FPA correction during Cold Weather approaches
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FPA correction during Cold Weather approaches
I fly the A330 and my Company has just renewed its guidance on how to fly NPA's with cold weather altimetry corrections. As with most guidance it issues important considerations have been left out, and when questioned nothing definitive has been given.
I would appreciate if any TRI/TRE's could share what they suggest in the following example.
Flying either a VOR or RNAV approach to an LNAV only minima with temp below 0'C: Our Company guidance is to correct both minima and FAF crossing altitude in FMC. What they will not say is what correction to FPA should be applied to stay on the profile. Mathematically I would say the same correction as that applied to the altitudes, based on FPA being derived from barometric sources not purely IRS, and so also needing correcting for the cold temperatures.
ie if modifying FAF and Minima by 10% then correct FPA also by 10%. The best I could get from our training dept is "increase it a little".
So, Vilas or any other TRE out there, is there any guidance you have in this scenario?
I fully understand if temp still allows to fly to an LNAV/VNAV minima, then don't correct FAF altitude and stay in FINAL APP mode with the coded FPA if possible. What I am looking for is guidance when having to fly APPNAV/FPA modes.
I would appreciate if any TRI/TRE's could share what they suggest in the following example.
Flying either a VOR or RNAV approach to an LNAV only minima with temp below 0'C: Our Company guidance is to correct both minima and FAF crossing altitude in FMC. What they will not say is what correction to FPA should be applied to stay on the profile. Mathematically I would say the same correction as that applied to the altitudes, based on FPA being derived from barometric sources not purely IRS, and so also needing correcting for the cold temperatures.
ie if modifying FAF and Minima by 10% then correct FPA also by 10%. The best I could get from our training dept is "increase it a little".
So, Vilas or any other TRE out there, is there any guidance you have in this scenario?
I fully understand if temp still allows to fly to an LNAV/VNAV minima, then don't correct FAF altitude and stay in FINAL APP mode with the coded FPA if possible. What I am looking for is guidance when having to fly APPNAV/FPA modes.
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Thanks Underfire,
Your formula relates to the altitude correction for cold weather.
My Company already specifies conservative corrections to be added to altitudes depending on ISA dev, what they don't say is if the FPA is also corrected by the same amount. These figures are 10% for ISA-15 and 20% for ISA-30, so are quite significant corrections.
ie In your first example for a temp of 0C at sea level you would increase altitudes (both FAF and MDA) by 6%. So if the normal FPA was 3degrees, do you fly from the corrected FAF altitude at 3x1.06? to stay on profile? This would make sense to me.
Your formula relates to the altitude correction for cold weather.
My Company already specifies conservative corrections to be added to altitudes depending on ISA dev, what they don't say is if the FPA is also corrected by the same amount. These figures are 10% for ISA-15 and 20% for ISA-30, so are quite significant corrections.
ie In your first example for a temp of 0C at sea level you would increase altitudes (both FAF and MDA) by 6%. So if the normal FPA was 3degrees, do you fly from the corrected FAF altitude at 3x1.06? to stay on profile? This would make sense to me.
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I think I'm probably being thick here, but.......
if you correct the platform altitude for temperature, you will now be flying at the correct altitude that you would be at if it was ISA temps.
Therefore, do you need to correct the FPA at all?
Dunno. Will try it in the sim.
mcdhu
if you correct the platform altitude for temperature, you will now be flying at the correct altitude that you would be at if it was ISA temps.
Therefore, do you need to correct the FPA at all?
Dunno. Will try it in the sim.
mcdhu
mcdhu,
Although the actual FPA remains the same, the FPA calculated by the aircraft will be different, because the ADIRS uses barometric altitude in its calculations. For example, to fly a 3° glidepath you might need to set an FPA of 3.3° on the MCP/FCU. The aircraft will think it's flying an FPA of 3.3°, but the true FPA will be 3°.
CX-HOR,
Your approximation is correct. If you need to correct the altitudes by 10%, then you should also correct the FPA by 10%. The actual calculation is:
FPA correction = arctan (altitude correction/distance to the field)
For example, let's say the glidepath is 3° and the FAF is at 1300ft, 4nm from the airfield. If you need to correct the FAF by 10%, then the correction is 130ft. According to the formula above, the FPA correction would be:
arctan (130/4 x 6076) = arctan (130/24304) = arctan (0.005349) = 0.306466° ≈ 0.3°,
so the corrected FPA would be 3° + 0.3° = 3.3°
You can get the same result using the approximation method:
10% x 3° = 0.3°
The same method also works for a 20% correction.
