why does it suddenly drop out of the sky?

So you are just flying along like a champ, and suddenly: BOOM! You hit the ground without warning.
This can have a few reasons, lets break down the most common ones:

- the model is out of control range
- the battery is too low/weak
- a physical connection problem

If you have a RC connection problem, the video reception is usually not affected.
When you see your copter go down with good video and a good battery, you should inspect RC antenna on quad and transmitter!

If you have a power problem, the video reception usually suffers at the same time like the control link. If you see video issue right before or while you crash, I suggest to inspect/check/replace the battery and power connector.
Power connectors can get loose or corrode. They might still work, but not pass as much current as previously because their resistance is going up.

If you have chosen a too strong motor for the batteries you are using, you might see the drop-out when you hit the throttle hard, especially when you do that after 1 or 2 minutes of flying.
When you see that, you need better/larger batteries, smaller/slower motors, or smaller props.

You should also know how to time your flights! Read on..