Altimeter accuracy

[Jim Stewart]

How close should an altimeter be to field elevation
when set to the pressure indicated by the field's
AWOS? I'm seeing a 50' error.

[karl gruber[_1_]]

75'



"Jim Stewart" > wrote in message
...
> How close should an altimeter be to field elevation
> when set to the pressure indicated by the field's
> AWOS? I'm seeing a 50' error.

[Rip]

Jim Stewart wrote:
> How close should an altimeter be to field elevation
> when set to the pressure indicated by the field's
> AWOS? I'm seeing a 50' error.
+/- 75 feet for IFR

Rip

[Ron Rosenfeld]

On Fri, 04 May 2007 01:42:07 GMT, Rip > wrote:

>Jim Stewart wrote:
>> How close should an altimeter be to field elevation
>> when set to the pressure indicated by the field's
>> AWOS? I'm seeing a 50' error.
>+/- 75 feet for IFR
>
>Rip

It depends on your field elevation.

You should be able to check and see what the error was at the last
altimeter certification for IFR.

The tolerances, at the lower altitudes, are: (Appendix E 14 CFR 43)

0 ±20'
500 ±20'
1000 ±20'
1500 ±25'
2000 ±30'
3000 ±30
4000 ±35
6000 ±40
8000 ±60

For a VFR only a/c, I do not believe there is any particular requirement.
--ron

[Capt. Geoffrey Thorpe]

"Jim Stewart" > wrote in message
...
> How close should an altimeter be to field elevation
> when set to the pressure indicated by the field's
> AWOS? I'm seeing a 50' error.

What's the accuracy of the AWOS?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Jim Stewart]

Capt. Geoffrey Thorpe wrote:
> "Jim Stewart" > wrote in message
> ...
>> How close should an altimeter be to field elevation
>> when set to the pressure indicated by the field's
>> AWOS? I'm seeing a 50' error.
>
> What's the accuracy of the AWOS?

Glad you asked. I called some friends at Allweather
(which made the AWOS at my field) and they said the
units are factory calibrated to +/- 5 feet and that
FAA specifications are +/- 20 feet.



>
> --
> Geoff
> The Sea Hawk at Wow Way d0t Com
> remove spaces and make the obvious substitutions to reply by mail
> When immigration is outlawed, only outlaws will immigrate.
>
>

[Jim Carter[_1_]]

I think what you've cited is for bench testing during certification. Once
installed in the aircraft then comparison against the pressure as recorded
by the official weather observation is +/- 75'.

An interesting discussion point came up recently that exposed the difference
in teaching over the past 30 years. 30 years ago we were taught to set the
altimeter to the known field elevation and record the difference between the
official pressure and the indicated pressure, then apply that difference to
every setting you received along your route of flight.

Today they teach to set your altimeter to the official pressure and that's
it.

So my question becomes, when executing a precision approach to a minimum DH
of 200' and then executing the missed procedure, the aircraft is allowed to
descend slightly below the DH as things spool up. If you are already 75'
lower than you think because of altimeter error, and you descend only 20'
more (one gradient on the altimeter) aren't you really only 105' off the
deck?


--
Jim Carter
Rogers, Arkansas
"Ron Rosenfeld" > wrote in message
...
> On Fri, 04 May 2007 01:42:07 GMT, Rip > wrote:
>
>>Jim Stewart wrote:
>>> How close should an altimeter be to field elevation
>>> when set to the pressure indicated by the field's
>>> AWOS? I'm seeing a 50' error.
>>+/- 75 feet for IFR
>>
>>Rip
>
> It depends on your field elevation.
>
> You should be able to check and see what the error was at the last
> altimeter certification for IFR.
>
> The tolerances, at the lower altitudes, are: (Appendix E 14 CFR 43)
>
> 0 ±20'
> 500 ±20'
> 1000 ±20'
> 1500 ±25'
> 2000 ±30'
> 3000 ±30
> 4000 ±35
> 6000 ±40
> 8000 ±60
>
> For a VFR only a/c, I do not believe there is any particular requirement.
> --ron
>

