One of the most highly touted features of modern drones is “return to home” (RTH), a safety feature which, in the event of an emergency, causes the drone to return to its launch point and safely land. So it probably comes as a surprise to learn that the RTH feature is one of the primary reasons why drones crash.
If you think about it, it makes sense. Even an amateur pilot has vastly more situational awareness than the small microprocessor in the drone controller, so the drone should be under the control of the pilot as much as possible. Many people think of RTH as a convenience feature, like power windows. In reality it’s a safety feature, much more like air bags.
To understand how RTH gets pilots into trouble, we need to take a moment and understand exactly what it does. Let’s review how the Phantom series RTH works. The short description is this:
At takeoff, a home point is set in the aircraft’s memory. The aircraft is also programmed for a return-to-home altitude. When RTH is activated, the aircraft rises to the RTH altitude and heads directly towards the home point. Once it is positioned over the home point, the aircraft descends until it lands.
RTH is automatically activated anytime the aircraft loses touch with the controller for more than 3 seconds, or when it detects a low battery condition, or it decides that it is getting too far away to safely return on the available power remaining. RTH can also be activated manually by pressing the return-to-home button on the controller.
RTH, the Fine Print
EXACTLY what the craft does when RTH is activated depends on where the craft is at the time. Sometimes these details are what gets a pilot into trouble. For example, if the craft is within 65 feet of the home point, it just begins to descend immediately right where it is, even if that is the edge of a lake. If the craft is further away, the craft will check its altitude against the RTH altitude. (The default is 90 feet, but you can, and should, change this before you take off.)
If its altitude is below the RTH altitude, the craft will climb to the RTH altitude. If it is already above RTH altitude, it does not make an altitude adjustment. Then it will orient towards the home point and fly in a straight line to the home point. After hovering for 5 seconds, the aircraft will initiate the landing sequence.
Seems simple enough, but let’s look at how this sequence can get you into trouble. Assume that you take off with the default return to home altitude set to 90 feet . You fly up to an altitude of 100 feet and start to take pictures of a 15 story building. As you fly around the building, and get to the backside, the building blocks the signal to your aircraft. The aircraft detects the loss of signal, notes that it is above the RTH altitude, and heads directly towards the launch point. Unfortunately, that point happens to be on the opposite side of the building leading to “unplanned building impact”.
Set Your RTH Altitude
The best way to protect against this scenario is to set the return to home altitude to be higher than the tallest object which could come between you and the aircraft. In this case, it’s the 15 story building. Setting the RTH altitude to 200 feet would cause the craft to fly over the building when it lost the signal, instead of into it.
But setting a high RTH altitude isn’t the best practice in all cases. For example, assume that you are standing on a bridge and decide to fly the craft underneath the bridge to get dramatic video of the beautiful canyon below. When the aircraft passes underneath the bridge, it’s likely that the bridge will block the signal. When that happens, the craft will rise to its RTH altitude and crash into the bottom of the bridge.
In this case, you are better off launching from somewhere other than the bridge itself and setting the RTH altitude low enough to clear the bridge. You can also set the aircraft’s loss of signal behavior to “hover” instead of “return to home,” which would give you time to move to a different position and retake control of the aircraft. A third possibility is to set ground control points on either side of the bridge and fly the aircraft underneath the bridge using those waypoints. Because the craft will be on a programmed route, it will not discover the loss of signal until the mission is completed. (Beware that big bridges can also block your GPS signal, making them super dangerous missions.)
Landing Nearby the Home Point
Another scenario which often happens to new pilots is that they become disoriented, lose track of which direction their aircraft is facing, panic and press the RTH button. There are two ways this can crash the aircraft: (1) the craft can fly into something as it is changing altitude and/or direction, and (2) the craft can land somewhere other than exactly where it took off from.
We’ve already talked about the hazards associated with the first condition, but how could the craft land somewhere other than the home point? There are a couple of ways this could happen. First of all, the home point and the aircraft’s position are both determined by GPS signals. GPS can easily be off by 10, 20 or 30 feet, even beneath a clear sky. This means that even though the home point is set for your driveway, the aircraft can attempt a landing on the edge of your roof, the top of a fence, or the tree in your front yard. Also, if the aircraft is within 65 feet of the home point, pressing RTH will cause the aircraft to initiate the landing sequence exactly where it is at the moment.
