Meteorological briefing

The winter season is slowly retreating, and the flying season will soon be upon us. Now is the time to reacquaint yourself with one of the most important elements of pre-flight preparation: the aviation briefing from the meteorological office.

(Article in AeroRevue 03/2012 - Oliver Baer)

In an investigation into an aviation accident in the Swiss Alps in 2007, in which the four passengers were fatally injured, the Office for Aviation Accidents drew the following conclusion: "The accident occurred as a result of the aircraft colliding with the terrain, owing to the fact that the flight was continued in spite of there no longer being adequate visual reference." One reason for this was the "insufficient pre-flight preparation", since "the weather conditions were not conducive to flying over the Gotthard Pass in the way which had been planned." Moreover: "The weather information used by the pilot did not include any specific details about the weather conditions in the Alpine region."

This tragic example - just one of many - is a poignant reminder of how inadequate pre-flight briefing and a cavalier attitude towards the elements can, in extreme cases, have fatal consequences.

(Picture: Kaspar Bucher)

Analysis of meteorological pre-flight preparation

The pilot who had intended to fly from Zurich to Florence in the late afternoon that day had used the METAR and TAF bulletins for the Florence, Parma, Milan-Linate/Malpensa and Zurich airports for his meteorological briefing.
From the content of the bulletins, the following weather picture can be pieced together: current visibility around Florence and Milan was over ten and six kilometres respectively, with scattered mid-level clouds and a higher layer of thicker cloud; Zurich CAVOK. Forecast: Slight increase in cloud cover on both sides of the Alps.
There appears to have been a complete lack of information to permit a reasonable assessment of the situation in the Alps.

The first indication of any untoward conditions appears in the METAR from Lugano:
FEW040, SCT070, BKN100.
Three layers of cloud, lying over the terrain ascending in a northerly direction.

For a flight over the Alps, the localised information from bulletins such as METAR and TAF is woefully inadequate. If the pilot had used GAFOR for his pre-flight briefing - a product which is specially designed for such flights, the situation would have been absolutely clear:
GAFOR for 15:00 to 21:00 hours UTC: Gotthard route: XXX, Lukmanier route: XMX.

The pilot did not request the GAFOR from ATC until very late on, once he was being confronted with ever-worsening conditions. Even then, he continued with the flight, looking for a pass that was navigable under VMC.

Interpretation of the aviation weather forecast

Another product which is indispensable for every flight preparation is the aviation weather forecast. In this particular case, the forecast stated: "The flat but extensive area of high pressure over central and western Europe has shifted its focal point to the Balkans. However, it is still influencing today's weather here. In the lower air layers, it is pushing somewhat damper air towards the southern side of the Alps. South side of the Alps and Engadine: 3-6/8 Basis 5,000-7,000 ft MSL, above this, 4-7/8 Basis 8,000 to 10,000 ft MSL. Hazards: Alpine passes from the south partially in cloud." This also clearly points to critical conditions in the Alps. A glance at a satellite image or webcam would have also given a clear indication of the situation.

One important initial lesson to be learned from this case is the need for employing a combination of different meteorological products and information in order to gain a comprehensive overview of the current and forecast weather situation.

Recipe for a meteorological briefing

(Picture: Oliver Baer)

In the cockpit, every pilot works systematically and with precision, often with the help of a check list. This principle should also be applied to a meteorological briefing. Why? Because if a systematic approach is taken, nothing will be forgotten.
One tried and tested way is to distinguish between the different types of bulletin or report: Current > Forecast > Hazards. We can illustrate this process using a concrete example of a VFR flight from Wangen-Lachen to Grenchen with Bern as alternate airport. The "current" part means obtaining an overview of the current weather situation, and then studying the weather conditions at the start of the flight, en-route and at the destination.
The products we use to do this are:

  • Aviation weather forecast: Weather conditions, clouds, visibility and weather (consulting an isobar chart is also helpful)
  • Satellite and radar images
  • Webcams along the route
Black and white satellite image of the Alpine region
Close examination of the satellite image clearly shows the accumulation on the southern Alpine ridge, which is confirmed by the weather reports that state that the clouds are lying over the rising terrain.

For the forecasting element, the process again begins with a study of the large scale weather developments, followed by a detailed look at the take-off and landing sites as well as the route:

  • Aviation weather forecast: Outlooks, wind and temperature (optional: GAMETSection II)

And now we come to the most important part, namely the possible hazards that could be awaiting us:

  • Aviation weather forecast: Hazards section
  • GAMET Section I)
  • AIRMET (optional: SIGMET)
  • optional: Low Level SWC

A correct approach to the weather

Finally, on the basis of all this information, the decision is taken as to whether the flight from Wangen-Lachen to Grenchen can be undertaken as planned.
In certain circumstances it is sufficient to adjust the flight altitude according to the conditions (in the case of stratus cloud cover it may sometimes be possible to fly lower than planned) or to modify the route (often the Alpine foothills are clouded over, but it is still possible to make the flight by going further north over the Swiss midland region).
If a thorough meteorological briefing is obtained along with the consequences that can be extrapolated from this information, situations like the Florence to Zurich flight can be avoided.

The question remains as to whether the pilot would have decided not to make the flight in the first place had he known about the difficulties in crossing the Alps from his pre-flight preparations.
After all, it is not the weather that causes an aviation accident, but incorrect action in response to the weather.