Alternate Airport Designation & Legal Minimums

Direct Answer

An alternate airport (alternate aerodrome) is a suitable airport selected in the flight plan in case the aircraft cannot safely land at the destination or must divert. It must meet the required weather, runway, approach, equipment, and operational minima at the expected time of use.

Types of Alternates

1. Take-off Alternate

Used when the aircraft cannot return safely to the departure airport after take-off because of weather, runway closure, aircraft performance, or other limitations.

1. En-route Alternate

An airport available along the route if an emergency, technical problem, medical issue, or severe weather requires diversion.

1. Destination Alternate

Used when landing at the planned destination becomes unsafe or impossible. Under ICAO IFR planning principles, at least one destination alternate is generally required unless specific exceptions apply. Two alternates may be required when destination weather is expected to be below the applicable minima or reliable weather information is unavailable. (icao.int)

Main Legal and Planning Requirements

Before designating an alternate, the pilot or operator checks:

• Forecast weather: Conditions at the alternate during the expected period of use must meet the applicable alternate-planning minima.
• Runway suitability: The runway must be long and suitable for the aircraft, considering wind, runway condition, elevation, temperature, and aircraft landing performance.
• Approach availability: A suitable instrument approach or other approved means of approach should be available, depending on the operation.
• Approach and landing minima: The alternate must meet the operator’s published minima. Alternate-planning minima can be higher than the normal landing minima for the same airport. (icao.int)
• Facilities and services: Navigation aids, communications, lighting, rescue and fire-fighting services, operating hours, and other required facilities must be available.
• NOTAMs and airport status: Runway closures, unserviceable navigation aids, restricted operating hours, or other limitations must be considered.
• Fuel requirements: The aircraft must carry sufficient fuel to reach the destination, proceed to the required alternate, and complete the legally required final reserve.
• Aircraft performance: The aircraft must be capable of safely approaching, landing, and stopping within the available landing distance. ICAO standards also require appropriate obstacle clearance and landing-performance consideration at the destination and alternate. (icao.int)

Important Difference

• Landing minima determine whether an approach may be continued and a landing made.
• Alternate minima determine whether an airport is legally suitable to be nominated before departure.

Therefore, an airport may be suitable for landing under certain conditions but not suitable for designation as an alternate if the forecast weather is below the required planning minima.

Simple Example

If the destination forecast shows deteriorating weather around the estimated arrival time:

1. Select a suitable alternate airport.
2. Check its forecast weather and alternate minima.
3. Confirm runway, approach, facilities, NOTAMs, and operating hours.
4. Calculate the additional fuel required.
5. Include the alternate in the operational and ATS flight plan.

Exact legal values depend on the applicable DGCA regulations, AIP, aircraft category, type of operation, instrument approach, and operator’s approved operations manual. ICAO Annex 6 requires operators to publish aerodrome operating minima for airports used as destinations or alternates. (icao.int)

Conclusion

Alternate designation is a safety and legal requirement, not merely a choice of the nearest airport. A proper alternate must be operationally usable, forecast to meet the required minima, and reachable with the legally required fuel.

This topic is closely connected with Navigation and Air Regulations, which are taught at Aero Pilots Academy using practical flight-planning examples. For further assistance, feel free to contact Aero Pilots Academy directly.