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ASN Wikibase Occurrence # 236959
Last updated: 13 June 2020
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Date:30-JUN-1994
Time:19:40 LT
Type:Silhouette image of generic S76 model; specific model in this crash may look slightly different
Sikorsky S-76B
Owner/operator:United Technologies Corporation
Registration: N76UT
C/n / msn: 760362
Fatalities:Fatalities: 0 / Occupants: 3
Other fatalities:0
Aircraft damage: Substantial
Location:Narragansett Bay near Newport, RI -   United States of America
Phase: En route
Nature:Executive
Departure airport:
Destination airport:
Narrative:
Forced landing and ditching. No injuries.
Probable cause: the failure of the left hand helical gear in the main gear box (MGB), due to undertorqued attaching bolts installed by the manufacturer, which resulted in a partial MGB failure. This failure resulted in the loss of the number one engine, illumination of the MGB chip light and a ditching to salt water on emergency floats.

Investigation noted:
The MGB removed from N76UT, was manufactured by Sikorsky Aircraft and designed to accumulate 3,000 hours before requiring overhaul. At the time of failure, N76UT and this MGB, had accumulated 1,972.1 hours.

A review of the aircraft maintenance records revealed that this MGB experienced four MGB chip detector light illuminations during it's service life. The first two chip lights occurred at 97 and 174 hours of service, where small amounts of "fuzz" were observed during the maintenance inspection. The third light occurred at 1,011 hours of service, where a small piece of garlock seal spring was observed on the left hand chip detector. The fourth chip detector light occurred at 1,965 hours of service, 7 hours before the MGB failure. After performing the Sikorsky serviceability check, in accordance with S-76B Maintenance Manual 66-10-00, no contamination was noted and the helicopter was returned to service.

The MGB oil filter has a differential pressure indicator (DPI) installed to measure the pressure difference between the input and output lines. An increase in differential pressure will pop the DPI red button out, which then requires a maintenance procedure to be performed. On six occasions, the DPI red button was found popped on this MGB. The first five occasions occurred at 834, 1,059, 1,265, 1,603 and 1,865 hours of service. The last occasion occurred at 1,962 hours of service, 10 hours prior to the MGB failure. During all six occasions the transmission filter was replaced, and no defects were noted.

The MGB contains a left and right oil pump. The oil path to each pump is through its respective chip detector assembly. The pumps then provide pressure to a common line to lubricate the MGB. Each chip detector assembly is surrounded by a screen to catch material and protect the oil pumps. Although there were several pounds of metal particles in the MGB sump, and both of the oil pumps were seized from the ingestion of metal particles, there was a low volume of material collected on the screens. Examination of the chip detector/screen assembly revealed that the screen did not completely protect the oil path to the pumps. A designed alternate oil path existed, allowing unscreened oil to pass to both MGB oil pumps. This design is common to all S-76 MGB's.

During the meeting on July 8, 1994, the certification of the MGB, without oil pressure, was discussed. A representative of the Transmission Design Engineering Department stated, the original certification of the MGB without oil pressure, was accomplished with both engines providing approximately 60 percent engine power to the MGB, with a load applied to the main and TTO output shafts. After approximately 18 minutes, the left and right bevel pinion gears failed from overheating. During the MGB failure on June 30, 1994, after the left engine dropped off line, the right engine was operated as necessary to accomplish the water landing and taxi. The total time of the single engine operation was less then 6 minutes. During this period of time the right bevel pinion failed due to overheating. This eliminated all engine power from the MGB.

Also, during the meeting on July 8, 1994, the requirements for a technical inspector (TI) reviewing the torquing of nuts and bolts was discussed. It was stated the required inspection stamps on the build-up records, are those of the certified mechanic who actually does the torquing, the mechanic who completes the assembly (often the same individual), and the inspector who verifies that these steps have been completed. The inspector is not required to witness the torquing process.

Sikorsky Aircraft issued an alert service bulletin, covering all S-76 models with the 76351-09600 series main gear box, as a result of the accident. The purpose was to perform a one-time inspection to check the torque on the 12 attaching bolts and nuts which secure the tail take-off pinion to the helical gear. In addition, an improvement in the fastener configuration was to be accomplished.

As of October 1, 1994, 45 main gear boxes had been inspected. There were no helical gear nuts and bolts found with reduced torque. Seventeen of the gear boxes had in excess of 1,500 hours of operation.

ASRS:
AFTER COMPLETING THE AFTER TKOF CHKLIST, WE HEARD A BUZZING SOUND, THEN A VIBRATION, FOLLOWED BY MAIN XMISSION CHIP CAUTION LIGHT ILLUMINATION. CAPT EXECUTED MEMORY ITEM CHKLIST, 'LAND IMMEDIATELY.' CAPT COMMANDED. 'ARM THE FLOATS,' THEN AT THE RIGHT TIME, HE COMMANDED BLOW THE FLOATS. ACFT MADE A GENTLE LNDG ON THE WATER. THERE WERE NO INJURIES OR AIRFRAME DAMAGE. DURING THE DSCNT, I WAS XMITTING ON 121.5 'MAYDAY' AND BROADCASTING OUR POS. UPON LNDG, I USED OUR HANDHELD CELLULAR PHONE, AND CALLED 911. A POLICE DISPATCHER ANSWERED AND I DESCRIBED OUR LOCATION AND SIT. THE DISPATCHER CONTACTED THE COAST GUARD, APPROX 10 MINS LATER, THEY ARRIVED ALONGSIDE AND WE TRANSFERRED TO THE COAST GUARD BOAT. WE THEN TOWED THE ACFT TO SHORE WHERE IT WAS RECOVERED.

Sources:

ASRS ACN: 277062
https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20001206X01604&AKey=1&RType=HTML&IType=FA


Revision history:

Date/timeContributorUpdates
13-Jun-2020 15:41 Aerossurance Added

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