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Incident analysis: Dji Air 2S emergency landing, full battery depletion and crash

fernadosilva

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Last weekend I had a scaring experience when flying from a hilltop near the coast. The take-off spot was at 350m of altitude, with a quite steep hill down to the sea level. I had flown a few times in the same location with the wind facing the hill and I never had any major problem.

In this specific day there was a light wind blowing from behind the hill, but from my previous experience I had full confidence in my Air 2S wind resistance, and I thought that, with some extra care, this unfavourable wind would be manageable. I am usually quite conservative on battery management, and I often land with 30% of battery or more.

After a few turns and takes, I reached 40% of battery and about 8’ of estimated remaining flight time and I started to return. The drone was about 1.5Km away in LOS. This was when everything became darker. As soon as I start to return, the estimated remaining time dropped sharply, since I am returning with head winds much stronger than I anticipated. Since the drone speed was quite low, I switched to sport mode. The speed remained low, and at a given time the drone was barely progressing at 10Km/h.

At 28% battery, and yet slightly far from the home point, I got the RTH message. I cancelled it, since I was already returning home. After that I start to receive “strong wind warnings” and when the drone was 240m away from the landing site, the battery dropped to 10% and the inevitable happened. I got the “emergency landing” warning. It is a 10s advance warning, and no cancel option exists. After 10s, the aircraft enters landing mode and starts descending. It is still possible to operate the aircraft in the horizontal in slow speed, but manoeuvrability is limited, and you are not able to change the global descending trend.

For 50s or so I useless struggle to return home, wasting precious battery reserve. Finally, I gave up, pointed the gimble downwards and start looking for an alternative landing site. The problem was that the cliff was quite steep and I had more than 200m below. Given the dense vegetation and difficult terrain, any land or crash here would mean that the aircraft would be almost surely irrecoverable.

81m below the take off point I reached zero battery, but the drone was still flying. I tried to reach a small road that runs 273m below the home point (192m below the zero-battery point). Finally, I reached the vertical of the road. I wrongly tried to avoid the vegetation and I attempt to land between a nearby water reservoir and the road, since the land seemed flatter. I understood later that it was a bad and risky choice, given the road proximity. The vegetation would be safer and a better choice for a soft landing, even if it seemed to be more difficult to recover the aircraft. As I came to understand later, my target landing spot was much more problematic for drone recovery, but more on that later.

At this stage I was at about -205m (about 65m above the road) and I start to lose line of sight (because the drone was quite close to the hill), and the radio signal was degraded. At -231m (42m high) I lost image and one or two seconds later I lost connection with the drone.

It took me about 20 minutes to walk and drive anxiously to the target landing site.... It was just 600m away and 270m below, but the windy downhill road is about 8Km long Above all, I was concerned that a potential fall could have been the cause of third-party damage or even hurt someone – even if nobody seemed near my target landing spot while I had visual contact.

Upon arrival, there was no signs of accident, no crowd around and in fact nobody nearby. Excellent news and relief! At the same time, a chilling discovery: the water reservoir had a security metallic fence all around (as it would be expected!), of course not visible from above. And the landing site I targeted was inside the fence.

Anyway, I should be able to see the drone somewhere inside the protected area, but near the fence. But I was not seeing nothing. As I was inspecting the fenced area from outside, and trying to figure out how many phone calls and days I would need to try find who had access to the protected area, a couple passed by and commented "oh, it is that not a drone??? It seems to be in bad shape." I looked in the direction they were pointing... While I was looking for the drone on the ground, it was in fact tangled on metallic spikes exactly on the top of the fence, which was about 2.0m high. I could not believe my good luck: in spite of my target landing spot and vertical alignment to be inside the fence, the aircraft had fallen exactly on the top of the fence. Moreover, the metallic spikes on the top of the fence had drilled the drone body and secured it safely there, avoiding any potential slip to the inner area. Besides the deep body holes from the metallic spikes, it had a broken arm and some further damage, showing that it had fallen from some height, but at least it had fallen in a safe place and I was able to recover the body (and activate my Dji care…) and the SD card.

