Airbus FPA cold Weather
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Yes. You set what you need, be it: a corrected FPA but not necessarily the exact one that you extracted from a chart.
What I am arguing is to use a corrected FPA from the FDP, directly proportionate to the altitude corrections applied, as an initial reference value. Subsequent (corr) alt vs distance checks will dictate how to adjust the FPA.
I think we’re clear on that.
Before, I was merely trying to explain the rationale of Airbus removing the FPA correction table from both A320 and A330 documentation:
-to avoid leading people into thinking that the single extracted ‘table FPA’ will 100% ensure correct vertical guidance without any further input and/or monitoring/crosscheck. (Subsequent litigation also)
It was decided that it is more important that pilots understand the temp effect on the FPA as a concept and apply corrections by anticipation without the use of a table (proportional to altitude corrections) and subsequent crosschecking.
What I am arguing is to use a corrected FPA from the FDP, directly proportionate to the altitude corrections applied, as an initial reference value. Subsequent (corr) alt vs distance checks will dictate how to adjust the FPA.
I think we’re clear on that.
Before, I was merely trying to explain the rationale of Airbus removing the FPA correction table from both A320 and A330 documentation:
-to avoid leading people into thinking that the single extracted ‘table FPA’ will 100% ensure correct vertical guidance without any further input and/or monitoring/crosscheck. (Subsequent litigation also)
It was decided that it is more important that pilots understand the temp effect on the FPA as a concept and apply corrections by anticipation without the use of a table (proportional to altitude corrections) and subsequent crosschecking.
OPEN DESCENT
Using a cold temperature corrected FPA and not doing D/Ht check are two different things. Both are required. If you set ISA chart FPA and you find you are in error at the first check then after correcting when you go back to the same FPA you will again be in error because FPA is not appropriate. So you start the descent at corrected FPA but still check after I nm. If in error do the required correction and when correct reset the corrected FPA. This way you have better chance of tracking correct vertical path.
Using a cold temperature corrected FPA and not doing D/Ht check are two different things. Both are required. If you set ISA chart FPA and you find you are in error at the first check then after correcting when you go back to the same FPA you will again be in error because FPA is not appropriate. So you start the descent at corrected FPA but still check after I nm. If in error do the required correction and when correct reset the corrected FPA. This way you have better chance of tracking correct vertical path.
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FPA as a flight guidance target is a performance target, not a geometric target. As you said.....
Off-topic
PS: hence the preference to use ALT p/b iso Push to Level Off in the A330 for EFATO-3rd segment and Circling. ALT is a geometric mode, well barometric really (locked to an ALT) whilst PTLO merely targets vs=0, prone to disturbances: parallel tracking.
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Thanks for the answers about the cold weather corrections.
It looks like you are correct in Airbus wanting the pilots to do it all themselves with this statement from the FCTM....
"FPA Correction
When the temperature is lower than ISA, the FPA that the aircraft actually flies is less steep
than the FPA that the ADIRS (ISA referenced) computes.
In vertical selected mode FPA, to correct the FPA for this ISA deviation effect, the flight crew
should select on the FCU a FPA slightly different from the FPA that the aircraft needs to fly."
Bottom line, you have to guess at the descent angle correction.
It looks like you are correct in Airbus wanting the pilots to do it all themselves with this statement from the FCTM....
"FPA Correction
When the temperature is lower than ISA, the FPA that the aircraft actually flies is less steep
than the FPA that the ADIRS (ISA referenced) computes.
In vertical selected mode FPA, to correct the FPA for this ISA deviation effect, the flight crew
should select on the FCU a FPA slightly different from the FPA that the aircraft needs to fly."
Bottom line, you have to guess at the descent angle correction.
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I would keep it simple by just setting the FPA showed on the chart and adjust accordingly (0.1 degree per 10 feet deviation) when checking the altitude every nautical mile. Never been a problem so far.
