If either the ceiling or visibility is forecast to be less than 2,000 feet or 3 statute miles during that arrival window, you are required to file an alternate.
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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).
Let's say the weather at your airport is IFR but clear a few miles away. While the VFR-only pilot will have to wait for the weather to improve, the IFR pilot can depart on an IFR flight plan, then cancel the IFR flight plan when they are in VFR conditions.
The lowest routinely used takeoff visibility minimum is 600 feet RVR, which is about one-tenth of a mile. Pretty low. It's the lowest visibility typically used in simulator training for taxi and takeoff practice.
This brings us to IFR (instrument flight rule) pilots. IFR flying is astronomically more challenging than VFR flying, but those pilots who achieve this distinction are invariably more equipped to fly IFR and VFR. Aviating under IFR, a pilot is authorized to fly into clouds in what is called zero visibility.
IFR flying provides certain protections which are not readily available to those flying under VFR. Even if you never fly into a cloud, having an IFR-equipped airplane and pilot will make for a safer and confident journey. Flying through and above the clouds provides for safer travels.
Why Do Pilots Prefer IFR Over VFR? IFR provides pilots with greater flexibility when it comes to controlled airspace, weather conditions, and career opportunities.
In aviation medicine, the 1% rule is a risk threshold that is applied to the medical fitness of pilots. The 1% rule states that a 1% per annum risk (See also risk management) of medical incapacitation is the threshold between acceptable and unacceptable.