Sometimes vacuum pumps fail in ways you might not expect. Vacuum pump-related failures caused a loss of engine oil pressure in several cases. In four cases, the vacuum pump housing literally split in two. In three instances, the vacuum pump adapter cracked. In two other incidents the vacuum pump gasket was defective.
Failures result from installation errors as well as from product defects.
A new Grob 115C suffered camshaft drive gear damage because the factory had mis-installed the vacuum pump. In two other cases, vacuum pumps were incorrectly installed in the field resulting in a losses of engine oil. Operator error is also a factor. Dual pneumatic pumps yield redundancy only so long as the pilot is able to detect the failure of the first pump. In another case, because there was no annunciator installed on this system, the pilot was unaware that the primary pump had failed and flew for hours with the standby pump as his only available source of pneumatic air.