Cold Weather Approach Correction NOTAM

I'm surprised this isn't taught during the instrument rating. This is standard military practice anytime the temp drops below 0 Celcius.
 
It's in the AIM, but one of those things a lot of people don't think about
 
Cold temperature compensation has been used outside of the US. Inside the US, the FAA has not provided guidance on the topic other than to note the correction table. The ACF-IPG has had an open issue requesting guidance from the FAA since 1992. The FAA commissioned MITRE to do a study of airports and procedures to determine if any US airports were affected. They determined that many airports in the lower 48 and in Alaska had issues in one of three areas, the MDA/DA, and or the intermediate leg, and or the missed approach hold. Over 200 such segments were determined to have potential issues and they are now documented in the NOTAM, identifying the segments and the temperatures. With this NOTAM, if an airport is on the list and the temperature is at or lower than the specified value, the pilot must adjust the altitudes according to the ICAO table. Since this is a change to a part 97 procedure, it will be regulation and pilots must comply under the stated conditions or they would be violating 91.175. Over time, the approach charts will be updated if they have a cold temperature adjustment requirement by adding a snow flake icon to the briefing area and indicating the warmest applicable temperature that will require adjustments to the charted altitudes on the approach chart.
 
I'm confused. Are you adjusting even if you receive a local altimeter setting? ?
 
ARFlyer said:
Looks like some more math that we get to do! :D I hope ATC is informed, since we have to report the corrected altitudes to ATC.
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Seems to me ATC should be reporting the corrected altitudes to pilots, wouldn't ya think?

dtuuri
 
JasonM said:
I'm confused. Are you adjusting even if you receive a local altimeter setting? ?
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Yes. The local altimeter setting will have taken into account temperature at the airport surface, but as you get higher above the airport elevation, the altimeter error in the aircraft altimeter continues to increase, putting you lower than it indicates. So at 200 feet above the airport, the correction might only be 30 feet, by the time you are at 3000 feet above airport elevation, the altimeter will be off by 420 feet. That is why you will see most of the affected segments are in the intermediate segment of the approach where there is only a 500 foot required clearance of obstacles.
 
dtuuri said:
Seems to me ATC should be reporting the corrected altitudes to pilots, wouldn't ya think?

dtuuri
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That is not a bad idea, at least for the intermediate segments and the missed approach segments. The adjusted altitudes will still vary by which approach is being flown at the time. It would probably be more cumbersome when a DA/MDA needs to be adjusted as there are more of these on a given approach and which one applies to an aircraft is based on the category of aircraft, the speeds being used, and which procedure is being executed (on an RNAV, it may be LPV or LNAV/VNAV or LNAV straight in or LNAV circling). Multiply this times the number of approaches and it could lead to a lot of communication to establish the correct altitude that is going to be used. This is more easily performed by an EFB function or as part of an FMS input.
 
John Collins said:
Yes. The local altimeter setting will have taken into account temperature at the airport surface, but as you get higher above the airport elevation, the altimeter error in the aircraft altimeter continues to increase, putting you lower than it indicates. So at 200 feet above the airport, the correction might only be 30 feet, by the time you are at 3000 feet above airport elevation, the altimeter will be off by 420 feet. That is why you will see most of the affected segments are in the intermediate segment of the approach where there is only a 500 foot required clearance of obstacles.
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Would they ever consider allowing GPS WAAS altitude as alternative to baro altitude for approaches, which would be vastly more accurate and temperature-independent? I suppose one could always use the highest of the two, in a pinch.
 
RotorDude said:
Would they ever consider allowing GPS WAAS altitude as alternative to baro altitude for approaches, which would be vastly more accurate and temperature-independent? I suppose one could always use the highest of the two, in a pinch.
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I don't think this would be in the cards. Baro Altitude is still used for vertical separation between aircraft. When colder than ISA, GPS altitude indicates above Baro Altitude and the other way around during the warmer times. Baro is still used for step downs and MDA/DA determination. Fortunately, the closer to the ground one gets, the temperature induced error decreases. Temperature limitations don't apply to WAAS when it is used for a GS as it is unaffected, but the DA or MDA still uses Baro.
 
