Aviation Investigation Report A98H0003

2.9  Potential Effect of High-Intensity Radiated Fields

The MD-11 certification process included tests to demonstrate an acceptable level of aircraft systems protection against the effects of high-intensity radiated fields (HIRF). The test conditions represented radiated field strengths that vastly exceed the maximum field strengths produced by all known commercial and military radars that were operating in proximity to the occurrence aircraft. Similarly, no hypothetical combination of known emitters and realistic distance separation geometry can be shown to exceed the field strength criteria used during MD-11 HIRF certification testing.

After leaving the airspace of JFK airport, the most significant HIRF environment encountered by SR 111 was in the vicinity of Barrington, Nova Scotia. The HIRF field strength near Barrington, in the environment external to the aircraft, was approximately 100 times weaker than the estimated peak field strength encountered by aircraft during normal approach and landing conditions at typical large, well-equipped airports. Therefore, it is probable that the normal operating environment around JFK airport was the most severe HIRF environment encountered by the aircraft during any portion of the occurrence flight.

The normal HIRF environment at JFK airport does not represent a hazard to aviation, as demonstrated by the uneventful arrival and departure of many aircraft each day, including previous flights by the occurrence aircraft. In addition, the minimum field strength required to induce an electrical discharge between exposed conductors (31 kilovolts per centimetre at sea level) is more than 1 000 times greater than the peak field strength associated with normal airport HIRF environments, and about 430 times greater than the theoretical worst-case HIRF environment for a commercial aircraft.

Resonance effects could not have produced localized field gradients of sufficient strength to induce an electrical discharge between exposed conductors. The required gain factor to the ambient field strength is approximately three orders of magnitude (about 1 000 times greater), whereas resonance gain factors rarely exceed one order of magnitude.

Radio frequency (RF) spectrum allocations are developed to ensure that authorized high-power RF sources will not interfere with aircraft radios and radars; therefore, it is unlikely that the 13-minute communication gap was caused by interference from a HIRF emitter. In any case, no technically feasible link exists between HIRF-induced VHF radio interference and an electrical discharge event leading to the ignition of flammable materials. Therefore, HIRF was considered not to be a factor in this occurrence.

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Date modified :
2012-07-27