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Indeed (ballpark)
delta 1.0 fpa = delta 100 ft/nm
hence
delta 0.1 fpa = delta 10ft/nm (the correction you’re talking about)
However these are corrections RELATIVE to your ‘base’ FPA. ISA deviation affects the ‘base’ FPA,
ISA deviation has to be taken into account before starting the approach.
The reactive correction on FPA caters for small errors but does not address a systematic error such as a large ISA deviation.
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Hi Open Des.
I undestand your point but how much deviation could we see? I’m not familiar flying in extreme cold places. The few cold place we fly it’s around -25 degrees from ISA and we are doing ILS under radar vector so we only apply a correction on the DA.
Assuming it’s really cold like -40 degrees from ISA. We shoot a VOR selected approach with a standard descent angle of 3 degrees. We start the descent at the FAF at the corrected altitude but use a FPA of only 3 degrees. The table for table correction shows it should be 3.4 degrees. It’s only .4 degrees difference. Surely at the first DME check we will notice we are too high and correct it accordingly. Then when back on profile, we put it back to 3 degrees and yes we might get slightly above the profile again. Maybe I’m missing something. What’s the worst case than can happen? We might be slightly high when visual?
I think there is a good reason they removed that table: Better be slightly high than too low. Imagine the crew use that FPA. corrections but did not correct the FAF altitude. That could be a dangerous situation if no proper DME checks. Just speculating here. Don’t throw rocks at me. This is quite new to me and it’s interesting.
I undestand your point but how much deviation could we see? I’m not familiar flying in extreme cold places. The few cold place we fly it’s around -25 degrees from ISA and we are doing ILS under radar vector so we only apply a correction on the DA.
Assuming it’s really cold like -40 degrees from ISA. We shoot a VOR selected approach with a standard descent angle of 3 degrees. We start the descent at the FAF at the corrected altitude but use a FPA of only 3 degrees. The table for table correction shows it should be 3.4 degrees. It’s only .4 degrees difference. Surely at the first DME check we will notice we are too high and correct it accordingly. Then when back on profile, we put it back to 3 degrees and yes we might get slightly above the profile again. Maybe I’m missing something. What’s the worst case than can happen? We might be slightly high when visual?
I think there is a good reason they removed that table: Better be slightly high than too low. Imagine the crew use that FPA. corrections but did not correct the FAF altitude. That could be a dangerous situation if no proper DME checks. Just speculating here. Don’t throw rocks at me. This is quite new to me and it’s interesting.
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Hi!
in your example where 3.4 would be the corrected FPA. If you’d set 3.0, you will probably gain around 40ft/nm.
40ft high at the first DME check would entice you to set 3.4. This would lead to you paralleling the vertical profile, being 40ft high once again at the next DME check. Now you’d select 3.8, back on profile at the next DME check where you’d set 3.4 back (50% of crew would set 3.0 probably).
So you can see the need of anticipation, especially when ISA deviation is large.
note: the exact opposite corrections are required in hot conditions, especially with temp inversions etc.
Anyhow, the discussion is quite academic. The most important thing is that we understand how the FPA behaves in non-ISA conditions and make due allowance for this by understanding.
in your example where 3.4 would be the corrected FPA. If you’d set 3.0, you will probably gain around 40ft/nm.
40ft high at the first DME check would entice you to set 3.4. This would lead to you paralleling the vertical profile, being 40ft high once again at the next DME check. Now you’d select 3.8, back on profile at the next DME check where you’d set 3.4 back (50% of crew would set 3.0 probably).
So you can see the need of anticipation, especially when ISA deviation is large.
note: the exact opposite corrections are required in hot conditions, especially with temp inversions etc.
Anyhow, the discussion is quite academic. The most important thing is that we understand how the FPA behaves in non-ISA conditions and make due allowance for this by understanding.
Pineteam - remember there is no great harm in setting a FPA and it being slightly too steep. It is permissible to go below profile on a NPA - as long as you are above the floor limit for that distance. Set what you reckon (a good start is to start with the FAF correction of +X%, increase the FPA by X%). Then adjust accordingly after the corrected alt / distance cross check. Watch the floor limits like a hawk of course (if any) and If you get a bit low, reduce the FPA or vice versa.
