More about altimeter calibration
flyer
Our Cessna 182S also mysteriously developed this condition, although
it is four years old now. It had been absolutely perfect until then.
Fearing the worst, I had the altimeter calibration checked through the
range of altitudes but it was only the zero adjustment which was
wrong, and that was readily rectified by the instrument technician.
Funny, our's was out by about 60 feet too. 60 feet is the required
altimeter tolerance for IFR here in Australia.
Definitely do not try to adjust it with the small screw next to the
Kollsman. I asked the same question as you and was told by the
technician that this only releases the adjustment mechanism, and you
will almost certainly make things much worse. If you turn it too far
and it disconnects internally, you will probably be unable to put it
back again. (and the aircraft will be immediately U/S).
Sounds like a very reasonable warranty item to me.
Andrew/.
I don't thing that it more than likely makes things worse, and it's
actually quite hard to f-up the altimeter unless you are a really bad
klutz.
It's not hard at all to reset the "zero adjustment" to the altimeter.
By that I mean getting it to read the field elevation when the
barometric pressure reported by the ATIS is set in the Kollsman
window.
First, set the altimeter to read exactly the field elevation. Compare
the barometric setting in the Kollsman window with what it should be.
If it is incorrect, here is how to make it correspond. When the
little screw next to the Kollsman adjustment knob is removed with a
small blade screwdriver and examined (you didn't drop and lose it, I
hope), you will see that it is a brass screw with an smalled diameter
unthreaded portion extending past the thread.
This is a custom screw that you will not find at Home Depot, i.e.
that's way you shouldn't drop it. Look inside the altimeter in the
hole where the screw was just removed and you will see a horizontally
oriented solid metal cylinder with a hole through it. The untreaded
extension of the brass screw came out of that hole. The only purpose
of the brass screw is to hold the cylinder from moving side to side by
capturing it through the hole. Using something like a slender awl, or
a small finishing nail, or a very small jeweler's screwdriver, insert
it through the hole in the horizontal cylinder of the altimeter and
move the cylinder to the side. I can't remember if it is toward or
away from the adjustment knob. It will only go one way. What this
has done is disengage the adjustment knob and barometric pressure
scale from the gears that turn the hands of the altimeter. You can
now rotate the Kollsman adjustment knob to change the barometric
pressure setting next to the arrow without moving the altimeter hands.
You will probably only have to move it a little to get the right
pressure set next to the arrow. If you have an error, I would suggest
only removing half of it since you are only doing this adjustment at
one altitude (on the ground). That might give the best compromise of
"correctness" for the altimeter on the ground and at altitude. Now
move the cylinder back into position so that the hole is centered in
the larger hole where the brass screw came out of. You sometimes have
to rotate the cylinder up and down to get the hole to line up. Rotate
the adjustment knob back and forth and make sure that the hands on the
altimeter once again move. Feed the brass screw into the threaded
hole in the altimeter, feeling to make sure the unthreaded extension
of the brass screw is going into the hole in the cylinder. Do not
force anything here, remember we are dealing with soft brass parts.
If it doesn't line up, move the cylinder a little bit one way or the
other and do a trial and error thing until it does. When you are sure
it lines up, tighten the brass screw down and Voila, you have now made
the Kollsman pressure window correspond to field elevation.
Caveat, doing this probably voids the pitot/static certification, as
if anyone really cares !!! That whole thing is a crock. I had to
replace an altimeter because the guy said it had too much friction.
He said he had to tap it too hard when changing altitudes with his
test box to get it to settle on the altitude. When my airplane is
flying, the whole instrument panel shakes! There is no way my
altimeter is going to stick in flight. So I managed to talk him out
of sellng me a new altimeter. He also found a minute pressure leak at
the connection of the pitot tube to the pitot line going to the
airspeed indicator using a soap bubble solution. But the airspeed
indicator was reading the perfectly correct speed during the test.
How is this ever going to be a problem in flight? You have an
infinite supply of air going into that tube.
The airspeed indicator reads the correct value. It corresponds to the
GS readout on my GPS when flying in calm winds.
What would a little leak matter? Also, he tried to fail the
transponder for low power output. He said it was marginal. What he
was really testing was the aircraft battery, which after he spent 2
hours dicking around with the pitot leak with the master switch on had
dropped to about 10.3 volts. I got a regulated DC power supply from
work, set it to about 14.4 volts and ran the aircraft off of it
instead of the aircraft battery. The transponder was now putting out
200 watts, just like it was supposed to.
The moral of the story, you should know a little physics and a little
about how stuff works before spending lots of extra money to pass the
pitot/static test.