Date: 201205
Local time of day: 0601-1200
Location: LGA.Airport, NY
Flight conditions: IMC
Weather elements / visibility: Rain; Thunderstorm; 2
Light: Daylight
Aircraft: PC-12
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E N90
ATC / advisory: TRACON N90
Aircraft component: Weather Radar
Reporter organization: Fractional
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Human factors: Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Weather
Primary problem: Ambiguous

Narrative: During our climb; our filed and assigned route took us through a line of depicted precipitation as seen via NEXRAD weather on a multi-function display. Our initial plan was to deviate around the cells to the north and then rejoin our route but as we got closer to the precipitation; all we saw on our on-board radar was an area of light to moderate precipitation to our east with heavy precipitation more to our north. It was discussed between the Captain and I and we decided to ask for vectors towards the area of light to moderate precipitation to our east. As we entered the area of precipitation we began to see small areas of red begin to appear on the radar; leading us to request a deviation more to the north to avoid the heavy precipitation. After being established on the new vector; and having started a climb to a new assigned altitude; our radar depicted that we were nearing an area of heavy precipitation. Around that time both of our radios began to receive static; and we entered an area of light to moderate turbulence. The Captain and pilot flying initiated an unrequested turn further to the north of our assigned heading in order to avoid the area of heavy precipitation. When I was able; I contacted NY TRACON to report the heading change; stating it was for weather. I estimate that the heading change was about 30 degrees further to the north of our assigned heading. The controller acknowledged our heading change and approved us to stay on the new heading until clear of the weather. The rest of the flight continued without incident. At the time; we were using the NEXRAD display for macro planning; and then as we got closer to the area of precipitation; relied more on our on-board radar. The area of precipitation was fairly narrow; maybe 10-15 NM; so I didn't believe that we were prone to any of the common on-board radar errors such as attenuation. This was not the case and it seems in this instance the radar was suffering from attenuation because as we got closer; the cells which were being depicted did seem to strengthen. In the future; I will plan on avoiding areas of depicted heavy or greater precipitation whether depicted on the NEXRAD; on-board radar; or both by the recommended 20 NM. In our specific case the NEXRAD depicted heavy precipitation while the on-board radar was depicting light to moderate. The more conservative action would have been to avoid the area because one source was depicting heavy precipitation; instead of looking for a confirmation from the on-board radar display.




Synopsis: PC12 First Officer reports a planned weather deviation based on NEXRAD weather displayed on the MFD. On board radar indicates only moderate precipitation and no deviation is requested. Once in the weather; more red is depicted and the crew elects to deviate more in agreement with the NEXRAD picture.