Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
SW-07-21
March 21, 2007
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) advises you,
owners and operators of Chet Raspberry Inc. (CRI) Model CH-54A, Heavy
Lift Helicopters Inc Model CH-53D, and Erickson Air-Crane Company Model
S-64E helicopters of a likely cause of a recent CH-54A accident and
recommends certain practices when performing maintenance and overhaul
of the tail rotor head assembly.
Background
One accident involving a CH-54A helicopter is believed to be the
result of in flight loss of a tail rotor blade due to failure of a tail
rotor spindle. The failure of the spindle is attributed to a fatigue
crack which developed at the root of a snap ring groove. The snap ring
groove is located on the spindle shaft at the inner end of the stack
bearing. The fatigue crack in the failed spindle was not detected
during the previous tail rotor head overhaul due to the presence of a
spacer. The spacer is pressed over the snap ring and can obscure crack
indications present in the snap ring groove. Some maintenance manuals
are not specific about the need to remove the spacer when performing
inspections of the spindle and, in some cases, manuals only note that
removal of the spacer is required if the spacer no longer conforms to
dimensional requirements. Additionally, military operators of CH-53
helicopters have noted deformation of the spindle at the shoulder area
of the snap ring groove for some tail rotor spindles which have been
subjected to ground strikes.
Recommendations
We recommend that you perform the periodic overhaul maintenance of
the tail rotor in accordance with the overhaul maintenance schedule and
procedures applicable to your helicopter. In addition, we recommend
that you remove the spacer which covers the snap ring grooves on each
tail rotor spindle when doing the scheduled overhaul maintenance of the
tail rotor head and when doing nondestructive inspections of the
spindles. When you inspect the tail rotor spindles, we recommend that
you inspect the snap ring grooves to verify the proper root radii
dimensions and to determine if there is evidence of fretting or cracks,
scratches or gouges that exist in this area. If any of these conditions
exist, perform the necessary maintenance actions prescribed in the
maintenance procedures applicable to your helicopter to correct the
condition before further flight. If you did not remove the spacer which
covers the snap ring groove during the last tail rotor head overhaul
maintenance, we recommend that you remove the spacer and perform the
inspection of the snap ring grooves as soon as possible.
Additionally, we recommend that you remove and inspect any tail rotor
that has experienced a ground strike in accordance with the spindle
inspection recommendations discussed above and in accordance with the
“Tail Rotor Ground Strike (Sudden Stoppage)” special inspection
procedures applicable to your helicopter. If you find any damage to the
tail rotor, perform the necessary maintenance actions prescribed in the
maintenance procedures applicable to your helicopter to correct the
condition before further flight.
For Further Information, Contact
Roger Durbin, Aerospace Engineer, Los Angeles Aircraft
Certification Office, ANM-120L, 3960 Paramount Blvd, Lakewood CA 90712;
phone: (562) 627-5233; fax: (562) 627-5210; email: roger.durbin@faa.gov