wx planning minima

[beardy]

I understand that, in the UK, at the planning stage, for a non ETOPS flight, when selecting an alternate airfield for the destination you can (if you wish) ignore weather which is forecast as either or both prob 30 and tempo below the applicable procedure minima.

What I can't recall is the justification for this, can anyone help me?

[BOAC]

I believe the 'justification' is JAROPS, based on that wonderful 'statistical analysis' - but I'm sure 'TEMPO' (and 'BECMG') have to be considered. PERSONALLY I exercise extreme disbelief in the 'PROB' side of the forecast, having been 3 times seriously caught by bad met. Many's the time I've arrived at a PROB 30 airfield to find PROB 100, but that is YOUR decision. Likewise whether you 'consider' a TEMPO or BCMG which claims to be outwith your operating window.

[beardy]

As I understand the definition of Tempo, the condition is expected to last less than one hour and is expected to agregate less than 50% of the period indicated and is expected to be temporary. Becmg is unnequivocal, the condition is to be treated as being present from the beginning of the time period indicated and is a permanent change. Prob is either 30% or 40% since, as I understand it, 50% becomes tempo.

Be that as it may and bearing in mind that sound judgement may lead you to not selecting an airfield that is prob30 or tempo below limits, I believe that you can legally use a div that is prob30 and/or tempo below limits. I just can't remeber where I learnt it.

[BOAC]

I see you are a 'line pilot'. It will be in your Ops Manual.

[beardy]

If it were as simple as that I wouldn't be asking. I ask because it is not in the Ops manual; the wording says that met reports & forecasts will indicate that the weather will be at or above the applicable planning minima. This does not preclude tempo nor prob30 since they do not indicate what the weather will be (only a probability of a temporary deterioration). Should your Astraeus ops manual be more specific I would be grateful if you would let me know, and whether it is a company introduced rule or if it is mandated somewhere. I agree that arriving in the tempo and/or prob30 causes problems, but nothing that suitable holding fuel shouldn't help solve.

The consensus (overwhelmingly so) at work is that tempo and/or prob30 below minima need not necessarily be taken in to account, but like me they can't recall where it is written and if it isn't specifically written then what the logic is for the derivation.

[normal_nigel]

prob 30 need not be taken into account for ETOPS planning either in BA.

NN

[beardy]

We only take prob30 and/or tempo into acct when it is below app minima for ETOPs diversions. We can ignore it if it is below the enhanced ETOPs minima (but above app mins). That rule is written, but nothing for dest alts.

[JW411]

For that matter and for as long as I can remember, I have always been taught that "Tempo" and "Prob 30" can be ignored for flight planning purposes but there is an "unwritten convention" around the phrase "Prob 40" that means "you had better believe that it will happen!"

In my long experience a "Prob 40" usually happens.

Where is this convention written down?

[normal_nigel]

BA

FCO's state that only prob 40 and above used in selection of an alternate.

TEMPO and BCMG are qualified by a time band must be considered.

TEMPO= 1 hour or less and the aggregate of all the occurances less than half of the indicated period.

Mean wind used and gusts only for ETOPS alts.

[beardy]

Should anyone be interested I have found the answer in JAROPS 1 subpart D (Chapter 1,01.03.98.)

Briefly; tempo, tempo fm, tempo tl, prob30 or 40 conditions are not applicable to destination alt wx (nor to dest, take off alt and en route alt) unless they are persistent conditions when they are applicable (quoted exapmles are haze, mist, fog dust/sandstorm and continuous precipitation.) Prob tempo deteriorations may be disregarded, improvements should be disregarded.