En route glide slope?

[Andrew Gideon]

I was staring at the VOR-head during a flight a while back, and started
wondering why we only use half of it most of the time. Whether by GPS or
VOR, the CDI provides us with lateral navigation. But the glide slope
needle sits unused until the very end of the flight (if then {8^).

Why?

Using older technology, why not have an altitude bug and let the bug and
altimeter feed the GS needle, providing VNAV information en route?

Will WAAS-ified GPSs be able to drive the GS needle en route?

Curiously...

Andrew

[Nathan Young]

On Fri, 19 Nov 2004 15:08:56 -0500, Andrew Gideon >
wrote:

>I was staring at the VOR-head during a flight a while back, and started
>wondering why we only use half of it most of the time. Whether by GPS or
>VOR, the CDI provides us with lateral navigation. But the glide slope
>needle sits unused until the very end of the flight (if then {8^).
>
>Why?
>
>Using older technology, why not have an altitude bug and let the bug and
>altimeter feed the GS needle, providing VNAV information en route?
>
>Will WAAS-ified GPSs be able to drive the GS needle en route?

Not trying to be argumentative, but a simple look at the altimeter
provides all the info you need to know. I do not see much value add
by having a GS needle tell me I am 100 feet too high (enroute), when I
can look at the altimeter and get the same info.

Another issue: The setting of the enroute cruise altitude introduces
an opportunity for error - where the pilot sets the wrong cruise
altitude.

Of course, this issue already exists with the more advanced altitude
preset autopilots. I wonder if setting the wrong altitude is a
problem with those?

-Nathan

[Stefan]

Nathan Young wrote:

>> Will WAAS-ified GPSs be able to drive the GS needle en route?

> Not trying to be argumentative, but a simple look at the altimeter
> provides all the info you need to know.

Even more to the point, a look at the altimeter provides you with the
correct information, namely pressure altitude.

Stefan

[Icebound]

"Stefan" > wrote in message
...
> Nathan Young wrote:
>
>>> Will WAAS-ified GPSs be able to drive the GS needle en route?
>
>> Not trying to be argumentative, but a simple look at the altimeter
>> provides all the info you need to know.
>
> Even more to the point, a look at the altimeter provides you with the
> correct information, namely pressure altitude.


Uh... Stefan..., a look at the altimeter provides you with *indicated*
altitude. It only shows *pressure* altitude if the altimeter setting is set
to 29.92

[Michelle P]

Andrew,
The software on my Pocket PC provides you with a highway in the sky if
you feed it the altitude you want for each leg.
Michelle

Andrew Gideon wrote:

>I was staring at the VOR-head during a flight a while back, and started
>wondering why we only use half of it most of the time. Whether by GPS or
>VOR, the CDI provides us with lateral navigation. But the glide slope
>needle sits unused until the very end of the flight (if then {8^).
>
>Why?
>
>Using older technology, why not have an altitude bug and let the bug and
>altimeter feed the GS needle, providing VNAV information en route?
>
>Will WAAS-ified GPSs be able to drive the GS needle en route?
>
>Curiously...
>
> Andrew
>
>
>

--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity

[Dave S]

Icebound wrote:
> "Stefan" > wrote in message
> ...
>
>>Nathan Young wrote:
>>
>>
>>>>Will WAAS-ified GPSs be able to drive the GS needle en route?
>>
>>>Not trying to be argumentative, but a simple look at the altimeter
>>>provides all the info you need to know.
>>
>>Even more to the point, a look at the altimeter provides you with the
>>correct information, namely pressure altitude.
>
>
>
> Uh... Stefan..., a look at the altimeter provides you with *indicated*
> altitude. It only shows *pressure* altitude if the altimeter setting is set
> to 29.92
>
>
Sounds like his enroute flight takes place above FL 180

Dave

[Mike Rapoport]

Many, if not most, turbine airplanes have this but it requires an airdata
computer system. You can set it to arrive at a particular point at a
specific altitude. You need an airdata system as long as you are climbing
or descending to a pressure altitiude. WAAS could only guide you to a GPS
altitude.

