Purchased this M20 for it's range and speed to economy index. So today I planned and started a four hour flight from Maryland to Indiana, USA.  About two hours into the flight, at 8,000 and an OAT of -5C, I noticed my M20 was showing decreased ground speed on the GTN 750. IAS confirmed the loss of speed. I immediately confirmed that I had Pitot Heat set to On. Then I started scrambling.  IAS continued to decline, and my GTN GS slowed to as low as 69 knots. I could not determine what was amiss. Looking below the ground everything seemed to be moving at a normal pace.  Chase plane view showed the M20 in a normal attitude for cruise speed. With the indicated speeds at the time the M20 should have had a significant nose up attitude and should have been at stall speed. It did not meet either of those characteristics.  ?????

I switched FSX to the Milviz C310 and caught up by loading my flight plan back into the GTN.  The C310 gained back to a normal cruise speed.  So at that point I reloaded the M20 and it too maintained a normal cruise speed at the top of its cruise range in IAS.  Everything stayed normal for about an hour, then the speed bleed occurred again.  This time it bled down to about 110 knots GS, then it was time to descend from 6,000 to 3,000 for initial approach altitude.  After entering descent the M20 came back up to a more normal speed and maintained it through the duration of the flight.

Has anyone else experienced an unexplained decrease in IAS and GS well into a long flight, say two hours or more in?  Really perplexed.

My guess is that Pitot Heat is not active, even though the switches may indicate otherwise...

Do you have my mods installed?  If so, I am going to investigate.. Let me know!

Edit: It is actually Engine Anti-Ice that is the culprit..

If the pitot heat is off/inop, doesn't the IAS drop to zero in icing conditions?

Happened to me on a flight from KSFF to 77S while I was passing Mt. Hood in IMC, which started as I approached the eastern slope of the Cascades.  ATC had me at 9200' (throughout the flight) and I started loosing power.  MAP started dropping as well as IAS.  I cleared the problem by leaning the mixture to the point of almost starving the engine and then adding mixture while jockying the throttle forward and back.  Engine started running fine again but I requested a lower altitude.  My OAT at the time of trouble was -2C, and of course the Pitot Heat was on (for the entire flight).  This was a final flight with the Ovation on an ORBX FTX Global airport install verification series I did after upgrading to P3Dv4.1 and the final release of ObjectFlow.  Started at KSEZ and worked CCW around the ORBX payware airports without a problem until this flight.  No problems after that.

I have a over 40 hours with the Ovation and this was the first and only time I had problems.  A few days ago I did a flight from KPVU to KLHM (almost 3 hours) in CAVU at 12500" without any problems (temps varied between -5C and -9C).

HTH,

Greg

3 minutes ago, J35OE said:

If the pitot heat is off/inop, doesn't the IAS drop to zero in icing conditions?

Invariably that is always the case in flight simulators, not so much in real aeroplanes, where icing can partially block the pitot tube and reduce the airspeed reading rather than completely drop it to zero. I seriously doubt a Carenado/Alabeo add-on features that kind of detailed simulation of partial icing and I'm fairly sure programs such as Active Sky leave the icing effect on the pitot up to the simulator to handle, so I'd guess it was an issue rather than something intentionally simulated.

P3D (and FSX before it) has a Pitot Heat variable and an Engine Anti-Ice variable.

The "H" key controls Engine Anti Ice.  (Shift-H is Pitot Heat).

If you lose power in flight, try pushing the "H" key.. it will likely solve the issue.

I have just updated my mod to turn on Engine Anti Ice when Pitot Heat is switched on in the panel. 

13 minutes ago, Bert Pieke said:

The "H" key controls Engine Anti Ice.

Didn't know that.  Thanks.

43 minutes ago, Chock said:

Invariably that is always the case in flight simulators, not so much in real aeroplanes, where icing can partially block the pitot tube and reduce the airspeed reading rather than completely drop it to zero.

I was talking about FSX/P3D.  Concerning RW flying; Depending on the blockage of the pitot/static system there's a great variety of errors that can occur concerning IAS, from reading way too low to reading way too high.