Horizontal Stabilizer Fitting – Loose Bolts - Civil Aviation Safety Alert (CASA) No. 2020-10


Owners, operators and maintainers of Bombardier Inc. Model CL-600-2B19 Aeroplanes including Canadian Regional JET (CRJ) 100/200/440 Series and Challenger CL850 Series

File Classification No.: Z 5000-35
RDIMS No.: 15292682
Document No.: CASA 2020-10
Issue No.: 01
Effective Date: 2020-10-14


The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness of a potential for loose mounting bolts on the horizontal stabilizer forward steady fitting going undetected during a scheduled maintenance inspection.


During an aging aircraft inspection, the mounting bolts for the forward steady fitting, part number (P/N) 600-21402 were found loose. Following this discovery, two additional aircraft in the hangar were inspected and found to have the same condition. The drawings for the forward steady fitting call for very tight and specific dimensional tolerances for the mounting holes; the bolts should not have any free play and be securely torqued. Teardown revealed that some of the holes in the fitting and the bracket were elongated beyond the required dimensional tolerances. These elongated holes were the result of loose bolts allowing movement of the fitting. The root cause as to why these particular bolts became loose is currently unknown.

Elongation of the mounting holes can cause the horizontal stabilizer pivot fitting assembly, which is a structurally significant item (SSI), to be overloaded due to asymmetric load input. It can also induce free play in the assembly, leading to reduced functionality of the flutter damping system and inadequate stiffness.

At the time the defect was found, no specific manufacturer recommended inspection was being performed. The inspector was performing a Detailed Inspection (DI) of all structures as part of an aging aircraft inspection which was above and beyond Bombardier’s recommendations.

Bombardier amended task card # 000-55-320-517 for the DI of the top canted rib and horizontal stabilizer trim actuator pivot structure using Temporary Revision 767 on 26 May 2020 to include an additional step at paragraph 4.A.(2) to “do a tactile check of the forward steady fitting bolts (8) for looseness.”

Recommended action:

Transport Canada (TC) recommends the following for all Bombardier model CRJ 100/200/440 and CL850 series aeroplanes:

Note: If the bolts have already been replaced with Hi-Lok fasteners as per a previous repair, this inspection is not required.

  1. Perform the DI in task card # 000-55-320-517 by applying a wrench to each of the mounting bolts checking for looseness. Using a wrench, attempt to turn each bolt with a moderate amount of torque to check if the bolt turns easily or is secure. (Note: Standard torque for NAS6203 is 20-25 in-lb. and for NAS6204 it is 50-70 in-lb.)
  2. If a mounting bolt is found loose, inspect the bolt hole to determine if it is within the required dimensional tolerance.
    1. If the bolt hole is within the tolerance, inspect the bolt and replace as required. Install a new nut and re-torque in accordance with the Aircraft Maintenance Manual (AMM) requirements.
    2. If the bolt hole is not within the dimensional tolerance, conduct the repair found in Generic Repair Engineering Order (GREO) 601R-55-31-321. Please note the GREO recommends replacing all the bolts to ensure future loose bolts do not occur. This also eliminates the future inspection recommendations of this CASA.
  3. Within Canada, owners, operators and maintainers should submit a Service Difficulty Report (SDR) to TC for the discovery of each reportable service difficulty (Please note that if any bolt is found loose during this inspection, a SDR should be submitted). Outside Canada, report any loose bolt in accordance with local reporting requirements.

TC continues to monitor events related to this issue and might issue recommended or mandatory corrective action, as necessary.

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact Marie-Claude Cardinal, Continuing Airworthiness in Ottawa, by telephone at 1-888-663-3639, by facsimile at 613-996-9178 or by e-mail at cawwebfeedback@tc.gc.ca.

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.