Airbus FPA cold Weather [Archive]

tcasblue

18th Mar 2020, 03:29

I know it has been discussed before.

I was just reading an article about cold weather corrections in general and this statement was made...

"In 2015 there was a crash of an Airbus where the crew employed a company sanctioned procedure to increase their Flight Path Angle (FPA) to compensate for their increased Final Approach Fix altitude, which was temperature compensated. Flying the charted FPA from the higher indicated altitude at the FAF would result in arriving over the runway at too high an altitude. In theory, computing a higher FPA based on the expected indicated altitudes would get them over the threshold at the correct height. The problem with this theory is that the corrected FPA needs to be constantly updated. Airbus has since abandoned this procedure. If you've picked it up somewhere, you also need to abandon the procedure. Fly the charted FPA, even when using temperature compensated altitudes."

https://code7700.com/altimetry_temperature_correction.htm#fpa

I don't believe the writer is an Airbus pilot. Is this paragraph accurate, especially about the change of procedure and that no flight path angle change is made anymore?

itsnotthatbloodyhard

18th Mar 2020, 07:38

compressor stall

18th Mar 2020, 08:34

That author is wrong.

From the current A319 FCOM:
When the temperate is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA that the aircraft actually flies, is less steep than the FPA that the ADIRS computes.

If appropriate, the flight crew should therefore apply corrections on the altitudes and on the FPA (in vertical selected FPA mode), and they should be vigilant on the parameters that are displayed.

....

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.

In any case, the check "altitude (corrected in temperature) versus distance" remains the reference.

I actually shot an RNAV approach in FPA at sea level at -27°C just 5 days ago. If I'd used the charted FPA (-2.54°) I'd have missed for sure.

I don't use the following in anger because of the variables but I got bored one day many moons ago and worked it out theoretically (cut and paste from my old notes):

•H = FAFcorr – MINIMAcorr (in feet)
•D=(Distance FAF to MINIMA )nm * 6072 (turns miles into feet)

•On your iPhone calculator – turn sideways for scientific….
a)Find H/D
b)Press tanh gives angle in radians
c)*180/pi

This is the Actual FPA that needs to be dialled into the FCU

vilas

18th Mar 2020, 09:56

The article appears to be from aviation journalist and is vague. In any NPA using barometric altitude the correction will have to be applied. Airbus having FLS uses automatic correction and aircraft capable of GLS or SLS use geometric altitude so the correction is not applicable. However these approaches are performed ILS like with FDs and presently are not available in Airbus narrow bodies or A330.

Nick 1

18th Mar 2020, 10:44

www.airbus-win.com/wp-content/uploads/2019/02/what-about-temperature-condition-1.mp4

compressor stall

18th Mar 2020, 11:00

tcasblue and Vilas - it appears his mistake is to assume that the aircraft gemoetrically descends at the increased FPA. He analysed the AC624 crash here https://code7700.com/altimetry_temperature_correction.htm#fpa

Half way down he states that the aircraft descends geometrically at the increased FPA from the uncorrected altitude, ending up low.

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/913x519/screenshot_2020_03_18_22_00_46_c0a2cc9269a0331f17c13d8321fcd f99c45091f5.png

In any case, TC states that the aircraft ended up low due to momentary windshears and does not correct (which it won't). The crew then did not monitor the profile as per the FCOM. The requirement to refine the FPA during the descent has nothing to do with the cold (assuming a uniform temperature gradient and level ground under the approach)

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1000x600/a15h0002_figure_15_3bb09e3f127fde0ff9df7ecd5d1b99309cc1deaf. png

vilas

18th Mar 2020, 16:19

The real problem was their SOP wrongly asked the crew not carry out Distance/ altitude checks. What the article meant by corrected FPA needs to be continuously updated was height needs to be checked Vs Dist and adjusted as required which is done even at normal temperature. But the conclusion is opposite and wrong. They had also sued Airbus on this. What came out of it I don't know.

tcasblue

18th Mar 2020, 16:54

We train to use a corrected FPA, although I’ve only ever had to do it in the sim (where it worked very well). I’ll attach the relevant part of the A330 QRH, which contradicts the claim that ‘Airbus has since abandoned this procedure’.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1479x952/1bda849f_1cb0_4172_90bb_f0e91e78b5e1_c67fa08cfd69cabb137d49b db9f957837433c582.jpeg
Thanks,

This is not in my QRH(320 series). I see the date on your page is a 2012 issue date. The accident happened in 2015. What is the date of your QRH publication?

itsnotthatbloodyhard

18th Mar 2020, 20:41

Thanks,

This is not in my QRH(320 series). I see the date on your page is a 2012 issue date. The accident happened in 2015. What is the date of your QRH publication?

