SA 18: Installation of an alternator filter subject to a PMA granted by the FAA
In the course of the safety investigation concerning the serious incident involving the Robinson R22 Beta II helicopter HB-ZGO on 6th April 2016 near Worb (BE), the Swiss Transportation Safety Investigation Board (STSB) identified a safety deficit and issued safety advice N° 18 in its final report no. 2319.
Target group(s)
Aircraft owners, operators and maintenance companies
Safety advice
In all aircraft that have been retrofitted with an alternator filter mounted on the alternator subject to a PMA (part manufacturer approval) granted by the FAA, it should be ensured that no short circuit can occur between the earthed alternator filter housing and the electrical system.
Safety deficit
An open fire in the engine compartment of a Robinson R22 Beta II helicopter occurred in cruise flight, which had been caused by a short circuit between a retrofitted alternator filter and the alternator terminal.
According to a PMA7 granted by the FAA, the installation of a Lone Star Aviation Corp. LS03-01004 alternator filter is approved for various light aircraft (e.g. Cessna C172). However, the installation instructions do not describe how the alternator filter must be mounted to the alternator. As a result, the alternator filter can be mounted in such a way that a short circuit may occur between the earthed alternator filter housing and the alternator’s A+ terminal.
In general, PMA-approved components as granted by the FAA with approval for installation in certain aircraft types do not always come with detailed installation instructions. The STSB considers it to be a fundamental risk that installations are performed which may carry a hidden or long-term hazard potential.