What is the difference between, BECMG & FM?

[jfkjohan]

Dear All,

My name is Johan and I am one step away from getting my professional pilot's license converted from australian to malaysian. All the flying is done, all the hours obtained and other requirements obtained, just a CAA/ATPL exam this end of the month and, providing which I pass, will be able to breathe for a tad bit with a brand spanking new frozen ATPL license!

Now on to business.

I have asked almost all of the commanders I know, and have yet to be given a proper straight answer. I am now seeking your help to enlighten me on the actual difference between a BECMG and FM when reading a TAF/TTF.

While learning in Australia, I have never come across the term BECMG particularly when planning my flight(s). For the exams as well (ie. CASA CPL MET), Under Operational Met, I have become accustom to appreciating and understanding the FM tag (ie.WX to presume change/take place - if it gets worst of course - 30mins before actual forecast time). Amongst other rules of course coupled with INTER/TEMPOs etc.

What is BECMG?

And is there a difference?

One school of thought is teaching me that a BECMG is coupled with a FM (ie. the weather will BECOME this and that FROM this and that), another school of thought is teaching me a BECMG will only take effect if the weather worsens (ie. for planning purposes etc) and supersedes a FM.

Then there is another school of thought which says they have abolished BECMG all together. Then there is another school of thought who believe that BECMG & FM is the exact same thing.

Please, could you help me? Thank you so very much.

Regards,
Johan

Johan Farid Khairuddin.com

[WELCO]

FM in a trend forecast means "FROM", and that denotes the time at which the change in Wx would take place. FM here is usually mentioned with "TL" which is "UNTIL". In TAF, FM is different in which it denotes a quick change that will take place in less than 60 minutes during the prevailing weather conditions in a certain time frame.

BECMG, as you may guess, stands for "BECOMING", and it denotes a permanent change in the prevailing Wx conditions. If the term BECMG is followed by a time frame, for instance BECMG 0710, then it's self-explanatory i.e between 7 and 10. If there is no time frame for the change in the Wx condition, you may expect that change to happen over the next 2 hours after the last Wx reported.


Good luck.

[cavortingcheetah]

Have a look at section 9.

METAR TAF DECODE

[Tinstaafl]

FM is a relatively quick change FROM what existed prior to the stated time to what is expected after that time

BECMG is a relatively slow change over a period of time where the weather starts to change at the beginning of the period, BECOMING the expected weather by the end of the period.

[roger henshaw]

FM plus a time signifies a total change to a TAF (ie all previous info is superceded by the data following the abbreviation FM from the time given.)

BECMG followed by a time period means that some elements of the preceding data are expected to change during the period given, in other words some of the previous data might remain and some of it may change.

[jfkjohan]

Dear All,

Thanks so much for the replies. Now, for the UK CAA papers, MET PRACT which I am sitting for on the 29th of September, let me give you an example of a question.

You are planning to schedule your landing at EGGW on the 30th at 1045Z. You require 2400 meters of visibility & clouds not more than SCT below 1500ft.

Can you land?

(Again, this is during the flight planning stage). And also, this is for the UK CAA MET PRACT ATPL papers.

Here is the TAF.

FCUK33 EGGY 300900
EGGW 301019 23010KT 9999 SCT010 BKN018 BECMG 1114 2000 -RA BKN012
TEMPO 1418 2000 DZ OVC004 FM1800 30020G30KT 9999 -SHRA BKN015CB=

What say you? If you had to answer this question in the test.

A. You can plan to land without an alternate
B. You can plan to land with an alternate
C. You can plan to land with 60minutes holding fuel
D. You can't land.


Regards,
Johan.

[bfisk]

Neither. You have to have two alternates as the weather will be below your requirements within +-1hr ETA. You will be restricted both by vis (can't convert to RVR since you say metvis required) and cieling from 11z onwards.

Don't know what airport EGGW is but it sure has quite ****ty approach aids according to this example.

The rules on how to interpret these groups planningwise are not written in stone anywhere else than your OM, that I am aware of. Where I work the basic rule about a BECMG group is to consider a deterioration and disregard an improvement.

[jfkjohan]

Hi Bfisk,

Thank you for your thoughts. I think where I am coming from is EXAM-LAND.

Not practical, but fully theory.

The TAF was made up and the requirements purely hypothetical.

Now, in exam land, and in the actual CAA ATPL NAVIGATION theory exam, what should I put as the answer?

Please help if able - thanks so much!

Regards,
JFK.

[mad_jock]

looks like a summers day TAF for EGPA that taf you gave.


