Narrative:

Encountered severe gyro magnetic compass precession while positioned on the roof top helicopter pad at a hospital. Following the patient drop off leg; I completed the start and before takeoff checklist. Prior to lift off I completed an instrument operational check and noted that the horizontal situation indicator (HSI) had precessed from 310 degrees to 212 degrees in a matter of 27 minutes while the aircraft was parked on the roof top pad. While this anomaly can be attributed to magnetic resonance imaging (mri) equipment; in this case the mri is located many floors beneath the helicopter landing pad. Numerous exhaust turbines; and intake turbines are located on the roof top pad; which may contribute to magnetic anomalies. Normally the precession can be corrected by manually slewing the HSI through the gyro magnetic compass control synchronization switch control panel. Aircraft does not have this panel installed; and the HSI cannot be manually controlled between magnetic mode (magnetic-the gyro slewed to the flux valve heading) or dg mode; operating the gyro as a free (uncontrolled) gyroscope; essentially rendering the HSI useless. This presents a significant safety issue. Both in not being able to manually control the HSI to make compass corrections; it creates a significant inability to fly accurate navigation heading and routes. There is no capability to fly IFR at this point. Furthermore; if the pilot was to encounter an inadvertent instrument meteorological conditions (iimc) the precession of the gyro could induce a loss of situational awareness. This base is an IFR capable base; however with an inability to manually slew the HSI; and input corrections; this capability and the safety of flight is severely compromised install gyro magnetic compass control synchronization switch control panel. Enabling the pilot to manually slew the gyro magnetic (HSI) compass.

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Original NASA ASRS Text

Title: An ECD-EC135 helicopter compass precessed nearly ninety degrees while parked on a hospital roof for 27 minutes but because this particular aircraft had no compass slew installed to correct the heading error; the aircraft was rendered non-IFR capable.

Narrative: Encountered severe gyro magnetic compass precession while positioned on the roof top helicopter pad at a Hospital. Following the patient drop off leg; I completed the start and before takeoff checklist. Prior to lift off I completed an instrument operational check and noted that the Horizontal Situation Indicator (HSI) had precessed from 310 degrees to 212 degrees in a matter of 27 minutes while the aircraft was parked on the roof top pad. While this anomaly can be attributed to Magnetic Resonance Imaging (MRI) equipment; in this case the MRI is located many floors beneath the helicopter landing pad. Numerous exhaust turbines; and intake turbines are located on the roof top pad; which may contribute to magnetic anomalies. Normally the precession can be corrected by manually slewing the HSI through the gyro magnetic compass control synchronization switch control panel. Aircraft DOES NOT have this panel installed; and the HSI cannot be manually controlled between Magnetic mode (MAG-the gyro slewed to the flux valve heading) or DG mode; operating the gyro as a free (uncontrolled) gyroscope; essentially rendering the HSI useless. This presents a significant safety issue. Both in not being able to manually control the HSI to make compass corrections; it creates a significant inability to fly accurate navigation heading and routes. There is no capability to fly IFR at this point. Furthermore; if the pilot was to encounter an Inadvertent Instrument Meteorological Conditions (IIMC) the precession of the gyro could induce a loss of situational awareness. This base is an IFR capable base; however with an inability to manually slew the HSI; and input corrections; this capability and the safety of flight is severely compromised Install gyro magnetic compass control synchronization switch control panel. Enabling the pilot to manually slew the gyro magnetic (HSI) compass.

Data retrieved from NASA's ASRS site as of July 2013 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.