That’s current as of right now. I guess the chart itself hasn’t been updated since 2012.

vilas

19th Mar 2020, 03:42

tcasblue

19th Mar 2020, 05:57

tcasblue
First low temperature correction has to be applied to final descent altitude and the minimum. To track this corrected vertical profile you use corrected FPA. The responsibility to ensure you are tracking correct vertical path is with crew by normal DME/altitude (corrected) checks.

Thanks,

Reading the report here...

https://www.tsb.gc.ca/eng/rapports-reports/aviation/2015/a15h0002/a15h0002.html

The published descent angle was almost 3.1 degrees with a temperature of -5C. Yet they chose to use an FPA of 3.5 which the report says was in agreement with the company procedures(which were different than Airbus procedures). But it seems like the chart published on this thread(which is not in my QRH) shows that one might use 3.2 degrees. I this correct.

vilas

19th Mar 2020, 08:15

tcasblue
The following faulty procedure of Air Canada was an accident waiting to happen:
Air Canada's practice was that, once the aircraft was past the FAF, the flight crews were not required to monitor the aircraft's altitude and distance from the threshold, nor to make any adjustments to the FPA.

pineteam

19th Mar 2020, 09:08

I'm a bit confused. I understand that on cold weather the real aircraft altitude is lower than the indicated one. I never knew that the FPA that the aircraft actually flies, is less steep than the FPA that the ADIRS computes. How is that possible? I would think the opposite to be true.

compressor stall

19th Mar 2020, 09:30

FPA is calculated from Baro inputs. When showing 2200 (and it’s actually at 2000) it “thinks” it’s higher then it is. Ergo it needs a higher descent angle to compensate to the threshold where the correction is zero.

Remember the correction amount is relative to height over ground. The correction at 2000’ is twice that at 1000’ etc. Rather than the complex formula and apps, just use 4•ISAD•height over ground/1000.

pineteam

19th Mar 2020, 09:41

Sorry Compressor stall, I still don't understand. Could you give an example using your formula?

You quote from the FCOM:
When the temperate is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA that the aircraft actually flies, is less steep than the FPA that the ADIRS computes

The way I understand: when it's colder than ISA: If you select FPA-3.0, and it's minus 30 degrees outside, the actual FPA flown by the plane will be lower than 3.0 degrees, so according to the correction table above, the crew needs to set FPA -3.3 degrees on the FCU. Am I correc?

That's why I don't understand when you say: It ''thinks'' it's higher then it is.
Thank you.

compressor stall

19th Mar 2020, 11:09

Essentially yes I think your summary is correct. Re it "thinking" it is higher that it is - let me explain my logic and terminology.

Assume:

Sea level airport
METAR calling -20°C at the runway
That's ISA-35
Uniform atmosphere, and approach overwater.

NPA FAF@ 2000' at 6.3nm from threshold (3°slope).
Minima 250 (ignoring PEC) 0.8nm from threshold (I know there's another 50' over the threshold but it doesn't really matter here)

If you did no temp correction, the aircraft would fly at 2000 on the FCU, but be actually around 1720' on the RADALT. (The 1720 is calculated by 2000-(4*35*2)). If you set -3° on the FPA, the aircraft "thinks" it's at 2000' (its getting baro info, not RADALT or geometric info) and would fly a profile to the threshold quite successfully (assuming it didn't hit anything on the way :ouch:). It, however, has not flown a geometric 3 degree slope, rather it has flown a slope of 1720' over 6.3*6072' which is about 2.57° geometric.

If you temp correct:

For the FAF: to have the RADALT at 2000' you'd need to set 2000+(4*35*2)=2280' on the FCU. Call this FAFcorr. The cold weather correction is 280' at the FAF
For the minima: to have the RADALT at 250' you'd need to be at 250+(4*35*.25) = 285. Call this MINcorr The cold weather correction at the minima is 35' (the correction is less the closer to the ground).

