FPA correction during Cold Weather approaches
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Thank you Buzz for the reference.
Sonic and Underfire: I understand you saying there is no geometrical reason to increase published values in order to maintain the obstacle clearance. Is that right?
The DA, pardon my ignorance, is not published in the AIP and not calculated by the procedure designer.
Sonic and Underfire: I understand you saying there is no geometrical reason to increase published values in order to maintain the obstacle clearance. Is that right?
The DA, pardon my ignorance, is not published in the AIP and not calculated by the procedure designer.
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I think that needs more detail. If you are within the NA abv/bel, then no, you do not need to add or subtract anything. This is calculated by the procedure designer to work within the min GPA to maintain obstacle clearance. Depending on the design, temperature and turn corrections may be included in the design.
DA not published in the AIP? I would guess that you are looking at a particular design. DA/MDA are always calculated by the designer using obstacles in either the approach or the missed approach.
If you are saying that when MDA is shown, DA is not calculated, I suppose that is true, and it leaves it up to the pilot to make sure that the decision altitude is high enough so that with momentary descent, MDA surface is not encroached.
DA not published in the AIP? I would guess that you are looking at a particular design. DA/MDA are always calculated by the designer using obstacles in either the approach or the missed approach.
If you are saying that when MDA is shown, DA is not calculated, I suppose that is true, and it leaves it up to the pilot to make sure that the decision altitude is high enough so that with momentary descent, MDA surface is not encroached.
Hi Sonicbum,
That quote comes from a section titled 'Effect of temperature on VPA'. I agree, the low temperature limit ensures obstacle clearance during the final segment by stopping the flight path angle from going below 2.5° (PANS-OPS). But what happens when you arrive at the DA? Under PANS-OPS, the DA is based on a minimum OCA/H of 75m/246 ft. The minimum OCA/H is based on a minimum obstacle clearance (MOC) of 75m/246 ft during the final approach. If you don't correct the DA for low temperature, then you won't have the required MOC when you arrive at the minima.
Some of the information published by the various regulators is confusing and contradictory regarding temperature correction requirements. However, Transport Canada's Advisory Circular 700-024 Required Navigation Performance Authorization Required Approach is very clear. It states the following:
https://www.tc.gc.ca/media/documents...24_issue_2.pdf
"The low temperature limit assures obstacle protection for the lowest expected temperature and prevents the effective VPA from going below 2.5 degrees.
ISA for the airport may be calculated using the following formulas. (etc..)"
ISA for the airport may be calculated using the following formulas. (etc..)"
Some of the information published by the various regulators is confusing and contradictory regarding temperature correction requirements. However, Transport Canada's Advisory Circular 700-024 Required Navigation Performance Authorization Required Approach is very clear. It states the following:
5.2 (10) Navigation System without Temperature Compensation. For aircraft without temperature compensation capabilities, the operation must occur within the temperature limits (TLim) published on the approach chart. Despite being uncompensated, the baro-VNAV path in the final segment will provide the required obstacle clearance. The flight crew members must correct, as required, for the cold temperature effects on minimum altitudes such as IAF, IF, FAF, DA, MA and MSA. In below ISA temperatures the baro-VNAV path will cross the FAF at an altitude below the temperature-corrected FAF crossing altitude. It is permissible to follow the baro-VNAV path from the temperature-corrected intermediate segment altitude.
Last edited by BuzzBox; 30th Jul 2016 at 08:06.
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The DA taken into account the cold temp limit GPA with the associated obstacle clearance.
The operative word here is minimum. In reality, it is a 200' ROC with a 50 foot momentary descent, that is a DA. (slightly different for PANS-OPS) The ROC is a sloping surface, different from the GPA, sloping from the TCH (150' ROC with 50' TCH) to 500 feet at the FAF. As noted by the text, the cold temp limit is accounted for in the calculations.
Minimum Sector Altitude
Currently, there is not a European-wide common procedure to deal with adjustments to Minimum Sector Altitudes (MSAs). Some regulators do not specify adjustments to MSAs and consequently ATC providers do not apply a temperature correction to published MSAs for cold temperatures. It is the flight crew reponsibility according to the provisions of ICAO PANS OPS referred above.
