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TTF Decode
sydpilot
Topic Author
sydpilot created the topic: TTF Decode
Hi,
I am studying for the CPL Met. exam using the Met. book (Issue 6, July 2011) and have a few questions about the TTF.
Aircraft arriving between 0900 and 1200 should plan for the conditions described in the METAR...
Aircraft arriving after 1200 should plan for the conditions described in the original forecast.
In an hour's time, another TTF METAR will be issued to cover the period from 1000 to 1300.
What does the second paragraph mean? A TTF is issued every hour? First at 0900Z then again at 1000Z. Can there be a gap in the time when a TTF is issued?
The TTF that is issued at 0900 replaces the TAF until 1200 [three hours]. Anyone arriving between 0900 and 1200 must plan according to that TTF and ignore the TAF. However when the next TTF comes out at 1000, it overrides the first TTF and remains valid until 1300. All arrivals between 1000 and 1300 must now plan according to the most recent TTF. Arrivals after 1300 refer to the TAF for flight planning purposes.
Thank you, Bob. I've understood your reply and thought I'd give it a go with a Canberra TAF,TTF from today. Can you let me know if my decode is correct.
For aircraft arriving between 2300Z on 29th and 0200Z on 30th, the TTF is valid, the TAF can be ignored and no signifigant change in the TTF is expected for the 3 hours validity of the TTF.
If the aircraft arrival time is 0230Z on 30th, the TAF is valid and can expect visibility of 9999 and FEW045?
Looks like you have the gist of it. Important to note with the TTF the NOSIG means no significant change from the observed conditions in the METAR (or SPECI) over the next 3 hours.
The other thing to note is that the vast majority of sites where a TTF is issued will actually issue them half-hourly these days, as most sites issue half-hourly METARS. They may even be issued more frequently if a non-routine SPECI observation is issued because conditions have deteriorated below minima in between half-hourly reports.
Just to complicate things further, not all TTF sites are staffed 24 hours, and this means that there may be periods (usually overnight) where AUTO METAR and SPECI observations continue to be issued, but without the attached TTF. The TTF's for the last 3 hours prior to them no longer being issued will tell you to use the TAF after the non-issue time.
You might also find the knowledge centre at BoM to be useful. They have information sheets on the different Aviation weather services (including TTFs) as well as fact sheets on various hazards as well as meteorology references such as a glossary and a list of abbreviations:
Q: The lowest cloud base forecast for an arrival at YBBN at 0700 is
A: 2000 ft AGL
B: 2500 ft AGL
How do I work out the weather for arrival at 0700 when the TTF and the TAF both end at 0600? Just a little confused with this one. Have I missed something? Sorry if this is a silly thing to ask.
The answer is (b). The TAF is valid from 2400 to 2506 so an arrival at 0700 must be referring to 0700 on the 24th since there is no TAF for 0700 on the 25th, The TTF issue time is ambiguous and is actually an errata (thanks for bringing it to our attention!). TTFs, METARs and SPECIs represent their issue times as a 6-figure date-time group (AIP GEN 3.5 12.3.1)
For this question it doesn't matter anyway since the TTF would have expired whether the TTF was issued on the 24th or the 25th.
For an arrival at 0700, you would need to use the TAF which forecasts SCT at 2500ft.
