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L35A Pitchs Up at 29000ft

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A friend of mine has the FSX L35 ver 2.6 on a Win8 system. When he climbs through 29000ft using the AP SPD mode, the L35 suddenly pitches up to max VS on the VS dial and the airspeed falls off towards a stall. He tried deleting and reinstalling the L35, and it happens both in the DVD and Steam sims.  Anyone else experience anything like this or have any suggestions what the problem might be?

Thx,

Al

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I just finished a flight and the same thing happened to me using FSX.

 

Gerry

Gerry,

What Op system are you using, and does this happen without any 'fix' for the SPD light bug in ver 2.6?

Thx.

Al

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I found the the Mach hold to be a bit aggressive when changing attitude in order to maintain the mach number.

Up to now, I have only seen excessive pitch down in a Mach hold climb which temporarily resulted in a descent, but not the other way round.

 

Maybe the auto pilot needs some more fixing so that it won't descend during a climb or vice versa when trying to chase the assigned IAS or mach.

Because I found the mach hold to be generally a bit unstable with regards to attitude, I usually do a V/S climb after FL290.

 

Depending on how you did your SPD light fix (I assume you did it the obvious way), it should not have anything to do with that at all.

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Hi Dave,

My friend experiences the pitch up problem every time he climbs in SPD mode, both in FSX DVD and Steam. This happens with and without any fix for the IAS/MACH - VS mode light problem.

I haven't seen this pitch up problem yet despite trying climbs in SPD mode at different speeds and power settings, thus different VS rates. I do sometimes see the FD 'jump' up a tiny bit at the 29000ft boundary, but it immediately returns to where it was. I run Win7 and my friend Win 8 so was wondering if that might be a factor although I don't see why that would be.

 

Al

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Hi Al,

 

I'm using Windows 7.  I did do the "fix" for 2.6, but I believe this sudden pitch up happened once before the fix.  I usually use spd hold during climb.  I haven't tried v/s through 29,000 but will do so next flight.

 

Regards,

 

Gerry

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I had overall smooth transitions from SPD to MACH during beta, however the aggressive pitchup occured one or two times (out of 20-30 flights). I was unable to determine the cause of them, so it's not related to the OS or installation, it must be something within the sim.

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From what I can discern by looking at the xml code is that when SPD mode is engaged, the current VS, IAS and Mach# are recorded -- these are the AP reference values, When crossing 29000ft the current applicable speed value, depending if you are climbing or descending, is compared to the reference IAS or Mach#  and an offset VS value is added or subtracted to/from the reference VS value. So for example, if the airspeed when crossing 29000ft is high, increasing the required VS will cause the aircraft to pitch up a bit (and so the FD to move up a bit) until the airspeed (and eventually the FD)  return towards the reference value. With constant power, pitch is used to control airspeed. It is this feedback mechanism that seems to be failing for those who are experiencing extreme pitch/FD changes.

 

Al

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If your theory is correct, why do you suppose that the feedback mechanism is failing for some and not others as we are all using the same code?

 

Gerry

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If your theory is correct, why do you suppose that the feedback mechanism is failing for some and not others as we are all using the same code?

 

Gerry

Wish the heck I knew the answer to that! If may have something to so with the operating system. So far the only person reporting that the pitch up happens frequently (or every time) is running Win8. It others are using Win8 and not having this problem, it would be good to know.  It could be some kind of timing issue, although FSX XML files typically are executed 18 times/sec.

 

From the AP code I looked at (and I am very much an amateur at this), at the 29000ft boundary for every 1 knot of airspeed, or 0.001 of Mach#, that the current airspeed differs from the AP reference values set when SPD was activated, 400ft/sec of VS is added or subtracted from the reference VS to force a return to the reference airspeed. If a much larger VS delta was erroneously being added or subtracted, maybe that would account for the observed behavior. But the fact that the pitch up (haven't heard of a pitch down issue) only happens occasionally for some is what makes me think of timing.

