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Aircraft Systems Topic 17.

Weather Radar - Part 3

Scanning a storm area

The conical beam of the weather radar sweeps up targets left and right of the aircraft centreline. This positions the azimuth of the storm area, and that position is incorporated with the distance to “paint” a two dimensional image on the screen. Through the tilt control knob, the pilot can better examine the storm in the vertical, relating various degrees of precipitation to the aircraft’s current level. The tilt knob is arguably the most important control function available to the pilot. Tilt knob use is essential if ground returns are not to be painted at differing aircraft attitudes and altitudes. The use of the tilt knob is a study in itself (refer Wx radar 14).


Fig Wx radar 14. Radar provides slices of storm characteristics.

When radar energy encounters a precipitation particle some of the energy is absorbed, some returns to the receiver, and some is scattered in various directions. Different types of precipitation have widely differing rates of reflectivity. Weather radar does NOT detect non rain bearing cloud, fog, or clear air turbulence (CAT). It does detect wet hail and large raindrops very well. A large amount of microwave energy is reflected by wet hail and large raindrops, whereas dry snow does not reflect energy, scattering it instead, mainly away from the receiver. Knowledge of the degree of reflectivity of the various precipitation types is a must for the ATPL exam.

Refer Wx radar 15.


This describes degrees between the edges of the beam. At 60 nm distance from aircraft a 3 degree beamwidth is 3nm across, and at 120 nm it is 6 nm across. This can lead to two distant storms that are close together being painted as one large storm. As the aircraft gets closer to the storms, they eventually resolve into two separate storms, once the distance between them is greater than the beamwidth at that range.

(refer fig Wx radar 16)


Fig radar 16. Beamwidth can effect storm resolution.

Thunderstorm avoidance distances

You should NOT attempt to fly over the top of a thunderstorm. If no other option is available, the minimum vertical clearance is generally accepted as 1, 000 ft for every 10 kt of wind blowing at the cloud top.

Any lateral clearance of an area painting contours should be at least 20 nm. Preferably pass on the upwind side of the storm, as storms can move faster than you may think, and hail from the anvil can extend many miles downwind. Fig’s radar 17a and 17b show the hail damage to a Cessna Citation which occurred outside the cloud in supposedly clear air. Images supplied by BASI.

Ground Mapping

In addition to scanning for hazardous precipitation indicative of turbulence, you can operate in ground mapping mode. In those systems using a parabolic dish, the beam is changed to a fan shape (refer Wx radar 20). Those using a flat plate antenna are not re-shaped. Prominent features such as lakes, cities, coastlines, islands and mountains can be distinguished, which can prove to be a good gross error navigation check. The relative brightness or colour of returns varies considerably dependant on the type of terrain. Water is generally planar, and largely reflects radar energy away from the aircraft. This means that water paints black on the screen (refer fig Wx radar 18). On the other hand, land shows up varying degrees of return brightness/colour, and so coastlines, rivers, and lakes show up with good clarity. Strongest returns are those from irregular objects such as buildings and steep rocky hillsides. Refer fig Wx radar 19.


One limitation to the use of weather radar for ground mapping is that when approaching a range of hills or mountains. The radar screen can show black areas (radar shadows) behind the hills which may appear to be lakes. This can be a little worrying when your planned track does not involve overflying any lakes. As the aircraft gets closer to the hills, the radar shadow which appears as lakes will be resolved, and the phantom lakes will disappear.

Best wishes

Rob Avery

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Marty says ... "Goodbye to GA".

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