A good rule of thumb is to never allow the RTH procedure to actually land the aircraft. Once it’s back within your visual control and you have a clear idea of its orientation, you should retake control of the aircraft by interrupting the RTH procedure. You can do this by holding down the RTH button on the controller or by tapping the dialog box on the screen.
In general, try not to use the RTH procedure unless it is absolutely necessary. If you become disoriented and lose track of which direction your craft is facing, stop what you’re doing. The aircraft will hover safely in the air while you figure it out. There is no need to panic.
Look at the screen. The map view will show you which direction your aircraft is pointing and it will draw a line between the aircraft and the home position. Rotate the aircraft so that it is pointing right down that line. Now, it’s headed towards you. Pressing the forward lever will cause the aircraft to travel along that line and come straight back to you.
RTH Best Practices
So let’s summarize the RTH best practices:
- RTH is a safety feature, not a convenience feature. It is not magic. It’s there for when you need it, but should not be relied upon.
- Always set the RTH altitude at the start of every flight for the particular mission you are flying.
- If RTH is initiated automatically by the aircraft, or if you press the button, interrupt the sequence as soon as you have your eyes on the aircraft and know which direction it is facing.
Following these simple guidelines will significantly reduce the chances of your drone winding up in a tree or a lake.
In my case the low battery warning sounded, so I initiated RTH, and disengaged it when the mavic pro was about 5′ above ground near me. Having plenty of battery to hover around a bit, I flew it over near some friends to show them some things about it. Suddenly the RTH engaged again and it flew straight up into trees, then spiraled down with a hard crash, breaking a couple of props. Now I can’t get the camera to show. Everything works fine, it says it is ready for GPS takeoff, but no picture.
Ouch! Sorry to hear that.
I always try to be extremely conservative about battery power, often landing with 20% left. The FAA teaches you to do “risk/benefit” assessment: everything you do has an associated risk, so ask yourself if the benefit is worth it. It’s a pain in the butt to land early and change the battery, but there’s a risk if you don’t. Battery monitoring circuitry has come a long way, but it isn’t yet as good as a gas gauge.
Regarding your craft, give Anita a call and she can probably give you a decent diagnosis over the phone.
I’m so very pleased to know that there is finally a excellent shop that can fix my crashed aircraft in the Bay Area. These folks are incredible!!! I say this from experience with other fix-it-shops. Their knowledge, willingness to work with you, and pricing are excellent. I can not recommended them enough!! Thanks so much.
Thank you very much! We love helping get pilots back into the air as quickly and inexpensively as possible.
I made the mistake of setting the Spark to RTH while my toddler wanted to pee all the sudden. >.< So I had to avoid a potential crash by performing a manual overwrite of landing. Needless to say I had to change someone's pants after the landing too. The RTH really fools you when you are just a beginner. Oye! Never using that feature again!
Thanks very much for sharing!
Thanks to you I have a better understanding of the RTH feature…and from this article I learned the importance of a well chosen launch point…My maiden flight was a great success…
Yay! Thanks so much for the feedback. Glad we could help!
Quick question for you guys . . . I was across a river in GPS mode, filming the Fort at St Augustine at dusk. I hit RTH, and the drone rose to return height, turned and faced me, but would not budge. Thankfully, when I placed it in Sport mode I was able to fly it back and land. Post flight analysis was that the obstacle sensors were seeing the sun behind me as a large obstacle. Sport mode turned off the obstacle avoidance and allowed me to fly it back. Do the bottom, downward looking sensors turn off when in Sport mode? Oh, and by the way, hitting the “Pause” button will cancel RTH too! 🙂
Hi! Thanks for the great question.
Yes, you should some quick thinking by turning on “sport” mode. Sometimes the sun will shine into those sensors and fool the drone into thinking that it is seeing an obstacle. Going into “sport” mode disables the sensors.
One other related anomaly can occur when the downward-facing sensors are not calibrated or when if the drone flies into a cloud: hitting the “home” button will cause the drone to start gaining altitude and continue gaining until it hits max altitude. The fog, or bad calibration, makes the drone think that it is flying too low. Once again, “sport” mode is the answer.
BTW, we probably don’t need to mention this, but those fluffy white clouds are made of water which is very bad for your drone, so definitely avoid them! 😉
We were flying in our farm in Colombia last week. The drone lost signal and activated the RTH feature while filming underneath some trees. Same as the bridge situation described, it crashed and stayed in the tree for two days until we found someone able to climb the tree and locate it. By the way, the spark is the most durable drone ever, you won’t believe all what this device endured and it still flies like new. I will be visiting you for some repairs.
