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 within 5 miles of the Aerodrome Reference Point 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 between 30 minutes and less 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 - i.e. its buffer is 30 minutes.

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

• A BECMG 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. The criteria for issue are;
  • Changes of 10 KT or more, the mean speed before or after the change being 30 KT or more
  • Wind gust • Gusts of 10 KT or more above a mean speed of 15 KT or more
  • Gust exceeds the last reported gust by 10 KT or more
  • Visibility When the prevailing visibility is below the aerodrome’s highest alternate minimum visibility or 5,000 m, whichever is greater
  • Weather When any of the following begins, ends, changes in intensity, or is occurring at a routine reporting time
  • Cloud When there is BKN or OVC cloud below the aerodrome’s highest alternate minimum cloud base or 1,500 ft, whichever is greater
  • Temperature When the temperature changes by 5°C or more since last report
  • Pressure When the QNH changes by 2 hPa or more since last report
  • Other:
    • Upon receipt of advice of the existence of wind shear (manual reports only)
    • The incidence of any other phenomenon likely to be significant

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 an Observation are below the minimums.

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.