AL 2003-07
17 July 2003
Horizontal Stabilizer Actuator Detached From Airframe Structure
Transport Canada has received Transportation Safety Board of Canada (TSB) notification of two occurrences involving a Beechcraft Model A100 King Air and a Model 99A Airliner aircraft, where an improperly installed horizontal stabilizer actuator became detached from the airframe structure during flight.
This Service Difficulty Alert is issued to preclude another occurrence attributable to the maintenance factors affecting this installation procedure.
The following illustrations and information are taken from the Beechcraft Airliner Model 99A TSB - Occurrence Bulletin. Figure 1 and Figure 2 show the airframe-to-stabilizer actuator attachments.
Post-accident inspection revealed that the horizontal stabilizer actuator had detached from the airframe structure, allowing the stabilizer (and the actuator) to move freely under the influence of air loads. During installation of the actuator, the two upper attachment bolts had been incorrectly installed behind the actuator upper attachment lugs, trapping the actuator lugs between the bolt shanks and the rivet heads in the channel and plate assemblies. Trapped in this manner, the weight of the actuator is suspended giving the false impression that the actuator is correctly installed.
Figure 1
Figure 2
Figure 1: Shows the channel and plate assemblies that provide the airframe mounting structure for the two upper attachment lugs of the horizontal stabilizer actuator. Plates, through which the bolts (arrows) are installed, are riveted in place on each channel. The rivet locations are uniform for all four plates. The actuator upper attachment bolts are shown installed in each of the paired channel and plate assemblies.
Figure 2: Shows the upper attachment lugs at the top of the horizontal stabilizer actuator. Each lug is fitted with a spherical bearing assembly. When installed correctly, the bolts seen in Figure 1 are inserted through these spherical bearing assemblies to attach the actuator to the airframe structure.
It is emphasized in the TSB 99A Occurrence Bulletin that the installation of the actuator requires the installer to hold the actuator in position with the upper attachment lugs in place between their respective channel and plate assemblies. The installer is on his knees, leaned forward, and holding the actuator (approximately 10 lbs.) in place ahead of him. During installation and when installed, the channel and plate assemblies hide the actuator upper attachment lugs from view, and the bolts must therefore be installed by feel.
The difficulty of installing this component requires that maintenance personnel be fully conversant with the applicable procedures in the Aircraft's Maintenance Manuals and confirmation of correct assembly, locking and sense of operation including an independent check.
Because these two reported cases have a human factor element traceable to difficulties that are encountered during the reinstallation of a component, a reassessment of the installation procedures, as they have been described in the Beech 99A and A100 Maintenance Manuals, is in progress.
Any further defects or occurrences should be reported to Transport Canada, Continuing Airworthiness, Ottawa via the Service Difficulty Reporting program.
For further information, contact a Transport Canada Center, or Mr. Don Nielson, Aircraft Maintenance and Manufacturing Branch, Ottawa, telephone (613) 952-0110, facsimile (613) 996-9178 or e-mail nielsod@tc.gc.ca.
References:
- TSB Occurrence Bulletin: A03C0094, 02 May 2003
- TSB Report Number: A99H0002, 14 June 1999, Titled: " Loss Off Control After Take-off".
- Beechcraft 99 Airliner
- Illustrated Parts Catalogue, P/N 99-590014 FS, (Revised April 10, 1987)
- Maintenance Manual, P/N 99-590015-1B21 (Revised June 01, 2001)
- Beech King Air Model 100 Series
- Maintenance Manual, P/N 100-590038-17, June 30, 2000
For Director, Aircraft Certification
R.A. Raoux
Acting Chief, Continuing Airworthiness