After looking at some Youtube videos re: Mavic Air on line and reading comments posted by viewers I’ve seen a couple people talk about how they nearly lost their drone because the drone initiated RTH because of low battery levels but evidently their wasn’t enough battery power left to return home so the drone just landed where ever it was as a fail safe.
I assumed that the RTH feature trigger was smart enough to calculate when it might need to return (based on remaining battery levels/%, distance from home and current battery burn rates on that flight up to that point).
Do you know what triggers a “battery level based” auto-RTH? Is it just based on remaining battery level (e.g., once battery level hits X% ) or does it have any more smarts per my suggestion above?
I’m just puzzled as to why people would run into scenarios where their drones don’t make it back to home and it auto-lands itself in their current position (unless the pilot just ignores/overrides RTH warnings)?
Can you think of what might trigger this type of “just land where ever I am” sort of drone behavior when auto-RTH is initiated?
Hi! Thanks for the great question.
The RTH process is complex and DJI hasn’t been fully transparent about what all is involved, but there is some stuff that we do know. For example, if you initiate RTH when you are close to the home point (around 10 meters) the craft will simply land where it is. This is problematic if you happen to be standing on the shore of a lake and your drone is nearby. (Ouch!)
Other than that, RTH does some simple calculations using the distance between home and its current location, its altitude, the battery level, and the flight time it took to get to where it is.
If the drone concludes that it is reaching the “point of no return,” the mid-point of a round-trip, it will initiate a standard return to home sequence. However, the calculation of that point may be optimistic. For example, we had a customer fly his drone out over water, failing to notice that he was traveling with an offshore tailwind.
When the drone decided it was time to go back, it was flying into a headwind and didn’t have enough power to fight it. He never saw that drone again.
The next level of automatic RTH is when the drone decides that it only has enough power to safely reach the ground from its current altitude. In this case, the drone will begin a controlled descent right where it is in order to avoid the inevitable uncontrolled descent when the batteries die.
These calculations are designed to be conservative, but it is very unwise to attempt to override them. Remember, you don’t REALLY know how much power is in the battery. There is actually no way to measure it, even though the smart batteries do an excellent job of estimating it.
Battery capacity is a function of the age of the battery, how much it was charged, how long since it was charged, the ambient temperature, the rate at which it was discharged, and probably some other factors I’ve overlooked here.
Smart pilots always land with a few percent in the gas tank, and always take off with a full tank.
Hope this helps, and happy flying!
Is it wise to hit the RTH function if you launched from a tall isolated building? I’m a new pilot with very limited hands on experience, and I live in a tall building at the edge of a golf course, and I wonder if I could launch from the roof and safely automatic RTH, or better use a manual RTH.
Hi! Thanks for your note.
No matter where you fly from, you will always have more situational awareness than the little microprocessor in the drone, so it’s almost always better to fly the craft yourself.
If you get disoriented or lose track of your craft. it’s fine to press the RTH button to get the craft back in sight and under your control PROVIDED THAT YOU HAVE SET THE RTH ALTITUDE HIGH ENOUGH TO CLEAR ANY SURROUNDING OBSTACLES. Then when you craft shows up, cancel the RTH operation and fly it by hand.
Hope this helps.
Hi all Drone experts.
I wanted to better understand the Return to home function.
I know that I am supposed to set the return to home than anything higher than my surrounding.
In case, let’s assume that if my Mavic 2 pro loses the signal behind a building and my RTH altitude is lower than the building, Obstacle avoidance and RTH obstacle avoidance is on and while flying back, a building comes in the way, what will it do? I’m assuming it should be point 1 but not sure.
1- Ascend till it clears the path and continues flying back. Also is there a limit to how high it can ascend while RTH is active in case if RTH is set lower than the obstacle in front of it.
2- Move right or left and clear the obstacle and fly back- Also what would happen if it moves right and is not able to see any clear path assuming there are tons of buildings, will it continue going left or right till it finds a clear path
3- Just crash into the building
Hi and thanks for your question.
You are correct, number (1) is the answer. Your drone will first ascend to the RTH height that you set in the app. If it then encounters an obstacle, it will attempt to ascend to avoid the obstacle.
The drone will not ascend higher than the specified maximum height limit (typically 400 feet), but in most cases this will be sufficient to clear the obstacle. However, if if this ceiling is reached and the drone is still blocked, it will attempt to land where it is without returning.