Say Altitude.

"William W. Plummer" wrote in message news:ctncc.82738
I was not using the GPS for altitude. Always use the altimeter
The altimeter was showing 7000.
ATC was showing 6700
Gps was 7060. However when I changed altitude the gps stayed frozen at
7060.

Aren't controllers supposed to tell you to turn off your altimeter and go to
VFR when an error that big is detected?

They can tell you to stop altitude squawk if they want. They have no business to
tell you to "go VFR"

I've never been told to turn off my altimeter!

Rich


William W. Plummer wrote:
"Hankal" wrote in message
...


Aren't controllers supposed to tell you to turn off your altimeter and go to
VFR when an error that big is detected?

Aren't controllers supposed to tell you to turn off your altimeter and go to
VFR when an error that big is detected?


The controller requested I turn off mode C.
In 25 miles he asked to turn back on.
Everything went fine from there on.
I could not have accepted VFR since I was in IMC condition.

William W. Plummer ) wrote:

Aren't controllers supposed to tell you to turn off your altimeter
and go to VFR when an error that big is detected?

Mode C is not always mandatory when flying IFR, at least in the US's
Northeast.

Last spring, I was returning from the AOPA Fly-In when I discovered I had a
mode C problem with my altimeter. The weather leaving Frederick, Maryland
(US) that day was low IFR, a really crappy day for a fly-in.

Upon calling for my clearance, I mentioned to the CD controller that I
suspected my mode C would not be functional. He had me wait while he
contacted others (perhaps the center controllers?). A moment later he gave
me the clearance and stated that I would be responsible for calling out my
altitude throughout the flight.

I then departed and as I was told, the departure controller had me call in
with my altitude every few thousand feet while climbing. From that point
on, I would receive a periodic "Cessna XXX, verify level at 9,000 feet"
call. I was impressed with the service.



--
Peter












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GPS is not nearly as accurate in the z dimension as in the horizontal
dimension. I often see mine (even with WAAS corrections) differ as
much as 3-400 feet from my altimeter.

On 05 Apr 2004 00:29:34 GMT, (Hankal) wrote:

Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?

Nathan,

GPS is not nearly as accurate in the z dimension as in the horizontal
dimension. I often see mine (even with WAAS corrections) differ as
much as 3-400 feet from my altimeter.

Reference for your altimeter is a pressure level (hence it's called altitude
and not height), reference for GPS height is an approximation of the earth
called the "geoid", usually in form of WGS-84. I'm not sure if WAAS takes
this into account. If not the difference is system immanent.

Regards

Kai Glaesner

You didn't specify if your GPS was connected to the altitude encoder or not.
If it's an IFR GPS, it'll be connected to the encoder, which only outputs the altitude
to the nearest 100 feet. Since you say it stayed at 7060 after minor altitude
changes, I'm guessing that it was connected... and the altimeter setting in the GPS
made the nice, round 100' increments "round" to 7060. Remember, the encoder only
outputs pressure altitude, which is then corrected to actual altitude on the ground
(or in an IFR GPS) by the equipment given the current altimeter setting. If it's
unusually differing temperatures where you are than where the altimeter is, the
plane's altimeter could be a bit different than the encoder's corrected output.

-Cory

Hankal wrote:
: Long CX IFR at 7000
: Controller comes on "say Altitude" My response "7000" He says you show 300
: feet lower and gives me the altimer reading. I confirm. He says "turn off you
: mod C"
: I comply.Look at my GPS which is at 7060.
: I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
: 7060.
: The controller comes back and says" I think this is due to a cold front in my
: flight area.
: 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
: C. He concurs my altitude at 7000.
: Request lower for decend for landing. The gps works fine now, ATC is happy.

: HAS ANYONE ELSE EVER EXPERINCED THIS?




--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

You didn't specify if your GPS was connected to the altitude encoder or not.

If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
altitude

My GPS is not connected.
Under most conditions it agrees with my
altimeter.
It was just strange that the GPS and ATC
did not not reflect the same as my altimeter.
Hank

OK... even still, though, the GPS isn't too accurate, and is based on absolute
altitude, not barometric altitude (mentioned previously). Also, the temperature
affects the altimeter... how high AGL were you? I read an interesting article (avweb,
I think) regarding temperature correction and obstacle clearance. When the altimeter
reading is down low and temps are cold, the higher you go the more error the altimeter
tells you. So, if the altimeter setting is down in the valley at 1000' MSL, but
you're shooting the approach with MEA at 10K', there can be issues with terrain
avoidance. Supposedly Canadian and AK folks get more indoctrination to this than
those in the lower '48.

-Cory

Hankal wrote:
:You didn't specify if your GPS was connected to the altitude encoder or not.
:
:If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
:altitude

: My GPS is not connected.
: Under most conditions it agrees with my
: altimeter.
: It was just strange that the GPS and ATC
: did not not reflect the same as my altimeter.
: Hank

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

It may well have been due to the cold temperatures. See the table at
this link;
http://www.faa.gov/ATpubs/AIM/Chap7/...ml#TBL%207-2-3

Your GPS may have had a correct altitude, and your barometric altitude
or mode C may have been reading a different altitude due to the cold air
temperature.

JPH

Hankal wrote:
Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?