Final answer:
To omit an alternate airport on an IFR flight plan, weather must be forecasted to have a ceiling at least 2,000 feet above airport elevation and visibility of at least 3 statute miles.
Step-by-step explanation:
Under specific weather conditions, a pilot can file an IFR (Instrument Flight Rules) flight plan without specifying an alternate airport. The circumstances allowing this include when the forecasted weather at the destination airport indicates a certain level of overcast ceiling and visibility throughout the flight duration.
Typically, for filing an IFR flight plan without an alternate airport, the weather forecast should specify that the airport's weather conditions will be at least a certain ceiling above the airport elevation and a certain visibility. This standard is outlined in the regulations and is usually stated as "1-2-3 Rule," where the forecasted weather should be:
- A ceiling of at least 2,000 feet above the airport elevation, and
- Visibility of at least 3 statute miles.
If these weather conditions are forecasted to be consistent at the destination airport for at least 1 hour before and after the estimated time of arrival (ETA), a pilot can file an IFR flight plan without designating an alternate airport.