Unable to reduce load

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Boatguy
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Unable to reduce load

Post by Boatguy »

Yesterday, I flew my DA40NG from my home airport in Petaluma (O69) to Lancaster (KWJF) to view the poppy fields, about 300nm. After crossing the Tehachapi’s and beginning my descent, I had an anomaly with the engine load.

I was cruising at 11,500’ when I began a steep descent on AP with VS at -800fpm in light to moderate turbulence. As usual I pulled the power back as the descent began. I wanted to slow down more due to the turbulence and saw the load was still around 50%. I pulled the power back all the way, watched the prop speed briefly exceed 2,100rpm, triggering the yellow flashing warning, and the load still said 31%. At this point I became alarmed as I continued the descent.

I pushed the power forward a bit and watched it climb to 50%, again triggering the brief prop speed over 2,100rpm, then pulled back all the way and the load dropped to maybe 28%, again triggering the flashing yellow RPM warning. The IAS was decreasing so I relaxed a bit. I tried this several times as I was descending and when pulled all the way back the load eventually went to zero. I may have also slowed my descent at some point. I know I looked quickly at the prop overspeed check list and tried toggling the ECUs from Auto to A then B, but that had no effect and there were no ECU fail warnings. By the time I was down to 6,000’ everything was responding normally.

We cruised around the poppy fields, descended to 5,000’ into much worse turbulence, went back up to 6,000', did another lap around the poppies and then descended and landed at KWJF without incident.

On the ground I checked the gearbox oil, which was fine, then called Diamond. They didn’t have too much of an explanation, but expressed interest in the ECU logs. As I was literally in the middle of a desert with no Austro engine shops around that wasn’t going to be immediately possible. They thought possibly the gearbox was slow to respond, then suggested that if all was good in my runup ECU test I should be fine to return home. My runup for the return trip was normal, we climbed to 12,500’ for the first half of the trip, descended to 6,500’ looking for less wind in the second half, and landed back at O69 without incident. Both descents on the way home were at 500fpm.

Diamond said one thing that caught my attention. They said the load reported was calculated by the ECUs based on prop speed and turbo boost. They speculated that the prop speed could have been higher than expected based on windmilling at the steeper descent angle.

This is all from memory and it was a long day so tomorrow I will go back to the plane and get the engine logs so I can more accurately describe the timeline and the behavior of the load.

Has anyone else ever experienced something like this with the Austro engines?
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Re: Unable to reduce load

Post by CFIDave »

Russ: What you experienced was perfectly normal! I'm really surprised Diamond didn't tell you this.

Austro engines are designed to prevent "combustion extinction" at higher altitudes, so they won't let you reduce power below about 20-30% above 10,000 feet (varying by altitude) so that the diesel "compression ignition" engines maintain sufficient heat to keep firing.

To show this, I've attached a graph from the Austro Engine manual that shows minimum and maximum power levels/load based on pressure altitude. What happened to you is referred to as "Low Power Correction" or "Idle Power Correction" in the bottom right of the chart:
Screen Shot 2020-04-24 at 2.18.57 PM.png
Where this might make things interesting is landing at an airport like Leadville/Lake County in CO (roughy 10,000 feet MSL), since you won't be able to bring the power back to idle while taxiing.
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chili4way
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Re: Unable to reduce load

Post by chili4way »

I learned something from Dave's explanation. (Thanks, Dave!) What's unclear to me is what happens to the setpoint for the propeller RPM when the power lever is set below the idle power correction level.

The windmilling explanation for the propeller overspeed makes sense to me. The relationship between the power lever position and the setpoint for the prop RPM (p 7-25 in the AFM) may have a contribution to what you experienced. Do you remember your IAS at the various points during the scenario? The ECU readout (or the G1000 readout) will provide this.

I experienced something similar one time when I was in level cruise at 13,000 MSL. When I tried to advance the power lever beyond what the engine could produce at that altitude (I think is was around 88%), I got a prop overspeed warning with no change in reported engine power. By pulling the lever back, the prop RPM and power level came back into balance.

I can see how something similar would happen in a descent where the IAS is faster than what the prop & engine settings would produce in straight & level flight at that altitude.
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Re: Unable to reduce load

Post by Boatguy »

Dave: Yes that totally explains it and why the "minimum" power kept dropping as I descended. Thank you very much for bringing that to my attention. I don't know why Diamond didn't give me that simple explanation as it would have reduced my anxiety on the way home.

Paul: I experienced something similar. We were cruising at 14,500 and decided to climb to see how far we could go. We set the power at 92% and started climbing. Somewhere around15,300' the load still read 92% but the prop started to flash a speed warning. I backed off the power a little to maybe 88% and the prop speed warning went away. We climbed to about 15,700' before descending back to our cruise. The altitudes are all indicated, not pressure as in the AFM, so the altitudes can't be compared specifically to the AFM. My interpretation was that the engine had the power, but the prop didn't have enough bite in the thin air.
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Re: Unable to reduce load

Post by neema »

I don't understand why the chart Dave posted isn't in the AFM. It's a pretty critical in understanding how the plane will behave at altitude. Also, some high altitude airports may be a no go. I heard a DA62 popped a tire trying to stop at Leadville because it was still making power on landing (the chart shows it'd be making just over 20% power at ~700 mb).

Jealous that when you pull the power back in DA40NGs you don't hear the gear buzzer going off! I'd love an STC to reconfigure the gear mute button to work in all circumstances on the Diamond twins rather than a split throttle regime
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Re: Unable to reduce load

Post by pilot_jon »

Not first hand experience yet, but I've heard that at high elevation airports it's not possible to perform an ECU test on runup due to the idle power correction preventing the engines from making idle power.
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Re: Unable to reduce load

Post by Boatguy »

The E4 Ops manual says the corrections start at 800mb which is about 6,500' depending on temperature. The highest airport I've departed from is KSAF at 6,350 where I had no problems.

There must be somebody on DAN who has departed from higher altitude airports.
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Re: Unable to reduce load

Post by ScottOHare »

I fly my DA40NG out of Driggs, Idaho, elevation 6200', have seen DAs as high as 8000' but have never had an issue on the ground or at pattern altitude with the idle power correction. I have had it activate at 8500' when I reduced power to idle to simulate an off field emergency landing, so I know I'm flying pretty close to where it kicks in.
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ScottOHare
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Re: Unable to reduce load

Post by ScottOHare »

I fly my DA40NG out of Driggs, Idaho, elevation 6200', have seen DAs as high as 8000' but have never had an issue on the ground or at pattern altitude with the idle power correction. I have had it activate at 8500' when I reduced power to idle to simulate an off field emergency landing, so I know I'm flying pretty close to where it kicks in.
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Re: Unable to reduce load

Post by chili4way »

There were no issues with the automated runup when my DA40NG was used for summertime flight demos at KASE/Aspen (7800 feet) with a DA of 11,000 feet. 10+ flights had no problems, and the runup was done on each flight as part of the demonstration. This is not to say the runup issue cannot happen, simply that it is very possible/probable to do the automated runup at high-altitude airports without issue.

Two things which may cause the runup to abort: a) power lever not fully back to idle, b) running the air conditioner (due to cabin pressure/external pressure imbalance or additional engine load via the supplemental alternator).

Diamond Tech Support has an unpublished (i.e. not in the AFM) alternate/backup manual runup procedure they make available to specific owner/operators based out of very high altitude airports with recurring runup issues.
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