ECU 'gotcha' in my DA62

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TimS
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Re: ECU 'gotcha' in my DA62

Post by TimS »

jast wrote: Tue Apr 20, 2021 5:21 am I‘d assume that the official performance charts are real values flown with this effect. Also keep in mind that the air is significantly thinner, so 13% in power does not result in the same thrust generated as at MSL.
Depends on how the percentage is calculated and also on how the prop governor is controlled by the FADEC.
Assuming the percentage is calculated based on fuel flow (which it should be), and this being a diesel engine, by definition 13% means the engines are producing 23.4HP per side. This is a fair amount of power, 46.8 HP total.

If the FADEC allows for a very fine pitch at the higher power settings, the engine will spin faster but waste a lot in prop tip drag. If however, the FADEC sets the pitch to something more course in order to maximize thrust (which would be the normal expectation), then you will be producing enough thrust to allow an old J3 Cub to take off (slight exaggeration to drive home the point).

Overall, I think this has the potential to put a lot of wear and tear on brakes, and may mean the AE-3XX series of engines has a design weakness which the owners and pilots should be aware and should consider if landing at a high altitude airport, especially the twins with SEO considerations. Could make for a very interesting landing.

It still would not stop me from buying one if everything else lined up, but it definitely is a weakness in the design. Not sure what/how to fix the issue without adding lots of complexity for very limited return.

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VickersPilot
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Re: ECU 'gotcha' in my DA62

Post by VickersPilot »

The above observations are certainly interesting but it should be noted the engine mapping that applies to high altitude operations, likely only applies to 1% of owners and to those owners, perhaps only applies to 1% of flights... so statistical significance would suggest the design compromise makes sense (in my view).

May I add how fantastic it is that the Austro Engines do what it takes to keep themselves running, unlike Lyco/Conti where power is never mathematically optimised and lean or rich cut scenarios are very real.
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photoSteveZ
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Re: ECU 'gotcha' in my DA62

Post by photoSteveZ »

jast wrote: Tue Apr 20, 2021 5:21 am Good discussion and important to know! I learned about this effect at my DA62 factory training in Austria. They made it sound as it is important to know, but practically speaking not a big issue.

I‘d assume that the official performance charts are real values flown with this effect. Also keep in mind that the air is significantly thinner, so 13% in power does not result in the same thrust generated as at MSL.
Diamond is (partially) right: "practically speaking [it's] not a big issue." Except at KLXV (9934' MSL) and KTEX (9070' MSL), where it is a big issue: landing MUCH longer than expected and wearing out the brakes taxiing around at 13% power. I was on a hair trigger to kill the engines should the brakes have faded or failed entirely.
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Re: ECU 'gotcha' in my DA62

Post by ScottOHare »

My DA40 NG is based at KDIJ - 6231' MSL and haven't had an issue with pulling power back at TPA of 7200'. I was surprised practicing an engine out, though, at 8500'.
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Re: ECU 'gotcha' in my DA62

Post by CFIDave »

Interesting photo to see Steve's DA62 parked with the blades on both engines feathered -- it looks like a turboprop! ;)

I wonder what would happen if Diamond tied the ECUs into one of the "weight on wheels" switches on the gear of DA42/DA62 twins (or airspeed dropped below a specific value on the DA40NG), so that the engines could be programmed to idle when the power levers are brought back and the plane is on the ground at airports like KLXV. The engines don't cool down as quickly on the ground compared to when the plane is flying, so would diesel combustion when taxiing still be assured?

Interestingly the glow plug controllers are designed so that glow plugs can't come on when the "weight on wheels" switches detect that the plane is airborne -- so there's no way to restart the engines in the air if they get too cold.
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Re: ECU 'gotcha' in my DA62

Post by Boatguy »

CFIDave wrote: Wed Apr 21, 2021 3:06 am I wonder what would happen if Diamond tied the ECUs into one of the "weight on wheels" switches on the gear of DA42/DA62 twins (or airspeed dropped below a specific value on the DA40NG), so that the engines could be programmed to idle when the power levers are brought back and the plane is on the ground at airports like KLXV.
Excellent suggestion.
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Re: ECU 'gotcha' in my DA62

Post by photoSteveZ »

jast wrote: Tue Apr 20, 2021 5:21 am
I‘d assume that the official performance charts are real values flown with this effect. Also keep in mind that the air is significantly thinner, so 13% in power does not result in the same thrust generated as at MSL.
This claim has been niggling at the back of my brain since I read it. Since the fadec system is programmed to hold a particular propeller rpm at a given power lever position, wouldn’t it just mean that at 13% load the prop pitch is adjusted to hold that rpm, regardless of air density? And wouldn’t thrust then also be independent of air density, at least to a first approximation (ignoring propeller efficiency differences at various pitch settings)?

Am I missing something?

[Update: I see TimS also addressed this question from a slightly different angle and came to a similar conclusion.]
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Re: ECU 'gotcha' in my DA62

Post by jast »

Hmm, you might be absolutely right that 13% power result in the same thrust at 9000ft as at MSL. But since the 13% is just a digital input into a control system (the ECU) it is not clear what this exactly means. Would be good to know though…
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Re: ECU 'gotcha' in my DA62

Post by photoSteveZ »

jast wrote: Wed Apr 21, 2021 6:52 am Hmm, you might be absolutely right that 13% power result in the same thrust at 9000ft as at MSL. But since the 13% is just a digital input into a control system (the ECU) it is not clear what this exactly means. Would be good to know though…
All I know for sure is that at 13% power the no-brakes taxi speed approaches 35 knots. :shock:
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Re: ECU 'gotcha' in my DA62

Post by Deanrobert »

I’ve had the same experience flying into TEX ( telluride) in da-42ng and our newly acquired da-62. This ecu functionality also does not allow you to do a normal run up in the pre- take off procedures as your engines are idling above normal idle rpms which precludes the run up buttons from engaging ...
Somewhat annoying , but I’d go back to TEX, knowing the handicap/ runway is plenty long
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