Check altitudes on LPV and GLS approaches.

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FARF

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I've noticed check altitudes being added to some FAA and foreign LPV and GLS Instrument approaches. I can understand the purpose of a check altitude on an ILS where an independent Glide Slope intersects a geographical fixed point and the altitude over this point should never change. So your barometric altimeter should closely correspond to that altitude at that point if you are on Glide Slope. But with the LPV and GLS approaches the Glide Slope is derived internally within the aircraft. Integrity checks are constantly comparing satellite altitude to barometric altimeter altitude and monitoring for deviation. Since the Aircraft GPS box is computing the altitude along your route of flight and with auto pilot engaged flying to the computed fix points and maintaining the altitude associated with each fix, what is the purpose of a Check Altitude along final approach? Without any warning flags indicating a deviation, your system should put you at that fix and at that altitude every time.

Comments?
 
If you have your barometric pressure set incorrectly, you could descend below minimums flying an ILS or GPSLPV approach. If you've got your needles centered on a precision approach you could possibly crash into the runway if you think you've still got a few hundred feet to go.

I don't really get your reasoning. An ILS and LPV approach arent really any different when it comes to flying the needles. Setting an altimiter wrong would have the same effect on either one.
 
Just Re-read your post. I think you were saying the check is to make sure the ILS signal is correct. From that standpoint, I get the question.
 
FARF said:
I've noticed check altitudes being added to some FAA and foreign LPV and GLS Instrument approaches. I can understand the purpose of a check altitude on an ILS where an independent Glide Slope intersects a geographical fixed point and the altitude over this point should never change. So your barometric altimeter should closely correspond to that altitude at that point if you are on Glide Slope. But with the LPV and GLS approaches the Glide Slope is derived internally within the aircraft. Integrity checks are constantly comparing satellite altitude to barometric altimeter altitude and monitoring for deviation. Since the Aircraft GPS box is computing the altitude along your route of flight and with auto pilot engaged flying to the computed fix points and maintaining the altitude associated with each fix, what is the purpose of a Check Altitude along final approach? Without any warning flags indicating a deviation, your system should put you at that fix and at that altitude every time.

Comments?
Click to expand...
Don't forget that barometric altitude is strongly affected by variations in air temperature, specifically by variations from standard temperature. So I would not necessarily expect my barometric altimeter to always read the same at a given point on a glide slope (either ILS or LPV). If your altimeter setting is from the airport you are flying into, then the effect will be smaller the closer you are to field elevation. But I know of at least one ILS where the charted GS intercept is several thousand feet above field elevation, and in that case, temperature can have a significant effect on indicated altitude. I'm sure there are plenty of ILS approaches where that's true.

Are you sure the GPS uses barometric altitude to check vertical integrity? Mine often prompts me for an altimeter setting on start up, but there is no convenient way to change that setting in flight when given a different altimeter setting (you'd have to go into the SYS page, scroll down to that setting, and enter the new value, not SOP in flight according to the Pilot's Guide). I've assumed that the GPS uses GPS altitude, as it is a WAAS unit and observes strict vertical integrity limits. My understanding is it uses WAAS for all integrity checks, though I haven't studied the method in detail - and I'd be very surprised if it compares anything to barometric altitude as the barometric information it has is generally not reliable.
 
I can't speak for other boxes but my 430W manual says nothing about a barometric input for integrity monitoring, unless I am blind.
 
I agree that an ILS check altitude is a valuable aid in determining inaccurate altimeter setting, temperature induced error or other altimeter malfunctions.

Possibly, there are only certain WASS/LASS enabled systems that perform a barometric/satellite altitude comparison? In that case, at the check point on a LPV/GLS if your barometric altimeter showed a concerning difference, a question arises, is it a barometric altimeter error? Do you choose to believe the barometric or the GPS box (assuming no GPS error indications). I am just not sure what conclusions to draw at a check altitude on LPV/GLS approaches if a deviation exists.
 
If you can't resolve an altitude discrepancy on approach, that's reason to go missed and try an ILS or divert to VMC.

Temperature errors can be anticipated. Hopefully, you can detect an iced/blocked static system from the symptoms and pull the alt static.
 
