A palavra que completa corretamente a lacuna ...[C]... é:

Instruções: As questões de números 53 a 55 referem-se ao texto abaixo.

Cumulonimbus (Cb)

Cumulonimbus is a heavy and dense cloud of considerable vertical extent in the form of a mountain or huge tower, often associated with heavy precipitation, lightning and thunder. The mature Cumulonimbus cloud has a distinctive flat, anvil shaped top.

Flight into a Cb is highly dangerous. The only sensible defence against the hazards associated with a Cb is therefore to avoid flying into one in the first place.

− Planning. Predicting an individual Cb cell is difficult but it is possible to predict the conditions which will trigger formation of a Cb. Forecasters are therefore able to advise flight crews and controllers of the likely timing, location, direction of movement, and height of cells and whether or not they may be embedded. Airport authorities can plan aircraft movements to take into account the disruption to operations caused by storms, and approach controllers can consider how they will manage en-route, departing, and arriving traffic when storms are in the vicinity. Flight crews can alter their routings to avoid forecast Cb activity or decide to carry extra contingency fuel in case they have to re-route in flight to avoid the storms or burn additional fuel because of the potential use of aircraft de/anti-icing systems.

− Awareness. Awareness of the conditions which lead to the formation of a Cb, recognition of a developing and mature Cb, and awareness of the signs which indicate the proximity of a Cb will help controllers and flight crews to plan operations to avoid the associated hazards.

− Weather Radar. In addition to visual recognition, Weather Radar is a particularly valuable aid to avoiding Cb clouds. Airborne weather radar enables the flight crew to identify the areas of the storm cloud which hold the largest water droplets, which indicate the areas with strongest updrafts. The area of the cloud with the most severe turbulence is where the updrafts adjoin the downdrafts; ...[C]... the pilot must avoid flying through the edge of the areas of cloud with the largest water droplets. It should be remembered that a large cloud will absorb a great deal of the radar pulse which may therefore not penetrate all of the way through the storm. This can give a false impression that there are no Cb cells beyond the cell immediately ahead of the aircraft.

− In flight avoidance. In certain circumstances, navigating through a line of Cb cells may be the only option open to a pilot, either because his destination is beyond the line of cells or because he is unable to climb over them. In such circumstances, the aircraft may have to diverge from track by many, perhaps hundreds of miles, in order to find a gap in the wall of Cb clouds. The aircraft captain will need to judge the least hazardous track to follow through a line of cells, something which will absorb the whole crew’s attention. The Weather Radar is invaluable in this situation.

If the Cb cell is situated over the destination aerodrome, then the pilot would be well advised to hold off or divert rather than attempt a landing.

(Adapted from http://skybrary.aero/index.php/Cumulonimbus)