First published in Flight Levels Online, Spring 2013 issue
Twin Commander Aircraft has drafted a Service Letter that advises owners of Dash-5-powered 690s, 690As, and 690Bs of a modification to correct a Beta light that may flicker in flight.
The publication, Service Letter 385, details an approved modification to the Beta Pressure System. The addition of a restrictor fitting before the Beta Pressure Switch will dampen pressure fluctuations in the system, which can cause the Beta Pressure Indicator Light to flicker in flight.
The modification is not mandatory, and can done at the owner’s discretion. It is possible that the modification had been done previously per 610494 Rwk. #6B. A check of the aircraft’s maintenance records will verify whether the rework was accomplished.
The Beta light is controlled by oil pressure, and illuminates when it is safe to put the propeller into reverse for braking on landing rollout. Fluctuations in oil pressure in the Beta Tube System can cause the Beta light to flicker. The modification basically involves the installation of a restrictor fitting in the tube leading to the Beta Pressure Switch.
The restrictor fitting serves to dampen fluctuations in oil pressure in the beta tube that potentially could cause false illumination of the Beta light.
The modification also repositions the NTS test valve to avoid any potential chafing against the cowling.
The Service Letter modification kit is relatively inexpensive and the work can be accomplished in about two hours by an authorized Twin Commander Service Center.
The modification is based on a redesign of the Beta Pressure System that was implemented for JetProp Commander models (the 840, 980, 900, and 1000); thus, those models are not affected by the Service Letter.