Some operators advise flight crews to add 1000 ft to the MSA when the temperature is - 30 °C or colder. (RAF FIH)
Minimum Vectoring Altitude
MVAs are established for use by the Air Traffic Controller (ATCO) when Air Traffic Control (ATC) provide a surveillance service (usually radar). Each MVA chart contains sectors large enough to accommodate the vectoring of aircraft within the sector at the MVA. The minimum vectoring altitude in each sector provides 1000 ft above the highest obstruction in non-mountainous areas and 2000 ft above the highest obstacle in designated mountainous areas.
According to ICAO PANS OPS, minimum vectoring altitudes shall be corrected for temperature. The temperature correction shall be based on seasonal or annual minimum temperature records. In turn, ATC authorities are required, as per ICAO PANS ATM, 8.6.5.2, Note 2, “to provide the controller with minimum altitudes corrected for temperature effect”
Under PANS-OPS, the DA is based on a minimum OCA/H of 75m/246 ft. The minimum OCA/H is based on a minimum obstacle clearance (MOC) of 75m/246 ft during the final approach
Minimum Sector Altitude
Currently, there is not a European-wide common procedure to deal with adjustments to Minimum Sector Altitudes (MSAs). Some regulators do not specify adjustments to MSAs and consequently ATC providers do not apply a temperature correction to published MSAs for cold temperatures. It is the flight crew reponsibility according to the provisions of ICAO PANS OPS referred above.
Some operators advise flight crews to add 1000 ft to the MSA when the temperature is - 30 °C or colder. (RAF FIH)
Minimum Vectoring Altitude
MVAs are established for use by the Air Traffic Controller (ATCO) when Air Traffic Control (ATC) provide a surveillance service (usually radar). Each MVA chart contains sectors large enough to accommodate the vectoring of aircraft within the sector at the MVA. The minimum vectoring altitude in each sector provides 1000 ft above the highest obstruction in non-mountainous areas and 2000 ft above the highest obstacle in designated mountainous areas.
According to ICAO PANS OPS, minimum vectoring altitudes shall be corrected for temperature. The temperature correction shall be based on seasonal or annual minimum temperature records. In turn, ATC authorities are required, as per ICAO PANS ATM, 8.6.5.2, Note 2, “to provide the controller with minimum altitudes corrected for temperature effect”
Underfire,
Call me thick, but I'm not sure I understand your explanation. I am, after all, a simple pilot.
Let's say we have an approach to a sea-level airport, where the low temperature limit is -20°C. Are you saying that the OCA/H is calculated to ensure the minimum obstacle clearance of 75m/246 ft at the OCA/H is achieved at -20°C, and at higher temperatures the actual obstacle clearance would be greater? In other words, assuming no obstacles, the OCA/H would be something like 286 ft, providing at least 286 ft obstacle clearance at temperatures greater than 0°C and a minimum of 246 ft at -20°C?
Call me thick, but I'm not sure I understand your explanation. I am, after all, a simple pilot.
Let's say we have an approach to a sea-level airport, where the low temperature limit is -20°C. Are you saying that the OCA/H is calculated to ensure the minimum obstacle clearance of 75m/246 ft at the OCA/H is achieved at -20°C, and at higher temperatures the actual obstacle clearance would be greater? In other words, assuming no obstacles, the OCA/H would be something like 286 ft, providing at least 286 ft obstacle clearance at temperatures greater than 0°C and a minimum of 246 ft at -20°C?
Last edited by BuzzBox; 30th Jul 2016 at 06:48.
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What I am saying is that the minimum obstacle clearance already takes into account several factors, one of which is the temperature correction.
Another factor, with the design criteria, there are no corrections for turns, which is why the FAA states no turns inside the FAF (unless specially designed). The taper to 500' ROC is one attempt to blanket in turn corrections before the FAF.
There are factors accounting for the accuracy of the nav system/gps.
With the ILS, many factors do not need to be accounted for, and that is why the min ILS is a 150' ROC with 50' momentary descent, or the 200' min.
