Civil Aviation Safety Alerts (CASA) No. 2017-02


Owners, Operators and Maintainers of Bell Helicopter Model 206A, 206B, 206B-3, 206L-1, 206 L-3 and 206L-4 Helicopters

Issuing Office: Continuing Airworthiness
File Classification No.: Z 5000-35
RDIMS No.: 12066419
Document No.: CASA 2017-02
Issue No.: 01
Effective Date: 2017-03-29

Loss of Hardware - Tail Rotor Drive Shaft Couplings


The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness of several occurrences, where critical hardware was found missing from tail rotor drive shaft flexible couplings and recommend strict adherence to Bell Helicopter maintenance practices that reduce the likelihood of this type of failure.


There have been a number of Bell Helicopter model 206 and 206L accidents involving tail rotor drive system failures. In several of these occurrences it was noted during the investigation that the hardware (bolt, washer and / or nut), that is used to assemble the flexible couplings in the tail rotor drive shafting, was missing. The flexible coupling is also known as a Thomas coupling. In some cases, this missing hardware was identified as causal to the occurrence, in other cases a causal relationship was not established.

During installation, torque applied to the nut is specified in the Bell Helicopter maintenance manual as 50 to 70 inch-pounds in addition to the tare torque of the nut. Tare torque, also known as run-on torque, is the rotational force required to turn the nut on the bolt, overcoming the friction of the self-locking feature.

Between 10 and 25 operating hours after installation of this hardware, there is a maintenance manual requirement to perform a torque check on the hardware. If, after application of the minimum torque (e.g. 50 inch-pounds) there is no movement of the fastener, the installation is considered acceptable.

There is also a requirement in the maintenance manual to perform the torque check every 100 hours.

Table 1 is a summary of the occurrences investigated by the National Transportation Safety Board (NTSB) of the United States of America where missing tail rotor drive shaft hardware was noted.

Table 1 - Model 206 / 206L Accidents Involving Missing Tail Rotor Drive Shaft Hardware
Occurrence Date NTSB Accident Number Synopsis
3 October 1984 NYC85FA001 In-flight loss of tail rotor authority, forced landing
17 July 1995 FTW95LA303 In-flight loss of tail rotor authority, forced landing
7 June 1998 FTW98LA260 In-flight loss of tail rotor authority, forced landing
23 March 2001 NYC01LA088 In-flight loss of tail rotor authority, forced landing
10 October 2001 NYC01LA168 In-flight loss of tail rotor authority, forced landing
6 June 2013 CEN13LA321 In-flight loss of tail rotor authority, forced landing

The NTSB investigations did not clearly identify a reason for the missing hardware in these accidents. The cause factors described in the reports included Under torque (insufficient torque) of a Thomas coupling self-locking nut for undetermined reasons and failure of maintenance personnel to ensure adequate torque of a tailrotor drive shaft coupling bolt.

Recommended Action:

  • Use a calibrated torque wrench when installing this hardware
  • Avoid re-use of this hardware
  • Verify that tare/run-on torque meets specification requirements. Before installing nuts, refer to Bell Helicopter Standard Practices Manual, Chapter 2 – Torque.
  • Perform torque check after 10-25 hours of operation, refer to Special Inspections in Chapter 5 of applicable Bell Helicopter Maintenance Manual.
  • Repeat torque check every 100 hours, refer to scheduled maintenance requirements in Chapter 5 of applicable Bell Helicopter Maintenance Manual.
  • Mark installed hardware with torque seal lacquer, Bell Helicopter consumable material C-049, after it has passed torque check. The marking allows visual detection of nuts that have loosened before those nuts fall off.

Contact Office:

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

Original signed by

Rémy Knoerr

Continuing Airworthyness

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.

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