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CirrusPilot

Saab 340 trouble with reverse thrust...

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Hello,

Magnificent aircraft, haven't had any problems running it until the last 2 flights. So when just touched down, and reverse thrust is toggled, after approx. 2 seconds of using said reverse thrust at max, the entire aircraft begins to shutdown... All power is lost to avionics, PFD, etc.. Displayed on the CWP is fuel, elec, hyd and aircon warnings. At this point in time, condition levers are at max, and I followed the digital checklist step by step. Anyone else having this problem? Or am I just a fool and i'm missing something completely obvious..?

Log.txt is attached.

Kind Regards,

CirrusPilot.

 

Log.txt

Edited by CirrusPilot
log.txt

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Did you reset the generators when you started the engines at your origin airport?  If you didn't reset them, you flew the whole trip on battery power, and they just died due to lack of power. 

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4 minutes ago, Goran_M said:

Did you reset the generators when you started the engines at your origin airport?  If you didn't reset them, you flew the whole trip on battery power, and they just died due to lack of power. 

Affirm, generators were reset, then left in the on position, and were untouched all flight.

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I should also mention that at the start of the flight, something failed, and scroll wheel decided to stop working. But this was simply fixed with a refresh of the aircraft, shown in the GizmoLog

GizmoLog.txt

Edited by CirrusPilot
added info

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Ok, when that happens, you also need to reload Gizmo.  That function is in the Gizmo tool tray on the side, with a lightning bolt.  However, best idea is to reload the whole sim.  Reloading the aircraft does not reload Gizmo.  I'm guessing that is likely the problem.  @Ben Russell or @JGregory might have more input into it.

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Ok, the scroll wheel problem only happened once, on today's flight, but the other problem has happened multiple times... I'm taking it on another flight right now, Ill see if it happens again. If so ill report back...

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When I've encountered this it's always been when I've been trying to slow down quickly after a forcing a landing in which I'm way overspeed. If I'm more gentle with going to reverse it doesn't occur; i.e. if I don't just immediately jam the power levels to full reverse it's fine.

I don't know if it's a bug of some kind, or if it's something like the sudden blade pitch change overloading the engines which then shut down to prevent damage.

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I've also been encountering this problem -- so I decided to investigate it in a bit more depth and record a video as well as data output, in the hope that this will shed some light.

The following video shows some trial runs:

https://youtu.be/80A6NaIvAVs

This is with X-Plane 11.21r2, Saab 340A v1.5, and all plugins except Gizmo64 were disabled by moving them to a different folder.

The video begins right after X-Plane has finished loading, to show the startup in case I'm doing something wrong. (I'm using the Auto-Start feature.)

In the trial runs, to keep it simple, I'm merely doing some high-speed taxis on the runway: Bring the engines to a high power setting (CTOT not used), accelerate to 80 knots, then bring the throttles to reverse and decelerate.

The video shows three runs, starting at the following times:

Run 1: 4:32
Run 2: 5:35
Run 3: 7:10

On the first two runs, I bring back the throttles very slowly and deliberately. I encounter no issues in these two runs. However, the throttle movement also seems excessively slow to me -- slower than I would expect on a rejected takeoff, for example. On Run 1, it takes 7 seconds from high power to reverse; on Run 2, 5 seconds.

On Run 3, I bring the throttles back more quickly. However, I wouldn't say I'm yanking them; it still takes 2 seconds from high power to reverse. On this run, the engines spool down shortly after the throttles reach the reverse range -- note turbine temperatures and RPMs.

Some observations:

  • There aren't any temperature or RPM exceedances.
  • At 7:35, shortly after the throttles reach the reverse range, the mixture on both engines goes to 0.0 -- see "mixt1" and "mixt2" data readout at the top left. I don't have any joystick axes configured for mixture controls, so something in the aircraft logic must be commanding the mixture to zero.
  • The mixture going to zero appears to be what causes the engines to spool down.
  • Even more interestingly, at 7:37, the mixture goes back to 1.0, but at this time, the engines have already flamed out.
  • Two seconds from high power to reverse doesn't seem excessively fast to me -- am I wrong? At least I'm surprised that it would cause a flameout. The fact that the mixture briefly goes to zero looks a bit suspicious to me -- maybe there's something in the airplane logic that could be fixed to prevent this?

I'm attaching my Log.txt, GizmoLog.txt, and a Data.txt that I captured during this run.

