Fig.6 Anatomy of a cumulonimbus

Fig.6 Anatomy of a cumulonimbus

The representation of the same cumulonimbus cloud will therefore be totally different depending on the part of the cloud that is scanned by the radar beam. Cloud mass reflectivity depends on the type of air mass and on the season. Cumulonimbus reflectivity is not the same in temperate regions and below the equator. An oceanic cumulonimbus reflects radar waves less than a continental cumulonimbus cloud of the same size and height.

Gain, tilt and the ND scale enable pilots to adjust the weather radar. Gain defines the level ratio between the signal received and the signal emitted according to the distance of the echoes. The CAL position of the gain control sets radar sensitivity at the standard calibrated level of reflectivity. The equivalence in precipitation is thus associated with a colour of the echoes presented on the ND:

Table 1 Extract from Air France A330/340 operations manual

The gain control allows the manual adjustment of radar sensitivity for a more precise evaluation of atmospheric conditions. Tilt is the angle between the horizontal and the center of the radar beam. The tilt control enables the range explored in the vertical plane to be varied manually. Depending on the altitude of the aircraft, at a specific tilt, the radar beam is reflected by the ground. Ground echoes are then present on the radar image. Adjusting the ND scale enables monitoring at varying distances of the aircraft. Heavy precipitation that returns most of the radar signal may also hide another disturbed area situated behind. Representation of the weather situation by crews is thus mainly linked to the use of the 3 setting parameters and their knowledge of radar, particularly of its limitations. Onboard radar does not directly detect dangers to be avoided and has specific limitations which require active monitoring from the pilots and constant analysis of the images presented to limit the risk of underestimating the danger of the situation. It should be noted that, at the time of the accident, the presence of ice crystals at high altitude was not considered to be an objective danger and that crews were not made aware of this.

2.5.2 On ground information sources

The weather information is also collected on-ground by several kinds of devices and provided to the pilot through the ATC (Aircraft Traffic Control) by different types of messages taken into account in the next paragraph. The main on-ground weather data sources are:

- On-ground weather radar - Meteorological stations - On-ground meteorological predictions The on-ground weather radar has the same behavior of the on-board weather data, the meteorological stations are composed by different sensors, similar to the on-board sensors. The meteorological predictions are based on the meteorological models described in the previous paragraphs.

CHAPTER 3 OVERVIEW ON CIVIL

AIRCRAFT FLIGHT

Aircraft can be divided in 3 main weight categories that have a different behavior respect the weather condition and trajectory. In this chapter are described and characterized the 3 aircraft categories, the phase of flight in which the aircraft trajectory can be divided and an overview of the trajectory optimization methods is provided.

3.1 Aircraft categories

The ICAO wake turbulence category (WTC) is entered in the appropriate single character wake turbulence category indicator in Item 9 of the ICAO model flight plan form and is based on the maximum certificated take-off mass, as follows [13]: - H (Heavy) aircraft types of 136 000 kg (300 000 lb) or more (i.e. long range aircraft like A320); - M (Medium) aircraft types less than 136 000 kg (300 000 lb) and more than 7 000 kg (15 500 lb) (i.e. regional aircraft like ATR72); and - L (Light) aircraft types of 7 000 kg (15 500 lb) or less (i.e. ultra-light aircraft like Cessna)