 

But this is all very much speculation.

 

Al

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I had overall smooth transitions from SPD to MACH during beta, however the aggressive pitchup occured one or two times (out of 20-30 flights). I was unable to determine the cause of them, so it's not related to the OS or installation, it must be something within the sim.

Matthias,

Check my logic (I use the term loosely) here.

 

If you set SPD mode at low altitude, say at 1500ft, with an IAS of 200kts, and then climb through 29000ft, at the 29000ft boundary your true airspeed has changed quite a bit although your IAS is still 200kts. Likewise, your Mach# has changed quite a bit from the time you set SPD hold. So when the AP compares the original Mach# to the Mach# at 29000ft, there will be a significant difference and so the a/c will pitch up to make the necessary correction. However, if the AP were to update the Mach# value as you approached 29000ft, it would not see much difference at 29000ft and so less correction would be required which might make the 29000ft transition smoother in general.  Same idea holds regarding updating IAS approaching 29000ft when descending although the altitude change would usually be less between when you set SPD hold and reach 29000ft, so less change in IAS.

 

So if you haven't tried it, to reproduce the aggressive behavior you saw just a few times you might try setting the SPD hold at low altitude and see what happens. However, all this does not explain why some see the aggressive behavior and some just see the FD pop up a bit as a speed correction takes place at 29000ft.

 

Al

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I will do some testing tomorrow and see what i can do. From previous beta testing the pitch would just adjust a small amount to hold the mach speed as it should. So i was thinking it could be weather as i test with no wind. But the code i wrote is designed to hold the recorded speed at the time it is turned on or passing through 29K. 

 

If i can't fix this issue the only other option is to go back to holding IAS only. 

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From previous beta testing the pitch would just adjust a small amount .........

 

Yes, this is what I usually see when climbing through 29000ft (the FD pops up a bit and then settles back somewhat), but as I understand it, what the a/c is doing here is adjusting the speed to match what the Mach# was when SPD hold was selected which could have been, for example, down low at 2000ft, etc. The a/c TAS has been increasing throughout the climb as IAS is held fixed, so it would seem to maintain a smooth transition when climbing through 29000 ft you would want to hold the current Mach#, not return to what the Mach# was at 2000ft.  Likewise, on descent, you would want to hold the current IAS as you pass through 29000ft for the rest of the descent. Or so it seems to me. I suppose the question is what is the intent when climbing through 29000ft -- to return to the initial 2000ft Mach# or to maintain the current Mach# for the rest of the climb? Perhaps Gary Stewart could provide some real world insight.

 

The other issue of why some, but not others, often see a 'violent' pitch up and major drop in airspeed when climbing through 29000ft is very strange. When this happens it seems as it the AP feedback system (PID?) has failed. From discussions with my friend, the bigger the adjustment required when crossing 29000ft the more likely the violent pitch up, at least in his case.

 

Thanks very much for looking into all this -- much appreciated.  And thanks for the L35A, it has become the a/c I fly 95% of the time.

 

Al

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After testing passing through 29K 10 times with wind and no wind the pitch was smooth and the system works as it should. I never once had any issues. So for now all we can do is wait to see why just a few users have an issue. So far most users, me and the beta team are no having issues. 

 

I think if your computer is not updating fast enough then this could happen because it can't adjust for the new recorded pitch when changing to mach.

 

 

The SPD mode under 29K records the IAS at the time you engage it.

The SPD mode above 29K records the MACH at the time you engage it.

 

When your passing through 29K in a climb it's holding the IAS and then disengages the SPD mode for 2 seconds then re engages the mode. This records the new MACH and it now continues to hold the MACH.

 

If it pitches way too much then it's using the MACH speed from the time you engaged the IAS and did not update the new MACH speed. So again this would be a personal sim issue. 

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Very interesting, I understand better now how SPD Hold works.

Thanks for looking into the issue and the feedback.

Al

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