Image: Someone looks scared - something must be going wrong.
An abnormal situation is one that was not intended or planned for, but that do not pose imminent danger of collision or harm. Examples are lost links, lost GPS, having to land at an alternate landing site.
Emergencies place people or property in imminent danger of harm. Examples include flyaways, loss of control, and battery fires.
With all abnormal and emergency situations, your first recourse is to follow aircraft manufacturer recommendations.
Lost Link
Control of the aircraft is through radio communication between the aircraft and the control station. An interrepution in that communication is called a lost link. Note that a lost link is not a flyaway and, on its own, is not considered an emergency.
Many drones have pre-programmed lost link procedures designed to allow you time to re-establish communication. Look to your user manual to know what your aircraft will do. For example, the DJI Mini-2 will return to home if the link is lost. Be sure to set the return-to-home altitude high enough to avoid obstacles. Also, brief your crew on lost link procedures.
Steps to take in the event of a lost link include ...
Keep visual contact with the aircraft
Make sure the controller is still powered on
Move closer to the aircraft
Try to initate the return-to-home procedure manually
Contact ATC as needed
Once (if) the link is re-established, land the aircraft as quickly as possible.
GPS Loss
As remote pilot in command, you should be ready to control your aircraft manually in the event GPS becomes unavailable. This is abnormal, but also not an emergency.
Even if you are flying manually already, loss of GPS can result in the inability of the aircraft to hover in a stable way, especially if there are winds. You may continue to operate the aircraft without GPS, if GPS is not mission critical. If you are having trouble controlling the aircraft, you should land as quickly as possible, including at an alternative landing site.
Before flying, check NOTAMS for potential GPS outages.
Flyaway
Flyaways are when aircraft control is lost - often starting as a lost link - and cannot be regained. This is an emergency. If pre-programmed failure routines are not properly set or also fail, the aircraft just flies off and will keep flying until it runs out of power. Then it will crash. Some aircraft allow you to turn off the propellers in mid-flight. Consider whether this is a better option than allowing the aircraft to keep flying away.
Take note of the battery level, speed, and direction of the aircraft when the flyaway begins. This will allow you to estimate where it will run out of power. Contact ATC if you are in or near controlled airspace. Use maps to pinpoint potential crash sites. You may chose to contact local authorites, though they will be unlikely to do anything about the situation until the aircraft crashes.
Flight Termination
Flight termination is ending a flight carefully and deliberately. As part of planning, you should know how you will terminate a flight if you encounter abnormal or emergency situations that you feel are dangerous or from which you cannot recover. Know where and how you will land and what you will do once you have landed the aircraft.
Collision
If your aircraft collides with anything, release all controls sticks right away. If the aircraft stabilizes, fly away from the object you collided with and land as soon as possible. If the aircraft remains unstable, strongly consider manually shutting down the rotors and allowing it to fall from the sky. In making this decison, evaluate what is directly underneath vs where the aircraft is otherwise going. Your main goals are to protect people, other living things, and property.
Battery Fires
Most modern uncrewed aircraft are powered by lithium batteries. These batteries are highly flammable, can self-ignite if overcharged, overheated, mishandled, defective, or damaged.
Batteries should be stored safely such that they cannot be short-circuited (i.e., metal cannot connect battery terminals). For extra safety, store them in a fire-proof battery bags.
When these batteries do catch fire, it's difficult to stop. Water does not help. Use a Type D extinguisher designed for chemical and metal fires. Cover the burning batteries with sand or soil to smother the fire.