PA31 captain reports being unable to comply with an ATC heading instruction due to a thunderstorm an...

Narrative:

Enroute line of frontal storms across track; approximately ZZZ-ZZZ1. Cleared to deviate right of track up to heading 020 deg M. Was holding heading 020 to fly through only available gap. Turbulence continuous moderate; but wasn't sure if there would be worse. In IMC and hand flying to minimize possible turbulence vs. Autopilot induced loads on airframe ('g' and IAS outside limits). Was right between cells with a large storm to the left. Mia center gave an instruction to turn left. Don't remember the new heading other than it would have put me into a ts cell. Instructed accompanying pilot to reply with 'unable;' then I queried the instruction in case I had misunderstood. Had understood correctly; but a left turn was not possible due ts. Advised center that I was still unable to turn as directed. Center advised that I would need to declare an emergency to disregard the instruction - so I declared. Unable to select txdr 7700 at the time due to a need to adjust power settings to control speed excursions. By the time I could change the code I was clear of the immediate storms with a clear path for the rest of the flight; so I cancelled the emergency and turned to a newly assigned heading of (I think) 340 deg M.

Original NASA ASRS Text

Title: PA31 Captain reports being unable to comply with an ATC heading instruction due to a thunderstorm and is instructed to declare an emergency; which is accomplished.

Narrative: Enroute line of frontal storms across track; approximately ZZZ-ZZZ1. Cleared to deviate right of track up to HDG 020 deg M. Was holding HDG 020 to fly through only available gap. Turbulence continuous moderate; but wasn't sure if there would be worse. In IMC and hand flying to minimize possible turbulence vs. autopilot induced loads on airframe ('g' and IAS outside limits). Was right between cells with a large storm to the left. MIA Center gave an instruction to turn left. Don't remember the new heading other than it would have put me into a TS cell. Instructed accompanying pilot to reply with 'Unable;' then I queried the instruction in case I had misunderstood. Had understood correctly; but a left turn was not possible due TS. Advised Center that I was still unable to turn as directed. Center advised that I would need to declare an emergency to disregard the instruction - so I declared. Unable to select TXDR 7700 at the time due to a need to adjust power settings to control speed excursions. By the time I could change the code I was clear of the immediate storms with a clear path for the rest of the flight; so I cancelled the emergency and turned to a newly assigned heading of (I think) 340 deg M.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1153151
Time
Date	201402
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	PA-31 Navajo/Chieftan/Mojave/T1040
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Flight Instructor Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 105 Flight Crew Total 7105 Flight Crew Type 1600
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Inflight Event / Encounter Weather / Turbulence

PA31 captain reports being unable to comply with an ATC heading instruction due to a thunderstorm and is instructed to declare an emergency; which is accomplished.