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Although I have worked as ATC more than 5 years(not US), I don't know the exact definition of VFR and IFR.

Of coure, I know that under VFR, pilot mainly use their eyes to navigate, but I want to know more exact and offical reference.

I tried to search these via google but failed to find offical reference such as ICAO annex or FAA regulations.

Could you guys help me to find the offical reference relating to VFR and IFR?

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The Rules of the Air are defined by ICAO and made applicable with few deviations in the ICAO member states.

ICAO Annex 2 Rules of the Air, defines in addition to the general rules, the Visual Flight Rules (VFR) and the Instrument Flight Rules (IFR). Flights shall be in compliance with either of them.

2.2 Compliance with the rules of the air

The operation of an aircraft either in flight or on the movement area of an aerodrome shall be in compliance with the general rules and, in addition, when in flight, either with:

a) the visual flight rules; or

b) the instrument flight rules.

Chapter 5 of Annex 2 contains the Instrument Flight rules.

Chapter 4 of Annex 2 contains the Visual Flight Rules.

4.1 Except when operating as a special VFR flight, VFR flights shall be conducted so that the aircraft is flown in conditions of visibility and distance from clouds equal to or greater than those specified in Table 3-1.

Table 3-1 is summarized below.

+---------------------------+-------------------+---------------------+-----------------------------+
|      Altitude band        |   Airspace class  |  Flight visibility  |     Distance from cloud     |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and above 3 050 m      | A*** B C D E F G  |        8 km         | 1 500 m horizontally        |
| (10 000 ft) AMSL          |                   |                     | 300 m (1 000 ft) vertically |
+---------------------------+-------------------+---------------------+-----------------------------+
| Below 3 050 m (10 000 ft) | A*** B C D E F G  |        5 km         | 1 500 m horizontally        |
| AMSL and above 900 m      |                   |                     | 300 m (1 000 ft) vertically |
| (3 000 ft) AMSL, or above |                   |                     |                             |
| 300 m (1 000 ft) above    |                   |                     |                             |
| terrain, whichever is the |                   |                     |                             |
| higher                    |                   |                     |                             |
+---------------------------+-------------------+---------------------+-----------------------------+
| At and below 900 m        |   A*** B C D E    |        5 km         | 1 500 m horizontally        |
| (3 000 ft) AMSL, or 300 m |                   |                     | 300 m (1 000 ft) vertically |
| (1 000 ft) above terrain, +-------------------+---------------------+-----------------------------+
| whichever is the higher   |        F G        |        5 km**       | Clear of cloud and with the |
|                           |                   |                     | surface in sight            |
+---------------------------+-------------------+---------------------+-----------------------------+
* When the height of the transition altitude is lower than 3 050 m (10 000 ft) AMSL, FL 100 should be
  used in lieu of 10 000 ft.
** When so prescribed by the appropriate ATS authority:
    a) flight visibilities reduced to not less than 1 500 m may be permitted for flights operating:
        1) at speeds that, in the prevailing visibility, will give adequate opportunity to observe 
           other traffic or any obstacles in time to avoid collision; or
        2) in circumstances in which the probability of encounters with other traffic would normally 
           be low, e.g. in areas of low volume traffic and for aerial work at low levels.
    b) HELICOPTERS may be permitted to operate in less than 1 500 m flight visibility, if manoeuvred 
       at a speed that will give adequate opportunity to observe other traffic or any obstacles in 
       time to avoid collision.
*** The VMC minima in Class A airspace are included for guidance to pilots and do not imply
    acceptance of VFR flights in Class A airspace

For Air Traffic Management purposes, ICAO Doc 4444 gives more detail of how both types of flights are treated by ATC.

National regulations may deviate from ICAO regulation. If they do deviate, the deviation have to be published in the Aeronautical Information Publication of the country.

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In the US, the definition of VFR depends on which airspace you are operating in. The rules for VMC (Visual Meteorological Conditions) are listed in FAR 91.155. The definition of IFR is easy. If conditions are less than the minimums listed for the type of airspace, you are in IFR conditions.

