Why has this apparently never been done on aircraft engines?
The oil level would under-read when the engine is running (because of the oil scooped up and making its way around the engine) but the level in the sump would still be highly representative of what margin you have.
It would be fairly easy to do, with a level gauge fitted to the bottom of the sump and sticking upwards into it, IMHO.
Has it been done on homebuilts?
Agreed, it is strange that no one has developed this
In a dry sump installation this would be very easy to implement in the oil catch tank.
Depending on the engine type, one could easily incorporate the gauge in the sump plug so installation would very simple .
I think the basic idea would be to get a baseline reading in cruise then set-up a threshold level that would put off an alarm.
One could measure the oil level by measuring the oil pressure at the bottom of the sump. I have that at home on the domestic oil tank
1 metre of oil is something like 0.085 bar. I used an off the shelf silicon bridge sensor for the above, and an off the shelf DVM module (200mV full scale).
So, no moving parts; just an off the shelf pressure gauge like this
It would have to be a “gauge” sensor (i.e. with a vent to ambient), because the measurement range would be much smaller than the ambient pressure variation.
I may be misunderstanding, but can’t you gauge this using oil pressure and temperature readings?
Roger wrote:
but can’t you gauge this using oil pressure and temperature readings?
Way too many factors affecting oil temperature to use it as a proxy for volume.
Peter wrote:
It would have to be a “gauge” sensor (i.e. with a vent to ambient), because the measurement range would be much smaller than the ambient pressure variation.
I think you may be forgetting about crankcase pressure which would probably interfere with any pressure readings as a proxy for volume.
On a slightly related note: the old radial engines had not only fuel tanks but also oil tanks. Apparently oil was added during flight. How was it decided when, or how much?
Radial engines are dry sump engines, just like Rotax. Scavenger pump below the crank shaft and external oil tank.
Well, the sump were actually the hanging cylinders 
Peter wrote:
Why has this apparently never been done on aircraft engines?
I think the level in the sump doesn’t tell you too much about how happy the engine is. The IO-520 in “my” aircraft normally runs on anywhere between 9-12 quarts. However the minimum safe level is 2.7 quarts … Couldn’t imagine there would be too much oil left in the sump at all, however the engine is happy and the manufacturer is even more happy to put it in the Aircraft Flight Manual.
Consequently a sump level gauge could tell you anything between 0 and 12…. and it would all be in the green range.
Michael wrote:
Way too many factors affecting oil temperature to use it as a proxy for volume.
I disagree. If you know your aircraft well, you’ll know how the engine behaves during the different phases of flight, and the oil temp and pressure gauge will tell you something…
