endotherm
 First Officer
Flight distance : 503241 ft
Australia
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Compass error messages get a bad rap around here. They are not as critical as you are making out. A "compass error" is just the system telling you that the data from the compass isn't making sense, and is unreliable. This could be because it was set up and calibrated incorrectly on the ground, or has flown into an area of disturbance. The aircraft's altitude is determined by a barometric altimeter, so it isn't going to change altitude due to a change in a different sensor in another sub-system. This doesn't cause the aircraft to catch fire or explode, or tumble out of the sky. If you think people are crashing drones and they are falling out of the sky because of a compass error, you would be very wrong. It would be due to flying in an unfamiliar mode and not being competent, i.e. pilot error.
Compasses don't get along with iron or ferromagnetic materials like steel and other metals and ores. If you get too close it will upset the compass, which is a device that reacts to and indicates magnetic fields. If you are hiking and using a handheld compass to navigate, you will be reading the magnetic influences of the Earth. If you suddenly walk next to a radio tower or under a structure, you might find your compass goes crazy for the duration. You ignore the readings for the moment because you know they are unreliable, and know that they will return when you clear the interference. You are not going to explode or fall to the ground! This is the same with your drone.
There are two main failures -- flying into a disturbance (e.g. getting close to a steel tower or power line pylon), or taking off from a disturbance (which has been accepted as "normal") and flying into an unaffected space. Both will return a "compass error". The first case is pretty self-explanatory. The second case usually occurs if you take off somewhere close to metal or ferro-magnetic materials. Taking off from your car's boot or hood, or even your sunroof is pretty stupid. Your campass is already being affected and returning bad data. Another bad launch site is when taking off from concrete footpaths/slabs, because they will have rebar reinforcement embedded in them. Same for other unknowns like roads and other manmade structures. It is recommended to setup and operate from a clear, natural area, where it is unlikely you will be affected by magnetic fields. What confuses a lot of people is why they can calibrate and set up on one of these "bad" areas but have little or no problem. The problem is that metal can be a strong or weak magnet, or their orientation may closely resemble the Earth's field and appear non-existent, or their orientation is highly contradictory to the Earth's (e.g. its S pole facing north, and N facing south). When calibrating on top of another magnet, you are basically offsetting the planet's N and S poles from their true position, to appear in say London and Sydney instead! This offset might be by a lot, or some insignificant amount. In the latter case, your aircraft will appear to calibrate correctly. When you take off and clear this disturbance, you are now only registering the planet's true field, which will be different from the reading you set when on the ground. The aircraft will tell you the compass data is unreliable, because it knows something is wrong, but doesn't know which one is right. It ignores the compass data and switches the aircraft to ATTI.
The problem is that we are spoilt by flying in GPS mode. The aircraft is doing 90% of the flying for you. It is making dozens of control changes every second, in response to wind etc. If you want to fly in a straight line, just point it in that direction and push the stick forward. Most people don't consider the micro corrections the aircraft is making to keep it level, and counteract the wind. When that assistance is removed, most people are thrown into an unfamiliar world. Their 10% ability isn't enough to pilot the aircraft properly! They will notice the aircraft moving by itself, not sitting still like it is used to. It is not running away with a mind of its own, it is being blown by the wind. You need to power and steer into the wind manually yourself to hold position, then worry about where you want to go on top of that. Your workload and stress levels go way up.
The compass is one of the sensors, along with GPS, which is used to determine your position, heading and course. It can be upset by magnetic interference. The system is smart enough to know when the compass data isn't making sense, and ignores it until it is reliable again. GPS mode requires a reliable compass and adequate GPS signal reception. If those conditions don't exist, it makes no sense to let the GPS flight mode to make decisions for flying the aircraft, does it? The aircraft will automatically switch to ATTI mode for the duration of the failure, then return back to GPS mode (assuming you aren't manually selecting and flying in manual mode). You don't have to manually switch it in and out of ATTI mode, it will happen automatically. If you leave it in GPS mode, it will resume by itself when conditions improve. It is perfectly fine and safe to fly it this way. There is no problem flying in ATTI mode either, it is essentially the same thing as GPS mode, except there aren't all the micro corrections being made for you to maintain position. You have to allow for, and make adjustments for the wind. An inexperienced pilot will say it is "uncontrollable" because they are probably flying in conditions close to the aircraft's capabilities, in wind that is too strong. The aircraft will appear to be not responding to their inputs. Even if the compass isn't reading properly, you can still fly. You can move forwards/backwards/left/right/up/down and rotate. You can counter the wind. You can fly in whatever direction you choose. You just won't be able to navigate properly.
In ATTI mode, the compass is effectively ignored and not used to "hold" the aircraft. You can move left, right, forward, backwards just like a simple RC helicopter (which is essentially flying in its own ATTI mode!) which would be much harder than flying one of our drones. So it is possible, and was already included by design.
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