Temperature correction on Baro-VNAV approach

[Dutchman95]

Originally Posted by underfire

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 .

Thanks for your reply. It's really not too complicated after all. The 737s in our fleet don't compensate so we'll just fly the Baro-VNAV approaches without correcting FAF and DA down to the minimum temperature, and below this minimum we'll go with the LNAV minima and correct all minimum altitudes.

With regard to your last remark, we don't correct while receiving radar vectors, unless (!) one of the minimum altitudes in the procedure is below the minimum vectoring altitude (MVA). Without checking I'm not entirely sure if this is stated in the FCOM or just company policy though.

@FlightDetent
That's exactly the reason why I believe adjusting the DA on a Baro-VNAV approach doesn't make sense in the first place, because of this increased distance to the threshold. But yeah, it's good for getting a better understanding of what we're actually doing, so cheers for thinking along with me!

[InSoMnIaC]

Whats all this nonsense? You should absolutely temperature correct your M(DA). The chart has been designed, as others have stated, to keep you clear of obstacles withinin the temperaure range. The chart designer choses a min temp based on historical min temps at the airport and uses It if he can (ie if the GP is not too shallow (min 2.5deg icao) and obstacles do not penetrate the obstacle clearance screen).

If these conditions cannot br met then either the GP is steepened (increase FAF alt), the minimum temp is increased or both.

the M(DA) has nothing to do with this. The designer could in theory also correct the M(DA) for us By selecting the most conservative M(DA), ie one that will allow us to be at the minimum geometric altitude at the lowest temp. This however will be overly restrictive for most of the year when the temperature is above that value. This is why we must amend the M(DA)

[de facto]

Listen to Matey

[de facto]

Originally Posted by de facto

Listen to Matey

Insomniac...lack of basics is a bummer no?

[172_driver]

Stop with the insults and give us something useful, de facto.

[Dutchman95]

Originally Posted by InSoMnIaC

Whats all this nonsense? You should absolutely temperature correct your M(DA).

As stated by someone else before, AMC2 CAT.OP.MPA.126 is quite clear about this:

For RNP APCH operations to LNAV/VNAV minima using Baro VNAV:

• when the temperature is within promulgated limits, the flight crew should not make compensation to the altitude at the FAF and DA/H;

Perhaps FAA regulation is different, I don't know about that.

[FlightDetent]

In addition, AMC stands for Acceptable Means of Compliance, and under EASA that actually is the only approved means of compliance. If you want to do different, you need to apply for an alternative means with EASA themselves (local national CAA won't do). Unlike in the days of EU-OPS / JAA.

Applies for all EU registered aircraft. Presumably, they did the homework rather well, and it is the right thing.
Until this thread, my understanding was the same as InSoMnIaC's.

[Sepp]

Originally Posted by FlightDetent

...
under EASA that actually is the only approved means of compliance. If you want to do different, you need to apply for an alternative means with EASA themselves (local national CAA won't do). Unlike in the days of EU-OPS / JAA.
...
.

Not so. An organisation can still agree an alternative, alternative means of compliance (referred to as AltMoC, presumably to avoid confusion with the original EASA AMC) with their national competent authority, which must then notify EASA of the situation. EASA approval is not a requirement. This EASA page explains the process.

EASA maintains a list of notified AltMoCs, which are available for use by other organidsations with the agreement of their competent authority.

[FlightDetent]

Sepp is perfectly correct: ARO.GEN.150.

Now that the book is opened - the relation between the approach angle, (M)DA and min RVR is explained in AMC5 CAT.OP.MPA.110. (Not that I really understand how to apply it, luckily not my job.)

[LEVC]

It is my understanding that you can fly it within the temperature limit stated on the chart, BUT the DA or MDA should be corrected because it is not taken in account when constructing the path, in fact you can use different minima depending on the onboard equipment and certification, the path flown will be the same whatever minima you use, and will be flatter if cold or steeper if warm, but the minima should be temperature corrected.

[OhNoCB]

I have had this discussion with my current UK airline too.

We currently correct everything below the temp corrected MSA, for all approaches. In my previous airline, we did not correct anything between and including the FAF - DA on a Baro-VNAV approach. I showed the AMC to the company who replied that they were within their right to be more conservative and correct anyway - I disagreed as it explicitly states that we should not do it - but was over-ruled.

