Embarrassed but Alive

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Mdm0515
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Embarrassed but Alive

Post by Mdm0515 »

Hi, and I continue to love this forum and all that can be learned.

I’m VFR only at this point, while steadily progressing on instrument training.

A recent round trip X-country for about 3HR total flight time indicated favorable weather on my planned return to home station.

During return, while en route at 8500’ I picked up ATIS for home station about 35 NM out that advised 3500’ broken. I was stupidly caught on top. Favoring legal ‘no cloud’ time over the various alternatives, in response I found a hole and dove through. I do realize this was a rookie mistake and a great time to learn, but my question is more airplane oriented. I was flying a new-ish DA 40 NG.

On dive/descent from 8500’ to 2500’ to punch through my hole I pulled throttle way back, and momentarily to idle. Prop sounded stressed/unnatural and general engine whining ensued until I input a little throttle and engine sounded happier.

Did I damage my prop governor and how would I know?

Made it back safe to station with no apparent issues through remaining end of flight.

Thank you
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pietromarx
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Re: Embarrassed but Alive

Post by pietromarx »

There won't be any damage to your airplane. You can throttle back and change the prop pitch at will ... but always good to be gentle and deliberative. The age-old recommendation on engines is to keep the CHT changes to less than 50°F per minute to avoid "shock cooling," but people will argue back and forth on this and I am sure that the diesel crowd will have additional thoughts.

Regardless, it doesn't hurt to be gentle...

If I can be so bold as to say, the absolute best thing you can do is to get your IFR, to use it regularly, and go from there.
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Boatguy
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Re: Embarrassed but Alive

Post by Boatguy »

The prop pitch will be changed as you change the power, as described in the AFM. You may have just have heard the change in the prop pitch. You may also have experienced what I did which is described in this thread:

viewtopic.php?f=22&t=7307&p=79257&hilit ... wer#p79257

CFIDave pointed out that beginning at and above about 6,000', the Austro engine enforces a minimum power level to avoid the shock cooling that Peter describes. Taking the power lever to 0% won't take the engine to 0% power. Here is the graph from the Austro Engine manual.

It's good to take about 2secs to move from 0% to 100%, or say 90% to 0%.
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chili4way
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Re: Embarrassed but Alive

Post by chili4way »

pietromarx wrote: Tue Jun 23, 2020 12:29 am The age-old recommendation on engines is to keep the CHT changes to less than 50°F per minute to avoid "shock cooling," but people will argue back and forth on this and I am sure that the diesel crowd will have additional thoughts.
The Austro engines are liquid-cooled and shock cooling isn't really a concern as with an air-cooled engine. (There isn't even a CHT gauge in the cockpit.) It's also not as important to line up into the wind when doing your run-up on a hot day.

As Russ points out, understanding how the power lever - engine - propeller/governor systems work together in your DA40NG in general and at higher pressure altitudes is a good thing to do so you are not surprised by normal, and otherwise unexpected, operational behavior.

The DA40NG is a great plane! And you'll enjoy it even more with your instrument rating.
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pietromarx
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Re: Embarrassed but Alive

Post by pietromarx »

chili4way wrote: Tue Jun 23, 2020 2:02 am It's also not as important to line up into the wind when doing your run-up on a hot day.
Only someone who hasn't flown a tail-dragger would say this ... :)

If you'll forgive me for being overly cautious, always keep an eye on the wind, keep the ailerons from getting lifted, and turn the airplane into the wind for run-ups. You'll thank yourself should you ever find yourself in an airplane that is less forgiving, on an icy taxiway, etc.

Same with keeping an eye on shock-cooling. While the NG may be forgiving (because it overrides your input to keep the engine from getting cooled too quickly), you may find yourself one day in an airplane that doesn't have these systems.

(I can just hear my inner flight instructor ...)
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Re: Embarrassed but Alive

Post by CFIDave »

Just to be clear, the reason the Austro engine of your NG will not let you reduce power back to idle (despite throttle lever position) at higher altitudes has nothing to do with shock cooling -- it's to prevent "combustion extinction."

Since diesel engines don't use spark plugs, they depend on very high compression ratios and cylinder heat to ignite the JetA fuel-air mixture. However, this becomes an issue at high altitudes with thinner air where even the Austro's high 18-to-1 compression ratio (combined with colder termperatures) may be insufficient to ensure ignition of the mixture. So the engine at high altitude is dependent on the turbocharger to pump in compressed air, and the turbocharger (in turn) is dependent on at least a minimal amount of exhaust gas pressure (roughly proportional to power level) to keep the turbo operational.

So at higher altitudes the Austro ECUs are programmed to maintain at least a minimal amount of power production despite the pilot pulling the power lever to idle -- to keep the turbo spooled up and compressing the air, and thus allowing continued diesel ignition of the fuel/air mixture.
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Colin
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Re: Embarrassed but Alive

Post by Colin »

You were probably in enough of a dive to have the propeller driving the engine. In the POH there is no prohibition against this. And the way to start a shutdown engine on the DA42 is to "airstart" it by having the propeller unfeather and drive the engine RPMs up enough to start.

DAI seemed to figure out this was a bad idea. The gearboxes were making metal, the clutches were needing service sooner... so now you are not meant to do that. I look back on the six times I did it in my plane with regret, but the gearbox has been changed since then so I should be okay.

I don't know how 3,500 broken is going to trap you on top. That means there are holes, right?

Before I had my IFR ticket, the few times I did what you are describing I had the plane on autopilot. That's a *really* good precaution for a VFR pilot because if you miss the hole you were aiming for an wind up inside a cloud you are a lot less likely to jerk the controls around while disoriented.
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Soareyes
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Re: Embarrassed but Alive

Post by Soareyes »

Mdm0515 wrote: Mon Jun 22, 2020 8:54 pm
On dive/descent from 8500’ to 2500’ to punch through my hole I pulled throttle way back, and momentarily to idle. Prop sounded stressed/unnatural and general engine whining ensued until I input a little throttle and engine sounded happier.
Another question about allowing the prop to drive the engine. On avgas planes it is OK to use the prop as a speed brake, for instance when high and fast on final you may want to pull the throttle all the way back and use drag from the prop to slow down and go down.

Two things make me think this might not be a good idea with an Austro engine. First is the proscription on using a windmilling prop for an air start. The other is something I read during my DA42 multi-engine training regarding emergency descents and not allowing the prop to drive the engine, "If you hear a whirring sound give it some throttle until the sound goes away."

They just pushed off my DA42 delivery for another 6 weeks but when I finally get it I want to take good care of the engines. Can I safely use prop drag as one of the tools in the kit to manage descent profile?
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Colin
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Re: Embarrassed but Alive

Post by Colin »

I do not. The only time I use prop drag is on short final if I have made a miscalculation or been slam-dunked by the controllers. Then it is a poor man's speed brake.
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Re: Embarrassed but Alive

Post by CFIDave »

Soareyes wrote: Tue Jun 23, 2020 5:11 pm Can I safely use prop drag as one of the tools in the kit to manage descent profile?
I'm not aware of any prohibition on this in any of the Diamond documents, although I find dropping the gear acts as a better speed brake.

One of the nice things about the DA42-VI and DA62 (compared to the earlier DA42 TDI and NG models) is that the wider "scimitar" prop blades create a lot more drag when the power lever is moved to idle. So much so that when landing, if you quickly move both power levers to idle it's like throwing out a "drag 'chute" that slows you down in a hurry -- just one more tool in your toolbox to use if needed.
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