I normally have the corrected altitudes for the on the day temp for the chart distances to hand and get the PM to call high or low off them. I can tell you it works just fine for LNAVs to ISA-57°C.
I normally have the corrected altitudes for the on the day temp for the chart distances to hand and get the PM to call high or low off them. I can tell you it works just fine for LNAVs to ISA-57°C.
Last edited by compressor stall; 26th Mar 2020 at 23:29.
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Thank you guys!
Compressor stall, Sorry I have a doubt about how you get the FPA of 3.41 degrees in your example quoted below. I saw you put the formula in an earlier post but I still don't get it.
I thought to find the angle we could simply take the height divided by the distance divided by 100. For example if you are 3000 feet at 10 nm: 3000/10 = 300 then 300/100 = 3 degrees.
But when I apply this formula with 2280 over 6.3nm /100 I get 3.61. But somehow you get 3.41...
Compressor stall, Sorry I have a doubt about how you get the FPA of 3.41 degrees in your example quoted below. I saw you put the formula in an earlier post but I still don't get it.
I thought to find the angle we could simply take the height divided by the distance divided by 100. For example if you are 3000 feet at 10 nm: 3000/10 = 300 then 300/100 = 3 degrees.
But when I apply this formula with 2280 over 6.3nm /100 I get 3.61. But somehow you get 3.41...
3000/10 giving a three degree slope is an easy approximation. But it’s not exactly three degrees - is a couple of hundred feet out. (Actually 3182’ over 10nm). Applying this approximation to FPA calcs gives the 0.2 difference between our answers.
Remember high school trig? SOH CAH TOA
In my earlier post you have a triangle of 2280 (opposite) and 6.3*6072 (adjacent).
that’s 3.41 degrees. Plenty of online trig calculators to help visualise.
And to clarify the 10% quick rule is say: FAF = 2000. FAF Corrected =2280.
2280/2000=1.14. Ie 14% higher. Times this by the chart FPA.
1.14*3= 3.42 degrees. Closer to the actual FPA than you can set of the dial.
Remember high school trig? SOH CAH TOA
In my earlier post you have a triangle of 2280 (opposite) and 6.3*6072 (adjacent).
that’s 3.41 degrees. Plenty of online trig calculators to help visualise.
And to clarify the 10% quick rule is say: FAF = 2000. FAF Corrected =2280.
2280/2000=1.14. Ie 14% higher. Times this by the chart FPA.
1.14*3= 3.42 degrees. Closer to the actual FPA than you can set of the dial.
Last edited by compressor stall; 27th Mar 2020 at 21:23.
Only half a speed-brake
@pinteam
https://1drv.ms/x/s!AiE0Si5ywRcJhK8a...Ur7oA?e=8Z087J This links an excel file on Onedrive storage. Provided here solely for studying the underlying logic, all the other usual disclaimers apply.
Somebody asked for a template - Jepp lookalike generator - for these things in a company I cared for. A page to insert into the route manuals. Guess the math still works 10 years onwards, fill the blue box and hit print, voila a sheet should come out. Won't work well in the Online Excel, need to download the file to disk. The original file contains no macros or VBA, heed any warnings about malware if shown!
The corrected altitude is calculated using a formula from Doc 8168, and the FPA needed just using ARCtang. Same as carefully explained above already, no magic.
https://1drv.ms/x/s!AiE0Si5ywRcJhK8a...Ur7oA?e=8Z087J This links an excel file on Onedrive storage. Provided here solely for studying the underlying logic, all the other usual disclaimers apply.
Somebody asked for a template - Jepp lookalike generator - for these things in a company I cared for. A page to insert into the route manuals. Guess the math still works 10 years onwards, fill the blue box and hit print, voila a sheet should come out. Won't work well in the Online Excel, need to download the file to disk. The original file contains no macros or VBA, heed any warnings about malware if shown!
The corrected altitude is calculated using a formula from Doc 8168, and the FPA needed just using ARCtang. Same as carefully explained above already, no magic.