I still disagree - but nonetheless, I suppose this shows how straight forward and unified this all is.

[underfire]

t is my understanding that you can fly it within the temperature limit stated on the chart, BUT the DA or MDA should be corrected because it is not taken in account when constructing the path, in fact you can use different minima depending on the onboard equipment and certification, the path flown will be the same whatever minima you use, and will be flatter if cold or steeper if warm, but the minima should be temperature corrected.

Sorry, but that is not correct. Procedures are designed for the lowest common denominator, uncompensated. Depending on what you mean "correcting" you can get yourself in a lot of trouble.

Again...

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 simply correct, while others are not, depending on when/what you correct, you run the risk of violating min separation.(or a host of other issues such as too high/too low)

[172_driver]

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?

I missed answering this part.

We use tables (or these days the EFB for more accurate numbers) and add to step-down and FAF altitudes in the FMC. So that'd make it a manual entry. Where our SOP (and they changed recently) seem to differ from others is that FAF should be corrected when using VNAV minima, and you are within the temp limits for the procedure. I am aware of the FCOM SP.16, the EASA AMC, your POV as a procedure designer(?) but I just go by our OM-B. It tells us to correct FAF. Not really sure what the big problem is here. We correct up a bit, even if the path has been validated down to typically minus 20 C. So we come in slightly steeper, but not steeper than we would have on an ISA day. If temp is outside the limit for the procedure we fly it down to LNAV minima instead and keep correcting. Not with AC, but another northern carrier in Europe. We correct from 5 deg C (so ISA-10). That changed, we used to correct only from 0 deg C, after a 737 got a hard terrain warning approaching a level off altitude on a non-precision appr. It was a few degrees above zero and they hadn't corrected. They did come down pretty fast being a bit hot and high, but nothing out of the extreme ordinary.

.

[underfire]

Well, messing with manual additions in the FMS can have unintended consequences (as you illustrated)

A. Following the release of OPSB 0166-17 Rev 4 in December 2017, Rockwell Collins has continued to discuss this issue with the FAA. These discussions have resulted in the release of the following publications. The FAA performed a review of these documents and provided feedback on their content before release.

• OPSB 0166-17 (523-0824828) : THE FMS MAY TURN IN THE WRONG DIRECTION AFTER SEQUENCING A “CLIMB TO” ALTITUDE THAT WAS MANUALLY EDITED OR TEMPERATURE COMPENSATED
  • Revision 5 has been released to clarify the Limitations for the aircraft, and the error conditions.
• Service Information Letter FMC-XX00-18-1 (523-0825523) : INSTRUCTIONS FOR AIRCRAFT FLIGHT MANUAL LIMITATIONS FOR ALTITUDE EDITS ON SPECIFIC PRO LINE 4 AND PRO LINE 21 FLIGHT MANAGEMENT SYSTEMS.
  • This document provides the list of applicable FMS part numbers which are affected by the issue and the FAA approved wording for the AFM Limitation.
• Service Information Letter CSU-XX00-18-1 (523-0825521) : INSTRUCTIONS FOR DISABLING OF AUTOMATIC TEMPERATURE COMPENSATION OPTION IN PRO LINE 4 AND PRO LINE 21 SYSTEMS.
  • This document provides a reference for those aircraft type which have previously enabled the Temperature Compensation option (in those which have the affected FMS part number)

[LEVC]

When correcting DA or MDA for cold temperature I understand it is on the PFD, not on the FMC, that it is not allowed according to the manuals.

From FAF/FAP onwards we are not allowed to modify anything, or else we are messing with the vertical path, that is clearly stated on the manuals (B737NG).

the path calculated by the FMC should, as pointed out by some, take in account the worse case within the limits of temperature on the chart, but the Minima at which you initiate your Go around needs to be increased on your PFD.

Do you correct DA for an ILS if it is -20C?, I take that the answer is a yes, same should be done for other type of approaches.

[Denti]

Originally Posted by LEVC

Do you correct DA for an ILS if it is -20C?, I take that the answer is a yes, same should be done for other type of approaches.

