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Temperature correction on Baro-VNAV approach

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Temperature correction on Baro-VNAV approach

Old 28th Oct 2018, 20:55
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Temperature correction on Baro-VNAV approach

Hello everybody,

I know Baro-VNAV approaches have been discussed on PPRuNe before, but I haven't been able to find a definitive answer to my question. Even after reading the FCOM as well as any relevant reference material available in my company, I'm still not sure what I'm really supposed to do.

Let me give you the following situation:

RNP APCH with VNAV minima
OAT -10 degr.
Note on plate: uncompensated Baro-VNAV not authorised below -20 degr.

"RNP APCH APV procedures are designed with a linear obstacle profile from the FAF to the DA. This obstacle clearance is assured down to the minimum temperature as mentioned in the minima box in the ‘notes’ section. This means that no temperature corrections in the final approach segment are necessary if the OAT is higher than the value mentioned in the note. Note that for temperatures below zero, corrections to MDA and DA should be applied for both types of approaches."

The information sounds quite contradictory to me. Yes, the temperature is below zero ("corrections to DA should be applied"), and yes, the OAT is higher than the value mentioned in the note ("no temperature corrections in the final approach segment (which includes the DA) are necessary"). So, should I correct the DA or not?

Thanks!
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Old 28th Oct 2018, 23:09
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There are two different questions. 1) Do I need to correct my DA/MDA 2) Am I allowed to follow the vertical path and related LNAV/VNAV minima.

To answer question 1. --> Every time the temperature drops below 0, the DA/MDA needs to be corrected. My company uses 10% of the height, to be added to the DA/MDA. So if the minima is 400'(300), where the height is in brackets, the new minima becomes 400' plus 10% of 300' --> 430'*.

The second question depends on the equipment that you have on board. On some airplanes you'll have temperature compensated systems, like FLS on some Airbuses. If you do NOT have temperature compensated equipment, you can only follow the vertical path until the published temperature on the chart, or 0ºC. In your example the published temperature is -20º, so you're authorized to follow the vertical guidance down to LNAV/VNAV minima (but these need to be corrected, see question 1).
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Old 29th Oct 2018, 00:08
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The information sounds quite contradictory to me. Yes, the temperature is below zero ("corrections to DA should be applied"), and yes, the OAT is higher than the value mentioned in the note ("no temperature corrections in the final approach segment (which includes the DA) are necessary"). So, should I correct the DA or not?
Why do you say the final approach segment includes the DA? I think this assumption is the source of the perceived contradiction. Yes, we always correct for DA/MDA when below zero regardless of actual minimum temperature stated in the notes. A thousand feet up you have a lot of buffer (not going to hit any terrain) whereas closer to the aerodrome, the same temperature error is unacceptable due to proximity of the terrain (The runway).
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Old 29th Oct 2018, 00:23
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For a new student, it might be a logical interpretation. "The profile takes you from FAF to 50' over the threshold (by definition) and indeed the DA is somewhere on it. With cold airmass, the whole slope gets flattened: yet there is no need to make corrections as the profile has been smartly designed to cater for this effect down to -20 deg.C - NICE. At the same time, the DA needs to be manually adjusted at -0 already ?! WTF...."

Dutch:
- the (M)DA is the minimum and we are not allowed to go lower than that, geometrically. Hence the correction.
- the profile is just the slope from FAF to THR. You are safe as the flattening had been taken care of, as long as you observe geometrical the (M)DA.
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Old 29th Oct 2018, 09:28
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Thank you for your suggestions. It sounds really natural to correct the DA since you don't want to go below the actual published minimum altitude, so I see what you guys are doing. However, in my company's latest documentation (August 2018) I can read the following statement:

"As per EASA regulation temperature corrections for the FAF and DA are no longer allowed when flying RNP approaches down to VNAV minima."

Could this mean that the published FAF and DA are determined using the minimum temperature for the procedure. Then no correction would be necessary, even if you're below zero (but not exceeding -20 in this case).

For the second question, we operate the 737 so it's uncompensated Baro-VNAV. Means we can fly the approach down to the minimum temperature, like you said.

