Pitot/Static/Transponder Problem

Building to the plans doesn't guarantee accurate static pressure
readings; the system is quite sensitive to tiny plane-to-plane
variations. And the official calibration check with the airplane
stationary wouldn't catch an in-flight static-pressure error.

Have an observer watch the altimeter during takeoff and landing ground
rolls: if it changes from a standstill to takeoff speed and then
changes in the opposite sense during the landing roll (the VSI would
also depart from zero at these times), then you'll know that there's a
static-pressure measurement error built into your installation. The
amount of the spurious altitude change would be about 200 ft times the
ratio of takeoff/landing speed to cruise speed; e.g., if the speed
ratio is about 1/3, then the change would be 60-70 ft.

The static port(s) wouldn't necessarily have to be relocated if this
error is present; you may be able to tweak the readings sufficiently
with a wire-and-tape "trip strip" just ahead of the port (to lower the
static reading) or behind it (to raise the reading). Even some store-
bought airplanes have such cheap-and-easy fixes near their ports.
Just keep experimenting with the height and position of the strip
until the airplane doesn't "climb" or "descend" while trundling along
a level runway.

On Sun, 05 Aug 2007 10:27:25 -0700, quietguy wrote:

Building to the plans doesn't guarantee accurate static pressure
readings; the system is quite sensitive to tiny plane-to-plane
variations. And the official calibration check with the airplane
stationary wouldn't catch an in-flight static-pressure error.

Yes, but: If the encoder and the altimeter are using the same static source,
both should be in error the same amount. Kyle's problem is a split indication;
the encoder is sensing a different altitude than the altimeter.

This tends to argue that the problem is a leak related to the altimeter itself,
not in the static system. Perhaps the altimeter has a leak that only manifests
itself when it's being vibrated (i.e., the engine running).

My Microair transponder gives me a readout of the altitude that's being sent to
ATC. I can set the A/C altimeter to 29.92 and compare. That's how I discovered
my encoder had gone south two months back; the altitude readout wasn't matching
the altimeter.

Ron Wanttaja

The encoder gives an uncorrected altitude. You altimeter is
corrected when you set it for barometric pressure at the beginning of
flight. ATC receives the signal from the encoder/transponder and
corrects it for the local pressure. If you are some distance from
their station, there could easily be a 0.2" Hg pressure differential
that would result in a 200' differential (one inch Hg per 1000' of
altitude).
So even if you are using the altimeter setting they give you, I
wouldn't expect perfect correlation if there's a pressure gradient
beteen you and them.

Dan

wrote in message
oups.com...

The encoder gives an uncorrected altitude. You altimeter is
corrected when you set it for barometric pressure at the beginning of
flight. ATC receives the signal from the encoder/transponder and
corrects it for the local pressure. If you are some distance from
their station, there could easily be a 0.2" Hg pressure differential
that would result in a 200' differential (one inch Hg per 1000' of
altitude).
So even if you are using the altimeter setting they give you, I
wouldn't expect perfect correlation if there's a pressure gradient
beteen you and them.

Dan

If it was an intermittent problem, your theory would be plausible, but since
I see the same problem virtually every time I deal with ATC, I'm thinking it
is a hardware problem. The only issue is which piece of hardware? ;-)

KB

Ron Wanttaja wrote:
On Sun, 05 Aug 2007 10:27:25 -0700, quietguy wrote:


Building to the plans doesn't guarantee accurate static pressure
readings; the system is quite sensitive to tiny plane-to-plane
variations. And the official calibration check with the airplane
stationary wouldn't catch an in-flight static-pressure error.


Yes, but: If the encoder and the altimeter are using the same static source,
both should be in error the same amount. Kyle's problem is a split indication;
the encoder is sensing a different altitude than the altimeter.

This tends to argue that the problem is a leak related to the altimeter itself,
not in the static system. Perhaps the altimeter has a leak that only manifests
itself when it's being vibrated (i.e., the engine running).

My Microair transponder gives me a readout of the altitude that's being sent to
ATC. I can set the A/C altimeter to 29.92 and compare. That's how I discovered
my encoder had gone south two months back; the altitude readout wasn't matching
the altimeter.

Ron Wanttaja

Are your transponder and altimeter physically close to each other?

(How far apart could they be in a Fly Baby??)

In larger planes it's not uncommon for the encoder to be mounted behind
the seats or such. Quite far from the altimeter - with a lot of extra
hose between them.

Just a thought...


Richard