Completely different case though, the ILS is a fixed geometric path in space, the VNAV approach is not. And if you, in direct breach of the relevant AMC, adjust your DA, you put it quite a bit farther away from the runway, whereas an adjusted DA on an ILS simply puts it where it should be anyway. And yes, the DA is taken into account for the minimum temperature of the procedure, in fact it is one major point there, choose a higher temperature and you can accept a lower DA, chose a lower temperature and you have to choose a higher one.

[underfire]

When correcting DA or MDA for cold temperature I understand it is on the PFD, not on the FMC, that it is not allowed according to the manuals.

We use tables (or these days the EFB for more accurate numbers) and add to step-down and FAF altitudes in the FMC.

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.

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 is my understanding that you can fly it within the temperature limit stated on the chart, BUT the DA or MDA should be corrected because it is not taken in account when constructing the path, in fact you can use different minima depending on the onboard equipment and certification, the path flown will be the same whatever minima you use, and will be flatter if cold or steeper if warm, but the minima should be temperature corrected.

Do you correct DA for an ILS if it is -20C?, I take that the answer is a yes, same should be done for other type of approaches.

Holy what is going on up there?!?!?!?!

When adjusted, they came in high and hot???? oh well, they landed....That seems okay?

Adjust in PFD, FMC or well, somewhere?

Where EXACTLY do you think the NA above and NA below temperatures come from? The DA/MDA NOT accounted for in the design???

Some adjust, some dont, some do it here, some do it there, some just think about it....no problems with the flightpath, obstacle clearances, missed approach, TCH, or min airspace separation????

Correct the MSA??? dont correct the MSA... What 1000 feet doesnt matter in the airspace? min sep????

This thread has been very enlightening on the basics of baro-vnav....damn.

finally.....phew....it is just this simple...

As stated by someone else before, AMC2 CAT.OP.MPA.126 is quite clear about this:
For RNP APCH operations to LNAV/VNAV minima using Baro VNAV:

• when the temperature is within promulgated limits, the flight crew should not make compensation to the altitude at the FAF and DA/H;

How about, correct the MSA after you get permission from ATC or they tell you to? Again, min sep in the airspace.

Again, procedures are designed with NA below at at 2.75 degree GPA (2.50 ICAO) and 3.5 GP NA above.

NO correction to the DA/MDA....

[172_driver]

underfire,

I, too, am somewhat surprised by the variety of answers.

Until recently we did not correct FAF. Now we do. The training department had a change of mind and I am quite sure they are knowledgeable about the AMC, the Boeing SP and general procedure design. What led them to that change, I do not know. I am still unsure as to why this is a big thing? We're putting the plane back onto a 3 degree path. Not that we need to, but we do. Correcting the DA is often no more than 50 ft, even on the coldest of days. If it's not necessary, as per procedure design, fine. Then we're just erring on the side of caution.

Come to think of it, we used to have slightly different SOP for NPA-approaches (VOR, NDB), LNAV, VNAV and RNP AR. It was confusing and standardization was poor. Now we fly them the same way whether it's a VOR or RNP AR. It's a valid argument, I'd say.

The MSA is part of your mental map of where you are. I don't go into Chicago, or New York, in blizzards with tight layers of traffic. Many of our destinations are fairly remote places, with terrain and cold temperatures. Separation to other traffic is not a factor. Descending procedurally on a FMC guided path is. Not correcting could be right out suicidal. And I am talking in general terms, not specifically about the VNAV approach.

[InSoMnIaC]

Some here must be quite happy to fly into terrain as long as they can quote the relevant AMC when asked at the pearly gates.

Noone is advocating changing the FMC numbers beyond the FAF. We are simply saying “decide earlier”. This does not change the calculated or flown VNAV angle.

[FlightDetent]

N.b. IF you correct the FAF and then descend continiously on a 3 deg geometrical profile, then the raise of (D)DA does not move the intersect point away from the threshold, so one of the undesirable effects is well mitigated.

If someone manages to merge the SOP for various NPA, err 2D approaches, into one drill for crews, it is a massive improvement on safety. The S word is being misapplied more often than not, but less workload and clearer standards is definitely the way to go. I can only imagine what a huge burden is lifted from the training system if the above is achieved. And how much good can be done with the freed-up resources!

As for correcting the MSA: if left uncorrected, the nubmer loses its meaning completely. The connection between MSA and traffic separation is completely lost on this guy.