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    $\begingroup$ There is no such thing as "IFR conditions"; alternatively, it could refer to both VMC and IMC (so would be a meaningless term). There's VMC, and there's less than VMC minimums (which is IMC). In VMC, a pilot may elect to fly either VFR or IFR, at convenience; in IMC, it is prohibited to fly VFR, so only IFR flight is allowed. $\endgroup$
    – user
    Dec 21, 2018 at 13:39
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    $\begingroup$ You negated your own argument above in your paragraph. IFR conditions are precisely what that means, and what you state, meteorological conditions under which only flight under instrument flight rules can be conducted. If you wish to disagree, that's fine, take it up with the FAA. "Step 1 – Review Weather Minimums. The regulations define weather flight conditions for visual flight rules (VFR) and instrument flight rules (IFR) in terms of specific values for ceiling and visibility. " faa.gov/news/safety_briefing/2015/media/SE_Topic_15_02.pdf $\endgroup$
    – Juan Jimenez
    Dec 21, 2018 at 14:04
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    $\begingroup$ The grammatical hang up for many (including myself) results from putting the word "conditions" behind IFR instead of in front of it. "Conditions (meteo) under which IFR (rules) must be followed" is much more clear than "IFR Conditions". By phrasing it the latter, it appears as if the conditions refer to a set of rules rather than to the weather. Although everyone knows what is meant by "IFR Conditions", I agree that this phrase is incorrect -- there is no such thing. But even official docs are written by humans, and "IFR Conditions" is found in numerous places throughout FAA material. $\endgroup$
    – Jimmy
    Dec 21, 2018 at 16:51
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    $\begingroup$ As long as the difference in terms is clearly understood I can't see anyone getting hung up on the difference between "conditions under which IFR must be followed" and "IFR conditions". The latter is simply a cleaner way to say the same thing. I occasionally get frustrated when the terms are intermingled, but tolerate it when context is clear and I believe the person knows the difference. Once I intervened on the radio, when my co-pilot reported we were VFR when in fact we were operating IFR in VMC. $\endgroup$
    – Michael Hall
    Dec 21, 2018 at 17:21
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    $\begingroup$ Except that phraseology exists for a reason. "When context is clear" is too subjective. For example, just look at the title of 91.151: "Fuel requirements for flight in VFR conditions". Your co-pilot (and probably others) understood your flight context to be operating "in VFR conditions", since this phrase is frequently (and incorrectly) substituted to mean VMC. However, he would be in violation if he planned his "IFR in VMC" flight to 91.151 standards instead of 91.167 standards. I firmly believe these two paragraph titles must be changed (and many other locations in many other docs). $\endgroup$
    – Jimmy
    Dec 21, 2018 at 17:55
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VFR is a combination of being in good conditions ( having 3SM visibility, and staying 500/1000/2000 ft clear of clouds), and being responsible for your own navigation, terrain clearance and traffic-avoidance (see-and-avoid).

Anytime you're not operating under VFR rules in VMC conditions, you are either IFR or operating illegally!

You can be following IFR rules even when conditions are good.
You cannot be VFR when you have an open IFR plan.

"In order to operate under Visual Flight Rules, visual meteorological conditions (3 Statute Miles (SM) visibility and at least 1,000' ceilings) must exist In addition to operating within VMC conditions, pilots must maintain specific distances from clouds, depending on airspace [Figure 1]"

https://www.cfinotebook.net/notebook/rules-and-regulations/special-vfr

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  • $\begingroup$ Your answer is incomplete. Any time the weather is not VMC you must be on an IFR flight plan. You may also be operating on IFR rules if you are on an IFR flight plan, regardless of the weather conditions. Then there is VFR-on-Top where you are operating on a combination of VFR rules and IFR rules. And see-and-avoid applies whenever conditions allow it—not just when flying VFR. $\endgroup$
    – JScarry
    Dec 21, 2018 at 16:06
  • $\begingroup$ There is also Special VFR. $\endgroup$
    – Vladimir F Героям слава
    Dec 21, 2018 at 17:30

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