EGPA 100850Z 13016KT 9999 -RA FEW002 SCT004 14/13 Q1006

EGPA 100805Z 1009/1018 11015G25KT 9999 FEW002 SCT008
TEMPO 1009/1015 8000 -SHRA BKN012
PROB30
TEMPO 1009/1014 4000 RA BKN006
BECMG 1009/1012 15023G35KT
BECMG 1015/1018 21015G25KT

Translated that means a sunny day in shetland with a mild breeze before lunch

[jfkjohan]

Hi Mad_Jock,

This is just a hypothetical question for an exam. It is not related to any weather phenomenon past or present (haha!)

Just would like to know what you would pick (questions & answer above) should you get presented that in the CAA UK ATPL MET PRACTICAL exam.

Will you believe it that until now, NOBODY has given me a straight answer? Haha!

Oh well... I pray someone will be able to answer the question in the near future. My exam is in 16 days! Gulp.

Regards,
Johan.

[jfkjohan]

Oh but thanks to Tinstaafl and Roger_Henshaw for giving me something to work with... !!!

Tinstaafl commented -

"FM is a relatively quick change FROM what existed prior to the stated time to what is expected after that time

BECMG is a relatively slow change over a period of time where the weather starts to change at the beginning of the period, BECOMING the expected weather by the end of the period."

Roger_Henshaw -

"FM plus a time signifies a total change to a TAF (ie all previous info is superceded by the data following the abbreviation FM from the time given.)

BECMG followed by a time period means that some elements of the preceding data are expected to change during the period given, in other words some of the previous data might remain and some of it may change."

Thanks Roger. So in the upcoming CAA UK ATPL MET PRACTICAL exam, I will forego everything that supersedes with a FM! Thats what I am accustomed to anyway learning that in Australia however; do the 30minutes buffer BEFORE apply? As in if the TAF's FM is expected to be TAF FM0204 CAVOK FM0410 +TSRASH, will the +TSRASH effect me in planning should I plan to arrive at 0330?

Also, confirm that FM's are (like above two times) 0400Z to 1000Z and not 04:10Z.

Thanks so much guys. Really, Thanks.

Johan

[Tinstaafl]

Yeah. I used to see the localsin Shetland out sunbathing during the sunny periods if they could find a solid masonry wall for a wind break that also had a sunny side getting the temp up to at least 15 deg C. Mad, they are. No appreciation of normal weather. They regard Gale & Strong Gale as a minor breeze, good for getting the laundry dry before it rains again in 15 minutes.

[Dick Whittingham]

JJ, I don't know about the CAA Overseas Exams, but I suspect they use the same syllabus as the UK exams. If this is so, you have been given some doubtful advice

Speaking of the UK exams, Met Practical only requires you to decode METARs and TAFs, not to know Ops Procedures about selecting destination and diversion weather. Use the link given by cavortingcheetah to memorise the decodes.

As a direct answer to your question, in TAFS, FM followed by a 4 figure UTC time, which is hours and minutes, introduces a permanent change to the group(s) listed that will occur at the time given and will then persist until overtaken by another later change. BECMG followed by a 4 figure UTC time, which is two whole hours, introduces a permanent change just as FM does, but which may occur as early as the first time and will certainly have occured by the later time.

In METARS FM and BECMG only occur in the TREND section at the end, with slightly different rules about timegroups

Dick

[mad_jock]

To be honest there is info missing.

But typical question!!!

I would go for B

Luton has ILS so minima on the ILS is 200ft

Wx +- 1 hour is at worst case is cloud base at 1200ft viz is 2000m and wind 10 knts.

And you always need an alternate.

[jfkjohan]

Hi Mad Jock,

Thanks for your thoughts - but it really is a hypothetical question, as a matter of fact I made up the minima requirements just to show the difference between REQUIRE/NOT REQUIRE (ALTERNATES) etc. Good spot though on your end!

Dick Whittingham,

Yes the CAA Overseas exam are EXACTLY as per the UK CAA syllabus. Therefore, I think your thoughts are one of the most meaningful I have in this thread which I will be studying very closely. If you have any other thoughts you would like to share with me with regards to this and/or any other "tips" you might have for the exam paper, please do send them my way as well!

This is actually a re-sit for me, personally I didn't have any other problem with ALL the other questions but since there were 7 questions which NEEDED me to have 110% understandings on the difference between BECMGs and FMs, clearly I flunked then.

Once again, thank you, for all your help in this. Really means a lot to me!

Regards,
Johan

[mad_jock]

If you take into account your minima

none of the answers are correct.

you need E) plan to go see what its like and have 2 alternates

[jfkjohan]

Maybe in the actual exam, when they mentioned "you were planning to land at... XYZ" they actually meant an actual flight and not as in the flight planning stage etc. I am going to throw out all "buffers" in the future when it comes to these questions.

Thanks to all that tried to help me with this!

Of course if there are more comments, do drop them in here. Thanks!

Johan