[Ron Rosenfeld]

On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter" >
wrote:

>I think what you've cited is for bench testing during certification. Once
>installed in the aircraft then comparison against the pressure as recorded
>by the official weather observation is +/- 75'.
>
>An interesting discussion point came up recently that exposed the difference
>in teaching over the past 30 years. 30 years ago we were taught to set the
>altimeter to the known field elevation and record the difference between the
>official pressure and the indicated pressure, then apply that difference to
>every setting you received along your route of flight.
>
>Today they teach to set your altimeter to the official pressure and that's
>it.
>
>So my question becomes, when executing a precision approach to a minimum DH
>of 200' and then executing the missed procedure, the aircraft is allowed to
>descend slightly below the DH as things spool up. If you are already 75'
>lower than you think because of altimeter error, and you descend only 20'
>more (one gradient on the altimeter) aren't you really only 105' off the
>deck?

I did cite the bench testing numbers, as they are the ones of which I am
aware.

I've heard of the 75' "allowance" for a field measurement, but I've not
seen a regulatory justification for that value. As someone who flies
instrument approaches to minimums, I, personally, would not be happy with a
75' error, and would have the system rechecked.

So far as you being only 105' off the deck in your hypothetical instance,
that is obviously the case.

By the way, for Category II operations, which may have a DH as low as
100'AGL, it is a requirement to have readily available the altimeter
calibration information from the last certification, (and to apply it
appropriately).
--ron

[Jim Carter[_1_]]

Isn't a radar altimeter required equipment for CAT II and CAT III work? We
were trying to get an aircraft and aircrew certified for CAT II in Seattle
many years ago and I thought that was the reason they had the RA installed.

--
Jim Carter
Rogers, Arkansas
"Ron Rosenfeld" > wrote in message
...
> On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter" >
> wrote:
>
>>I think what you've cited is for bench testing during certification. Once
>>installed in the aircraft then comparison against the pressure as recorded
>>by the official weather observation is +/- 75'.
>>
>>An interesting discussion point came up recently that exposed the
>>difference
>>in teaching over the past 30 years. 30 years ago we were taught to set the
>>altimeter to the known field elevation and record the difference between
>>the
>>official pressure and the indicated pressure, then apply that difference
>>to
>>every setting you received along your route of flight.
>>
>>Today they teach to set your altimeter to the official pressure and that's
>>it.
>>
>>So my question becomes, when executing a precision approach to a minimum
>>DH
>>of 200' and then executing the missed procedure, the aircraft is allowed
>>to
>>descend slightly below the DH as things spool up. If you are already 75'
>>lower than you think because of altimeter error, and you descend only 20'
>>more (one gradient on the altimeter) aren't you really only 105' off the
>>deck?
>
> I did cite the bench testing numbers, as they are the ones of which I am
> aware.
>
> I've heard of the 75' "allowance" for a field measurement, but I've not
> seen a regulatory justification for that value. As someone who flies
> instrument approaches to minimums, I, personally, would not be happy with
> a
> 75' error, and would have the system rechecked.
>
> So far as you being only 105' off the deck in your hypothetical instance,
> that is obviously the case.
>
> By the way, for Category II operations, which may have a DH as low as
> 100'AGL, it is a requirement to have readily available the altimeter
> calibration information from the last certification, (and to apply it
> appropriately).
> --ron

[dave]

I read somewhere on the web that your A&P can adjust your altimeter by
removing a pin or a screw that allows the barometric pressure to be set
independently from the altitude. Once it's set correctly, the pin or
screw is replaced.
Dave


Jim Stewart wrote:
> How close should an altimeter be to field elevation
> when set to the pressure indicated by the field's
> AWOS? I'm seeing a 50' error.

