Questions on altimetry correction on ILS and LPV approach.

J

John777

Pre-takeoff checklist
Joined
Apr 1, 2016
Messages
199
Display Name
Display name:
Louis
Hello everyone,

I just had a few question on making altimetry correction when flying into airport under extreme temperature(cold). Even though we are following the electronically guided(or by GPS) glide path down to the minimum, I thought without applying temperature correction, the DA(H) would be lower than usual.

If this is true, do we also apply correction to our DA(H) as well by adding certain correction number to it?

John.
 
John777 said:
Hello everyone,

I just had a few question on making altimetry correction when flying into airport under extreme temperature(cold). Even though we are following the electronically guided(or by GPS) glide path down to the minimum, I thought without applying temperature correction, the DA(H) would be lower than usual.

If this is true, do we also apply correction to our DA(H) as well by adding certain correction number to it?

John.
Click to expand...

The cold-weather correction chart is available as download in ForeFlight.
 
John777 said:
Hello everyone,

I just had a few question on making altimetry correction when flying into airport under extreme temperature(cold). Even though we are following the electronically guided(or by GPS) glide path down to the minimum, I thought without applying temperature correction, the DA(H) would be lower than usual.

If this is true, do we also apply correction to our DA(H) as well by adding certain correction number to it?

John.
Click to expand...
Only if the NOTAP specifies a correction for the final (not inalienable) approach segment.
 
Last edited:
Air Force guidance is to add the correction to altitudes inside the final approach fix. I think their guidance on when to apply the correction just changed, it has to be pretty cold to make a big difference.
 
Cooter said:
Air Force guidance is to add the correction to altitude inside the final approach fix. If I'm not mistaken their guidance on when to apply the correction just changed.
Click to expand...
Unless you're in the Air Force, flying an Air Force aircraft, I'd suggest following the instructions in the NTAP. ;)
 
John777 said:
Hello everyone,

I just had a few question on making altimetry correction when flying into airport under extreme temperature(cold). Even though we are following the electronically guided(or by GPS) glide path down to the minimum, I thought without applying temperature correction, the DA(H) would be lower than usual.

If this is true, do we also apply correction to our DA(H) as well by adding certain correction number to it?

John.
Click to expand...

I don't think so. The local altimeter setting will include temperature correction and give you accurate altitudes close to field elevation. It's how the cold weather affects the "column" of air as you get higher that causes problems. The colder it is, and the higher you get, the greater the error. Airplanes hitting terrain and obstructions a number of miles out on final while using the altimeter to be at the altitudes on the Approach Chart is the problem. Not on very short final close to field elevation.

EDIT: this is oversimplified. See 2 posts below
 
Last edited:
luvflyin said:
I don't think so. The local altimeter setting will include temperature correction and give you accurate altitudes close to field elevation. It's how the cold weather affects the "column" of air as you get higher that causes problems. The colder it is, and the higher you get, the greater the error. Airplanes hitting terrain and obstructions a number of miles out on final while using the altimeter to be at the altitudes on the Approach Chart is the problem. Not on very short final close to field elevation.
Click to expand...
Incorrect.
 
MauleSkinner said:
Incorrect.
Click to expand...

Like I said "I don't think so." What is correct?

Edit: I was over simplifying. There are differences in close and down low. At 200 feet they range from 10 to 60 feet from +10 to -60 degrees C. At 900 feet, roughly equivalent to a 3 mile final on a 3 degree glidepath, the differences range from 20 to 270 feet. So yeah, it sure seems you should adjust your DA/DH by somewhere from 10 to 60 feet if it's a 200 foot AGL one. And be extra careful out there a ways. The farther out and higher you are, the greater the error.
 
Last edited:
luvflyin said:
Like I said "I don't think so." What is correct?
Click to expand...
The Notices to Airmen Publication (NTAP) specifies which segment or segments need to be corrected, and below what temperature or temperatures, and which fix to use in calculating the altitude correction for the altitudes in each segment.

The corrections may need to be applied to intermediate, final, or missed approach segments singly or in any combination. The critical temperature is not necessarily the same for each segment, either, and if multiple critical temperatures exist, only the most limiting (highest) will show up on the chart.

