From an AAIB report after an A321 was badly damaged by hail in 2003
(
http://www.aaib.gov.uk/sites/aaib/cm...pdf_029049.pdf):
"The radar was set to a scale of 160 nm and with no significant returns ahead and no thunderstorm activity forecast the radar was switched OFF. The aircraft had been in clear skies above towering Cu for most of the flight and, in accordance with normal procedures, the radar had only been turned on when required."
""Recorded data concerning the use of the weather radar showed that it was switched on and displayed to the first officer for approximately 12 minutes as the aircraft climbed from 4,700 feet through 23,600 feet. The DFDR did not record any use of the weather radar during the remainder of the flight."
The following recommendations were made:
"Recommendation
Present guidance material not only suggests that, in areas of thunderstorm activity, readjusting the radar tilt frequently is the only way to monitor storm development but also that when the upper limit of the storm cell is determined it should be avoided vertically by at least 5,000 feet. The inability of weather radar to detect certain types of precipitation, associated with storm cells, in the upper levels of the atmosphere above 30,000 feet however make it impossible to determine with any accuracy the upper limit of a cell when its vertical development exceeds 30,000 feet. Calculations to determine the aircraft's clearance above the upper limit of a cell can therefore be inaccurate resulting in an aircraft entering the active element of a storm cell whilst attempting to safety over-fly it.
It is therefore recommended that:
Safety Recommendation 2004-47
The Civil Aviation Authority should consider reviewing their guidance material concerning the use and interpretation of airborne weather radar, with a view to highlighting the potential for displayed data to be unreliable when used for calculating the safe vertical clearance for overflight of active storm cells."
The CAA later accepted this recommendation:
"CAA Response
The CAA accepts this Recommendation.
The CAA has reviewed the guidance material contained in Aeronautical Information Circular (AIC) 72/2001 entitled ‘The Effect of Thunderstorms and Associated Turbulence on Aircraft Operations’ and has amended and updated it as necessary. In particular it covers the subject of the use and interpretation of airborne weather radar with regard to calculating the safe vertical clearance for the overflight of active storm cells. This includes highlighting the potential for the displayed data of the weather radar to be unreliable when used for these calculations."
The revised AIC was later published as AIC 81/2004 (Pink 66) on 19 August 2004. It can be viewed at
http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4P066.PDF if you are registered with UK AIS.