When corrected, the aircraft "thinks" it's flying from 2280' (but is actually only 2000') over 6.3nm. It needs to tell itself that it is flying 2280' over 6.3*6072 which is 3.41° (but it is actually geometrically flying 3°). I would set FPA-3.4° on the FCU - and have the PM calling out high/low from the chart at each nm to the threshold. In practice you should use the delta alt / distance between the FAFcorr and the MINcorr, but for simplicity you get the drift if I've explained it well enough!

That said, I think the figures in the table above are slightly conservative. I don't know why (nor why they are not in the A320 FCOM) The manuals seem to be updated in late 2017 for the A32F and interestingly not in the Main FCOM/FCTM changes documents from that period that I can find.

itsnotthatbloodyhard

20th Mar 2020, 04:53

The published descent angle was almost 3.1 degrees with a temperature of -5C. Yet they chose to use an FPA of 3.5 which the report says was in agreement with the company procedures(which were different than Airbus procedures). But it seems like the chart published on this thread(which is not in my QRH) shows that one might use 3.2 degrees. I this correct.

Almost. Because the chart’s based on ISA deviation rather than raw temperature, you’re looking at around ISA-20C (actually -19 for the elevation there, but near enough), so 3.1 degrees becomes 3.3. The airfield temperature would have to have been -25C (ISA-40) for 3.5 degrees to be correct.

pineteam

20th Mar 2020, 05:13

Compressor stall. It took me a while but I think I undestood it now haha. Thank you again for your valuable contribution. = )

tcasblue

20th Mar 2020, 10:02

Almost. Because the chart’s based on ISA deviation rather than raw temperature, you’re looking at around ISA-20C (actually -19 for the elevation there, but near enough), so 3.1 degrees becomes 3.3. The airfield temperature would have to have been -25C (ISA-40) for 3.5 degrees to be correct.

Thanks....shows the importance of reading carefully as I missed that.

I wonder if the 0.2 degree difference between this chart and what was actually used correspond with the flight path error.

The report seems to be careful to say that the correction they made was in accordance with company procedures(which were different from Airbus procedures but approved by the regulator) but they do not say if these procedures were accurate.

tcasblue

21st Mar 2020, 13:42

www.airbus-win.com/wp-content/uploads/2019/02/what-about-temperature-condition-1.mp4

Thanks,

A lot of interesting information(although they only seem to mention for A350 and A300) but nothing about adjusting FPA descent angle. Apparently, my company is not the only narrowbody Airbus operator without the FPA adjustment chart that was posted earlier from an A330 QRH.

pineteam

21st Mar 2020, 16:45

My company also does not have the table about the FPA correction table.

tcasblue

21st Mar 2020, 20:04

My company also does not have the table about the FPA correction table.

So what guys out there doing on FPA approaches when it comes to corrections to descent angle?

Vilas? You seem to be really knowledgeable about Airbus.

vilas

22nd Mar 2020, 08:39

tcasblue
I don't have any information on FPA correction.But cold temperature correction to Final Descent altitude and minima is a must otherwise you'll bust it. FPA if not corrected it will be in error but to safer side and as you carryout Dist/Ht check you will have to keep correcting it. All I can say is with.FLS option installed it is corrected through FMS and is flown in FINAL APP. So Airbus may be leaving it to the operator's.

Nick 1

22nd Mar 2020, 10:25

So basically AB say , ask your FltOps to calculate and produce the data or buy an a/c with FLS capability...

compressor stall

22nd Mar 2020, 10:50

FLS aside - with the advent of LNAV / VNAV there are fewer occasions where cold weather temp correction is required as these charts have a temperature range.

Nick - it's not that straight forward to get FLT OPS to generate a table that is really accurate as the temp correction changes with the height over ground, not height AAL - so it's different for every runway. And inversions (as are common in extreme cold high pressure systems) play with the math again as the cold temperature formulae rely on a uniform decrease in air temp.

So what guys out there doing on FPA approaches when it comes to corrections to descent angle?

Just dial in a few more tenths depending how cold it is and PM monitors high/low and PF adjusts - like any NPA. You are aiming for a CDFA but it doesn't always work, through chart design or adjusted FPA not quite accurate. I just checked some GoPro footage and I made 4 FPA adjustments between 2500' and minima (340') when it was ISA-40ish.

itsnotthatbloodyhard

22nd Mar 2020, 11:02

If it helps any, there’s an app called CoolAltitude which will calculate all the relevant corrections for you (screenshot attached). Obviously all the usual caveats apply - for guidance only, doesn’t replace official documentation, etc etc.

