Dispatch by actual weather

[ranklein]

Hi all,

Looking to find a specific rule (ICAO, FAA) that enables using the phrase NOSIG in a METAR report as some sort of a forecast (2 hours one) for the destination.Can a METAR even serve as a forecast?

I believe a TAF would overule it if forecast weather is below what's shown and expected for the next two hours in the METAR.

Thanks!

[bookworm]

Not sure about FAA/ICAO.

JAR-OPS 1.297 (a) and (b) use the following wording:

the appropriate weather reports or forecasts or any combination thereof indicate that, during a period commencing 1 hour before and ending 1 hour after the estimated time of arrival at the aerodrome, the weather conditions will be at or above the applicable landing/planning minima

AMC OPS 1.297 uses the following in its table header:

Application of aerodrome forecasts
APPLICATION OF AERODROME FORECASTS (TAF & TREND) TO PRE-FLIGHT PLANNING

Thus there is at least an implicit recognition that METAR trends may be taken into account, but no specific guidance as to how they rank against TAFs.

[bfisk]

When sector is less than 30 mins, METAR wx ok for dispatch w/o 2nd alternate if no TAF is avaliable.

[hikoushi]

With regards to FAA part 121 ops, If no TAF is available, we would use the Area Forecast for the geographical area of the airport in question to determine whether we can initiate the flight or not. With regards to METARs (to include other official weather such as ASOS info, etc.), an unfavorable METAR would prevent the flight from dispatching if the METAR would still be valid at the ETA.

Example for dispatch minimums: TAF forecasts MVFR conditions all day long. Our flight is 20 minutes long. Current METAR for destination airport, which will still be valid at our ETA, shows below mins conditions. Though the forecast is good, the METAR prevents us from departing. If the flight was longer and would put us on the ground after the next METAR change, then the forecast would become the controlling item. If the METAR trend for the last few hours has shown something markedly worse than forecast for a consistent period of time, then it would also be something to consider if the arrival time was still relatively soon. Obviously for a much longer flight the current METAR would be more or less irrelevant at our ETA, so would typically not be a consideration in normal situations.

It does not work both ways; e.g. on the same 20 minute flight our TAF forecasts zero-zero but the METAR (which will still be current at ETA) shows VFR, we still cannot begin the flight until the forecast changes.

Basically take all reports (METARs, AWOS reports over the phone, etc.) and forecasts (TAFs, or if not available Area Forecasts) that will be valid at your ETA, and the WORST one becomes controlling for dispatching the flight.

[ranklein]

Heard somewhere that when NOSIG appears in the METAR, it means the described weather is not going to change for 2 hours. Some use that, when the weather is good, as some sort of a forecast.

Also, I understand that the TAF always takes precedence over any METAR, right?

Thanks again!

[bfisk]

Anyways, your company manuals should address this.

[Noak]

As bfisk wrote:

Anyways, your company manuals should address this.

We have in our OM-C the following:

If available, the TREND forecast appended to a METAR or SPECI overrules any TAF for the period of validity of the TREND.

TREND - TRENDTYPE LANDING FORECAST
Definition:
A forecast intended to meet requirements of local users and of aircraft within about 1 HR flying time from the AD.
The period of validity is normally 2 HR from the nominal observation time of the report to which the TREND is appended.

[ranklein]

Thanks guys,

good info!