John Collins said:
Yes. The local altimeter setting will have taken into account temperature at the airport surface, but as you get higher above the airport elevation, the altimeter error in the aircraft altimeter continues to increase, putting you lower than it indicates. So at 200 feet above the airport, the correction might only be 30 feet, by the time you are at 3000 feet above airport elevation, the altimeter will be off by 420 feet. That is why you will see most of the affected segments are in the intermediate segment of the approach where there is only a 500 foot required clearance of obstacles.
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I assumed we should always use the local and ATC assigned alt settings, so when exactly are you supposed to deviate by this rule?
 
John Collins said:
I don't think this would be in the cards. Baro Altitude is still used for vertical separation between aircraft. When colder than ISA, GPS altitude indicates above Baro Altitude and the other way around during the warmer times. Baro is still used for step downs and MDA/DA determination. Fortunately, the closer to the ground one gets, the temperature induced error decreases. Temperature limitations don't apply to WAAS when it is used for a GS as it is unaffected, but the DA or MDA still uses Baro.
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Thanks. I assume that if aviation started today, altitude (and separation) would be GPS/WAAS based, but clearly transitioning to it from a baro-based legacy is complicated.
 
JasonM said:
I assumed we should always use the local and ATC assigned alt settings, so when exactly are you supposed to deviate by this rule?
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Only when the temperature is cold enough to be a factor. How do you know when that is? Once the approach charts are all updated it will be indicated by a "snowflake" icon following by the temperature at which you need to start compensating. The NOTAM has more information. You also need to tell ATC you're doing this.
 
JasonM said:
I assumed we should always use the local and ATC assigned alt settings, so when exactly are you supposed to deviate by this rule?
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You aren't adjusting the altimeter setting, you are adjusting the altitudes on the approach plate.
 
Like Gucci said, it's standard to make the calculations in the military below 0 C. D-15 in the Flight Information Handbook has the chart. Always a good question on instrument rides.
 
John Collins said:
I don't think this would be in the cards. Baro Altitude is still used for vertical separation between aircraft. When colder than ISA, GPS altitude indicates above Baro Altitude and the other way around during the warmer times. Baro is still used for step downs and MDA/DA determination. Fortunately, the closer to the ground one gets, the temperature induced error decreases. Temperature limitations don't apply to WAAS when it is used for a GS as it is unaffected, but the DA or MDA still uses Baro.
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This is a nit, and I might be missing something, but isn't this backwards? GPS should be indicating the true altitude above MSL to within a few meters, at least with WAAS. If Baro was reading lower than the actual altitude, there would be no need to add a correction to Baro to ensure adequate obstacle clearance. I think MikeOH gave the correct explanation earlier, in cold weather all the pressure levels are lower, so the Baro altimeter reads higher than true altitude.

Again, I might be missing something re what is meant by: GPS altitude.
 
azure said:
This is a nit, and I might be missing something, but isn't this backwards? GPS should be indicating the true altitude above MSL to within a few meters, at least with WAAS. If Baro was reading lower than the actual altitude, there would be no need to add a correction to Baro to ensure adequate obstacle clearance. I think MikeOH gave the correct explanation earlier, in cold weather all the pressure levels are lower, so the Baro altimeter reads higher than true altitude.

Again, I might be missing something re what is meant by: GPS altitude.
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Right. I had it backwards. :redface: GPS altitude is closer to true altitude and not affected by temperature, and the Baro altitude will indicate higher than true altitude in cold conditions.
 
azure said:
This is a nit, and I might be missing something, but isn't this backwards? GPS should be indicating the true altitude above MSL to within a few meters, at least with WAAS. If Baro was reading lower than the actual altitude, there would be no need to add a correction to Baro to ensure adequate obstacle clearance. I think MikeOH gave the correct explanation earlier, in cold weather all the pressure levels are lower, so the Baro altimeter reads higher than true altitude.

Again, I might be missing something re what is meant by: GPS altitude.
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What you say sounds correct to me. When it's cold, there is less airmass above the barometric altimeter, so it reads higher. OTOH, GPS doesn't care and always gives the correct altitude. So in cold air, GPS reading (which is correct) would be lower than the barometric reading, corrected to a nearby airport (unless they are both sitting on the ramp at the airport).
 
