Alternate Aerodrome
The International Civil Aviation Organization (ICAO) defines an alternate aerodrome as a location where an aircraft can land if it is not possible or advisable to continue to the intended landing site. This alternate must have the necessary services and facilities available, meet performance requirements for the aircraft, and be operational at the expected time of use. There are three types of alternate aerodromes: Take-off, En-route, and Destination alternates.
A Take-off alternate is designated for situations where landing at the departure aerodrome becomes necessary shortly after take-off. If the weather conditions at the departure site are below required minima, or if other factors prevent returning, this alternate must be specified in the operational flight plan. For two-engine aircraft, the alternate must be within one hour’s flight time at the one-engine-inoperative cruising speed. Aircraft with three or more engines are allowed a two-hour flight time under similar conditions, but they may use the all engines operating cruising speed.
An En-route alternate is primarily required for Extended Diversion Time Operations (EDTO) involving two-turbine-engine aircraft. If needed, it must also be included in both the operational and Air Traffic Services (ATS) flight plans.
A Destination alternate is crucial for flights under Instrument Flight Rules (IFR). At least one must be specified unless certain conditions are met. These include having reasonable certainty that visual meteorological conditions (VMC) will prevail for landing, multiple usable runways, or if the aerodrome is isolated, in which case extra fuel is required instead of a destination alternate. The Point of No Return (PNR) must also be established and not exceeded without a safe landing assessment.
If the destination weather forecast predicts conditions below applicable minima or no suitable forecast is available, two destination alternates should be designated in the flight plans. ICAO allows for variations from standard procedures based on specific safety risk assessments conducted by the operator. These assessments should evaluate the operator’s capabilities, aircraft systems, aerodrome facilities, meteorological information quality, and any additional risks associated with the alternate aerodromes, along with corresponding mitigation measures. Guidance on conducting these safety risk assessments can be found in ICAO Doc 9976 and Doc 9859.
