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.