Thanks again!
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Old 29th Oct 2018, 12:52
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Re read your SP 16.
n altitude correction due to cold temperature is not needed for the following conditions:
• while under ATC radar vectors
• when maintaining an ATC assigned flight level (FL)
when the reported airport temperature is above 0°C or if the airport temperature is at or above the minimum published temperature for the procedure being flown.
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Old 29th Oct 2018, 18:47
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Dutchman95, it could be though unlikely unless you have some very customised approach plates? However, I would really like to read the two lines before and after the above quote.
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Old 29th Oct 2018, 18:52
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AMC2 CAT.OP.MPA.126

(i) For RNP APCH operations to LNAV/VNAV minima using Baro VNAV:
(A) the flight crew should not commence the approach when the aerodrome
temperature is outside the promulgated aerodrome temperature limits for the
procedure unless the area navigation system is equipped with approved
temperature compensation for the final approach;
(B) when the temperature is within promulgated limits, the flight crew should not
make compensation to the altitude at the FAF and DA/H;

(C) since only the final approach segment is protected by the promulgated
aerodrome temperature limits, the flight crew should consider the effect of
temperature on terrain and obstacle clearance in other phases of flight.
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Old 29th Oct 2018, 20:22
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Thanks FlyingStone for the reference.

Despite that, my company just started with cold temp corrections to all published altitudes below corrected MSA on baro-VNAV..

Apparently there were two different schools, and someone's ego stronger than the others.
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Old 30th Oct 2018, 06:21
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172 driver,

Do you have an fcom?��

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Old 30th Oct 2018, 06:47
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Originally Posted by de facto View Post
172 driver,

Do you have an fcom?��

Yep, a generic one. I read what you qouted above, SP.16
I found it strange that Boeing would publish something like that. Though most, not all ATC units correct for low temperature. Oslo TMA for instance.
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Old 30th Oct 2018, 09:02
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I believe the confusion is between the applicable correction and the DA(DH) that pilots do actually "bug" on the PFD. No corrections are applicable between the FAF and DA(DH) but Your company procedures may require You to "bug" a temperature corrected DA which has obviously zero effect on the profile.
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Old 30th Oct 2018, 10:09
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Actually, this is a big discussion in our airline as well(another Dutch company). The initilal guidance in our manual when we first started flying RNP approaches was that you did have to correct the DA for low temperatures, even though you were within the temp bracket to fly to LNAV/VNAV minima. I did some research and have found several documents backing this up.

However, as a previous poster already posted, since 2016, EASA guidance states that when above the minimum temperature stated on the approach chart, temperature corrections are not to be made to the DA, even though the temperature is below 0!!

Last edited by flyburg; 30th Oct 2018 at 10:43.
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Old 30th Oct 2018, 16:10
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Originally Posted by flyburg View Post
Actually, this is a big discussion in our airline as well(another Dutch company). The initilal guidance in our manual when we first started flying RNP approaches was that you did have to correct the DA for low temperatures, even though you were within the temp bracket to fly to LNAV/VNAV minima. I did some research and have found several documents backing this up.

However, as a previous poster already posted, since 2016, EASA guidance states that when above the minimum temperature stated on the approach chart, temperature corrections are not to be made to the DA, even though the temperature is below 0!!
Exactly, I've also read several documents in which this was stated clearly, and not just in outdated documents for some reason. However, the fact that EU regulation currently prohibits these corrections to both FAF and DA indicates to me that all those documents can be disregarded.

Originally Posted by sonicbum View Post
I believe the confusion is between the applicable correction and the DA(DH) that pilots do actually "bug" on the PFD. No corrections are applicable between the FAF and DA(DH) but Your company procedures may require You to "bug" a temperature corrected DA which has obviously zero effect on the profile.
We don't do that, our manual clearly states "the uncorrected DA is set on the PFD". Using a corrected DA on the flatter vertical path would also increase the required distance to make visual contact with the runway.
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Old 30th Oct 2018, 16:14
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172driver....
Your lack of basic knowledge is frightening.
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Old 30th Oct 2018, 17:04
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172driver....
Your lack of basic knowledge is frightening.
Que?

