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.