Alternate due to weather
General alternate requirements CAR 239, CAR 234, CAAP 234
CAR 234 (3) (c) (i) requires you, as pilot in command, to make provision to carry adequate fuel so that, in the event of a forced diversion (due to prevailing weather conditions), you may navigate and land at an alternative aerodrome in safety.
The preceeding parts to this section of the VFRG address the other factors mentioned in CAR 234.
During your pre-flight planning for a proposed flight, CAR 239 requires you to study all available information appropriate to the flight. If you plan to fly beyond the vicinity of the aerodrome, pre-flight study must include the current weather reports and forecasts for the route to be followed and at aerodromes to be used (CAR 239 (1) (a)).
CASA publishes definitions of meteorological minima for various flying operations and provides direction and guidance that will ensure adequate fuel is carried that is commensurate to the studied meteorological conditions. For instance, CASA provides guidance regarding (CAR 240 (1)):
- the circumstances in which a pilot in command must plan for an alternative course of action,
- the grounds on which an alternate aerodrome is in itself suitable to land and
- the calculation of fuel that must be carried to ensure safety given the above factors.
This subsection of the VFRG outlines CASA’s guidance material regarding alternate requirements due to weather, which is published in AIP ENR 1.1 and CAAP 234-1(1).
Alternate aerodrome suitability AIP ENR 1.1
When nomination of an alternate aerodrome is required, any aerodrome may be nominated for that flight providing:
- it is suitable as a destination for that flight, and
- is not an aerodrome for which that flight would require to provide for an alternate aerodrome.
When an aerodrome forecast is ‘provisional’, the pilot in command must make provision for a suitable alternate that has a firm forecast.
Alternate minima AIP ENR 1.1
For flight by aeroplanes under VFR (day or night) and helicopters operating under VFR at night, the alternate minima are a ceiling of 1500 ft and a visibility of 8 km (AIP ENR 1.1). For VFR helicopter operations by day, the alternate minima are the same as for night unless the additional conditions specified below are met.
When operating a helicopter under VFR, and the use of helicopter VMC is permissible at the destination, the pilot in command must provide for a suitable alternate aerodrome when either of the following conditions is forecast at the destination:
- cloud – more than SCT below a ceiling of 1000 ft or
- visibility – less than 3000 m.
A flight permitted to operate under VFR at night (AIP ENR 1.2) must provide for an alternate aerodrome within a one hour flight time from the destination unless:
- the destination is served by a radio navigation aid (NDB/VOR) and the aircraft is fitted with the appropriate radio navigation system capable of using the aid or
- the aircraft is fitted with an approved GNSS receiver and the pilot and aircraft meet the requirements of AIP GEN.
Weather conditions AIP ENR 1.1
Except when operating an aircraft under VFR by day within 50 nm of the point of departure, the pilot in command must provide for a suitable alternate aerodrome when arrival at the destination will be during the currency of, or up to 30 minutes prior to the forecast commencement of, the following weather conditions:
- cloud – more than SCT below the alternate minimum
- visibility – less than the alternate minimum
- visibility – greater than the alternate minimum, but the forecast is endorsed with a percentage probability of fog, mist, dust or any other phenomenon restricting visibility below the alternate minima or
- wind – a crosswind or downwind component more than the maximum for the aircraft. Wind gusts must be considered.
In determining requirements for alternate aerodromes, forecast amounts of cloud below— the alternate minima are cumulative. For determining requirements, the cumulative cloud amount is interpreted as follows (AIP ENR 1.1):
- FEW plus FEW is equivalent to SCT;
- FEW plus SCT is equivalent to BKN and
- SCT plus SCT is equivalent to BKN or OVC.
When weather conditions at the destination are forecast to be as specified above, but are expected to improve at a specific time, provision for an alternate aerodrome need not be made if sufficient fuel is carried to allow the aircraft to hold until that specified time plus 30 minutes.
When weather conditions at the destination are forecast to be above the values specified above, but additionally, intermittent or temporary deteriorations in the weather below those values are forecast, provision of an alternate need not be made if sufficient additional fuel is carried to allow the aircraft to hold for:
- 30 minutes for intermittent deterioration (INTER) and
- 60 minutes for temporary deterioration (TEMPO).
When thunderstorms, or their associated severe turbulence, or their probability are forecast at the destination, sufficient additional fuel must be carried to permit the aircraft to proceed to a suitable alternate or to hold for:
- 30 minutes when the forecast is endorsed INTER or
- 60 minutes when the forecast is endorsed TEMPO.
INTER and TEMPO holding fuel requirements are not cumulative. When a forecast has a number of INTER or TEMPO deteriorations, holding fuel is required only for the most limiting requirement (AIP ENR 1.1).
When TAFs include an FM or a BECMG causing an operational requirement to either become effective or be removed, the timing for the change in operational requirement is as follows (AIP ENR 1.1):
- when the weather during the FM or BECMG is forecast to create an operational requirement, that operational requirement will become effective 30 minutes before the onset of the FM time, or 30 minutes before the start of the BECMG period; and
- when the weather during the FM or BECMG is forecast to remove an operational requirement, that operational requirement will remain effective until 30 minutes after the FM time, or 30 minutes after the end of the BECMG period.
The additional fuel required by the above conditions must be carried when the ETA of the aircraft at its destination or alternate falls within the period of 30 minutes before the forecast commencement time to 30 minutes after the expected time of cessation of these deteriorations. If the holding time required because of INTER or TEMPO or the probability of INTER or TEMPO requirements (as described above) extends past 30 minutes after the forecast cessation of these deteriorations, the aircraft need only carry sufficient fuel to hold until 30 minutes after the forecast cessation time (AIP ENR 1.1).
Due to the continuous weather watch provided by TTF, the 30-minute buffers required by the above conditions do not apply. Flights which will be completed within the time of validity of the TTF may be planned wholly with reference to the destination TTF (AIP ENR 1.1).
TTF may have either one visibility or two visibilities included in the report. Operational requirements will apply when (AIP ENR 1.1):
- the sole visibility is less than the alternate minimum or
- the higher visibility is less than the alternate minimum.
Flights which cannot use TTF will plan the flight on the current TAF until such time as the destination ETA falls within the validity periods of a TTF (AIP ENR 1.1).