FARF said:
I've noticed check altitudes being added to some FAA and foreign LPV and GLS Instrument approaches. I can understand the purpose of a check altitude on an ILS where an independent Glide Slope intersects a geographical fixed point and the altitude over this point should never change. So your barometric altimeter should closely correspond to that altitude at that point if you are on Glide Slope. But with the LPV and GLS approaches the Glide Slope is derived internally within the aircraft. Integrity checks are constantly comparing satellite altitude to barometric altimeter altitude and monitoring for deviation. Since the Aircraft GPS box is computing the altitude along your route of flight and with auto pilot engaged flying to the computed fix points and maintaining the altitude associated with each fix, what is the purpose of a Check Altitude along final approach? Without any warning flags indicating a deviation, your system should put you at that fix and at that altitude every time.

Comments?
Click to expand...

Do you have an example?
 
Somedudeintn said:
If you have your barometric pressure set incorrectly, you could descend below minimums flying an ILS or GPSLPV approach. If you've got your needles centered on a precision approach you could possibly crash into the runway if you think you've still got a few hundred feet to go.

I don't really get your reasoning. An ILS and LPV approach arent really any different when it comes to flying the needles. Setting an altimiter wrong would have the same effect on either one.
Click to expand...

I'd never thought of that before. Using the glide path altitude at FAF as a check on the altimeters accuracy. Not the other way around where you are using the altimeter as a check on the glideslope's accuracy, checking for false glideslopes. If your altimeter was reading 100 feet off of that altitude while you were on glidepath, then maybe you should adjust your DA a like amount. GPS, not having false glideslopes, would have made that altitude on the chart unnecessary because it doesn't have false glideslopes.

It would boil down to the altitude was originally used to check ILS glideslopes. Then along came WAAS and LPV whose glideslopes didn't need checking.
Then it was discovered that the glideslope could be used to check the altimeter.
 
Attached is an example.
 

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fiveoboy01 said:
I can't speak for other boxes but my 430W manual says nothing about a barometric input for integrity monitoring, unless I am blind.
Click to expand...
430 has baro-aiding, which is the integrity monitoring. Mine even yells at me if I turn my GTX327 off, since it's getting its baro from the altitude encoder. I've never had to feed it an altimeter setting, though.
 
Hmm. I'll need to look again. Is that a standard configuration when installing, or an optional thing? I also have a 327.
 
azure said:
Don't forget that barometric altitude is strongly affected by variations in air temperature, specifically by variations from standard temperature. So I would not necessarily expect my barometric altimeter to always read the same at a given point on a glide slope (either ILS or LPV). If your altimeter setting is from the airport you are flying into, then the effect will be smaller the closer you are to field elevation. But I know of at least one ILS where the charted GS intercept is several thousand feet above field elevation, and in that case, temperature can have a significant effect on indicated altitude. I'm sure there are plenty of ILS approaches where that's true.

Are you sure the GPS uses barometric altitude to check vertical integrity? Mine often prompts me for an altimeter setting on start up, but there is no convenient way to change that setting in flight when given a different altimeter setting (you'd have to go into the SYS page, scroll down to that setting, and enter the new value, not SOP in flight according to the Pilot's Guide). I've assumed that the GPS uses GPS altitude, as it is a WAAS unit and observes strict vertical integrity limits. My understanding is it uses WAAS for all integrity checks, though I haven't studied the method in detail - and I'd be very surprised if it compares anything to barometric altitude as the barometric information it has is generally not reliable.
Click to expand...

What approach is it that has the GS intercept several thousand feet above field elevation?
 
luvflyin said:
Looks about right. About 10 miles out, about 3000 feet above on a 3 degree glideslope.
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The point being, though, that on a day like today (about 25-30dF above standard), I'd expect the altimeter to be reading maybe a couple hundred feet low at GS intercept.
luvflyin said:
?????
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I don't get that one either. The ILS 15 at KBTV has its GS intercept at 2000 MSL, field elevation is 335. Maybe he meant the ILS or LOC/DME 33 ?? :confused:
 
azure said:
The point being, though, that on a day like today (about 25-30dF above standard), I'd expect the altimeter to be reading maybe a couple hundred feet low at GS intercept.
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Gotcha. Before reading this thread I had never considered using that altitude on the plate to "check the altimeter." If it's a hot day the altimeter is going to read a little high. But the glideslope is not affected by the temperature. If the altimeter reads 50 feet above the "Glide Slope Altitude at FAF" when your over the FAF and you are on the glide slope, then maybe you should consider adding 50 feet to the DA
 
luvflyin said:
Gotcha. Before reading this thread I had never considered using that altitude on the plate to "check the altimeter." If it's a hot day the altimeter is going to read a little high.
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I had to think about this again but I'm pretty sure what I wrote was correct. On a hot day the atmosphere expands, the pressure levels are all higher than at standard temperature, so at a given true altitude, you are at a pressure level that is "normally" lower and thus your altimeter reads low, not high.
But the glideslope is not affected by the temperature. If the altimeter reads 50 feet above the "Glide Slope Altitude at FAF" when your over the FAF and you are on the glide slope, then maybe you should consider adding 50 feet to the DA
Click to expand...
That's a good point, and also the basic reason behind the cold temperature correction that's required at quite a few airports including my home base (KMPV).
 
