Precipitation Static (P-Static)

3 January 1992; USAir Express (Commutair) Beech 1900; Gabriel, NY: The aircraft hit high ground on approach 3.9 miles (6.2 km) from the runway at about 1600 feet (490 meters) above minimum altitude at that point. The NTSB believes that the glide slope indicator may have been unreliable due to precipitation static. One of the two crew members and one of the two passengers were killed...[from: Airsafe.com, Fatal Events Since 1970 for US Airways and US Airways Express, http://www.airsafe.com/events/airlines/usair.htm]

Static charges accumulate on an aircraft from any number of sources. This static electricity may damage equipment and/or injure personnel. An electrostatic charge builds up as a result of fluid or air flow over the structure or components of the aircraft. Static is mainly a problem with the aircraft communications systems (e.g., AN/ARC-182 airborne radio).  P-Static requires special emphasis because of:

• Increased sensitivity of electronic systems
• Wider frequency spectrum for new communications systems
• Increased use of composite materials
• Degraded/loss of VHF communications
• Erroneous magnetic compass readings
• Audio degradation (i.e., rumble or squeal)
• Inoperable or degraded ADF, VLF, and/or VOR radio
• Unreliable ILS glideslope readings
• Aircraft windshield "St. Elmo's Fire"
• Degraded or inoperable navigation systems (LORAN-C)

The metallic skin of the aircraft and/or aircraft exterior mounted antennas are charged through various flight processes such as:

• Particles of rain, snow, ice, sand or dust striking frontal outer aircraft surfaces (triboelectric charging) and transfer charge
• Aircraft engine causing surface ionizing
• Flight below a cloud charged layer causing electric cross-fields

The following are three aircraft p-static discharge methods that include:

• Corona disharge from an aircraft's sharp-edged flight surfaces/extremities
• Streamer discharges on dielectric surfaces
• Arc-overs between electrically isolated or intermittently grounded flight surfaces

EUT=Equipment Under Test

DC=Direct Current

HV=High Voltage