This may help as well
Vertical Error Budget (VEB).
The VEB is a set of allowable values that contribute to the total error associated with a VNAV system. Application of equations using the VEB values determines the minimum vertical clearance that must exist between an aircraft on the nominal glidepath and ground obstructions within the OEA of instrument procedure segments. When the VEB is used in final segment construction, its application determines the OCS origin and slope ratio.
or just give you a headache!
This is from Eurocontrol:
Another factor, with the design criteria, there are no corrections for turns, which is why the FAA states no turns inside the FAF (unless specially designed). The taper to 500' ROC is one attempt to blanket in turn corrections before the FAF.
There are factors accounting for the accuracy of the nav system/gps.
With the ILS, many factors do not need to be accounted for, and that is why the min ILS is a 150' ROC with 50' momentary descent, or the 200' min.
This may help as well
Vertical Error Budget (VEB).
The VEB is a set of allowable values that contribute to the total error associated with a VNAV system. Application of equations using the VEB values determines the minimum vertical clearance that must exist between an aircraft on the nominal glidepath and ground obstructions within the OEA of instrument procedure segments. When the VEB is used in final segment construction, its application determines the OCS origin and slope ratio.
or just give you a headache!This is from Eurocontrol:
Last edited by underfire; 31st Jul 2016 at 01:48.
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buzz...wanted to show a basic for the DA and OCS.
The obstacle evaluation surfaces for an aerodrome are fixed. If the procedure is outside of this, you must provide evaluations. Procedure design wants to avoid this, as then, every cycle, the obstacles must be evaluated.
Note that for the OCS, it is a fixed surface. This surface takes into account the low temp limit (FAA 2.71 eff GPA, and 2.5 effective GPA ICAO) ie the surface does not vary with the effective glideslope.
The DA is set by the controlling obstacle. (there are seldom obstacles in the approach, so the DA is usually set by an obstacle in the missed)When there are, it is typical to raise the HAT, hence the DA as illustrated.
This shows an obstacle in the missed (as well as the DA vs OCS, with momentary descent)
In a design, when you have turns, you have to adjust the surface to take into account the turn. While this appears simple, it is actually incredibly complex due to the body geometry of aircraft in a turn and the turn methodology between aircraft.
Enjoy!
The obstacle evaluation surfaces for an aerodrome are fixed. If the procedure is outside of this, you must provide evaluations. Procedure design wants to avoid this, as then, every cycle, the obstacles must be evaluated.
Note that for the OCS, it is a fixed surface. This surface takes into account the low temp limit (FAA 2.71 eff GPA, and 2.5 effective GPA ICAO) ie the surface does not vary with the effective glideslope.
The DA is set by the controlling obstacle. (there are seldom obstacles in the approach, so the DA is usually set by an obstacle in the missed)When there are, it is typical to raise the HAT, hence the DA as illustrated.
This shows an obstacle in the missed (as well as the DA vs OCS, with momentary descent)
In a design, when you have turns, you have to adjust the surface to take into account the turn. While this appears simple, it is actually incredibly complex due to the body geometry of aircraft in a turn and the turn methodology between aircraft.
Enjoy!
Last edited by underfire; 1st Aug 2016 at 01:02.
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...none of which has anything to do with the OP which buzz-box correctly and succinctly answered with his very first reply in post #5 after you failed to read the question and gave a superfluous and incorrect answer.
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response to buzz box if you can read and comprehend.
Since the OP's question was about cold weather correction standards, the actual calculations should be considered.
Since the OP's question was about cold weather correction standards, the actual calculations should be considered.
Last edited by underfire; 1st Aug 2016 at 22:05.
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Bloggs, I read it differently. They suggest that if the (corrected) FPA would bring you below the minimum altitudes, you do something about it. Correct the corrected 
Why the need for such statement is altogether a different thing.
Now, with the ISA dev being different between the points of measurement i.e. airport thermometer vs. aircraft static port, you put quite a can of worms on the table!
