Locked up VSI (Sim practice)

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Jason608

Pre-takeoff checklist
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Mar 2, 2015
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Jason608
This is on a Sim using Pilot Edge ATC to practice for the IR.
Basically, VSI shows descent while ALT is frozen.

Situation:
OAT 71F, ASI correct at 140kts.

ATC cleared me to 3,000' from 4,000' in IMC on an IFR approach.
I set the AP preselect to 3,000' and confirm a 500' decent via the VSI and go on to other tasks.
After 1 minute or so ATC asks what altitude I am at, which is around 3,950' (confirmed via FF).
The VSI displays a 650' decent and the ALT is frozen at 3,950'. Then the ALT starts to work again showing a decent but not immediately to a lower altitude. I turn off AP and confirm instruments are now working correctly.

If the Static port is blocked, the VSI and ALT will "freeze" but going through the books/training/etc I always saw the VSI returns to 0 then freezes.

I remember reading you can have ice blockage at 70 degrees.

First learning point: (1) Don't stop the instrument scan.

Am I right here or was it a sim issue?
 
Jason608 said:
I remember reading you can have ice blockage at 70 degrees.
Click to expand...

Yeah, in the carburetor. The pitot-static are not going to ice up unless it's at or below freezing.

A VSI stuck on something other than zero is also highly unlikely.
 
MauleSkinner said:
Did you try breaking the glass on the VSI?
Click to expand...

Didn't help.

image.jpg
 
dmspilot said:
Yeah, in the carburetor. The pitot-static are not going to ice up unless it's at or below freezing.

A VSI stuck on something other than zero is also highly unlikely.
Click to expand...

To do that, the orifice in the instrument itself would have to plug up somehow. I agree that seems really unlikely.

For a truly iced pitot-static, you have pitot heat and alternate static.
 
MauleSkinner said:
Did you try breaking the glass on the VSI?
Click to expand...
That was the first thing I thought of! :)
And then I thought: the sim people would be very sad to hear "the CFII told me to break it if it freezes". :D
I don't think they could just replace the LCD for a few hundred, they'd have to buy a brand new one-piece $6k "simulator", no? :)
 
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