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Alternate Airport Requirements in Aviation: Understanding the 1-2-3 Rule and Beyond

When flying under Instrument Flight Rules (IFR), planning for the destination alone is not enough. Weather can change, approaches can become unavailable, and unexpected conditions can arise. To manage this risk, regulations require pilots to consider alternate airports under certain conditions.


Understanding when an alternate is required, how to apply the 1-2-3 rule, and what makes an airport legal as an alternate is essential for safe and compliant IFR operations.



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What Is an Alternate Airport?

An alternate airport is a secondary airport listed in an IFR flight plan that the pilot can divert to if landing at the intended destination becomes unsafe or impractical.


Alternates exist to ensure:

  • A safe landing option if weather deteriorates

  • Compliance with fuel reserve requirements

  • Reduced pressure to attempt unsafe approaches


The 1-2-3 Rule Explained

The 1-2-3 rule is the primary method used to determine whether an alternate airport is required for IFR flight planning.


The Rule (14 CFR §91.169)

An alternate airport is required if, for at least one hour before to one hour after the estimated time of arrival (ETA) at the destination, the forecast weather is less than:

  • 2,000 feet ceiling, and

  • 3 statute miles visibility


If either condition is below these values, an alternate must be filed.


Breaking It Down Simply

Ask this question:

  • From 1 hour before to 1 hour after my ETA, will the weather be at least 2,000 feet and 3 miles?

    • Yes → No alternate required

    • No → Alternate required


This applies to Part 91 IFR operations in the United States.


Weather Sources Used for the 1-2-3 Rule

The rule is based on forecast weather, not current conditions.


Acceptable sources include:

  • TAFs (Terminal Aerodrome Forecasts)

  • Area forecasts or equivalent

  • Weather briefings from Flight Service


Actual weather at the time of arrival may differ—but planning must be based on forecasts.


Choosing a Legal Alternate Airport

Not every airport qualifies as a legal alternate. Requirements depend on the type of approach available.


Alternate with a Precision Approach

If the alternate has a precision approach (e.g., ILS, GLS):

  • Forecast weather must be at least:

    • 600-foot ceiling

    • 2 statute miles visibility


Alternate with a Non-Precision Approach

If the alternate has only non-precision approaches (e.g., VOR, LOC, LNAV):

  • Forecast weather must be at least:

    • 800-foot ceiling

    • 2 statute miles visibility


Alternate with No Published Instrument Approach

If the alternate has no instrument approach:

  • Forecast weather must allow descent from MEA

  • Must allow landing under basic VFR

  • Pilot must determine that a safe descent and landing are possible


What About GPS Approaches?

RNAV (GPS) approaches count as:

  • Precision-like if LPV minimums are available and authorized

  • Non-precision if only LNAV minimums are available


Aircraft equipment and authorization determine how the approach is classified.


Fuel Requirements and Alternates

Filing an alternate affects fuel planning.


Under Part 91, the aircraft must carry enough fuel to:

  1. Fly to the destination

  2. Fly from the destination to the alternate

  3. Fly 45 minutes at normal cruising speed


Failing to plan adequate fuel is a regulatory violation—even if you never divert.


When an Alternate Is Not Required

Even when weather is good, pilots should consider:

  • Airport reliability

  • Single-runway airports

  • NOTAMs affecting approaches or lighting

  • Terrain or obstacle concerns


An alternate may be legal but still wise.


Standard Alternate Minimums vs. Non-Standard Alternate Minimums

Some airports have non-standard alternate minimums, published in:

  • Terminal Procedures Publication (TPP)

  • Chart supplements


These may require:

  • Higher ceilings

  • Greater visibility

  • Specific equipment (e.g., radar, GPS)


Pilots must always check for these restrictions.


Takeoff Alternates vs. Destination Alternates

A takeoff alternate is different from a destination alternate.


Takeoff alternates are required when:

  • Weather at the departure airport is below landing minimums

  • An immediate return after takeoff may be required


These are more common in Part 121 and 135 operations.


Common Training and Checkride Questions

Expect questions such as:

  • “Explain the 1-2-3 rule”

  • “What are alternate minimums for precision vs non-precision approaches?”

  • “Can GPS be used at an alternate?”

  • “What if the destination has no TAF?”


Clear, structured answers demonstrate strong IFR planning knowledge.


Common Pitfalls and Misunderstandings

  • Using current weather instead of forecasts

  • Forgetting to check non-standard alternate minimums

  • Assuming any GPS approach counts as precision

  • Planning fuel only to the destination


Best Practices for Alternate Planning

  • Always plan alternates conservatively

  • Choose alternates with multiple approaches

  • Consider terrain and lighting

  • Build extra fuel margins

  • Brief the alternate just like the destination


Conclusion

Alternate airport requirements are a cornerstone of IFR safety and risk management. The 1-2-3 rule provides a clear trigger for when an alternate is required, but smart pilots go further—choosing alternates that are reliable, accessible, and realistic given aircraft performance and weather trends.


Planning for the possibility of not landing where you intended is not pessimism—it’s professionalism.



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