In case of an emergency landing (non-fatal) on a road/highway or in a large field, what happens next?
There seem to be two parts to your question. First, what practical steps can I take on the ground immediately after landing? Second, which authorities must be notified?
For the first point, there are just too many scenarios to have a single answer but basically you should immediately check for injuries or damage to property and activate the emergency services if required. After that, secure the aircraft to prevent any further damage, call your insurance company and inform the aircraft owner and local property owner (as applicable). I'd also suggest giving your family a call rather than have them hear on TV news that you were "in a crash landing".
There are more dramatic possibilities, of course: if you ditched or landed in a remote area or in extreme weather then actual survival could be your highest priority. But that's a topic for another site.
For the second point, at least in the US it's possible that you don't have to notify anyone. If no one is hurt, nothing is damaged, and none of the incidents requiring immediate notification occurred (see 49 CFR 830.5 and the AIM 7-6-2) then there's no legal requirement to inform anyone. The authorities in this case would be the NTSB, not the FAA, by the way. There's a more detailed discussion of these requirements here.
An additional concern that I know nothing about is liability and other legal issues. Getting legal advice - e.g. from your insurance company or AOPA - would probably be a good idea whatever the circumstances.
Outlandings are a regular occurrence when gliding cross-country. During competitions with tens of participants, it is not uncommon for a few gliders to land out each day. For this reason, in the UK at least, glider pilots are taught how to land out, and think about this eventuality on every flight. Many will have experienced a landout with an instructor before going solo, and practice approaches to fields are a requirement of gaining the cross-country endorsement (part of the UK syllabus).
In practice, there isn't a lot to do after your beautifully-executed landing. In common law countries (including the US and the UK) on private land you are trespassing, and the landowner can ask you to leave. Of course this is already your intention, but it's complicated by the expensive machine you have just deposited there. Any damage you cause (or caused, on your way in) is your liability. Your aircraft is still yours, and the landowner cannot prevent you from removing it. If he tries, he is responsible for keeping it safe and will be liable for any damage. He can charge a reasonable fee for his time and the unauthorised use of his land, but often will not. Some landowners object to professional balloon companies charging large sums for taking parties of champagne drinkers into their fields and want a cut of the profits. In this case, remind him that you are a private individual and that you landed there without intending to in an emergency.
So your plan should basically be:
If you have approached over built-up areas or roads, someone will probably have seen you and is likely to call the emergency services on your behalf. In the UK, NATS (the National Air Traffic Service) coordinates with the police in cases of aircraft outlandings. You should call their distress and diversion (D&D) cell to inform them of the situation, even if there is no damage or injury.
If the landowner tries to impound your aircraft or becomes aggressive, call the police. There have been cases where landowners have caused damage to aircraft and recourse was only possible because of police attendance.
If you have caused any damage, try to come to an agreement on the cost. Often, young crops are not damaged by landing aircraft, and if you have landed in a long crop you are probably not capable of discussing compensation. Ploughed fields and grass fields are more likely to have damaged your aircraft than the other way round.
Outlandings are not the end of the world. If it worries you, why not hop along to your local gliding club and take a flight. Most instructors will be more than happy to show you round the local fields!
You are talking explicitly about an emergency landing, not about an outlanding (yes, this word really seems to exist...).
Here's the answer that is valid in Europe: In case of emergency, the federal office of aviation has to be informed. They will then check the conditions. If you start before their OK, it will be a criminal act.
If it was a security landing, you will need to ask the owner of the land for permission and you'll need somebody who's watching while you take off.
And no, you don't have to inform the police, but usually you will.
