Flight Rule Categories
Flight rules are regulations established by the FAA which specify cloud and visibility conditions for operating an aircraft. There are two basic sets of flight rules.
- Visual Flight Rules (VFR)
- Instrument Flight Rules (IFR)
Visual Flight Rules
Visual flight rules state the pilot must be able to operate an aircraft using visual cues for navigation, orientation and separation from terrain and other aircraft. This is why cloud ceiling and flight visibility are the most important variables for safe operations during all phases of a flight.
The minimum weather conditions vary by type of airspace (controlled or uncontrolled), and even from country to country. However, in the contiguous U.S. and Alaska, VFR conditions are cloud ceilings 1,000 FT or greater AGL and/or flight visibility 3 statue miles or greater.
Instrument Flight Rules
Instrument flight rules permit an aircraft to operate when the minimum weather conditions are lower than the weather minimums for VFR. The pilot is unable to safety navigate an aircraft using visual references. Pilots must hold an instrument rating license to fly under IFR conditions.
In the contiguous U.S. and Alaska, IFR conditions are cloud ceilings less than 1,000 FT and/or flight visibility less than 3 statue miles.
NWS Specific Flight Rules
There are two other flight rules categories that you may hear used in the NWS. They were created many years ago back when the NWS use to perform pilot weather briefings. They are Marginal Visual Flight Rules (MVFR) and Low Instrument Flight Rules (LIFR).
| Category | Cloud Ceiling (feet) | Visibility (statue miles) |
| MVRF | > 1,000 to < 3,000 | and/or > 3 to < 5 SM |
| LIFR | < 500 | and/or < 1 SM |
MVRF is used to alert general aviation VFR rated pilots that although conditions are still VFR, they may not be comfortable flying in lower ceilings or visibilities. At airports near mountains, MVFR conditions can mean to the pilot that there may encounter IFR conditions with closed passes between the mountains.