Is it correct that IFR can have visual flight(depending pilot's eye) not instrument in VMC? If yes, Does IFR include VFR?
IFR and VFR don't directly correlate to what the weather or visibility is. Each is a set of rules (the "FR" stands for "flight rules" in both cases) that you are going to follow as a pilot. Yes, in the case of VFR, several of those rules relate to meeting certain weather/visibility minima... but that's not the only difference, hence why there is a distinction between VFR and VMC (or IFR and IMC).
I know VFR flight can fly everywhere they want to go as maintaining VMC.
Careful there - not everywhere
. There are several of examples where VFR flight isn't allowed, such as:
- Above 17,999 ft MSL (and below 60,001 ft MSL).
- Any time the visibility drops below a certain number of miles or the pilot can't maintain a minimum number of feet above/below/horizontally from a cloud (the numbers change depending upon which airspace you're in or even what time of day it is).
- Any time the pilot can't meet minimum requirements to operate in a given classification of airspace (such as the inability to receive permission to enter it if it's controlled airspace).
None of those apply to flights operating under an IFR clearance.
so Is it possible IFR flight fly everywhere they want to go(not planned route) and when they encounter IMC, they use instrument(IFR) and just keep flying(in the IMC)?
why IFR flight must get clearance(including route), and fly as planned route?
How would the controller be able to provide positive separation if (s)he has no idea where you're going? How would (s)he know when you're providing your own separation and when you're depending upon the controller to do so?
The answer to both ("not possible") explains why the answer is: no, that situation is not (and could not be) permitted.
and I know there's vfr altitude(like 3,500ft, 5,500ft), and i have some question about it.
It sounds like you're referring to a "VFR cruising altitude" when you're above 3,000 ft AGL. In other words, you're referring to 14 CFR 91.159
I believe VFR flight are free in flying, and they just see and avoid some obstacle. But when they fly as vfr altitude and encounter cloud, i think they must keep VMC so they should descend or climb to avoid cloud. it means they have free in choosing altitude. So i think the vfr altitude disturbs VFR flight to maintain VMC. Why is there vfr altitude which restrict them maintaining VMC ?
Someone could probably come up with a few reasons, but two good ones would be:
- They help lessen the probability that someone is going to appear at your 12 o'clock, opposite direction, same altitude, converging very quickly.
- They help you (and the controller) maintain appropriate vertical separation between IFR traffic in Class B or Class C airspace (minimum is 500 ft).
And again, I'd be hesitant to agree with a wide open statement such as "they have [freedom] in choosing altitude." If you don't have permission to enter controlled airspace and suddenly climb into it to avoid a cloud, it's great that you avoided the cloud... but you still violated the Class B airspace and might end up receiving a phone number to call to discuss a possible pilot deviation.