AMOSSG/5-SN No. 12

16/3/05

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FIFTH MEETING

Montreal, 11 to 14 April 2005

Agenda Item	6:	Aerodrome forecast requirements

CRITERIA FOR TAF CANCELLATION

(Presented by W. Woveris)

SUMMARY
This paper outlines the user's views that TAF cancellation criteria are not clear and should be better defined, using currently available capabilities to limit the need for cancellations.

1.	INTRODUCTION

1.1	Annex 3 contains provisions governing the cancellation of TAF. These provisions are not defined precisely and result in practices varying among States MET service providers. Diverging practices and not fully using current review capabilities lead to inefficiencies in the industry.

2.	DISCUSSION

2.1	TAF cancellation upon issuance of a later TAF

2.1.1	Annex 3, paragraph 6.1.2 reads: "The issue of a new forecast by a meteorological office, such as a routine aerodrome forecast [TAF], shall be understood to cancel automatically any forecast of the same type previously issued for the same place and for the same period of validity or part thereof."

2.1.2	This text is not always well understood by the users. For example, an 18H TAF, issued six hours prior to the start of the validity period, would possibly cancel, at issuance time, a current valid TAF of the same type, while in fact it should cancel the current TAF only after the start of its validity period.

2.1.3	A change in wording is suggested, similar to: "… and for the same period of validity or part thereof. The cancellation shall apply only to the part of the earlier forecast overlapping the validity period of the new forecast." NB: this considers the fact that both TAF are of the same type.

2.2	TAF constant review requirement

2.2.1	Annex 3, paragraphs 6.2.4 and 6.2.5 state that meteorological offices preparing TAF shall keep forecasts under constant review and that TAF not kept under constant review shall be cancelled. The meaning of "keeping TAF under constant review" is not precise. This leads to a divergence in policies among various States. Some cancel TAF when no METAR are issued, such as during non-operational hours when no MET observer is present. Some use AUTO METAR, some may even use other inputs than METAR to keep the TAF under review (e.g. weather radar, satellite data, etc.). A missing parameter of a METAR sometime leads to the cancellation. This lack of harmonisation highlights the need to better define how a meteorological office maintains a TAF under constant review and what tools and data can be used for doing so. It is clear that some situations require minimum data (e.g. Caribbean airports, most of the time), while others need more inputs (e.g. Northern Europe airports near seashore, frontal situations, etc.). States' TAF amendment procedures will likely have to be reviewed when they begin using partial or totally automated reports. Therefore, a timely review of provisions for Annex 3 would be in order. Some States have already dealt with this and their procedures may be of use to illustrate the point1. IATA generally has found the implementation of these procedures acceptable.

2.3	Inefficiencies

2.3.1	TAF cancellations bring major inefficiencies to the users in terms of higher fuel reserves (alternate fuel for two airports). While some TAF cancellations are fully justified by a difficult weather situation combined with lack of adequate data, other TAF cancellations are unnecessary and negatively impact flight operations, with resulting higher costs and emissions.

3.	ACTION BY THE AMOSSG

3.1	The AMOSSG is invited to:

a)	note the content of this working paper;

b)	develop an Annex 3 amendment proposal for paragraph 2.1.2, to clarify that a new forecast will cancel the previous forecast, only for the overlapping period of validity;

c)	develop an Annex 3 amendment proposal for paragraph 6.2.4 to better define the meaning of "keeping TAF under constant review"; and

d)	develop an Annex 3 amendment proposal for paragraph 6.2.5 to harmonize the situations in which a TAF needs to be cancelled, while maximizing the capabilities for keeping TAF under review, as a function of individual conditions.

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