Although the actual FPA remains the same, the FPA calculated by the aircraft will be different, because the ADIRS uses barometric altitude in its calculations. For example, to fly a 3° glidepath you might need to set an FPA of 3.3° on the MCP/FCU. The aircraft will think it's flying an FPA of 3.3°, but the true FPA will be 3°.
CX-HOR,
Your approximation is correct. If you need to correct the altitudes by 10%, then you should also correct the FPA by 10%. The actual calculation is:
FPA correction = arctan (altitude correction/distance to the field)
For example, let's say the glidepath is 3° and the FAF is at 1300ft, 4nm from the airfield. If you need to correct the FAF by 10%, then the correction is 130ft. According to the formula above, the FPA correction would be:
arctan (130/4 x 6076) = arctan (130/24304) = arctan (0.005349) = 0.306466° ≈ 0.3°,
so the corrected FPA would be 3° + 0.3° = 3.3°
You can get the same result using the approximation method:
10% x 3° = 0.3°
The same method also works for a 20% correction.
Last edited by BuzzBox; 27th Jul 2016 at 02:08.
From the A330 FCTM (my emphasis):
In cold weather, the atmosphere differs from the International Standard Atmosphere (ISA) conditions. The parameters computed by the ADIRS are barometric and ISA-referenced. The ADIRS use inertial data and barometric altitude to compute FPA. When the atmosphere differs from ISA, the altitude and FPA computed by the ADIRS and the associated indications on the PFD (altitude, VDEV) are not accurate.
When the temperature is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA the aircraft flies is less steep than the FPA the ADIRS computes.
When the temperature is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA the aircraft flies is less steep than the FPA the ADIRS computes.
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Hi CX,
all the barometric altitudes associated with the procedures will be subject to an altimeter error in cold weather ops, i.e your true altitude will be lower than your indicated altitude as you know.
When the OAT falls at 0 deg C and below you will need to apply some corrections to the different altitudes of your procedures, that is as you say the FAF altitude, the minima but the step down fixes altitudes as well. You can get the temperature corrections tables for high and low altitude in your FCOM PER-OPD-GEN-ALTITUDE TEMPERATURE CORRECTION. Airbus has removed a while back the tables for FPA modification as its usage was not practical. It is obviously quite impractical as well to temperature correct all the step down altitudes after the FAF in a NPA, therefore one method that you could use is to correct the FAF and the MDA and from there build your own descent profile, i.e check how many miles you have between the FAF and the last DME check altitude before your MDA and find out how many feet you need to lose per NM. You will definitely have a slightly steeper angle of descent.
In the calculations are correct, you will end up at your last DME check fix altitude higher of the amount of feet that are required from the above mentioned table within +/- 20 ft we could say.
all the barometric altitudes associated with the procedures will be subject to an altimeter error in cold weather ops, i.e your true altitude will be lower than your indicated altitude as you know.
When the OAT falls at 0 deg C and below you will need to apply some corrections to the different altitudes of your procedures, that is as you say the FAF altitude, the minima but the step down fixes altitudes as well. You can get the temperature corrections tables for high and low altitude in your FCOM PER-OPD-GEN-ALTITUDE TEMPERATURE CORRECTION. Airbus has removed a while back the tables for FPA modification as its usage was not practical. It is obviously quite impractical as well to temperature correct all the step down altitudes after the FAF in a NPA, therefore one method that you could use is to correct the FAF and the MDA and from there build your own descent profile, i.e check how many miles you have between the FAF and the last DME check altitude before your MDA and find out how many feet you need to lose per NM. You will definitely have a slightly steeper angle of descent.
In the calculations are correct, you will end up at your last DME check fix altitude higher of the amount of feet that are required from the above mentioned table within +/- 20 ft we could say.
Last edited by sonicbum; 26th Jul 2016 at 14:35. Reason: Explanation error
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Procedure design calculates the low and high temps, based on the published FPA and ISA. NA below is 0.917 FPA, and NA above 1.13 FPA. (so, in reality, you need to correct to .917GPA to have obstacle protection, ie 2.75 instead of 3)
There are components of both temperature and altitude to consider.
There are components of both temperature and altitude to consider.
