Narrative:

Repeat occurrence: same condition previously reported; no corrective action taken. [I] encountered severe gyro magnetic compass precession while positioned on the roof top helicopter pad at [a] hospital. Following the patient drop off; 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 300 degrees to 212 degrees in a matter of 48 minutes while the aircraft was parked on the rooftop 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. The [aircraft I was flying] 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 in not being able to manually control the HSI to make compass corrections. It creates a significant inability to fly accurate navigation headings and routes. There is no capability to fly IFR at this point. Furthermore; if the pilot was to encounter 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. The federal aviation flight standards service issued a safety alert for operators date: 7/20/06 safo 06-007; regarding this matter. They recommend as a corrective action: 'pilots should be prepared to select the 'free' directional gyro function; if available; and 'slew' the compass heading to a correct magnetic heading. This aircraft as stated earlier is not so equipped.

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Original NASA ASRS Text

Title: EC135 pilot reported excessive HSI precession after landing on a hospital rooftop helipad and attributed the problem to the hospital Magnetic Resonance Imaging (MRI) equipment and/or machinery on the roof. The helicopter was not equipped with a gyro magnetic compass synchronization control panel to correct the problem.

Narrative: Repeat occurrence: Same condition previously reported; no corrective action taken. [I] encountered severe gyro magnetic compass precession while positioned on the roof top helicopter pad at [a] hospital. Following the patient drop off; 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 300 degrees to 212 degrees in a matter of 48 minutes while the aircraft was parked on the rooftop 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. The [aircraft I was flying] 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 in not being able to manually control the HSI to make compass corrections. It creates a significant inability to fly accurate navigation headings and routes. There is no capability to fly IFR at this point. Furthermore; if the pilot was to encounter 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. The Federal Aviation Flight Standards Service issued a Safety Alert for Operators DATE: 7/20/06 SAFO 06-007; regarding this matter. They recommend as a corrective action: 'Pilots should be prepared to select the 'free' directional gyro function; if available; and 'slew' the compass heading to a correct magnetic heading. This aircraft as stated earlier is not so equipped.

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.