The last video file on the drone was damaged, since due to battery depletion the file was not properly closed, but I was able to recover it. Upon video recovery, I found that the aircraft carried on the flying/landing for about 40s after connectivity was lost. Given the last image, I estimate that the last frame was recorded at an height of about 15m-20m and that the drone had probably entered in free fall shortly after.

A remarkable conclusion is that this specific Air 2S and battery combination had a battery reserve of about 2:20s-2:30s after reaching the 0% indication (but do not take this as granted or norm). Thanks Dji for being so conservative on battery reserve, which is fully understandable for safety reasons -- as it happened to me - and taking into account that the battery estimate is not fully reliable and it depends on many factors, including age, manufacturing, weather, and so on.

I committed a ton of errors in this flight, most of them well known and widely reported. Anyway, it is always worth to repeat some obvious lessons:

-- Never underestimate the strength of the wind when flying away on the wind direction.

-- Recall that when flying near a hilltop or crest, with wind blowing from behind the hill, the wind speed is strongly amplified due to crest factor, which may impair return. Moreover, don’t forget that you may find strong wind vortexes near the hill.

-- Never forget that wind conditions in mountainous areas may change quite fast.

-- Never overestimate the battery reserve and do not have the remaining estimated time as granted. Weight this rough estimate by the specific weather conditions. Under strong winds the battery consumption is much higher than in calm conditions.

-- When returning home in the conditions above, avoid the hilltop altitude, where the wind speed will surely be the most unfavourable. Try to descend to the middle of the hill and return at much lower level. If you are flying from the hilltop, try to postpone the ascend as much as possible to avoid the crest effect.

-- If the aircraft enters emergency landing mode, do not fight against it. Point the gimbal 90º downwards and start immediately looking for alternative landing sites.

-- When flying near a hilltop, take into account that you may have to descend much more than your current altitude to find a landing site.

-- Try to land near a road or easily accessible location, in order to ease aircraft recovery. But consider that the safety of people and goods in the ground is much more important than the drone value. Try to land or crash in safer places, irrespectively of the cost. It is only electronics and metal.

— Try to understand if your target landing spot is within a fenced area. In my case, it was silly do not guess that a spot close to a water reservoir (as any other facility) was surely a fenced area.

-- Keep your cell phone number in a label attached to your aircraft. Most people will call you immediately if they find your drone before you arrive at the landing or crash site.

The video below reviews this incident.


Log file below.

Fly safely.
 

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GFields

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Last weekend I had a scaring experience when flying from a hilltop near the coast. The take-off spot was at 350m of altitude, with a quite steep hill down to the sea level. I had flown a few times in the same location with the wind facing the hill and I never had any major problem.

In this specific day there was a light wind blowing from behind the hill, but from my previous experience I had full confidence in my Air 2S wind resistance, and I thought that, with some extra care, this unfavourable wind would be manageable. I am usually quite conservative on battery management, and I often land with 30% of battery or more.

After a few turns and takes, I reached 40% of battery and about 8’ of estimated remaining flight time and I started to return. The drone was about 1.5Km away in LOS. This was when everything became darker. As soon as I start to return, the estimated remaining time dropped sharply, since I am returning with head winds much stronger than I anticipated. Since the drone speed was quite low, I switched to sport mode. The speed remained low, and at a given time the drone was barely progressing at 10Km/h.

At 28% battery, and yet slightly far from the home point, I got the RTH message. I cancelled it, since I was already returning home. After that I start to receive “strong wind warnings” and when the drone was 240m away from the landing site, the battery dropped to 10% and the inevitable happened. I got the “emergency landing” warning. It is a 10s advance warning, and no cancel option exists. After 10s, the aircraft enters landing mode and starts descending. It is still possible to operate the aircraft in the horizontal in slow speed, but manoeuvrability is limited, and you are not able to change the global descending trend.