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Procedure design calculates the low and high temps, based on the published FPA and ISA. NA below is 0.917 FPA, and NA above 1.13 FPA. (so, in reality, you need to correct to .917GPA to have obstacle protection, ie 2.75 instead of 3)
There are components of both temperature and altitude to consider.
There are components of both temperature and altitude to consider.
Procedure design calculates the low and high temps, based on the published FPA and ISA. NA below is 0.917 FPA, and NA above 1.13 FPA. (so, in reality, you need to correct to .917GPA to have obstacle protection, ie 2.75 instead of 3)
There are components of both temperature and altitude to consider.
There are components of both temperature and altitude to consider.
http://www.faa.gov/documentlibrary/m...nd/8260_52.pdf
CX-HOR's question related to approaches that are flown to LNAV minima, without vertical guidance, where the pilot controls the vertical profile by selecting an FPA on the autopilot MCP/FCU. In cold temperatures, the aircraft's actual FPA will be less steep than the FPA computed by the ADIRS. In such cases, the pilot should select a higher FPA on the MCP/FCU to ensure the aircraft flies the required FPA, as described above.
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When flying such approaches, the DA must be corrected for temperature,
RNAV(RNP) usually always have a range of useable temperatures or at least the minimum useable temperature for the procedure, as for example in Kathmandu RNAV RNP RWY02.
No corrections are required to any altitude, include the DA.
Only half a speed-brake
No corrections are required .... include the DA.
Hey Sonicbum,
The rules might be different in the US, but my understanding is that when approaches are designed to PANS OPS criteria, the temperature limitation only prevents the angle of the final approach path from becoming too shallow. Pilots still need to correct the DA to ensure sufficient obstacle clearance at the minima.
According to PANS OPS Doc 8168 Vol 1:
The rules might be different in the US, but my understanding is that when approaches are designed to PANS OPS criteria, the temperature limitation only prevents the angle of the final approach path from becoming too shallow. Pilots still need to correct the DA to ensure sufficient obstacle clearance at the minima.
According to PANS OPS Doc 8168 Vol 1:
Part II FLIGHT PROCEDURES - RNAV AND SATELLITE BASED
Section 4 - Approach procedures with vertical guidance
Chapter 1 - APV/Baro VNAV approach procedures
1.4 OPERATIONAL CONSTRAINTS
1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s);
b) the DA/H; and
c) subsequent missed approach altitudes/heights.
Note.— The final approach path VPA is safeguarded against the effects of low temperature by the design of the procedure.
Section 4 - Approach procedures with vertical guidance
Chapter 1 - APV/Baro VNAV approach procedures
1.4 OPERATIONAL CONSTRAINTS
1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s);
b) the DA/H; and
c) subsequent missed approach altitudes/heights.
Note.— The final approach path VPA is safeguarded against the effects of low temperature by the design of the procedure.
Last edited by BuzzBox; 29th Jul 2016 at 01:58.
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Hi Buzz,
from the ICAO RNAV (RNP) procedure approach design :
"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..)"
According to the above you are obstacle protected at the promulgated altitudes, therefore I do not see the reason to correct the DA.
Reference RNAV RNP APPROACHES DESIGN MANUAL
from the ICAO RNAV (RNP) procedure approach design :
"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..)"
According to the above you are obstacle protected at the promulgated altitudes, therefore I do not see the reason to correct the DA.
Reference RNAV RNP APPROACHES DESIGN MANUAL
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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. (where ICAO goes to 2.5, FAA goes to 2.71)
Post #12, which no one seems to understand, includes the calculations, should anyone want to understand the temperature limits/protection, as well as how to compensate for temperatures and adjust the FPA accordingly. (ie if you want a 3 degree FPA and the temperature is this, here is the difference)
In the procedure design, you are protected within the limits of the procedure, but outside the procedure, you need to provide corrections. The includes temperature and the effect of turns. I suppose turn protection would be a matter for another topic.
Post #12, which no one seems to understand, includes the calculations, should anyone want to understand the temperature limits/protection, as well as how to compensate for temperatures and adjust the FPA accordingly. (ie if you want a 3 degree FPA and the temperature is this, here is the difference)
In the procedure design, you are protected within the limits of the procedure, but outside the procedure, you need to provide corrections. The includes temperature and the effect of turns. I suppose turn protection would be a matter for another topic.
Last edited by underfire; 29th Jul 2016 at 07:02.