Mike
MU-2


"Andrew Gideon" > wrote in message
online.com...
>I was staring at the VOR-head during a flight a while back, and started
> wondering why we only use half of it most of the time. Whether by GPS or
> VOR, the CDI provides us with lateral navigation. But the glide slope
> needle sits unused until the very end of the flight (if then {8^).
>
> Why?
>
> Using older technology, why not have an altitude bug and let the bug and
> altimeter feed the GS needle, providing VNAV information en route?
>
> Will WAAS-ified GPSs be able to drive the GS needle en route?
>
> Curiously...
>
> Andrew
>

[Mike Rapoport]

"Icebound" > wrote in message
...
>
> "Stefan" > wrote in message
> ...
>> Nathan Young wrote:
>>
>>>> Will WAAS-ified GPSs be able to drive the GS needle en route?
>>
>>> Not trying to be argumentative, but a simple look at the altimeter
>>> provides all the info you need to know.
>>
>> Even more to the point, a look at the altimeter provides you with the
>> correct information, namely pressure altitude.
>
>
> Uh... Stefan..., a look at the altimeter provides you with *indicated*
> altitude. It only shows *pressure* altitude if the altimeter setting is
> set to 29.92
>
All altitudes shown on an altimeter are "pressure altitudes" (as opposed to
true altitudes) since the altimeter is an air pressure guage marked in feet.

Mike
MU-2

[Julian Scarfe]

"Mike Rapoport" > wrote in message
k.net...
> Many, if not most, turbine airplanes have this but it requires an airdata
> computer system. You can set it to arrive at a particular point at a
> specific altitude. You need an airdata system as long as you are climbing
> or descending to a pressure altitiude. WAAS could only guide you to a GPS
> altitude.


Does it really drive the glideslope needle?

Some of the more sophisticated GPSs have VNAV functionality, and turbine
aircraft FMSs almost certainly do too, based on a barometric altitude input.
But I always thought there was a reluctance to put the information on the GS
needle because the glideslope of an ILS is generally associated with a
trajectory with terrain clearance -- something that simple VNAV can't
guarantee.

Julian Scarfe

[Stefan]

Icebound wrote:

> Uh... Stefan..., a look at the altimeter provides you with *indicated*
> altitude. It only shows *pressure* altitude if the altimeter setting is set
> to 29.92

Uh... Icebound..., the altimeter *always* indicates pressure altitude.
To get useful information, you must set the altimeter appropriately, of
course. Appropriately may or may not mean 1013.

On the other hand, your GPS always gives you GPS altitude, which is of
no value in flight.

Stefan

[Bob Moore]

Dave S > wrote
> Sounds like his enroute flight takes place above FL 180

Sounds as if you have never flown outside of the USofA.
Notice that he WAS posting from "ch".

Bob Moore

[Julian Scarfe]

"Stefan" > wrote in message
...
>
> Uh... Icebound..., the altimeter *always* indicates pressure altitude.
> To get useful information, you must set the altimeter appropriately, of
> course. Appropriately may or may not mean 1013.

You two are just arguing about terminology. I've always thought the norm is
to refer to an altitude measured on an altimeter (rather than another
instrument like a GPS) as "barometric altitude", with "pressure altitude"
reserved for the indicated altitude with 1013/29.92 set.

Julian Scarfe
..

[Andrew Gideon]

Mike Rapoport wrote:

> Many, if not most, turbine airplanes have this but it requires an airdata
> computer system. You can set it to arrive at a particular point at a
> specific altitude. You need an airdata system as long as you are climbing
> or descending to a pressure altitiude. WAAS could only guide you to a GPS
> altitude.

Perhaps I'm missing something basic, but don't we specifically adjust the
kollsman window to convert values provided by a barometric altimeter to a
true altitude (at least below 18,000')? Of course, you're quite right that
the GPS would need "correction" to yield the pressure altitude used above
18,000 (which I didn't consider because I've never flown there myself {8^).

