ICAO defines an Alternate aerodrome as "an aerodrome to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the aerodrome of intended landing where the necessary services and facilities are available, where aircraft performance requirements can be met and which is operational at the expected time of use".
Alternate aerodromes may be designated as Take-off , En-route or Destination alternates. ICAO defines each of these as follows:
- A Take-off alternate is an aerodrome at which an aircraft would be able to land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure.
- An En-route alternate is an aerodrome at which an aircraft would be able to land in the event that a diversion becomes necessary while en route.
- A Destination alternate is an aerodrome at which an aircraft would be able to land should it become either impossible or inadvisable to land at the aerodrome of intended landing.
Almost all flights require Destination Alternates but Take-off and En-route alternates are only required in specific circumstances and the departure aerodrome may also be an en-route or destination alternate for the same flight.
A take-off alternate aerodrome should be specified in the operational flight plan if either the meteorological conditions at the departure aerodrome are below the applicable landing minima or in case it should not be possible to return to the departure aerodrome for some other reason. For aeroplanes with two engines, such an alternate must be within one hour's flight time at the one-engine-inoperative cruising speed specified in the Aerodrome Operating Manual (AOM) or equivalent document and assuming the aircraft mass is the actual mass at take off and ISA and still-air conditions prevail. Aeroplanes with three or more engines are permitted two hours of flight time for the same purpose and under the same conditions except the all engines operating cruising speed may me used. An aerodrome may not be designated as an alternate unless the available information indicates that at the estimated time of potential use, the prevailing conditions will be at or above the applicable operating minima.
An en-route alternate aerodrome is only required for EDTO operations by aeroplanes with two turbine engines. If it is so required, it must be specified in the operational and ATS flight plans.
At least one destination alternate aerodrome must be designated and specified in the both operational and ATS flight plans if a flight under IFR is going to be made unless one of the following applies:
- after taking all relevant factors into account, it is reasonably certain that VMC will prevail for the approach and landing
- there is more than one independently useable runway is available which can be expected to be useable at the ETA and at least one of those runways has an instrument approach procedure
- the aerodrome is isolated, in which case a destination alternate is not required and instead additional fuel must be carried. a Point of No Return (PNR) must be determined and this must not be passed unless an assessment of the prevailing weather, traffic and other operational conditions indicates that a safe landing can be achieved at the ETA.
Guidance on the planning of operations to isolated aerodromes is included in ICAO Doc 9976, the "Flight Planning and Fuel Management Manual"
Two destination alternate aerodromes must be designated and specified in the operational and ATS flight plans if either the destination weather forecast for the ETA indicates that conditions will be below the applicable aerodrome operating minima or no appropriate destination weather forecast for the ETA is available.
ICAO guidance also provides for the State of the Operator, after considering the results of a specific safety risk assessment which one of their aircraft operators has conducted and which demonstrates how an equivalent level of safety could be maintained without following normal procedures, may approve a variation. Such a specific safety risk assessment must include at least reference the:
- capabilities of the operator;
- overall capability of the aircraft involved and its systems;
- facilities and procedures at any aerodrome to be designated
- quality and reliability of meteorological information;
- additional hazards or risks to normal safety standards which may arise from each alternate aerodrome variation and the corresponding mitigation measures.
ICAO notes that guidance on carrying out a safety risk assessment for this purpose is included in both Doc 9976 the 'Flight Planning and Fuel Management Manual' and in Doc 9859, the "Safety Management Manual".