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Strange instrument reading question


lanmancz
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Hello,

I just noticed a strange reading on my VSI and altimeter. I was in a nice and gentle 30 deg left turn, trying to keep the speed and altitude stable when I noticed that my VSI showed +10 m/s climb but my indicated altimeter altitude was actually going down. The weather was nice, very little wind, 26 deg temperature, no clouds. I was at about 7000m at the time at ~0.8M. Any idea what might be causing that please ? The first thing that comes to mind is "my pitot froze" but it was a nice summer day with almost no clouds, can that be the cause ?


Edited by lanmancz

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I'd say pitot freezing. I've seen that happen in other aircraft in DCS under less than realistic circumstances. If you still have the track you can try switching pitot to the backup one.

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Happened to me again. In a 45 deg turn at altitude about 6000m this time. VSI was reading +-0 m/s (no significant climb or descent) but altimeter was going down. Pitot heat was turned on as I was climbing to altitude well before it happened. Switching to second source solved it and altimeter climbed up to the proper reading. It is very strange though.


Edited by lanmancz

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So if I understand you correctly you turned the heating on a long time before it happened? The manual says it can take up to 2 mins for the heating system to work.

 

I dunno if what you saw is intended; some dev feedback would be nice perhaps.

 

EDIT: You weren't transonic, were you?


Edited by Corrigan

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Next Time you can change the Pitot static source. And don't forget Pitot Heat.

But when you takeoff with 26° and go up to 7000m there's a good chance for icing (Dewpoint would be interesting).

 

Not sure that icing would occur in clear air conditions though, like it does in DCS.

Maybe if you descend from cold air into warm moist air, it could happen.

http://www.masterarms.se A Swedish Combat Flight Simulator Community.
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Well, happened to me several times as well, and i had the main pitot heater (CL74) engaged right from the start up!

Had to engage secondary pitot heater (CL75) and after a while i had again good readings on altimeter and IAS, might be they are mixed up, have to check next time it happens.

Luckily the AOA is still working properly when your IAS goes down. So if you see your IAS drop but your AOA is still good, probably pitot frozen ...

 

Edit:

Perhaps the confusion stems from the pitot tube selector lever. When you start the plane, is the lever on main or on auxilary? To be honest, it is not quite clear what is what. I take it that when the upper tip points to the left, it is main pitot. That is also the startup position. Is that correct?


Edited by splashman
Pitot selector switch argument

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This is possibly a correctly modeled instrument error in the VSI that is caused by inertia/G force. When level, old VSIs (IVSIs) indicate a climb when positive G force is applied and a descent when negative G is applied. The use inertia of a weight in the instrument to provide a more instantaneous indication of climb or descent. Otherwise the instrument has to wait for a pressure change which takes some time and is therefore quite laggy. The downside is that they will indicate incorrectly when you apply G.

 

This would explain the examples above. In both cases you are descending with G. The G counters the descent rate on the VSI to indicate level or even a climb.

 

You can test this by flying very low and level (so you know your altitude is consistent), pull some G and note what the VSI says.


Edited by Kaiza
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True of course, but I'm not seeing that implementation in DCS.

 

As for now it seems that your pitot probe will freeze in the clear and dry air at 25 000 feet.

I'll buy it if you are flying into clouds.

 

(Please note that I haven't experienced this in the MiG with pitot heating on, so for me it's not a problem like in the Sabre where it always occurs.)

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After some more flying I noticed that it's happening even in level flight but interestingly enough around mach 1. Must be some instrument error while crossing the sound barrier.

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