Cracking of main rotor transmission bevel gears - Civil Aviation Safety Alerts (CASA) No. 2018-10


Owners, operators and maintainers of Bell helicopter model 206L-series, 407 and 505 helicopters

File Classification No. : Z 5000-35
RDIMS No. : 14269800
Document No. : CASA 2018-10
Issue No. : 01
Effective Date: 2018-11-16


The purpose of this Civil Aviation Safety Alert (CASA) is to improve awareness of the potential for cracking of main rotor transmission gear and recommend strict compliance with the operations and maintenance procedures associated with cracked gears


Transport Canada received a Service Difficulty Report (SDR) in late 2017 that a bevel gear from a model 407 helicopter main rotor transmission was found to have a broken tooth. The gear had a crack in the same area. The transmission containing that gear had been removed from the helicopter in response to two activations of the transmission chip detector system within 6 operating hours.

Further investigation has revealed that cracked bevel gears are not uncommon on the model 407 transmission: this was the sixth such occurrence reported by SDR since 2001. In addition, there have been 15 gears found cracked during overhaul inspections. In the cases of gears found cracked during overhaul, it seems likely that the crack in the gear did not produce enough metallic debris to activate the chip detector system in the helicopter.

Bell has developed a welded version of the bevel gear assembly for the transmission of the model 407 helicopter. The part number (P/N) of this gear is 407-040-043-101 and it replaces gear assemblies that are bolted together. In some cases the cracks in the gear originated from the bolt holes. It is believed that the P/N 407-040-043-101 gear is less vulnerable to cracking because it does not contain bolt holes.

Cracked gears on the model 206L-series are less common than on the model 407. This series of helicopters includes models 206L-1, 206L-3 and 206L-4. There have been three in-service reports of cracked gears and one instance of a 206L-series gear found cracked during overhaul.

There have been no in-service or overhaul reports of cracked gear on the model 505. However, since the model 505 helicopter uses the main rotor transmission design from the model 206L-4, it is considered vulnerable to this failure.

Cracking of main rotor transmission bevel gears is a failure mode that has been recognized by Bell for a considerable period of time. To ensure that customers were aware of this hazard and comply with recommended procedures in the event of this hazard, Bell issued Operations Safety Notice (OSN) GEN-96-28 “Transmission Spiral Bevel Gear Cracks” in 1996. The OSN is applicable to all Bell civil helicopter models.

Some of the information in recent SDRs indicates that operators may not be strictly following the Bell Flight Manual and Maintenance Manual procedures for responding to chip detector activation. In addition, it may not be clear to operators how they should respond if the serviceability check of the transmission produces satisfactory results but there is a subsequent chip detector activation.

Based on available information, Transport Canada has concluded that the potential for a cracked bevel gear is not in itself an airworthiness unsafe condition if the transmission chip detector system is functioning properly and if the flight manual and maintenance manual procedures are strictly complied with.

Recommended action:

To reduce the risk associated with bevel gear cracking, Transport Canada recommends the following:

  1. Ensure that the transmission chip detector system is always functional.
  2. Always follow the flight manual and maintenance manual procedures in response to transmission chip detector activation.
  3. If there is a chip detector activation and the serviceability check specified in the maintenance manual produces acceptable results but there is a subsequent chip detector activation within a short period of time, consider the transmission to be unserviceable: remove it from the helicopter and have it disassembled and inspected in accordance with the component repair manual requirements.
  4. For model 407 helicopters, consider voluntary installation of welded gear assembly P/N 407-040-043-101 during the next transmission shop visit (overhaul or repair).

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or Ross McGowan, Continuing Airworthiness in Ottawa, by telephone at 1-888-663-3639, facsimile 613-996-9178 or e-mail

Original signed by

Rémy Knoerr
Chief, Continuing Airworthiness
National aircraft certification

The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.