Case 1: -05 AD OAT, FG OVC001, temp. inversion at 1200 feet and 45 kt winds aloft at 3000 AAL. - - - no, I refuse to open that one
Why the need for such statement is altogether a different thing.Now, with the ISA dev being different between the points of measurement i.e. airport thermometer vs. aircraft static port, you put quite a can of worms on the table!
Case 1: -05 AD OAT, FG OVC001, temp. inversion at 1200 feet and 45 kt winds aloft at 3000 AAL. - - - no, I refuse to open that one
Last edited by FlightDetent; 19th Nov 2016 at 21:47.
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underfire:
The OP's question was specifically about adjusting the FPA when flying a CDFA to a non-precision approach with LNAV minima. All the stuff you have pasted is about ensuring obstacle clearance when designing an approach with vertical guidance, or simply applying a cold weather altimeter correction which is not what was asked.
Yes but the problem is that when buzzbox wrote...
...he was correct, whilst your response is incorrect:
The FAA states in NTAP US Cold Temperature Restricted Airports:
KTVC cold weather temperature restriction = -14ºC
RNAV RWY 10 uncompensated baro-VNAV NA below -19ºC
...correcting the DA is mandatory below -14 whilst you can still fly the baro-VNAV.
Since the OP's question was about cold weather correction standards, the actual calculations should be considered.
response to buzz box if you can read and comprehend.
"the temperature limitation only prevents the angle of the final approach path from becoming too shallow. Pilots still need to correct the DA"
...he was correct, whilst your response is incorrect:
You dont need to correct anything if you are within the temp limits, that is why the chart states uncompensated baro VNAV and there is NA below and NA above.
"Pilots must use the corrected MDA or DA/DH as the minimum for an approach...The temperature restriction at a "Cold Temperature Restricted Airport" is mutually exclusive from the charted temperature restriction published for "uncompensated baro-VNAV systems"
That is, pilots have to correct the DA anytime it is cold enough, as defined in the US by the list of Cold Temperature Restricted Airports, and none of the formulas for designing an APV approach you have pasted here override that simple requirement. eg:KTVC cold weather temperature restriction = -14ºC
RNAV RWY 10 uncompensated baro-VNAV NA below -19ºC
...correcting the DA is mandatory below -14 whilst you can still fly the baro-VNAV.
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Hi all! Sorry I'm busting in here, but seriously, this thread is pretty much the only useful one I found while googling 3 days straight looking for a formula on how to calculate FPA temperature corrections.
@BuzzBox
FPA correction = arctan (altitude correction/distance to the field)
Could you tell me where you got this? Thx in advance ...
@BuzzBox
FPA correction = arctan (altitude correction/distance to the field)
Could you tell me where you got this? Thx in advance ...
Only half a speed-brake
Trigonometry, no offence intended, just draw the two triangles. Mathematically precise: FPA corrected = arctan (height corrected/ distance to aimpoint).
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Well, I've been looking at it and isn't the correct formula like this:
https://docs.google.com/drawings/d/1...it?usp=sharing
?
But I suppose the more simple formula comes from small-angle approximation?
Edit: ok, I just saw you actually put height corrected instead of height correction. That kinda was the thing I was wondering about in BuzzBox's post.
Thx!
https://docs.google.com/drawings/d/1...it?usp=sharing
?
But I suppose the more simple formula comes from small-angle approximation?
Edit: ok, I just saw you actually put height corrected instead of height correction. That kinda was the thing I was wondering about in BuzzBox's post.
Thx!
Last edited by Kazume; 19th Nov 2016 at 23:12.
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Kazume looks like you're correcting height above FAF but not the threshold height.
Also I believe just proportionally correcting the FPA as suggested in the original post yields approximately the same answer... e.g. if the FAF correction was 10% then simply correct the FPA by 10% as well.
However the drawings aren't representative of what's actually happening, and I think these approximations may slightly underestimate the required FPA (so you'll tend to end up a little high).
Also I believe just proportionally correcting the FPA as suggested in the original post yields approximately the same answer... e.g. if the FAF correction was 10% then simply correct the FPA by 10% as well.
However the drawings aren't representative of what's actually happening, and I think these approximations may slightly underestimate the required FPA (so you'll tend to end up a little high).