Cmon then keyboard warrior... explain what I am lacking:

So far I've stated;

1. Our company applies cold temp corrections on baro-VNAV, as of recently, to all altitudes below temp corrected MSA. That includes, step-down altitudes, FAF, DA - despite what the FCOM Supp. Proc. says and the reference to AMC.CAT.OP.MPA.

It ain't dangerous, we're just flying slightly steeper than we would have otherwise done in say - 20 deg.

2. Regardless what the Supp Proc.says, there are ATC units that do not apply cold temp corrections to vector altitudes. We have it in our network. So for Boeing to generalize in such manner, I found rather puzzling. I thought the lawyers what have looked it through?
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Old 30th Oct 2018, 21:08
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Dutchman, What does the chart say? Does it say "uncompensated baro-vnav"?

When we design the procedure, there is the NA above and below temperatures. If you are between the NA above and NA below, the OCS has been validated to avoid the obstacles, from IF onwards.

In FAA land, the NA below takes you to a 2.75 degree glidepath (ICAO goes to a 2.5 GPA) The NA above takes to you a 3.5 GPA.

So uncompensated, you are between basically 2.5 and 3.5 degrees and the OCS has been validated for this.

Therefore, if you are within the temperature boundary, you do not need to correct the FAF nor the DA/MDA altitudes. Again, if the chart states uncompensated baro vnav, which I think they all do, or it is assumed uncomp.

IF you get outside the temperature limits, you must either not use the procedure or correct...

Bottom line, when you use the procedure, you do not have to compensate if you are within the temperature boundaries. IF your system has the ability to compensate, all the better, but not req'd. The FMS, not the driver outside of the FMS.
Keep in mind, if you simply add a factor to the altitude, say the FAF or DA, while your system remains uncompensated, that is a problem. This is why many ATC vectors will not allow for compensation, due to the blend of ac with the system ability or not, and not to rely on the drivers to be consistent in the correction .

Our company applies cold temp corrections on baro-VNAV, as of recently, to all altitudes below temp corrected MSA. That includes, step-down altitudes, FAF, DA - despite what the FCOM Supp. Proc. says and the reference to AMC.CAT.OP.MPA.
That is fine, as long as the ac system accepts the input. If it does not, and you are simply manually adding corrections, (not in the system) that is a problem. That means the ac thinks it it here, while you think it is there. Again, it depends on the level of automation and system capabilities. In your statement, who is providing or correcting the MSA temp?
If your FCOM says no, I am not sure what the question is here. I would be curious what your SOP then uses as the temperature limit before you decide to correct? You with AC?

Last edited by underfire; 31st Oct 2018 at 16:17.
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Old 30th Oct 2018, 23:32
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My understanding, and the procedure at a major UK operator with a large fleet of uncompensated B737s, is that you would fly the approach to LNAV/VNAV minima without any corrections provided the temperature was within the charted limit. If the temperature drops below the charted limit you can still fly the approach, but to LNAV minima only and with temperature corrections applied.
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Old 31st Oct 2018, 11:18
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Originally Posted by Dutchman95 View Post
We don't do that, our manual clearly states "the uncorrected DA is set on the PFD". Using a corrected DA on the flatter vertical path would also increase the required distance to make visual contact with the runway.
We do set the temperature corrected minima on the PFD, although I disagree with it, but it makes no difference on your profile as it is simply a "bug".
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Old 31st Oct 2018, 18:02
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It does not affect the profile, but should you raise the DA to the original geometric value, on a flatter profile the point where slope-x-DA would be further from the runway. That, in turn, should (?) force you to recalculate the minimum RVR / OPS 1.430. Shyte what a can of worms.
Originally Posted by underfire
Therefore, if you are within the temperature boundary, you do not need to correct the FAF nor the DA/MDA altitudes.
While that's exactly what the quoted regulation says, you're seeing only the obstacle side of it, underfire.

I trust you the slope covers me over OCA/H until the LO temp limit and that you guys did your math well. The OPS side has, in addition, some minima as well. Such as the DA is never less than 250 ft on RNP-baroVNAV. That's where the old method was more readily understandable.

Anyways Dutchman95 thanks for coming here, I learned something through your question.
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