Data_340a.txt

GizmoLog_340a.txt

Log_340a.txt

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Here are the conditions where the engine mixture will be set to zero (0):

1. Ng (N1) Overspeed when Ng >= 110.0

2. Np Overspeed when Prop RPM > 1573

3. Fire Handle has been pulled

4. Condition Lever is placed in "FUEL OFF" position.

I was unable to see the prop rpm gauge in your video to determine if that is where the problem is.  I suggest you re-test knowingly the above outlined parameters.

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Thanks -- I think that makes it a lot clearer to me what may be going on.

I'm currently on the road, and it will be a couple of weeks before I can do some more tests, but I'm fairly confident that it's a propeller overspeed that's causing the mixture to be set to zero -- it's visible in the video that the Ng didn't overspeed, and I didn't pull the fire handle or set the condition lever to "FUEL OFF". Also, a propeller overspeed would be consistent with the fact that the mixture returns to 1 after a few seconds (after the propeller has slowed down again).

There are some things I still don't understand though:

  • As I understand, the Np overspeed protection shuts off fuel but also turns on ignition so that, once Np reduces below the overspeed limit and fuel is turned on again, the engine lights up again -- which doesn't however happen in the sim. My reference for this is page 4 of this document. Admittedly this is for a 340B, but shouldn't the A be broadly similar?
  • Why does the overspeed occur in the first place? I assume what is happening is that, as the propeller pitch goes to through the "zero thrust" setting, the gas generator is still producing a significant amount of output. Because there is not much resistance on the prop, this causes it to overspeed. But shouldn't there be some form of protection against this? I find it hard to believe that it should be possible to overspeed a modern turboprop so easily -- am I wrong? This article seems to indicate that the PT-6, at least, contains specific measures to prevent overspeed when going into the beta range (see the paragraph "The inherent problem with a reversing propeller system"). Does the CT7 simply lack such protection?

At any rate, I'm learning a lot about turboprops!

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10 hours ago, YawString said:

As I understand, the Np overspeed protection shuts off fuel but also turns on ignition so that, once Np reduces below the overspeed limit and fuel is turned on again, the engine lights up again -- which doesn't however happen in the sim. My reference for this is page 4 of this document. Admittedly this is for a 340B, but shouldn't the A be broadly similar?

Yes, the "A" model does the same, and this is modeled.  The fact that the engine will not re-start may be an X-Plane thing, but it will require some investigation to confirm.  This will take some time and may actually end up being done for v2.0. 

 

10 hours ago, YawString said:

Why does the overspeed occur in the first place? I assume what is happening is that, as the propeller pitch goes to through the "zero thrust" setting, the gas generator is still producing a significant amount of output. Because there is not much resistance on the prop, this causes it to overspeed. But shouldn't there be some form of protection against this? I find it hard to believe that it should be possible to overspeed a modern turboprop so easily -- am I wrong? This article seems to indicate that the PT-6, at least, contains specific measures to prevent overspeed when going into the beta range (see the paragraph "The inherent problem with a reversing propeller system"). Does the CT7 simply lack such protection?

Not sure why, but i can tell you that moving the power levers quickly into reverse is extremely unrealistic.  The engines have no time to spool down, and that could be part of the problem.  Normally, you would be at "Flight Idle", or slightly above, at touchdown (low engine speed), and then into Beta and Reverse.  I'm not sure that our model is wrong in that respect.  

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On 6/13/2018 at 5:17 PM, JGregory said:

Yes, the "A" model does the same, and this is modeled.  The fact that the engine will not re-start may be an X-Plane thing, but it will require some investigation to confirm.  This will take some time and may actually end up being done for v2.0. 

OK, thanks for looking into this!

On 6/13/2018 at 5:17 PM, JGregory said:

Not sure why, but i can tell you that moving the power levers quickly into reverse is extremely unrealistic.  The engines have no time to spool down, and that could be part of the problem.  Normally, you would be at "Flight Idle", or slightly above, at touchdown (low engine speed), and then into Beta and Reverse.  I'm not sure that our model is wrong in that respect.

All of that makes sense -- and I've seen videos where the pilot will "pause" at ground idle for a second or two before selecting reverse. Part of the problem is definitely that I'm using a CH Throttle Quadrant, and pulling the throttles back into the detent already makes the engines go into reverse -- which makes it harder to do this "pause" at ground idle. I'll see what I can do to alleviate this -- I'm thinking of creating a small FlyWithLua script that will play a click sound when the throttles go into the ground idle detent.

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