Gavin Grimmer
As a matter of curiosity, at 5 degrees C, the propeller rpm needed to actually break the sound barrier is 3150.
I racked my brains for weeks trying to work out why Mike would have had the motor working this hard, until I stumbled upon an article on the web talking about runaway propellers.
To cut a long story short, the pitch of Constant Speed Propeller fitted to VH-MDX
was hydraulically operated by a Governor Control that had it’s own oil pump incorporated
in it and the oil feed to it came from the engine oil pump. The purpose of the Governor
oil pump was to increase the oil pressure in the propeller to make it more responsive.
In the event of the propeller losing oil pressure, this system was designed to go
to full fine pitch as if it went to full coarse, it would ‘bog’ the motor. The problem
was that in full fine, to maintain enough airspeed to maintain altitude in a heavily
loaded aircraft such as what MDX was, would mean running the engine at a very high
rpm, but incidently that doesn’t necessarily mean full power.
The problem then can
be two fold as to why the propeller lost the oil pressure in the first place. If
there was oil being lost overboard (and this may have been the source of the earlier
“thought we had a fire in the cockpit” smell) and the oil level had dropped to the
point that in the rough turbulent air, the main oil pump was picking up air, then
the air fed into the Governor would eventually cause the propeller to lose oil pressure
and start fineing up, but the problem is the motor itself is then also in danger...
not just because of the higher rpm needed to maintain flight, but the lack of oil
will eventually cause it to self destruct.
Another problem that can cause the loss
of oil pressure and that can be from a problem in the Governor itself and there is
at least one AD (I haven’t looked further) that I have found that shows this possibility...
https://www.casa.gov.au/files/awb01-036issue1-supmccauleypropellergovernoridlergearbearingspdf
Although
this is only a relatively recent article, it does show the possibility i.e. “consequential
internal leak in the oil transfer system causing the governor to lose some or all
control.”
In this situation, although the engine is no longer in danger of losing
it’s own oil pressure, it then becomes in danger of excessive heat generated by the
high revving engine which will eventually lead to seizing anyway. From an article
on the Net:
VH-MDX’s propeller had stops built into the hub that limited it’s full fine pitch to 11.5 degrees which theoretically gives it a forward movement of 51.13” for every full turn of the propeller. At 2920 rpm this calculates to an airspeed of 122 knots, but due to inefficiencies this is more likely much lower. As a rough rule of thumb, you can expect around 80% efficiency out of a propellor which means about 97 knots... so as you can see, to maintain a safe speed, Mike would have needed to run the motor at a high rpm... well past red line!
MDX pg 60