The proposed EASA amendments (NPA 2011-03 ~ Appendix P) have been added in green, plus 3 UAS events are plotted.
EDIT :: Some additional background:-
A common characteristic in most Unreliable Air Speed (UAS) events was the proximity to deep convective/tropical storm cloud formations; hence the EASA NPA 2011-03 Appendix P proposal is primarily a "deep convective cloud" extension to the existing JAR intermittent maximum envelope. This hypothesis is based on the often rapid onset of UAS events, the lack of airframe icing, some observations of strong moisture on the windscreen (e.g., rain reported but suspected to be melting ice crystals), and the proximity to such cloud formations, is that the aircraft are likely encountering an area of high ice water content[1]. The observations of low or no reflectivity on the pilot’s radar are consistent with flight through ice particles, possibly concentrated at small sizes[2].
Refs:- (BEA Interim Report No.3 - p.77)
[1]
The background noise changed rapidly around 2 h 09 min 46. This change in the background noise was identified as possibly being characteristic of the presence of ice crystals but did not give rise to any specific comments from the crew, the phenomenon being little known to pilots at the time.
[2]
The conversations in the cockpit did not reveal any malfunction of the weather radar and indicate that the latter displayed a usable image.