I am very new to flying drones and still very much in the process of learning. On my third or fourth flight, I was using Map Pilot, flying at an altitude of 335 ft with terrain awareness enabled. My Mavic Pro was about 2,000 feet out and only a quarter of a mile into its mission when out of nowhere, I hear a the sound of a small plane approaching. I panicked and immediately hit the return to home button on the controller. The drone was at 865 feet above the home point and immediately started climbing, reaching 913 feet, though it was all down hill to the home point. I knew this would happen as I hadn't yet reached the summit of the highest hill in the survey area. I used the left stick to manually bring the vehicle down on its way back, but was overly conservative as I was afraid that I'd get too low and run into the trees. I ended up with the Mavic at 700 feet AGL when I reached flat land at the base of the hill, which was 500 feet from the home point, and had brought it down to 500 feet by the time it was overhead. Luckily the plane, which was flying pretty low, passed a little past the area my drone was flying, but the incident sure gave me a scare.
I found the following FAA rules governing low flying aircraft:
Title ·14, ·Code of Federal Regulations, Section 91.119 of the General Operating and Flight Rules which specifically prohibits low-flying aircraft.
·91.119 Minimum safe altitudes; general
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes;
(a) ·Anywhere. ·An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) ·Over congested areas. ·Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2.000 feet of the aircraft.
(c) ·Over other than congested areas.
An altitude of 500 feet above the surface except over open water or sparsely populated areas. In that case, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) ·Helicopters. ·Helicopters may be operated at less than the minimums prescribed In paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with routes or altitudes specifically prescribed for helicopters by the Administrator.
I interpret paragraph (c) as meaning that a plane can fly lower than 500 feet over water or sparsely populated areas, which is scary because I'd think it's where probably the majority of UAV flights take place. But even 500 feet is only a hundred feet higher than 400 feet!
Looking back, I think the most prudent thing to do in that situation, instead of returning to home, would have been to temporarily stop the mission and hover, bringing the drone down to a hundred feet or so above the tree tops and wait for the plane to pass. If I had known the Mavic's actual height above the terrain as it was descending, I would have been able to do this.
My question is, in the case of a paused or cancelled mission, or on the way back to the home point from a terrain aware mission, would it be possible, either graphically or numerically, to display the drone's actual height above the ground based on SRTM or whatever elevation data is being used? This would allow the pilot to have the drone "hunker down" at a safer altitude until an aircraft passed, or for manually lowering the craft to a safe altitude when returning from a terrain aware flight, which using the default return to home behavior, may be many hundreds of feet higher than what is safe or legal.