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/640x480/414e6a04_99d6_4b12_94f9_0b88ea911d2c_888e9dc4dcd664c26c62555 b8712cfb3412702d3.png

Bus Driver Man

22nd Mar 2020, 11:10

So basically AB say , ask your FltOps to calculate and produce the data or buy an a/c with FLS capability...
If you use the same correction like the altitude correction, it should more or less work out. 4% for every 10 degrees below ISA. That seems to be the case in the table posted by itsnotthatbloodyhard.
E.g. OAT -10 = ISA -25 gives a correction of 10%.
If the approach starts at 3000’ AGL, you correct it to 3300’. About 3.3 FPA should bring you at the (corrected) MDA at the right place.

Just remember that (in the 10% case) initially you’ll be 300’ high according the altitude vs. distance table. At 2000’ you’ll be 200’ high etc.
Or you do a bit of extra work prior the approach and you correct the altitude vs. distance table. Remember to calculate the % from the height, not altitude, as this is a common mistake.

Edit:
By the way, at my previous company, we didn’t correct any altitudes down to -15 OAT. Except for the MDA. I think that at EasyJet it’s down to -10 OAT. I assume it’s because the margin on the published altitudes is probably sufficient down to those temperatures.

tcasblue

22nd Mar 2020, 14:09

deltahotel

22nd Mar 2020, 15:15

I’ve just put some numbers into the cold wx app that we use - temp, elevation, VDA, MDA and it comes out with 3.32deg

HtH

tcasblue

22nd Mar 2020, 16:14

Thanks,

Maybe you can input the altitudes and temperature for the approach that was used by the accident aircraft and see what descent angle is required.

I’ve just put some numbers into the cold wx app that we use - temp, elevation, VDA, MDA and it comes out with 3.32deg

HtH

Thanks,

The crew used 3.5 degrees. That might explain why they were low on approach if 3.3 degrees was the angle that should have been used. As we know, there was no crosscheck of altitude versus distance and their company procedures manual(used instead of Airbus issued manuals) did not require them to crosscheck altitude versus distance.

It would be interesting to see the airlines formula table(or whatever it is) that was used to come up with 3.5 degrees as the crew apparently followed this table properly..

TheEdge

23rd Mar 2020, 08:25

Guys why doing so difficult ? There is no table, FCOM or QRH that can exactly tell you what FPA you must use in case of low temperature correction during NPA...keep it easy, do your corrections at minimum and FDP, the rest as usual check your DME/elevation against charted figures.

deltahotel

23rd Mar 2020, 09:14

Mmmm. The problem is that as it gets colder the range/height figures get less and less accurate. At what point are you going to decide that they need correcting? -5, -10, -15? Doing the calculations manually and writing them on a scrap of paper is a right ballsache; using a modern cold wx app is really easy so why not do it?

tcasblue

23rd Mar 2020, 09:58

The problem is that as it gets colder the range/height figures get less and less accurate. At what point are you going to decide that they need correcting? -5, -10, -15? Doing the calculations manually and writing them on a scrap of paper is a right ballsache; using a modern cold wx app is really easy so why not do it?

It appears that this is what one has to do. Your own corrections for each distance/altitude point. If you are fortunate, your company has provided you with a table or an app to make it easy.

But I still don’t understand why the descent angle table is in the A330 manual and not the A320 manual.

deltahotel

23rd Mar 2020, 10:06

Sorry - can’t help with that. Boeing pilot me, in fact I didn’t even know that FPA was a thing until recently!

OPEN DES

23rd Mar 2020, 17:57

The FPA correction chart in your QRH is a company specific DU.
Airbus abandoned the FPA correction chart because they don’t want crew to blindly select an FPA and then stop monitoring ‘alt vs distance’. They wanted to avoid the false sense of safety after having extracted a single number from the chart. In the same sense we should not blindly rely on a nominal FPA -3.0 to always do the trick in ISA conditions.
What about hot temperature corrections? And so on......
-You select what you need and adjust, by active monitoring and periodic crosschecking.
-The FPA is baro-inertial and only correct in ISA conditions. (Baro-long term)

We train to use a corrected FPA, although I’ve only ever had to do it in the sim (where it worked very well). I’ll attach the relevant part of the A330 QRH, which contradicts the claim that ‘Airbus has since abandoned this procedure’.