azure said:
I had to think about this again but I'm pretty sure what I wrote was correct. On a hot day the atmosphere expands, the pressure levels are all higher than at standard temperature, so at a given true altitude, you are at a pressure level that is "normally" lower and thus your altimeter reads low, not high.

That's a good point, and also the basic reason behind the cold temperature correction that's required at quite a few airports including my home base (KMPV).
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I don't think you have that quite right. When it gets hot, the air gets less dense. Thinner air makes the altimeter think it is higher above the earth. High to low, watch out below is about the altimeter reading "higher." You fly that altitude and the airplane is what is "lower." The rocks altitude does not change. Neither does the glide slopes angle.
EDIT: the temperature part of this seems to be wrong
MORE EDIT: AIM 7-2-3
 
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luvflyin said:
Gotcha. Before reading this thread I had never considered using that altitude on the plate to "check the altimeter." If it's a hot day the altimeter is going to read a little high. But the glideslope is not affected by the temperature. If the altimeter reads 50 feet above the "Glide Slope Altitude at FAF" when your over the FAF and you are on the glide slope, then maybe you should consider adding 50 feet to the DA
Click to expand...

Assuming the altimeter and pitot/static system are being properly maintained, any error seen at the PFAF will not be present at DA. As you descend towards the altimeter setting source the errors should decrease significantly. Even on a very cold day at a "cold temperature" airport the corrections required are to keep you from losing all obstacle clearance in the intermediate segment, not at a 200-300' DA.
 
luvflyin said:
I don't think you have that quite right. When it gets hot, the air gets less dense. Thinner air makes the altimeter think it is higher above the earth. High to low, watch out below is about the altimeter reading "higher." You fly that altitude and the airplane is what is "lower." The rocks altitude does not change. Neither does the glide slopes angle.
Click to expand...
Yes, air density decreases but the altimeter reads pressure, not density. Atmospheric pressure is due to the weight of the air above you. As the atmosphere expands, at any given true altitude there is more air above you and thus the pressure is greater. The altimeter reads the higher pressure as a lower altitude.

Also, think about cold temperature correction: you have to add the safety factor because the altimeter is reading higher than actual altitude when it's cold.
 
aterpster said:
Assuming the altimeter and pitot/static system are being properly maintained, any error seen at the PFAF will not be present at DA. As you descend towards the altimeter setting source the errors should decrease significantly. Even on a very cold day at a "cold temperature" airport the corrections required are to keep you from losing all obstacle clearance in the intermediate segment, not at a 200-300' DA.
Click to expand...
Whoops, good point. As long as you're using the local altimeter setting, by the time you're down to DA the correction should be negligible. Thanks.
 
Looks like I've been getting my apples and oranges mixed up. Let's say your on the ramp at a sea level airport and it is a standard day, 29.92, 59F. Altimeter is set to 29.92 and reads 0. Temperature drops to zero, nothing else changes. What does the altimeter read?
 
Doesn't high to low look out below apply to pressure AND temperature?

So if it was colder than standard, altimeter will read higher, if it's warmer, it will read lower.
 
fiveoboy01 said:
Doesn't high to low look out below apply to pressure AND temperature?

So if it was colder than standard, altimeter will read higher, if it's warmer, it will read lower.
Click to expand...

That's what I knew and the AIM confirms it. But it does seem to get more complicated close to the surface and with big temperature differences. AIM 7-2-3.
 
luvflyin said:
I don't think you have that quite right. When it gets hot, the air gets less dense. Thinner air makes the altimeter think it is higher above the earth. High to low, watch out below is about the altimeter reading "higher." You fly that altitude and the airplane is what is "lower." The rocks altitude does not change. Neither does the glide slopes angle.
EDIT: the temperature part of this seems to be wrong
MORE EDIT: AIM 7-2-3
Click to expand...

The plot thickens
 
luvflyin said:
That's what I knew and the AIM confirms it. But it does seem to get more complicated close to the surface and with big temperature differences. AIM 7-2-3.
Click to expand...