- Andrew

[Mike Rapoport]

"Andrew Gideon" > wrote in message
online.com...
> Mike Rapoport wrote:
>
>> Many, if not most, turbine airplanes have this but it requires an airdata
>> computer system. You can set it to arrive at a particular point at a
>> specific altitude. You need an airdata system as long as you are
>> climbing
>> or descending to a pressure altitiude. WAAS could only guide you to a
>> GPS
>> altitude.
>
> Perhaps I'm missing something basic, but don't we specifically adjust the
> kollsman window to convert values provided by a barometric altimeter to a
> true altitude (at least below 18,000')? Of course, you're quite right
> that
> the GPS would need "correction" to yield the pressure altitude used above
> 18,000 (which I didn't consider because I've never flown there myself
> {8^).
>
> - Andrew
You are right and as Julian points out, I should use the term barometric
altitude. Barometric altitude (with the correct kollsman setting) is only
correct if the temperature lapse rate is 2C/1000'. It is also affected by
airflow over mountains and can be off by 1,000' in extreme conditions (I've
read). My point is that there is no way to "convert" GPS altitude to
barometric altitude since the barometric altitude is subject to a host of
errors. Everybody needs to be on the same "system" and that is barometric.

Mike
MU-2

[Mike Rapoport]

It drives the flight director and the autopilot follows the flight director.
I don't know for certain if it drives the GS needle. I am pretty sure it
does.

Mike
MU-2

"Julian Scarfe" > wrote in message
...
> "Mike Rapoport" > wrote in message
> k.net...
>> Many, if not most, turbine airplanes have this but it requires an airdata
>> computer system. You can set it to arrive at a particular point at a
>> specific altitude. You need an airdata system as long as you are
>> climbing
>> or descending to a pressure altitiude. WAAS could only guide you to a
>> GPS
>> altitude.
>
>
> Does it really drive the glideslope needle?
>
> Some of the more sophisticated GPSs have VNAV functionality, and turbine
> aircraft FMSs almost certainly do too, based on a barometric altitude
> input.
> But I always thought there was a reluctance to put the information on the
> GS
> needle because the glideslope of an ILS is generally associated with a
> trajectory with terrain clearance -- something that simple VNAV can't
> guarantee.
>
> Julian Scarfe
>
>

[Dave S]

Bob Moore wrote:

> Dave S > wrote
>
>> Sounds like his enroute flight takes place above FL 180
>
>
> Sounds as if you have never flown outside of the USofA.
> Notice that he WAS posting from "ch".
>
> Bob Moore

Nope.. Never have.

Dave

[Stefan]

Julian Scarfe wrote:

> You two are just arguing about terminology. I've always thought the norm is
> to refer to an altitude measured on an altimeter (rather than another
> instrument like a GPS) as "barometric altitude", with "pressure altitude"
> reserved for the indicated altitude with 1013/29.92 set.

Ah! I wasn't aware of that. Of course I meant barometric altitude.

Stefan

[Darrell S]

It would require extensive (read expensive) modifications to achieve what
you wish. First of all the CDI is driven by a VHF omni range or Localizer
signal while the GS indicator is driven by a UHF signal from the glide slope
transmitter. In the beginning use of ILS there were 2 separate receivers
to be tuned.... one for the Localizer VHF frequency and one for the GS UHF
frequency. Then they paired up LOC/GS frequencies so that tuning the
localizer automatically tunes for the GS. Much the same way that tuning in
VHF VOR stations automatically tunes the paired UHF tacan DME receiver for
VOR stations that have DME.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"Andrew Gideon" > wrote in message
online.com...
>I was staring at the VOR-head during a flight a while back, and started
> wondering why we only use half of it most of the time. Whether by GPS or
> VOR, the CDI provides us with lateral navigation. But the glide slope
> needle sits unused until the very end of the flight (if then {8^).
>
> Why?
>
> Using older technology, why not have an altitude bug and let the bug and
> altimeter feed the GS needle, providing VNAV information en route?
>
> Will WAAS-ified GPSs be able to drive the GS needle en route?
>
> Curiously...
>
> Andrew
>