Alternate Aerodrome: Definition, Requirements, and Best Practices in Aviation Operations

Introduction

An alternate aerodrome is a cornerstone of aviation safety, designed to ensure that pilots have a suitable backup airport if they cannot land at their primary destination. This comprehensive guide explores the concept, regulatory frameworks (ICAO, FAA, EASA), types, requirements, exceptions, practical considerations, and best practices for selecting and documenting alternate aerodromes.

What Is an Alternate Aerodrome?

An alternate aerodrome is an airport designated in a flight plan as a potential diversion point if it becomes impossible or inadvisable to land at the intended destination. This concept is enshrined in international aviation regulations and is a critical part of risk management for all IFR (Instrument Flight Rules) operations.

Key Points:

• Provides a safe, pre-planned option in case of bad weather, airport closure, or emergencies.
• Required for regulatory compliance and fuel planning.
• Not automatically assigned by Air Traffic Control (ATC); the pilot in command chooses the diversion aerodrome in real time if needed.

ICAO Regulatory Framework

The International Civil Aviation Organization (ICAO) sets global standards for alternate aerodrome requirements in Annex 6 of the Convention on International Civil Aviation.

Core Provisions:

• For IFR flights, at least one suitable alternate must be specified in operational and ATS flight plans unless exceptions apply.
• Exceptions: No alternate is needed if there is “reasonable certainty” of good weather (visual meteorological conditions—VMC) at ETA and for a reasonable period before and after.
• ICAO distinguishes between take-off, en-route, and destination alternates, with requirements for each phase.

Source: ICAO Annex 6, Part I, Section 4.3.4.3

FAA Regulatory Requirements

The Federal Aviation Administration (FAA) governs alternate aerodrome requirements via 14 CFR Part 91.169 and the Aeronautical Information Manual (AIM).

Key US Rules:

• For IFR flights, an alternate is required unless, from 1 hour before to 1 hour after ETA, the weather at the destination is forecast to have a ceiling of at least 2,000 feet and visibility of at least 3 statute miles (the “1-2-3 Rule”).
• If these minima are not met, the flight plan must include a suitable alternate with met minima at ETA.
• Take-off alternate required if the departure airport’s weather is below landing minima.
• En-route alternates are assessed for long-range or extended operations (ETOPS).

Source: FAA AIM 5-1-9 , 14 CFR 91.169

EASA Regulatory Requirements

The European Union Aviation Safety Agency (EASA) enforces alternate aerodrome requirements under Commission Regulation (EU) No 965/2012, particularly in Part-CAT.

Key EASA Points:

• Destination alternate required for all IFR commercial operations unless there is reasonable certainty of VMC at ETA or operating to an isolated aerodrome.
• Two alternates may be required if weather at both the destination and first alternate is below minima or unavailable.
• Take-off and en-route alternates are addressed for extended and remote operations.

Source: EASA CAT.OP.MPA.180 , AMC1 CAT.OP.MPA.180

Types of Alternate Aerodromes

1. Take-off Alternate

A take-off alternate is required if the departure airport is below landing minima at take-off. It must be within a specified distance (usually one hour’s flight time for twin-engine aircraft, two hours for three or more engines) and must meet suitability criteria for the aircraft type.

Reference: Skybrary: Take-off Alternate

2. En-route Alternate

An en-route alternate is an airport along the planned route, used in case of in-flight emergencies such as technical failures or medical issues. It is particularly important for long-haul, polar, or oceanic flights, and for ETOPS/EDTO operations.

Reference: ICAO Annex 6, Section 4.3.4.3.2

3. Destination Alternate

A destination alternate is a backup for the planned destination. Required for most IFR flights, unless reliable VMC is forecast or the destination is an isolated aerodrome.

Reference: EASA CAT.OP.MPA.180

When Is an Alternate Aerodrome Required?

• IFR Operations: Almost always, unless weather is reliably good at ETA.
• Exceptions: Not required if “reasonable certainty” of VMC exists, or for isolated aerodromes with special fuel planning.
• Multiple Alternates: Required if both destination and first alternate are forecast below landing minima or if weather data is unavailable.

FAA Example: The 1-2-3 Rule—if, for 1 hour before/after ETA, the destination forecast is at least 2,000 ft ceiling and 3 miles visibility, no alternate is required.

EASA Example: Two alternates required if both destination and first alternate are below minima.

Exceptions to Alternate Aerodrome Requirements

• Good Weather: No alternate needed if VMC is forecast at destination for defined period.
• Isolated Aerodromes: No suitable alternate; special fuel/holding requirements apply.
• Multiple Runway Destinations: Some authorities allow exceptions if the destination has multiple independent runways and reliable weather reporting.

Operators must document any exception and ensure all criteria are met.

Filing Procedures and Documentation

• Operational Flight Plan (OFP): Must list all alternates (take-off, en-route, destination) and support each choice.
• ATS Flight Plan: Filed with ATC, alternates appear in appropriate ICAO/FAA form fields.
• ATC Role: ATC does not automatically divert to the filed alternate; diversion decisions are made by the pilot, coordinated with ATC.
• Documentation: Required for regulatory review, dispatcher oversight, and search & rescue reference.

Fuel Planning Requirements

Choosing an alternate directly impacts fuel calculations. Regulations require enough fuel to:

• Fly to the intended destination,
• Conduct a missed approach,
• Proceed to the alternate,
• Hold for the required period (usually 30–45 minutes),
• Include contingency and discretionary fuel as needed.

Standard Fuel Segments

References:

• IVAO Fuel Planning Guide
• EASA AMC1 CAT.OP.MPA.150(b)

Best Practices in Selecting and Planning Alternate Aerodromes

1. Assess Suitability

• Runway length and surface
• Approach aids (ILS, RNAV, VOR, etc.)
• Weather reporting and forecasting
• Operational hours and emergency services

2. Monitor Real-Time Conditions

• Review latest NOTAMs, ATIS, and METARs for all alternates.
• Adjust plans if new risks arise.

3. Document Thoroughly

• Clearly state reasons for each alternate choice.
• Ensure all regulatory requirements are met and documented.

4. Plan Fuel Conservatively

• Build in extra margins for unforeseen events.
• Review recent dispatch and in-flight fuel consumption data.

5. Coordinate With Dispatch and ATC

• Ensure clear communication regarding alternates and fuel status.
• Be prepared to amend alternates in-flight if conditions change.

Special Scenarios: Multiple Alternates and Isolated Aerodromes

Some flights (e.g., over remote or high-risk areas) require two destination alternates or special planning for isolated aerodromes. Regulatory rules differ; always consult the latest guidance for your region and operation type.

Conclusion

Alternate aerodromes are an essential part of safe, compliant, and efficient flight operations. Thorough understanding, documentation, and planning for alternates—supported by robust fuel calculations and regulatory knowledge—are critical for every pilot, dispatcher, and operator.

Key Takeaways:

• Always know your alternates and their suitability.
• File, brief, and document all alternates before flight.
• Monitor real-time conditions and be ready to adapt.
• Ensure fuel planning is robust and meets all regulatory standards.

For more information, operational support, or aviation safety training, contact our experts today.

References & Further Reading

• ICAO Annex 6: Operation of Aircraft
• FAA AIM 5-1-9
• EASA Regulations Part-CAT
• Skybrary: Take-off Alternate
• IVAO Fuel Planning Guide

Related Terms

• IFR (Instrument Flight Rules)
• ETOPS (Extended-range Twin-engine Operations)
• Flight Plan
• NOTAM (Notice to Airmen)
• METAR (Meteorological Aerodrome Report)

Last updated: June 2024