For 50s or so I useless struggle to return home, wasting precious battery reserve. Finally, I gave up, pointed the gimble downwards and start looking for an alternative landing site. The problem was that the cliff was quite steep and I had more than 200m below. Given the dense vegetation and difficult terrain, any land or crash here would mean that the aircraft would be almost surely irrecoverable.

81m below the take off point I reached zero battery, but the drone was still flying. I tried to reach a small road that runs 273m below the home point (192m below the zero-battery point). Finally, I reached the vertical of the road. I wrongly tried to avoid the vegetation and I attempt to land between a nearby water reservoir and the road, since the land seemed flatter. I understood later that it was a bad and risky choice, given the road proximity. The vegetation would be safer and a better choice for a soft landing, even if it seemed to be more difficult to recover the aircraft. As I came to understand later, my target landing spot was much more problematic for drone recovery, but more on that later.

At this stage I was at about -205m (about 65m above the road) and I start to lose line of sight (because the drone was quite close to the hill), and the radio signal was degraded. At -231m (42m high) I lost image and one or two seconds later I lost connection with the drone.

It took me about 20 minutes to walk and drive anxiously to the target landing site.... It was just 600m away and 270m below, but the windy downhill road is about 8Km long Above all, I was concerned that a potential fall could have been the cause of third-party damage or even hurt someone – even if nobody seemed near my target landing spot while I had visual contact.

Upon arrival, there was no signs of accident, no crowd around and in fact nobody nearby. Excellent news and relief! At the same time, a chilling discovery: the water reservoir had a security metallic fence all around (as it would be expected!), of course not visible from above. And the landing site I targeted was inside the fence.

Anyway, I should be able to see the drone somewhere inside the protected area, but near the fence. But I was not seeing nothing. As I was inspecting the fenced area from outside, and trying to figure out how many phone calls and days I would need to try find who had access to the protected area, a couple passed by and commented "oh, it is that not a drone??? It seems to be in bad shape." I looked in the direction they were pointing... While I was looking for the drone on the ground, it was in fact tangled on metallic spikes exactly on the top of the fence, which was about 2.0m high. I could not believe my good luck: in spite of my target landing spot and vertical alignment to be inside the fence, the aircraft had fallen exactly on the top of the fence. Moreover, the metallic spikes on the top of the fence had drilled the drone body and secured it safely there, avoiding any potential slip to the inner area. Besides the deep body holes from the metallic spikes, it had a broken arm and some further damage, showing that it had fallen from some height, but at least it had fallen in a safe place and I was able to recover the body (and activate my Dji care…) and the SD card.

The last video file on the drone was damaged, since due to battery depletion the file was not properly closed, but I was able to recover it. Upon video recovery, I found that the aircraft carried on the flying/landing for about 40s after connectivity was lost. Given the last image, I estimate that the last frame was recorded at an height of about 15m-20m and that the drone had probably entered in free fall shortly after.

A remarkable conclusion is that this specific Air 2S and battery combination had a battery reserve of about 2:20s-2:30s after reaching the 0% indication (but do not take this as granted or norm). Thanks Dji for being so conservative on battery reserve, which is fully understandable for safety reasons -- as it happened to me - and taking into account that the battery estimate is not fully reliable and it depends on many factors, including age, manufacturing, weather, and so on.

I committed a ton of errors in this flight, most of them well known and widely reported. Anyway, it is always worth to repeat some obvious lessons:

-- Never underestimate the strength of the wind when flying away on the wind direction.

-- Recall that when flying near a hilltop or crest, with wind blowing from behind the hill, the wind speed is strongly amplified due to crest factor, which may impair return. Moreover, don’t forget that you may find strong wind vortexes near the hill.

-- Never forget that wind conditions in mountainous areas may change quite fast.

-- Never overestimate the battery reserve and do not have the remaining estimated time as granted. Weight this rough estimate by the specific weather conditions. Under strong winds the battery consumption is much higher than in calm conditions.