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1479x952/1bda849f_1cb0_4172_90bb_f0e91e78b5e1_c67fa08cfd69cabb137d49b db9f957837433c582.jpeg

TheEdge

24th Mar 2020, 07:35

The FPA correction chart in your QRH is a company specific DU.
Airbus abandoned the FPA correction chart because they don’t want crew to blindly select an FPA and then stop monitoring ‘alt vs distance’. They wanted to avoid the false sense of safety after having extracted a single number from the chart. In the same sense we should not blindly rely on a nominal FPA -3.0 to always do the trick in ISA conditions.
What about hot temperature corrections? And so on......
-You select what you need and adjust, by active monitoring and periodic crosschecking.
-The FPA is baro-inertial and only correct in ISA conditions. (Baro-long term)

Could not agree more. Even during normam ISA standard NPA we do monitor DME/altitude during final and make needed corrections as needed. I cannot understand how you can just get a figure for an FPA from a book and blindly applying it.

Goldenrivett

24th Mar 2020, 10:52

Could not agree more. Even during normam ISA standard NPA we do monitor DME/altitude during final and make needed corrections as needed. I cannot understand how you can just get a figure for an FPA from a book and blindly applying it.
I agree. We don’t just estimate the drift angle at the descent point, adjust the heading once and then wait for a runway to appear! Or maybe some operators do?

vilas

24th Mar 2020, 11:06

OPEN DESCENT
-You select what you need and adjust, by active monitoring and periodic crosschecking. 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.

Uplinker

24th Mar 2020, 11:11

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1000x600/a15h0002_figure_15_3bb09e3f127fde0ff9df7ecd5d1b99309cc1deaf. png

I wonder if the above is adequately explained to pilots; i.e. if disturbed, the aircraft will not regain the original "glide-slope" but will parallel it, either above or below.

Certainly on any NPA where visibility is low, (but in limits for the NPA); I would feel extremely uncomfortable NOT checking my altitude at every mile on the way down. How bizarre for an airline to say it was not necessary. Basic piloting and SA, surely?

FlightDetent

24th Mar 2020, 12:49

I wonder if the above is adequately explained to pilots; i.e. if disturbed, the aircraft will not regain the original "glide-slope" but will parallel it, either above or below.
To pilots, such a thing need not to be explained at all.

Moreover, in case when a company detailed a specific automation drill in their manuals (such as LNAV/FPV 2D APCH in sub-zero temps) and omitted to-reiterate distance/altitude principle for the sake of brevity or whatever, pilots would do it anyhow.

Not to mention any pilot examiner would need to discontinue the bi-yearly check-ride should the applicants ignore such basic protocol.

There is something rotten west of the Denmark's largest island.

OPEN DES

24th Mar 2020, 13:03

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.

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.

OPEN DES

24th Mar 2020, 13:10

I wonder if the above is adequately explained to pilots; i.e. if disturbed, the aircraft will not regain the original "glide-slope" but will parallel it, either above or below.

You’d be surprised.... few people realize.
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.

tcasblue

26th Mar 2020, 00:00

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.

pineteam

26th Mar 2020, 03:00

TheEdge

26th Mar 2020, 07:57

Bottom line, you have to guess at the descent angle correction.

No guessing, just normal standard NPA approach SOP, with DME/altitude check every NM and FPA adjust accordingly as per above post.

OPEN DES

26th Mar 2020, 09:20

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.

I think you’re missing the point here.
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.

pineteam

26th Mar 2020, 09:50

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.

OPEN DES

26th Mar 2020, 10:23

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.

pineteam

26th Mar 2020, 10:38

Hi Open Des. Thank you very much for your time! We need more threads like this one! = )

compressor stall

26th Mar 2020, 11:57

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.

OPEN DES

26th Mar 2020, 15:19

Hi Open Des. Thank you very much for your time! We need more threads like this one! = )

You’re welcome.
be safe!

pineteam

27th Mar 2020, 14:47

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...

When corrected, the aircraft "thinks" it's flying from 2280' (but is actually only 2000') over 6.3nm. It needs to tell itself that it is flying 2280' over 6.3*6072 which is 3.41°

compressor stall

27th Mar 2020, 21:08

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.

FlightDetent

28th Mar 2020, 02:41

@pinteam

https://1drv.ms/x/s!AiE0Si5ywRcJhK8abKmOrJAmAUr7oA?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.