One occurrence noted in your report is a Mooney VH-KUE which
suffered a fatal accident after the vacuum system to the
instruments failed. This was subject to your draft safety
occurrence report 9703221 of 22 May 1998. The CASA response to that
report concurred that excessive operating hours since new or
overhaul can contribute to vacuum pump failure. The CASA response
also noted an extensive range of advisory material issued by CASA
to educate the aviation industry on the problem.
To implement a blanket overhaul requirement would probably increase
the reliability of operation of these vacuum pumps but would not
prevent failures.
The aircraft is, however, intended to be safe even with a vacuum
pump failure. This is facilitated by CAO 20.18 Appendix IV which
requires that specific instruments have duplicated sources of
power. The result is that a full vacuum failure results in
part-panel operations. As you note, part-panel operations are not
desirable, but under normal expected conditions are considered to
provide an acceptable 'get-you-home' capability.
You will note that Discussion Paper DP 9902RP was issued covering
General Operating and Flight Rules. That Discussion Paper included
proposed alternate instrument power supply requirements at section
91.345 which closely align with the current requirements in CAO
20.18. The review of comments received against that Discussion
Paper will be reflected in the Notice of Proposed Rule Making that
will be circulated for comment before CASR 91 is issued. ATSB may
care to consider providing a specific comment against this
requirement in the NPRM which can be considered with other public
comments on this important rule.
