Odd VSI behavior at higher altitudes

[Nathan Young]

I have a PA28-180. Whenever the winds allow, I like to fly at high
altitudes. On my last few flights, I have noticed that the VSI tends
to show a slight descent when I am at high altitude. The altimeter
will stay constant/steady (as does my Mode C readout). The VSI does
not exhibit this behavior at lower altitudes.

So for example: At 3-5kft, the VSI seems to be dead on. At 10k and
above, the VSI shows a 30-50fpm descent to maintain level flight.

Thoughts?

Thanks,
Nathan

[Michael]

Most likely, it's really a temperature issue. At lower temperatures,
something gets less pliable and doesn't seal quite right, and you're
getting a slight case leak. The symptom ( a slight indicated descent
in level flight) is consistent with a case leak.

Michael

[Paul kgyy]

In level flight, there's no pressure differential in the VSI, so I
don't see how a leak could matter. Doesn't the case pressure in level
flight always match the pressure in the static line?

[Frank Ch. Eigler]

"Paul kgyy" > writes:

> In level flight, there's no pressure differential in the VSI, so I
> don't see how a leak could matter. Doesn't the case pressure in level
> flight always match the pressure in the static line?

Well, if the leak is toward the cabin, then that is a source of
slightly lower pressure. Still, gazing at a diagram of how a VSI
works (http://www.ae.su.oz.au/aero/instruments/VSI.html) it seems to
me that a leak between the case and the cabin would result in a
*climb* indication if anything. But maybe that diagram is not
general.

Perhaps the explanation is something like this. When at high
altitude, the pressure inside and outside the canister are both
relatively low. Subtracting two such small pressures may have
exaggerate mechanical limitations/errors (sort of similarly how
adding/subtracting two large floating point numbers magnifies relative
errors).

- FChE

[Michael]

> In level flight, there's no pressure differential in the VSI, so I
> don't see how a leak could matter.

Actually, that's not right. The pressure inside the fuselage is
generally not equal to the outside static pressure. In general, since
there are openings, the pressure inside the fuselage is actually lower
than static pressure, due to venturi effect. That's why IFR airplanes
(usually) have static ports, rather than venting into the cabin, and
why the use of an emergency static source (which vents into the cabin)
causes altimeter and airspeed errors.

In a descent, the pressure in the static system is higher than the
pressure in the VSI case, so there is a continuous flow of air from the
static system into the VSI. The VSI reads this flow of air as a
descent.

In level flight, the pressure in the VSI case is the same as the
pressure in the static system (it has equilibrated) and thus a little
higher than the pressure outside the VSI case. If there is a case
leak, the air leaks out of the case. This means replacement air comes
in from the static system - just like in a descent - and the VSI
indicates a descent.

Michael