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Altimeter accuracy



 
 
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  #1  
Old May 7th 07, 11:45 PM posted to rec.aviation.owning
Ron Rosenfeld
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Posts: 264
Default Altimeter accuracy

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
  #2  
Old May 8th 07, 04:38 AM posted to rec.aviation.owning
Jim Carter[_1_]
external usenet poster
 
Posts: 403
Default Altimeter accuracy

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



  #3  
Old May 8th 07, 01:18 PM posted to rec.aviation.owning
Ron Rosenfeld
external usenet poster
 
Posts: 264
Default Altimeter accuracy

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
  #4  
Old May 8th 07, 05:03 PM posted to rec.aviation.owning
Al G[_2_]
external usenet poster
 
Posts: 112
Default Altimeter accuracy


"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


  #5  
Old May 9th 07, 04:22 AM posted to rec.aviation.owning
Ron Rosenfeld
external usenet poster
 
Posts: 264
Default Altimeter accuracy

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
  #6  
Old May 9th 07, 05:45 PM posted to rec.aviation.owning
Al G[_2_]
external usenet poster
 
Posts: 112
Default Altimeter accuracy


"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


  #7  
Old May 10th 07, 08:27 PM posted to rec.aviation.owning
Ron Rosenfeld
external usenet poster
 
Posts: 264
Default Altimeter accuracy

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
 




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