What's an Alternate Landing Site?
Simply put, an alternate landing site (or an ALS) is an aerodrome that an aircraft can rely on if the aircraft is unable to land at its primary destination. The International Civil Aviation Organisation - the global aviation safety regulator - defines a destination alternate as:
"An alternate aerodrome to which an aircraft may proceed should it become either impossible or inadvisable to land at the aerodrome of intended landing"
What does this mean for airlines?
"Trip Fuel" is the minimum fuel required for the flight to make the journey from the departure airport, to the intended destination airport. The calculation to determine the trip fuel required for a flight includes the total mass of the aircraft, including its total reserve fuel payload.
When airlines plan their alternates for any given flight, there are a multitude of considerations. Fuel is a major factor. The Captain must plan the fuel required to reach the intended destination, but then continue on to the alternate. This additional fuel is called either alternate reserve (ALT RES) or island reserve (IRES).
The Captain must load this ALT RES / IRES fuel at the departure airport, and carry it for the duration of the flight. But because an aircraft burns fuel to carry its own flying mass, the heavier the aircraft, the more fuel it requires. So, the more ALT RES / IRES on board, the more trip fuel is consumed. Conversely, the less ALT RES / IRES on board, the less trip fuel is consumed.
What causes a diversion from Perth Airport?
Generally, it's two things - either an incident on the runway, or bad weather. For Perth, it's typically weather-related. In winter it's low visibility caused by fog, and in summer it's strong winds and surface turbulence. Perth Airport has some fancy approach and landing aids which reduce the risk of visibilty-related issues, but it still occurs. Likewise, no fancy landing aids can combat localised thunderstorms or very strong winds - both of which Perth Airport suffers, and causes flights to divert.
Our solution is pretty simple. Build a facility close to Perth Airport (in flying time) that suits airlines' criteria for an ALS, yet isn't subjected to the same weather that Perth Airport sustains.