[Ron Rosenfeld]

On Tue, 08 May 2007 03:38:30 GMT, "Jim Carter" >
wrote:

>Isn't a radar altimeter required equipment for CAT II and CAT III work? We
>were trying to get an aircraft and aircrew certified for CAT II in Seattle
>many years ago and I thought that was the reason they had the RA installed.

It is definitely not required for Category A aircraft flying under Part 91
for CAT II. I don't know about other categories.

There was a period of time when I and my Mooney were CAT II qualified and
certified.

The inner marker can substitute for a radio altimeter for 100' DH. The RA
is not necessary at all for the 150 DH. The altimeter and static system
has to have had the IFR check within the past 12 months; and altimeter
correction data must be available to the pilot, including both the scale
error and, wheel height correction if the wheel to instrument height is
greater than 10 feet.

There are a very few CAT II approaches that require a RA for use of the
100' DH, because of absent or siting problems with the IM.

I don't know of any guidance for Category A Part 91 a/c under CAT III, or
if any authorizations have ever been issued for that.
--ron

[Al G[_2_]]

"Ron Rosenfeld" > wrote in message
...
> On Tue, 08 May 2007 03:38:30 GMT, "Jim Carter" >
> wrote:
>
>>Isn't a radar altimeter required equipment for CAT II and CAT III work? We
>>were trying to get an aircraft and aircrew certified for CAT II in Seattle
>>many years ago and I thought that was the reason they had the RA
>>installed.
>
> It is definitely not required for Category A aircraft flying under Part 91
> for CAT II. I don't know about other categories.
>
> There was a period of time when I and my Mooney were CAT II qualified and
> certified.
>
> The inner marker can substitute for a radio altimeter for 100' DH. The RA
> is not necessary at all for the 150 DH. The altimeter and static system
> has to have had the IFR check within the past 12 months; and altimeter
> correction data must be available to the pilot, including both the scale
> error and, wheel height correction if the wheel to instrument height is
> greater than 10 feet.
>
> There are a very few CAT II approaches that require a RA for use of the
> 100' DH, because of absent or siting problems with the IM.
>
> I don't know of any guidance for Category A Part 91 a/c under CAT III, or
> if any authorizations have ever been issued for that.
> --ron

Ron is correct. We once had a 182 and pilot approved for the 150' DH for
fog seeding.

Al G

[Jim Stewart]

dave wrote:
> I read somewhere on the web that your A&P can adjust your altimeter by
> removing a pin or a screw that allows the barometric pressure to be set
> independently from the altitude. Once it's set correctly, the pin or

It's under warranty and the dealer says his A&P
can't break the seal.


> screw is replaced.
> Dave
>
>
> Jim Stewart wrote:
>> How close should an altimeter be to field elevation
>> when set to the pressure indicated by the field's
>> AWOS? I'm seeing a 50' error.

[[email protected]]

On May 7, 2:41 pm, "Jim Carter" > wrote:
> I think what you've cited is for bench testing during certification. Once
> installed in the aircraft then comparison against the pressure as recorded
> by the official weather observation is +/- 75'.

The installed altimeter should give the same reading error
on the ground as it did on the bench unless the static system is way
out of whack, in which case the airplane shouldn't be flown. The
Canadian specs for scale error are here:
http://www.tc.gc.ca/civilaviation/RegServ/Affairs/cars/Part5/Standards/t571s02.htm


Dan

[Ron Rosenfeld]

On Tue, 8 May 2007 09:03:07 -0700, "Al G" > wrote:

> We once had a 182 and pilot approved for the 150' DH for
>fog seeding.

That wouldn't happen to have been in Medford, OR?
--ron

[Al G[_2_]]

"Ron Rosenfeld" > wrote in message
...
> On Tue, 8 May 2007 09:03:07 -0700, "Al G" > wrote:
>
>> We once had a 182 and pilot approved for the 150' DH for
>>fog seeding.
>
> That wouldn't happen to have been in Medford, OR?
> --ron

*Ding*, We have a winner!

