Alternate air related load drop

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ememic99
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Alternate air related load drop

Post by ememic99 »

When flying at higher altitudes (e.g. 14.000 ft or higher) I noticed significant drop in right engine load when alternate air is opened. This doesn't happen at lower altitudes (IIRC I haven't seen this at 12.000 ft) and it's always right engine.

Here are the pictures I took few days ago.

With alternate air closed:
AAoff.jpg
With alternative air opened:
AAon.jpg
No change in IAS/TAS so it doesn't seem to have real influence on power available, it looks more like FADEC/firmware issue. The engines are CD-155.

I haven't found any explanation for this and I would like to hear if anybody has experienced this or has an explanation.
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Re: Alternate air related load drop

Post by meowmeow »

ememic99 wrote:When flying at higher altitudes (e.g. 14.000 ft or higher) I noticed significant drop in right engine load when alternate air is opened. This doesn't happen at lower altitudes (IIRC I haven't seen this at 12.000 ft) and it's always right engine.

Here are the pictures I took few days ago.

With alternate air closed:
AAoff.jpg
With alternative air opened:
AAon.jpg
No change in IAS/TAS so it doesn't seem to have real influence on power available, it looks more like FADEC/firmware issue. The engines are CD-155.

I haven't found any explanation for this and I would like to hear if anybody has experienced this or has an explanation.
Same effect when you switch to ECU B on that engine? (Also, just out of curiosity, did you turn on the alt air for experimental reasons or because you had to?)


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ememic99
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Re: Alternate air related load drop

Post by ememic99 »

First time when I noticed that, I was in severe icing in descend, so you can imagine the thoughts in my head :)

Later on I confirmed that experimenting at different altitudes.

Why would ECU-B make any difference when everything is ok on engine test?
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Re: Alternate air related load drop

Post by meowmeow »

ememic99 wrote:First time when I noticed that, I was in severe icing in descend, so you can imagine the thoughts in my head :)

Later on I confirmed that experimenting at different altitudes.

Why would ECU-B make any difference when everything is ok on engine test?
Not sure, just thinking if perhaps a difference in software version on the ECUs could be causing it - just speculation. I guess looking at the ECU data makes sense.
Since you flew through icing, perhaps a difference in how much TKS gets to the prop and wing parts on the affected side vs the other side could play a role.
(All that said - this is just brain storming; I'm not a system specialist :)


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Re: Alternate air related load drop

Post by ememic99 »

meowmeow wrote: Mon Apr 20, 2020 10:08 pm Not sure, just thinking if perhaps a difference in software version on the ECUs could be causing it - just speculation. I guess looking at the ECU data makes sense.
Since you flew through icing, perhaps a difference in how much TKS gets to the prop and wing parts on the affected side vs the other side could play a role.
Both engines feature same ECU SW version and the behaviour has been confirmed many times in smooth and dry air. And no, the amount of TKS can't play any role :)
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Re: Alternate air related load drop

Post by meowmeow »

ememic99 wrote:
meowmeow wrote: Mon Apr 20, 2020 10:08 pm Not sure, just thinking if perhaps a difference in software version on the ECUs could be causing it - just speculation. I guess looking at the ECU data makes sense.
Since you flew through icing, perhaps a difference in how much TKS gets to the prop and wing parts on the affected side vs the other side could play a role.
Both engines feature same ECU SW version and the behaviour has been confirmed many times in smooth and dry air. And no, the amount of TKS can't play any role :)
Not sure why not - if e.g. less TKS reaches one side due to a clogged pipe it could to a different rate of de-icing on prop blades and leading edge, leading to different load. Just saying - maybe a long shot, just trying to pitch in ideas.


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Re: Alternate air related load drop

Post by ememic99 »

Thanks but it’s not how it works. ECU data analysis makes sense.
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Re: Alternate air related load drop

Post by Boatguy »

I think there are two separate questions. Why does the alternate air effect a difference in the two engines? Why does a reduction in power in one engine not effect a change in IAS/TAS?

The first question could be a simple mechanical issue which may be possible to observe on the ground, though requiring removal of the cowling.

The second question of why the engine reports less power, but there is no change in IAS/TAS can be tested. Without the alternate air enabled, if you pull the power back to 68% on only the right side, is there any change in IAS/TAS? It's a pretty small change from 76% > 68%. In my single it wouldn't have a huge effect. In a twin, changing only one side by 8% may not have much effect on IAS/TAS. It may also take 3-5 min to see the change. To my simple mind there should be some yaw effect. Can you observe any change in yaw? In any case, it's easy to test the power change effect on IAS/TAS independent of alternate air.
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Re: Alternate air related load drop

Post by ememic99 »

Boatguy wrote: Tue Apr 21, 2020 3:32 am The second question of why the engine reports less power, but there is no change in IAS/TAS can be tested. Without the alternate air enabled, if you pull the power back to 68% on only the right side, is there any change in IAS/TAS? It's a pretty small change from 76% > 68%. In my single it wouldn't have a huge effect. In a twin, changing only one side by 8% may not have much effect on IAS/TAS. It may also take 3-5 min to see the change. To my simple mind there should be some yaw effect. Can you observe any change in yaw? In any case, it's easy to test the power change effect on IAS/TAS independent of alternate air.
If I pull power back from 75% to 68% on the right side only, RPM will drop and instant yaw and IAS decrease (2-3 kts) will be experienced. In my case (opening alternate air) RPM remain constant, while reported engine load and fuel flop drop.
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Re: Alternate air related load drop

Post by chili4way »

My guess is that there is a difference in ALT AIR intake air availability at high altitudes & higher power settings between the left and right engines. This is when air availability becomes the limiting factor to power production. Since the engines are oriented the same way in both nacelles, the ALT AIR intakes are asymmetric relative to the airframe. Airflow differences between the inboard and outboard side of the nacelles may create differences in interior air pressure that limits available air at the ALT AIR box. This happens at high altitudes and higher power settings because the turbocharger is ‘maxed-out' and cannot make up this difference.

The power lever experiment doesn’t produce the same results because the ECU controls the propeller pitch based on the power setting (and apparently, not the power produced) – ref page 7-29 in the DA42-TDI AFM. Pulling the power back changes both power and propeller pitch resulting in a larger thrust asymmetry.

One way to test this hypothesis is to open the ALT AIR at altitude and symmetrically adjust power settings to best available. Then close the ALT AIR and see if any asymmetry develops. It probably will not. Next, symmetrically increase power above this level (e.g. to max available) and open ALT AIR to see if power asymmetry develops. It probably will.
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