Thanks. My post was half-question too haha. I thought I had it down in my head, but the conflicting posts had me second-guessing myself:)
 
luvflyin said:
Looks like I've been getting my apples and oranges mixed up. Let's say your on the ramp at a sea level airport and it is a standard day, 29.92, 59F. Altimeter is set to 29.92 and reads 0. Temperature drops to zero, nothing else changes. What does the altimeter read?
Click to expand...
If the barometric pressure doesn't change, the altimeter reading doesn't change.

As long as you're on the ground.

But if the whole column is now 59dF below standard, then as you go up, the air thins more quickly because the atmosphere contracts because of the cold. All the pressure levels - 925mb, 800mb, 750mb, 500mb, etc. - are found at lower true altitudes. Because the altimeter is calibrated in altitude units assuming the decrease in pressure with altitude is what it would be on a standard day, it reads higher for a given altitude.

The difference between indicated and true altitudes increases the higher you go, and is pretty much negligible very near the ground. That's why, as Wally said, the correction to DA on a precision approach in cold weather is small, and in most cases practically negligible.

Edit: see Table III-1-4-1a in this link.

Umm, okay, that was in meters. Table III-1-4-1b is in feet. :)
 
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azure said:
If the barometric pressure doesn't change, the altimeter reading doesn't change.

As long as you're on the ground.

But if the whole column is now 59dF below standard, then as you go up, the air thins more quickly because the atmosphere contracts because of the cold. All the pressure levels - 925mb, 800mb, 750mb, 500mb, etc. - are found at lower true altitudes. Because the altimeter is calibrated in altitude units assuming the decrease in pressure with altitude is what it would be on a standard day, it reads higher for a given altitude.

The difference between indicated and true altitudes increases the higher you go, and is pretty much negligible very near the ground. That's why, as Wally said, the correction to DA on a precision approach in cold weather is small, and in most cases practically negligible.

Edit: see Table III-1-4-1a in this link.

Umm, okay, that was in meters. Table III-1-4-1b is in feet. :)
Click to expand...

Think I'm getting it now. It being about altimeters are built and calibrated to normal conditions tells the story. Extreme conditions make a significant difference. While high to low watch out below is true for pressure and temperature, it has nothing to do with this. It's the difference between standard atmosphere and the actual atmosphere "right here, right now." Not the difference between here and the air you last set your altimeter to a hundred miles ago. My bad for even bringing that up.
 
Wasn't their that MU2 that piled in in Canada that they thought might have been due to a bad altimeter setting?

LPVs are basically a ILS as far as I'm concerned, heck in many ways they are better, just the FAA doesn't consider them "precision".
 
James331 said:
Wasn't their that MU2 that piled in in Canada that they thought might have been due to a bad altimeter setting?

LPVs are basically a ILS as far as I'm concerned, heck in many ways they are better, just the FAA doesn't consider them "precision".
Click to expand...

This isn't about bad altimeter settings. It's about the actual altitude of the airplane differing from the altitude the altimeter says, with a good setting, due to temperature extremes.
 
fiveoboy01 said:
Hmm. I'll need to look again. Is that a standard configuration when installing, or an optional thing? I also have a 327.
Click to expand...
Good question. It was that way when I took ownership! If I turn off the 327 entirely (off, not standby), I get a msg annunciation and the message is complaining about no baro. I can't recall if it's on the 429 or rs232 side, but it complains until you turn it back on. Next time I'm out at the airplane, I can verify.
 
James331 said:
Wasn't their that MU2 that piled in in Canada that they thought might have been due to a bad altimeter setting?

LPVs are basically a ILS as far as I'm concerned, heck in many ways they are better, just the FAA doesn't consider them "precision".
Click to expand...

It is more ICAO than the FAA. LPV uses exactly the same TERPs protected airspace as does CAT I ILS.
 
James331 said:
Wasn't their that MU2 that piled in in Canada that they thought might have been due to a bad altimeter setting?

LPVs are basically a ILS as far as I'm concerned, heck in many ways they are better, just the FAA doesn't consider them "precision".
Click to expand...

I googled and found an MU2 accident that happened a couple months ago. The altimeter was 28.84. That's a pretty severe case of "high to low" if they had been up in the flight levels and it was left on 29.92.
 
Meanwhile, back at the ranch (the original post), what was the original intent of the "Check Altitude", called "Glide path altitude at FAF" on the chart"? Was it to detect false glideslopes? Or to detect bad altimeters?
 
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