A PC12 at FL250 being vectored by ATC to avoid weather experienced an ahrs (attitude and heading ref...

Narrative:

In cruise flight at Fl250; there was a line of thunderstorms. I was unable to cross XXX1 as usual; so I was vectoring around the line to the east with the last big cell right near XXX2 and some smaller cells around XXX3 and XXX4. There was a big open clearing to stay VMC; so I began our turn south between. ATC then asked for a turn and decent to Fl200. As this was requested; attitude and heading reference systems (ahrs) 1 flagged and I had black instruments with red X and loss of autopilot chime. As I began to hand fly the airplane and try to stay VMC; ATC asked for a turn and decent; to which I replied 'unable'. I then referred to the emergency checklist; still maintaining VMC and hand flying. ATC then responded again; demanding I turn and descend; again I replied 'unable'. I was then queried as to the reason I would not comply with an ATC clearance; and I responded 'instrument malfunction and working the checklist and need to stay VMC till problem solved'. I was then told to squawk emergency; I responded 'I am not declaring an emergency; I just need some time and to stay VMC till I get the problem sorted out'. I was told by ATC; 'you are an emergency aircraft and you are required to squawk emergency'. I squawked 7700 to comply. I then ran the checklist using ahrs #2 for the remainder of the flight; then returned to assigned squawk code after all was resolved.

Original NASA ASRS Text

Title: A PC12 at FL250 being vectored by ATC to avoid weather experienced an AHRS (Attitude and Heading Reference System) failure requiring hand flying the aircraft to remain VFR. An emergency declared by ATC when turn and descent clearances were not followed.

Narrative: In cruise flight at Fl250; there was a line of thunderstorms. I was unable to cross XXX1 as usual; so I was vectoring around the line to the east with the last big cell right near XXX2 and some smaller cells around XXX3 and XXX4. There was a big open clearing to stay VMC; so I began our turn South between. ATC then asked for a turn and decent to Fl200. As this was requested; Attitude and Heading Reference Systems (AHRS) 1 flagged and I had black instruments with red X and loss of autopilot chime. As I began to hand fly the airplane and try to stay VMC; ATC asked for a turn and decent; to which I replied 'unable'. I then referred to the Emergency Checklist; still maintaining VMC and hand flying. ATC then responded again; demanding I turn and descend; again I replied 'unable'. I was then queried as to the reason I would not comply with an ATC clearance; and I responded 'Instrument malfunction and working the checklist and need to stay VMC till problem solved'. I was then told to squawk emergency; I responded 'I am not declaring an emergency; I just need some time and to stay VMC till I get the problem sorted out'. I was told by ATC; 'You are an emergency aircraft and you are required to squawk emergency'. I squawked 7700 to comply. I then ran the checklist using AHRS #2 for the remainder of the flight; then returned to assigned squawk code after all was resolved.

Data retrieved from NASA's ASRS site as of April 2012 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	844329
Time
Date	200906
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PC-12
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	AHRS/ND
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 150 Flight Crew Total 4800 Flight Crew Type 375
Events
Anomaly	Aircraft Equipment Problem Less Severe Inflight Event / Encounter Weather / Turbulence

A PC12 at FL250 being vectored by ATC to avoid weather experienced an ahrs (attitude and heading reference system) failure requiring hand flying the aircraft to remain VFR. An emergency declared by ATC when turn and descent clearances were not followed.