Hello,
Since my last visit, I am experimenting some problem with the display of MP on the g1000 (the plane is a NA C182).
I should add that during the visit, the hose used to sense MP was changed due to wear signs.
Since then, I noticed two things:
- when idle, MP value is higher than usually (15 vs 12")
- when applying some power adjustment (increasing or decreasing), MP does have a huge inertia/slowness to update to the right value.
If waiting long enough, values seems to be never absurd, (except for idle, clearly too high).
This inertia is quite annoying ‘cause you cannot input any power setting correctly.
And when taking off (sea level), you generally rotate with 18", slowly reaching 27-28" when reaching 1000’ AGL…
Power is there, but not really optimistic feedback.
Obviously I thought there could be a leak after the work done on the hose.
But the workshop said they tried to tighten everything again without any result. I also thought about a different diameter of the hose.
Of course, the workshop want to change the MP sensor, but not sure it will change anything.
What do you think about all that mess ?
(I do have some graphic logs I could post eventually)
Thanks for your advices,
Regards
Alexandre.
Does it indicate correctly with the engine not running?
Is the hose from the manifold to the sensor being pinched anywhere?
That would give a slow response.
When the engine is off, it displays (as usually), atmospheric pressure minus 1 (average 29.5"). I didn’t notice anything weird.
But when the engine runs up, the pressure decreases very slowly from 29.5 to 15-16.
For what I have seen, I don’t think the hose was pinched. It is running through a collar which was not over tighten.
Is it the correct hose material? If it is no suction hose but a softer type it could be squezed by the lower pressure inside compared to ambient pressure.
This sounds like a blockage in the pipe – maybe insects?
It takes a LOT to collapse a small-diameter pipe – say 6mm ID. I doubt even vacuum would collapse it, and anyway not completely. However some installers have a habit of using a very low grade clear PVC pipe and if they used one with a really thin wall then it might be possible.
Has this problem started suddenly, or was it possibly always like that during your ownership?
It is not uncommon that the sensor get dirty from fuel, water condensation, oil which cause the same sluggish results. Some aircraft have even drains on these lines, or tampon like filters to prevent this.
Thin walled hose as Peter indicates is also a fair point, some low quality hoses will colapse under vacuum make it restrictive.
If your shop has a good pitot static test set they would also be able to test the sensor directly. I can switch my pitot static tester to "Hg to troubleshoot MAP systems, and check their calibration.
This has been discussed other places. The MAP tube is often just a tiny hair tube. Maybe not even a mm internal diameter. The MAP tube is however, just like any other static pressure tube, susceptible for water contamination (and fuel in this case). According to AC 43-13 static tubes should be at least 3/8" diameter, or 10 mm to prevent water being a problem. Any diameter less than this, and the tube will not be self drainable due to capillary forces.
Maybe it is just blocked also, by dirt or insect.
I second that the most likely cause it that you have an orifice in the connection between the MP probe and the place it samples. Orifice means there is a time lag.
When the engine is off, it displays (as usually), atmospheric pressure minus 1 (average 29.5").
That is weird, it should display exact atmospheric pressure because the system is not air tight and over time, pressure equalization takes place.
When I read the title of this thread I thought it was yet another UK political scandal 
