Forecast, Observations and Alternates

Forecasts, Observations and minimas

IFR takeoff minima

• 300 feet for single-engine IFR

Forecast

• A TAF is a forecast of expected conditions at an aerodrome with 5 miles of the Aerodrome Reference Point <ref.http://www.bom.gov.au/aviation/data/education/awp-taf.pdf</ref> with the following types:
  • TAF AMD = amended
  • TAF CAN - cancelled
  • TAF NIL - none will be issued
  • PROV TAF - provisional Aerodrome forecast
• A TEMPO is a forecast of weather change that may occur betweee\n 30 minutes and kess than 60 minutes - i.e. its buffer is 60 minutes.
• An INTER is a forecast of weather change that may occur between 30 minute or less.

When the TEMPO and INTER is over the weather reverts to the previous conditions of forecast (in a TAF) after the buffer period.

• A BECMING in a forecast is a permanent expected change in the weather within 2 hours and not more than 4 hours.
• PROB is only used for Thunder Storms
• A SPECI is a special report when actual conditions deteriorate

An Operational Requirement to divert or hold occurs when airport arrival overhead is within buffer period of the forecast.

• when Thunderstorms are predicted in the TAF and arrival time falls within the buffer of 30 mins

Observations

• A METAR is an observation of conditions made on the hour or half-hour by a human, or automatially by an AWS .
• A SPECI is a special report of meteorological conditions, issued when one or more elements meet specified criteria significant to aviation. SPECI is also used to identify reports of observations recorded 10 minutes following an improvement (in visibility, weather or cloud) to above SPECI conditions

OPREC

An operational requirement occurs when weather conditions are forecast to be bad and the aircraft arrival is within the buffer period of the Forecast, or Observation. BAD covers:

• wind worse than aircraft and pilot limitations,
• visibility
• cloud base below IAP MDA, and
• forecast of thunderstorms.

Alternate

An alternate is required when their is an OPREC and the aircraft has insufficient fuel for holding outside the end of the OPREC. There may be multiple OPREC - so the holding period must cover to after the buffer of the last OPREC.

An alternate is required at night if there is no standby power, no one on call for a PAL system, unless the aircraft can hold to daylight and after the buffer of the last OPREC - if any.

A nominated alternate must not require an alternate.