Does a pilot need to communicate with ATC while flying VFR (during departure, flight or departure) or does it depend on type of flight (like private, commercial, etc.)?
Or is it required for pilot to inform ATC only during arrival and departure for runway clearance and altitudes?
If a pilot does not remain in contact with ATC during the flight, how does ATC know what pilot's intentions are?
It depends on which class of airspace you are flying in. For airspace classes A, B, C and D, a continuous two-way radio contact with ATC is required for all VFR flights. In airspace classes E, F and G, there is no such requirement.
The below table shows requirements for both IFR and VFR to fly in different airspace classes (source: ICAO Annex 11, Appendix 4). You should notice the second column from the right, which specified the radio communication requirements for a given airspace class.
These are general rules, that apply to all ICAO member states (basically, every country in the world), unless a specific state has decided to deviate from the ICAO standards. Such deviations will be listed in the national AIP, section GEN 1.7. For example, in some countries, two-way radio communication may be required for VFR traffic within certain airspaces classified G, even though that is generally not the case. A table with rules specific for your country will be available in your national AIP, section ENR 1.4, and will probably look quite similar to the above.
At controlled airports, a control zone will be established, which will be class A, B, C or D (cf. ICAO Annex 11 para 2.6, a control zone cannot be classified E). This means that, at a controlled airport, VFR traffic must establish a two-way radio contact with ATC, just like IFR.
At uncontrolled airports, however, this is generally not required.
During the approach, departure and en-route phases of flight, whether or not you are required to be in contact with ATC depends entirely on where you are flying. Very generally speaking, if flying close to controlled airports or above a certain level, you can expect the airspace to be controlled, and thus, contact with ATC is required. At low altitude, away from major airports, the airspace will often be uncontrolled, and no radio contact required.
The above is a very general description, and you really must refer to local charts and procedures to figure out if you are required to contact ATC in a specific area.
As a point of advice, ATC is there to help you. It is a good idea to contact ATC even if you are not required to do so, in order to receive flight information service and alerting service. This allows you to receive important information about traffic, weather, military activity, etc., and if something happens to you, a search and rescue mission can be initiated without delay.
Note: This answer is specifically about procedures in the USA; it does not apply to the rest of the world, although most countries are very similar.
It depends "where" you are flying resp. in which airspace. The airspace is divided into types, according to the picture attached, and is an international classification. It is then up to the countries to split their airspace (including the one in the vicinity of the airfields) into the shown ones (some use just two, some use more). On specific charts, you can see if it is necessary or not. Summarizing, in controlled airspace (so if the airfield lies inside of an airspace class E or higher) it is mandatory. If the airspace around an airfield is of class F (missing in the chart) or G, it is not. This is quite often the case for smaller airfields.
Here are some straight-forward answers assuming you're asking about flights operating in the FAA jurisdiction.
- Pilots are not required to communicate with ATC while operating VFR unless they are operating at a towered airport or within certain airspace which is usually associated with a towered airport. Take a look at chapter 3 of the Aeronautical Information Manual for more detail on airspace. I think it's also a point of confusion that not all controlled airspace requires communication with ATC. I have often told students to think of class E airspace as "controllable" airspace. ATC does not exercise control over all the aircraft in controlled airspace.
- Airports without control towers have a common frequency on which aircraft can broadcast their intentions, but pilots aren't required to do so. Some aircraft don't even have radios. Towered airports will always require communication with ATC but the specific procedures vary slightly depending upon the type of airspace.
- ATC often doesn't know the intentions of the pilot. If an airplane violates airspace ATC will usually try to contact them on all their broadcast frequencies or on guard (121.5). On en route frequencies it's not uncommon for ATC to give traffic advisories for controlled aircraft pointing out traffic that is not talking to to ATC.
Whether VFR aircraft need to talk to ATC depends on the class of airspace and country it is in; sometimes it's required, sometimes it's optional, and sometimes it's not available at all (due to distance, terrain or other factors). We'd need more specifics to give a useful answer.
If an aircraft isn't talking to ATC (whether required or not), ATC probably doesn't know their intentions. This may require them to notify other traffic of a potential conflict or even vector them away to provide extra separation. In some cases, they will notify the military, which may intercept it.
To give you a complete answer to all your questions: AFAIK the jurisdiction does apply to all countries in the world (ICAO members). There are probably additional, national rules (which I estimate to not be a lot as aviation is quite international; only regarding the airspaces and mandatory radio-contact).
It only depends on the airspace you are in. As mentioned by others, there are 7 of them, from A to G where as A is the "strictest" and G is the least regulated one. Countries are free to split their airspace into the different classes (which often results in countries only using certain classes but not all). Radio contact is mandatory for VFR traffic in A, B, C and D. Every airspace (except E (has, but only mandatory for IFR), F and G) therefore also has a corresponding radio-frequency to contact. As a simple conclusion, only so-called controlled airfield (airfields inside of an airspace A, B, C or D) have an ATC. Others are "free to land" (there are rules which you should respect, see VAC or similar approach informations, but there is no mandatory radio-contact required; there also is no one having the permission to give you any clearance). For more information regarding this answer, have a look at an ICAO chart or the airspace list posted by others.
Basically, he is required to contact the ATC on the corresponding frequency before entering the airspace. Additionally, some airports require you do contact them at specific points a certain time (5 minutes) before approaching (so not just before you enter their airspace). This is written on the VAC (Visual Approach Charts) of every airport and is not a "flight-restriction" but a "condition" to get a clearance to enter their airspace. Summarized: There is only obligation to contact an ATC if you want to enter it's airspace. Probably, the obligation is to call it early enough from outside, but this is an additional requirement IF you want to enter it's airspace. Stay outside and you never ever have to contact them.
That's why radio-contact is mandatory inside certain airspaces (A, B, C, D), the others (E, F, G) do not have an "ATC". The only possibility to switch the frequency of your current airspace is on approval by the ATC (which sometimes makes sense, imagine you want to cross a neighboring airspace: you first have to contact that one before you can enter, but you're still in your first one. In this case, the ATC you are in will allow you to change frequency). As the pilot talked to the ATC before, the ATC knows know very well (even dough they are not on the same frequency anymore) what he is going to do.
For all the people complaining about the wrong rules of the airspaces: If you want to know them exactly with all the details, please refer to one of the posted pictures, my answer is regarding to the question of mandatory VFR radio-contact. Nothing more and nothing less.
edit: airspace E does not have mandatory VFR radio-contact.