It was indeed. United Airlines hired us to "Clear it up". $15/hr for
Night/Single Engine/IFR below minimums. My wife later bought me some "Small
Flowers" from Wal-Mart, so I would always have "Mini-Mums". ;)

Al G

[Ron Rosenfeld]

On Wed, 9 May 2007 09:45:04 -0700, "Al G" > wrote:

>
>"Ron Rosenfeld" > wrote in message
...
>> On Tue, 8 May 2007 09:03:07 -0700, "Al G" > wrote:
>>
>>> We once had a 182 and pilot approved for the 150' DH for
>>>fog seeding.
>>
>> That wouldn't happen to have been in Medford, OR?
>> --ron
>
> *Ding*, We have a winner!
>
> It was indeed. United Airlines hired us to "Clear it up". $15/hr for
>Night/Single Engine/IFR below minimums. My wife later bought me some "Small
>Flowers" from Wal-Mart, so I would always have "Mini-Mums". ;)
>
>Al G
>

I have waited many an early morning for the fog to clear. (Our kids used
to live in Ashland). Once or twice I noted the Cessna taking off, when the
commercial jets couldn't land.
--ron

[Al G[_2_]]

"Ron Rosenfeld" > wrote in message
...
> On Wed, 9 May 2007 09:45:04 -0700, "Al G" > wrote:
>
>>
>>"Ron Rosenfeld" > wrote in message
...
>>> On Tue, 8 May 2007 09:03:07 -0700, "Al G" >
>>> wrote:
>>>
>>>> We once had a 182 and pilot approved for the 150' DH for
>>>>fog seeding.
>>>
>>> That wouldn't happen to have been in Medford, OR?
>>> --ron
>>
>> *Ding*, We have a winner!
>>
>> It was indeed. United Airlines hired us to "Clear it up". $15/hr for
>>Night/Single Engine/IFR below minimums. My wife later bought me some
>>"Small
>>Flowers" from Wal-Mart, so I would always have "Mini-Mums". ;)
>>
>>Al G
>>
>
> I have waited many an early morning for the fog to clear. (Our kids used
> to live in Ashland). Once or twice I noted the Cessna taking off, when
> the
> commercial jets couldn't land.
> --ron

The conditions had to be just right. 25-31 degrees F, and the heavier
the fog the better. We would fly a 150' pass, and dribble about a 1/2 pound
of crushed dry ice over the centerline, making several passes. Within 5
minutes, it was snowing. Within 15-20, most of the fog was on the ground,
and RVR's went from -600 to 6000. The 182 would build ice while doing the
low passes, but the normal 90-270 was done above pattern altitude where it
was 40 F and the ice would leave. We had an "Unofficial" backcourse/Loc only
with a 150' DH as well. The Loc/Dme was at the far end of MFR's 14 ILS, so
it gave us a distance to the approach end of our backcourse. We figured if
the engine went, we would just keep the energy up and shoot one of the
approaches. We actually did this under the hood for practice, and I once did
it with the GADO inspector who authorized me, on board. Sounds pretty dumb
now. At the time it was just like going out for touch & go's, except you
couldn't see anything outside.

Al G

[Mike[_17_]]

"dave" > wrote in message
. ..
> I read somewhere on the web that your A&P can adjust your altimeter by
> removing a pin or a screw that allows the barometric pressure to be set
> independently from the altitude. Once it's set correctly, the pin or
> screw is replaced.
> Dave

Not legally. Adjustment of an altimeter in this manner is considered a
calibration. Calibration of instruments is considered a major repair and A&P
mechanics are prohibited from performing repairs (minor or major) to
instruments. In addition, the data correspondance between the altimeter and
encoder (if equipped) would be nullified and require retesting by an
appropriately-rated repair station.

Mike