If the MP dropped but performance did not change, clearly the MP indication was faulty.
This happens a lot with EGT probes. The probe tips get eaten by the hot gases and initially the reading goes higher and higher (because the instrument has a pullup resistor inside, to implement the open probe detection) and eventually the instrument decides it is too high to be real and flags that probe input. At the same time the engine runs fine and the CHT in that cylinder remains constant.
But that does not make the indicators have no value. For example I have two oil pressure gauges. The historically unreliable Socata one plus one which I added a few years ago. If both indicate low, it’s time to look for an airport FAST because there is a leak for sure. The leak may be in the pipe leading to a T-piece feeding both gauges (getting two separate oil pressure takeoff points on the engine was a bit messy) but that will empty the oil system pretty quick too despite having a very small orifice.
My MP gauge is also currently broken (JPI EDM830), showing funny oscillations. Haven’t gotten around to fixing it (have the factory builtin gauge as well which is primary), I bet it is this damn JPI screw connector that shakes loose every now and then. Had it with EGT and CHT sensors before. This time it will be Loctite on it.
I guess more than 50% of such problems are indicator failures. However, I have to admit that it takes quite a bit of time for the pilot to realize. When my MP suddenly started to drop to 9" and then to 0", I was focused on finding a field for a forced landing. Only a few seconds later I realized that the engine sound, airspeed or attitude hadn’t changed.
So the most important clue was missing from your description: airspeed/engine sound did not change after the incident. Then I would have continued as well in your case. With the original description, I would have landed, maybe even with a Mayday.
2 faults developing on the same flight, how are the odds for this?
For how long did you fly upside-down?
Fuel flow was constant.
Ted is right it was a leak in the line to the MP gauge, the line ruptured.
The 2 symptoms were caused by different causes, just an coincidence.
Oil came out of a 45 degree angle connector at the back of the engine where the pressure line to the oil cooler is connected.
2 faults developing on the same flight, how are the odds for this?
Could the oil leak have blocked the air into the turbo, e.g. clogged an air filter? Or is it not filtered?
We are assuming that the MP drop and the oil overboard are from the same cause. As there is no mention of oil being all over the engine the vent seems likely. The MP drop is likely an unduction leak, I’m assuming that there was a reduction in power/fuel flow – if not its a gauge fault, a leak in the pressure line to the instrument.
On the other hand sometimes the same cause can produce quite different faults, which can be very confusing.
How would that cause the MP drop? That is essentially a turbo failure or a leak into the inlet manifold.
Iced up crankcase vent pipe
The waste gate is manual, by a cable.
Oil pressure and oil temp was normal.
Is your wastegate oil pressure actuated?
If so, it probably sprung a leak.
Otherwise, something came off fairly dramatically and cut one of the many oil pipes, but not dramatically since you lost oil very slowly. Cutting one of the pipes which lead up to the rocker covers might do that. It would not show up on the oil pressure indication whose sensor normally comes off one or the other end of the main oil gallery.