-- When returning home in the conditions above, avoid the hilltop altitude, where the wind speed will surely be the most unfavourable. Try to descend to the middle of the hill and return at much lower level. If you are flying from the hilltop, try to postpone the ascend as much as possible to avoid the crest effect.

-- If the aircraft enters emergency landing mode, do not fight against it. Point the gimbal 90º downwards and start immediately looking for alternative landing sites.

-- When flying near a hilltop, take into account that you may have to descend much more than your current altitude to find a landing site.

-- Try to land near a road or easily accessible location, in order to ease aircraft recovery. But consider that the safety of people and goods in the ground is much more important than the drone value. Try to land or crash in safer places, irrespectively of the cost. It is only electronics and metal.

— Try to understand if your target landing spot is within a fenced area. In my case, it was silly do not guess that a spot close to a water reservoir (as any other facility) was surely a fenced area.

-- Keep your cell phone number in a label attached to your aircraft. Most people will call you immediately if they find your drone before you arrive at the landing or crash site.

The video below reviews this incident.


Fly safely.

Well, I'm glad you were able to retrieve your drone. I was going to suggest posting your fly logs here so @Meta4 could analyze them. But, I think you know the mistakes you made.

1. Flying out with a tailwind and not understanding coming back would be hard.
2. Not realizing that wind speeds are greater the higher you fly.
3. Using sport mode using more battery than N mode
4. Not lowering altitude when you saw the drone struggling to return due to headwinds.

At any rate, you got your drone and will get a replacement. Lesson learned.
 

fernadosilva

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Well, I'm glad you were able to retrieve your drone. I was going to suggest posting your fly logs here so @Meta4 could analyze them. But, I think you know the mistakes you made.

1. Flying out with a tailwind and not understanding coming back would be hard.
2. Not realizing that wind speeds are greater the higher you fly.
3. Using sport mode using more battery than N mode
4. Not lowering altitude when you saw the drone struggling to return due to headwinds.

At any rate, you got your drone and will get a replacement. Lesson learned.
You are 100% right. And I was well aware of most of them as a former paraglider pilot... No excuses :-( Just excess of confidence.
 

Yorkshire_Pud

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ou are 100% right. And I was well aware of most of them as a former paraglider pilot... No excuses :-( Just excess of confidence.
When you are actually flying, if you touch one of the corner buttons on the map you might bring up the attitude indicator.
That would give you a live view of how much wind the drone was in i.e. if the horizon is near the centre of the circle, little or no pitch is being use to fly in accordance with your commands. A side wind or sideways fight is indicated by the slant of the 'horizon'.
I have seen this "eyes on the drone", when a Mavic Mini was flying at full elevator down wind but the drone was near enough in the hover position.
 
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fernadosilva

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When you are actually flying, if you touch one of the corner buttons on the map you might bring up the attitude indicator.
That would give you a live view of how much wind the drone was in i.e. if the horizon is near the centre of the circle, little or no pitch is being use to fly in accordance with your commands. A side wind or sideways fight is indicated by the slant of the 'horizon'.
I have seen this "eyes on the drone", when a Mavic Mini was flying at full elevator down wind but the drone was near enough in the hover position.
Yes, you are right, I use sometime the atitude indicator to estimate the wind speed. But in this situation it would only provide the confirmation of what I already knew from the drone reaction. All the errors were committed before :-(
 
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Yorkshire_Pud

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My point was that it would, on the out bound fight, probably have shown the drone to be in a near hover position, though fighting a side wind, whilst, at a guess, going near full pelt.
Alternatively, if I have any suspicion about wind strength, I switch the gimbal to FPV mode and do a 360deg yaw whilst close to home.
 

fernadosilva

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I was going to suggest posting your fly logs here so @Meta4 could analyze them.
Thanks for the suggestion. I added them at the end of my post anyway, just in case someone wants to make some more detailed analysis. Of course, I have only Dji fly logs prior to lose connection.
 
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