The FAA's Aircraft Certification Service has issued a Special Airworthiness Information Bulletin an Enstrom 480 was destroyed in a ground resonance accident, injuring the pilot and his passenger.

According to the FAA, this "nonmandatory recommendation" was prompted by an August 5, 2005, incident in Goshen, Indiana. In that incident, an Enstrom 480 helicopter encountered ground resonance after landing, causing the pilot and front seat passenger to be thrown from the helicopter, which was destroyed.

The FAA notes that, "We are still investigating the accident. The preliminary information indicates that a cracked transmission mount strut, the elastomeric damper damage, or improperly serviced oleo struts could have resulted in ground resonance."

Applied to the Enstrom 480 and 480B, the manufacturer has issued an Immediate Action Required Letter (I/L), dated August 5, 2005, as a result of the accident. The crashed Enstrom 480 model was equipped with an elastomeric damper, part number (P/N) J237172.

Enstrom’s I/L requires that you remove certain elastomeric dampers from service immediately, until they complete further investigation. If the helicopter has previously experienced ground resonance, you must ground the helicopter until you replace certain elastomeric dampers with airworthy hydraulic dampers. Enstrom doesn’t permit ferry flights.

The FAA has recommended that operators "take immediate action if you encounter ground resonance. When the RPM is near operational speed, such as during landing, IMMEDIATELY pull the aircraft to a hover. After the vibrations have dampened out, perform a hovering autorotation. If you encounter ground resonance at low rotor RPM, such as when the rotor RPM is being increased from idle, IMMEDIATELY close the throttle.

The FAA also recommend(s) the following actions before further flight:

• Per the I/L, remove any elastomeric damper, P/NJ237172, and replace with an airworthy hydraulic damper, P/N 28-14375-10.
• Inspect the transmission mount for any crack.
• Inspect the elastomeric damper for any visible deformation and cracking.
• Inspect each oleo struts prior to take off for proper servicing.

If any owner or operator of ENSTROM Model 480 and 480B helicopters experiences ground resonance or detect any damage described above, please contact us immediately at:

• FAA Chicago Aircraft Certification Office, Mr. Shawn Malekpour at (847) 294.7837
• FAA Rotorcraft Directorate, Mr. Uday Garadi, at (817) 222.5123.

FMI: www.faa.gov/aircraft/safety/alerts/SAIB