within the margin of drift of the barometric sensor) relying on the OSD height to stay dry. That's exacerbated by the lack of depth perception in the camera view of the water surface and the difficulty of estimating height above water for a distant object.

Yes .. Just the same as flying over land.

 

[sar104]

Yes .. Just the same as flying over land.

From the aircraft point of view certainly. I think it is harder for the pilot to judge height over water, and sometimes the VPS height doesn't seem to work quite as well either, judging by some of the logs we've seen.

The OP was also wrong about the ultrasonic sensors - ultrasonic waves are largely reflected since there is a very large discontinuity in acoustic impedance at the air/water interface. The IR sensors fare worse though, because they are not optimized for specular reflecting surfaces.

 

[thispilothere]

Is it possible that, over running water like a fast moving creek with ripples, the drone thinks it's flying forward and tries to back up to maintain a hover? This has happened several times with my MA2. It backed up about 6-9 feet before I took the controls and forced it forward again.

 

[sar104]

Is it possible that, over running water like a fast moving creek with ripples, the drone thinks it's flying forward and tries to back up to maintain a hover? This has happened several times with my MA2. It backed up about 6-9 feet before I took the controls and forced it forward again.

That might happen in opti mode, but not if the FC has a GNSS solution.

 

[Dave Maine]

Flying low over ground or water the most reliable height knowledge is the pilot’s eyesight.

 

[Squidinc]

It's common and normal to see the indicated height drift over the duration of a flight and errors of 10 ft or more are common.

Minor errors in the indicated height don't cause the drone to fly into the ground or water.
If a pilot flies their drone into the ground or water, that's a pilot error.

Drones don't lose control over water.
Drones don't simply "fly into the water" without the pilot pulling the left stick down to fly the drone into the water.
This thread is just contributing to the misinformation around flying over water.
If you are flying close to any obstacle, whether horizontally or vertically, whether the obstacle is dry or wet, don't fly into it.

Couldn’t agree more

 

[twickers14]

As a newbie dare I just say that so long as LOS is maintained and you are quick on the controls you should be alright. I have over 300 flights on my Spark and it took a long time to pluck up the courage to fly over water but when I did I just kept the craft in sight and a decent height above the water's surface. Slightly tight feeling in the nether regions but all was well! Highly unscientific contribution but hope it helps. I will be flying my new MA2 over water when the opportunity arrives.

 

[Squidinc]

I don’t believe drones just “fly themselves into water”. I fly 90% or more over rough ocean and surf. Sometimes high above and other times a metre or so above breaking waves. Never had an issue over a year now with the MA2.