OPERATIONS
ALTERNATE AIRPORTS: THE MATH BEHIND IN-FLIGHT SAFETY
Every flight plan, reserves one or more backup runways. It's not extra caution: it's a calculation the rules require.
When you review the ICAO flight plan of a business aircraft, one or more secondary airport designations almost always appear. It's neither routine paperwork nor over-caution: the alternate airport is an operational calculation required by regulation to guarantee a safe way out in any contingency.
When you review the ICAO flight plan of a business aircraft, one or more secondary airport designations almost always appear. It's neither routine paperwork nor over-caution: the alternate airport is an operational calculation required by regulation to guarantee a safe way out in any contingency.
What is an alternate airport?
It's a pre-selected, pre-assessed aerodrome where the aircraft can land safely if the approach to the destination isn't possible, due to weather below minimums, runway closures, ramp supply failures or air-traffic congestion.
The fuel rule and reserves
The alternate airport directly drives how much Jet A-1 is loaded. At a minimum, the fuel plan must cover:
Trip fuel: from the origin to the destination airport.
Alternate fuel: from a missed approach at the destination to the alternate's runway.
Contingency reserve: a margin for the unexpected, usually around 5% of the trip fuel.
Final reserve: fuel to hold for about 30 minutes over the alternate, at standard altitude and speed.
Trip fuel: from the origin to the destination airport.
Alternate fuel: from a missed approach at the destination to the alternate's runway.
Contingency reserve: a margin for the unexpected, usually around 5% of the trip fuel.
Final reserve: fuel to hold for about 30 minutes over the alternate, at standard altitude and speed.
When is a destination alternate mandatory?
In business aviation it's good practice to always include one. The U.S. rule known as 1-2-3 (FAR 91.169) makes it legally mandatory if forecasts (METAR/TAF) show that, in the window from one hour before to one hour after the estimated time of arrival (ETA), the ceiling will be below 2,000 feet or visibility below 3 statute miles. Other states, under ICAO criteria, set their own thresholds, so the applicable minimum depends on the country of operation.
En-route and takeoff alternates
On intercontinental flights, or when the departure airport's weather is below the aircraft's landing minimums, dispatch must name a takeoff alternate. That way, if a technical emergency occurs just after takeoff and returning to the origin isn't possible, the crew has a nearby protected runway within an equivalent flight time (usually one hour with one engine inoperative).
Selection criteria
Choosing an alternate isn't just finding the nearest runway. The flight-support team weighs runway length and strength (PCN/ACN), the availability of instrument approach procedures (ILS/RNAV), the FBO's operating hours and unrestricted fuel supply. The alternate itself must also forecast its own landing minimums (for example, a 600-foot ceiling and 2 statute miles with a precision approach, under the U.S. standard).
Frequently Asked Questions
Does the alternate fuel limit baggage or passengers?
On trips near the aircraft's range limit, yes. Carrying fuel to reach a distant alternate adds to the airframe weight and may require adjusting the payload or planning a tech stop to refuel.
What if the alternate's weather also deteriorates in flight?
During real-time flight monitoring, the dispatch center tracks how the weather evolves. If the designated alternate drops below minimums, an en-route replanning amendment is issued to assign a new valid alternate.
Can the alternate airport be changed in flight?
Yes. The crew, coordinating with the dispatcher and air traffic control (ATC), can change the designated alternate if weather or ramp availability points to a better option.
How does a destination alternate differ from a takeoff alternate?
The destination alternate protects the arrival. The takeoff alternate is assigned when the origin is open for departure but closed for landing (dense fog, for example): if a technical problem arises on departure, the aircraft heads to that nearby backup runway.
Can an airport without a control tower be an alternate?
Only if it meets the authority's requirements: a published instrument approach procedure, valid official weather reports (such as AWOS/ASOS stations) and the right infrastructure for the aircraft category.
On trips near the aircraft's range limit, yes. Carrying fuel to reach a distant alternate adds to the airframe weight and may require adjusting the payload or planning a tech stop to refuel.
What if the alternate's weather also deteriorates in flight?
During real-time flight monitoring, the dispatch center tracks how the weather evolves. If the designated alternate drops below minimums, an en-route replanning amendment is issued to assign a new valid alternate.
Can the alternate airport be changed in flight?
Yes. The crew, coordinating with the dispatcher and air traffic control (ATC), can change the designated alternate if weather or ramp availability points to a better option.
How does a destination alternate differ from a takeoff alternate?
The destination alternate protects the arrival. The takeoff alternate is assigned when the origin is open for departure but closed for landing (dense fog, for example): if a technical problem arises on departure, the aircraft heads to that nearby backup runway.
Can an airport without a control tower be an alternate?
Only if it meets the authority's requirements: a published instrument approach procedure, valid official weather reports (such as AWOS/ASOS stations) and the right infrastructure for the aircraft category.
