What is the 3 1 rule IFR? In aviation, the rule of three or 3:1 rule of descent is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent.
If a usable precision approach is available: 600' ceilings AND 2 SM of visibility. If only a non-precision approach is useable: 800' AND 2 SM of visibility. If no instrument approaches are available: Descent from the MEA and landing must be conducted under basic VFR.
You need to name an alternate airport on your IFR flight plan if, for the period of time one hour before and one hour after your estimated time of arrival, the weather is forecast to include a ceiling of 2,000 feet agl and/or visibilities less than three statute miles (the 1-2-3 rule).