While the final segment correction applies to Baro VNAV DA, there will be another temperature limit published on the chart below which you cannot use Baro VNAV.
 
MauleSkinner said:
The Notices to Airmen Publication (NTAP) specifies which segment or segments need to be corrected, and below what temperature or temperatures, and which fix to use in calculating the altitude correction for the altitudes in each segment.

The corrections may need to be applied to intermediate, final, or missed approach segments singly or in any combination. The critical temperature is not necessarily the same for each segment, either, and if multiple critical temperatures exist, only the most limiting (highest) will show up on the chart.

While the final segment correction applies to Baro VNAV DA, there will be another temperature limit published on the chart below which you cannot use Baro VNAV.
Click to expand...

Ah. Makes sense. Publishing what you need segement by segement. I found that NTAP. Always thought that was about high altitude airports and Alaska. KACV caught my eye. Practically at sea level in California and it is one of them.
 
luvflyin said:
Ah. Makes sense. Publishing what you need segement by segement. I found that NTAP. Always thought that was about high altitude airports and Alaska. KACV caught my eye. Practically at sea level in California and it is one of them.
Click to expand...
I looked up KACV airport charts and did you mean that 'ice crystal' symbol?

John.
 
luvflyin said:
Ah. Makes sense. Publishing what you need segement by segement. I found that NTAP. Always thought that was about high altitude airports and Alaska. KACV caught my eye. Practically at sea level in California and it is one of them.
Click to expand...
Yes, plenty of airports in northern CA and the PNW, the northern tier of US states and New England - very much New England. My home base, KMPV, is one of them, and it doesn't have to get that cold to trigger the corrections, not even below zero Fahrenheit. Been a while since I looked, but I believe it's -10C at my airport, which is 14F.
 
Everyone, thanks for your participation and good pilot is always learning !
Have a great weekends, and safe flight!

John777
 
azure said:
Yes, plenty of airports in northern CA and the PNW, the northern tier of US states and New England - very much New England. My home base, KMPV, is one of them, and it doesn't have to get that cold to trigger the corrections, not even below zero Fahrenheit. Been a while since I looked, but I believe it's -10C at my airport, which is 14F.
Click to expand...

I just looked and MPV is not on the list nor does it show on the Approach Charts. Maybe they raised some altitudes on the approaches recently making it not necessary anymore. Thing I find interesting about ACV is that it is for all of the Aproaches there and for the intermediate segment only. Not final or missed approach. That makes sense for RW32 because there is some wicked terrain out there. But the RW01 and RW14 approach intermediate segements are offshore where there is no terrain.
 
luvflyin said:
I just looked and MPV is not on the list nor does it show on the Approach Charts. Maybe they raised some altitudes on the approaches recently making it not necessary anymore. Thing I find interesting about ACV is that it is for all of the Aproaches there and for the intermediate segment only. Not final or missed approach. That makes sense for RW32 because there is some wicked terrain out there. But the RW01 and RW14 approach intermediate segements are offshore where there is no terrain.
Click to expand...
Hmmm... I linked to a copy of the list that I've referred to in the past and KMPV is definitely (still) on the list, though the temperature restriction kicks in at -20C, not -10 as I incorrectly remembered (though not far away, at KRUT the temperature restriction is -4C which is not very cold at all, the point I was trying to make).

(Unless that list is obsolete?...)

The only (somewhat) recent change I'm aware of to the charts at KMPV is to eliminate the VOR/DME 35 and to make rwy 35 NA at night from instrument approaches (for sure on straight-ins, I haven't checked the circling minimums to 35 from 17 lately). Those changes aren't new though, they were in effect already a year ago. I'm not aware of any recent changes to the charted altitudes.
 
I think that NTAP is old. It got linked through NBAA.org. The one I have is dated JAN 07, 2016. ACV is not on the old one. There are 6 California airports now where there were only 2 then. Vermont had 9 then, there are only 6 now. Back then the rule was it applied to all procedures and runways at an airport unless exceptions were made and it looks like MDW was the only one where it applied to runways 22L and 22R only. Near as I can tell the rule now is it applies to all procedures at an airport. Seems excessive to me, especially at ACV, where it applies to the intermediate segements only and it has Approaches with intermediate segements that are over the ocean without obstructions.
 
You must log in or register to reply here.
Back
Top