John Collins said:
Baro Altitude is still used for vertical separation between aircraft. When colder than ISA, GPS altitude indicates above Baro Altitude and the other way around during the warmer times.
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azure said:
This is a nit... If Baro was reading lower than the actual altitude, there would be no need to add a correction to Baro to ensure adequate obstacle clearance.
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With respect to the vertical separation of aircraft I see nothing wrong with John's statement. The correction is for obstacle clearance when terrain becomes the issue. That begs the question, "What's next, MEAs?"

RotorDude said:
What you say sounds correct to me. When it's cold, there is less airmass above the barometric altimeter, so it reads higher.
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I think the mass of the air above is the same (altimeters measure the weight of the atmosphere above), but the pressure levels are closer togather, so the altimeter is closer to the earth.

dtuuri
 
dtuuri said:
I think the mass of the air above is the same (altimeters measure the weight of the atmosphere above), but the pressure levels are closer togather, so the altimeter is closer to the earth.

dtuuri
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No, if the air is unusually cold, more of it settles under the aircraft than normally for a given actual MSL altitude. This means there are fewer molecules above, i.e. less mass in the column above it, and therefore less weight, which leads to lower pressure reading, which makes the barometric altimeter indicate higher than it really is. The GPS altimeter would keep chugging along with no change, since it doesn't care about air pressures or temperatures.
 
RotorDude said:
No, if the air is unusually cold, more of it settles under the aircraft than normally for a given actual MSL altitude. This means there are fewer molecules above, i.e. less mass in the column above it, and therefore less weight, which leads to lower pressure reading, which makes the barometric altimeter indicate higher than it really is. The GPS altimeter would keep chugging along with no change, since it doesn't care about air pressures or temperatures.
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With respect to the the vertical separation of aircraft then, you agree with me? That's the context of John's statement as I understand it. Aircraft at a colder than standard pressure level, by definition, have the same pressure (weight/mass) of air above it as normally. GPS altitude stays constant; cold air contracts vertically, lowering all pressure levels.

dtuuri
 
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RotorDude said:
What you say sounds correct to me. When it's cold, there is less airmass above the barometric altimeter, so it reads higher. OTOH, GPS doesn't care and always gives the correct altitude. So in cold air, GPS reading (which is correct) would be lower than the barometric reading, corrected to a nearby airport (unless they are both sitting on the ramp at the airport).
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AIM 1-1-18: "GPS altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided."


Bob Gardner
 
dtuuri said:
With respect to the the vertical separation of aircraft then, you agree with me? That's the context of John's statement as I understand it. Aircraft at a colder than standard pressure level, by definition, have the same pressure (weight/mass) of air above it as normally. GPS altitude stays constant; cold air contracts vertically, lowering all pressure levels.

dtuuri
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Sure, for separation purposes all you need is for all barometric altimeters to be properly calibrated and indicate the same reading under the same conditions (and of course must have the same altimeter setting). And yes, when an aircraft follows a "pressure level", it would descend/climb relative to true MSL altitude, depending on the pressure and temperature variations. And also yes, at a given pressure setting, that corresponds to the weight of the atmosphere above the barometric altimeter, so that weight will remain constant if the pressure indication (i.e. indicated altitude) is constant.
 
bobmrg said:
AIM 1-1-18: "GPS altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided."
Bob Gardner
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I don't think that statement is specific to WAAS, but GPS in general.
WAAS GPS is considerably more accurate than GPS alone, and has built-in integrity validation. According to this comparison table, WAAS GPS is spec'ed to be accurate (both vertically and horizontally) to 7.6 m (about 25 feet) at least 95% of the time. The actual performance is apparently much better, measured to be better than 1.3 m (about 4.3 feet). Again, these are both vertical and horizontal variations. Barometric altimeter accuracy is far worse, subject to many errors, and dependent on a manually-entered altimeter setting which is known to be correct in only one spot on Earth, and even then only to perhaps +/- 75 feet.
So the issue is not accuracy but traffic separation. Obviously all aircraft must be on the same page for separation purposes, be it barometric or GPS/WAAS, and if someone follows their GPS/WAAS, even if 100% accurate as far as true MSL altitude, it could easily run them smack into traffic legally following their barometric altimeter.
 
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The faa is running a bunch of safety seminars on the Notam, or don't fly where it gets that cold.
 
Where do I go to sign up for my cold temp compensation again? :dunno:
 
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