Jump to content

uncompensated baro-vnav in cold temperature


Recommended Posts

as we all know , we need to apply cold temperature correction to our airbus and

there are many different discussions on this topicūü§Ē:

-someone is sure that it is necessary to correct only DA/MDA

-someone suggests correcting from FAF

-someone suggests all points below MSA

-someone claims that there is no need for an adjustment if the temperature is not lower than the minimum permissible specified in the charts

 

we have real pilots here, how is it right?)))

 

 

 

Link to post
Share on other sites
  • radeon29 changed the title to uncompensated baro-vnav in cold temperature
  • Deputy Sheriffs
6 hours ago, radeon29 said:

as we all know , we need to apply cold temperature correction to our airbus and

there are many different discussions on this topicūü§Ē:

-someone is sure that it is necessary to correct only DA/MDA

-someone suggests correcting from FAF

-someone suggests all points below MSA

-someone claims that there is no need for an adjustment if the temperature is not lower than the minimum permissible specified in the charts

 

we have real pilots here, how is it right?)))

 

 

 

Try to find that document in the internet...and you have all you need...
image.png

Link to post
Share on other sites
11 hours ago, masterhawk said:

Try to find

unfortunately not found, if you have a link please send via PM.

P.S

i doubt you read this document every time in cold weather;)

if you can summarize the main points it would be great!

Link to post
Share on other sites
On 1/6/2021 at 6:55 AM, radeon29 said:

as we all know , we need to apply cold temperature correction to our airbus and

there are many different discussions on this topicūü§Ē:

-someone is sure that it is necessary to correct only DA/MDA

-someone suggests correcting from FAF

-someone suggests all points below MSA

-someone claims that there is no need for an adjustment if the temperature is not lower than the minimum permissible specified in the charts

 

we have real pilots here, how is it right?)))

 

 

 


*This is how we do it in the US* 
 

If approach plate says:


1.thumb.png.2c68c243e465b520aa4e7061116d87cf.png

This means that you need to compensate when temp is below -16C or above 54*C for ONLY points from the FAF to the MDA, not including the missed approach segments. You also don't have to tell ATC.

If it says:

 1.png.7a69ff93cea84c13f2d7f90edf8778a8.png

You must compensate ALL points on the approach from the IAF/IF to the missed approach hold, advise ATC, and also adjust your MDA/DA.

 

 

  • Thanks 1
Link to post
Share on other sites
  • Deputy Sheriffs

Basically the scientific explanation of cold temperature corrections is that because the altimeter is calibrated on ground to ISA conditions (QNH 1013.25 hPa and temperature +15 ¬įC) when the temperature gets very cold at the sea level, the air density at sea level is also higher than it would be in warmer temperatures. At higher altitudes the temperature difference from ISA is smaller and thus the pressure gradient is steeper (air gets less dense faster as you climb). This results in your altimeter to give a reading that is higher than what your true altitude is and is the reason why a temperature correction is needed. So essentially because you are actually at a lower altitude than your altimeter says that you are, there is a risk that you are actually below the minimum safe altitude determined for certain area around the airport and closer to the ground and obstacles than you think. So a correction needs to be applied to ensure that you are flying at a safe altitude.

One problem with this is that since the gradient is not constant but depends on the altitude, thus also the correction depends on the altitude. One could think that they could just apply a correction by setting the altimeter to a QNH setting that takes this into account, but this would assume that the correction is constant at all altitudes and thus does not work. So essentially the thinking needs to be reversed from thinking through the altimeter gauge but rather correct all the reported altitudes in charts to a safe level and then fly that safe level indicated on the altimeter. Essentially this has to be done to all published altitudes on the chart below transition altitude/level (above that every pilot levels their readings to show ISA conditions because terrain and obstacles should no longer be an issue). So saying that correction is needed only below MSA can turn out deadly because if you don't adjust MSA to begin with, you can fly with altimeter indicating MSA but your true altitude being below the MSA.

With some procedures (like mentioned above) the authorities have left some buffer in them so that you can fly the procedure without temperature corrections and still be at a safe altitude up to certain temperature. In this case altitudes related to the procedure have enough of margin so that authorities have determined the flight being safe without temperature corrections.

Good rule of thumb which is determined safe by ICAO for temperatures above -15 ¬įC is the 4% rule. This means that you add a 4% of correction in the height (altitude measured from ground level, not Mean sea level) for every 10 degrees below the standard temperature. So if the airport is for example at 500 feet MSL (close enough to sea level that ISA 15 ¬įC can be assumed), the temperature is -5 ¬įC and you need to correct for an altitude of 2500 feet MSL (height of 2000 feet) you would calculate first calculate the temperature difference which in this case is 20 degrees. Apply 4% for each 10 degrees, in this case total of 8% and then take 8% from 2000 feet (height) and add it to the altitude. So we get 2000*0.08 = 160 and then add that to altitude of 2500 feet and the final altitude is 2660 feet which we round up to 2700 feet which is the temperature corrected altitude in this case. For minimums at 700 feet (200 feet height) we apply the same logic with 8% which leads to 16 feet correction, so we add 16 feet to the 700 feet minimums altitude and get 716 feet corrected minimums.

When the airport elevation (and thus terrain around it as well) raises, the ISA temperature at the ground level decreases. A good rule of thumb for this is decrease of 2 ¬įC for every 1000 feet. For better accuracy, this could be taken into account as well, but because the 4% rule is already a conservative estimate, and use of 15 degrees gives higher altitude difference, it's basically safe to use 15 degrees as the ISA in all cases. Just thought mentioning this out as well in case someone wonders about this as well.

So there is a quick scratch by me into the altitude corrections. Living in cold area myself, this stuff has always fascinated me personally.

Link to post
Share on other sites
  • Deputy Sheriffs
On 1/7/2021 at 3:46 PM, atav757 said:

This means that you need to compensate when temp is below -16C or above 54*C

 

This is party incorrect. Not authorized means just that. You may not fly the LNAV/VNAV if you have an uncompensated Baro Vnav system and you fall outside of the temperature restrictions. If there are other minima listed you may/might fly those when you compensate but not the approach with LNAV/VNAV minima.

Link to post
Share on other sites
1 hour ago, The Dude said:

 

This is party incorrect. Not authorized means just that. You may not fly the LNAV/VNAV if you have an uncompensated Baro Vnav system and you fall outside of the temperature restrictions. If there are other minima listed you may/might fly those when you compensate but not the approach with LNAV/VNAV minima.

 

I appreciate your input, but I was not partially incorrect. He's asking in reference to the Aerosoft Airbus.. a BARO VNAV system IS considered compensated if you apply altitude corrections electronically. I pulled this right from my company's SOP manual. It's very simply described and a pretty easy concept. Trust me, I've shot many approaches with that line stating that, at temperatures below and above the range, all legally.

 

In other words, you CAN use the VNAV minima as long as you compensate.

 


 

image.png

Link to post
Share on other sites

 

1 hour ago, The Dude said:

 

This is party incorrect. Not authorized means just that. You may not fly the LNAV/VNAV if you have an uncompensated Baro Vnav system and you fall outside of the temperature restrictions. If there are other minima listed you may/might fly those when you compensate but not the approach with LNAV/VNAV minima.

 

I realized you may be talking about procedures outside of the US. Again I did say "This is how we do it in the US".

Looking at some European charts, I never see this line. It seems to just say "VNAV approaches not authrozed below .." which of course means you just can't use those minima. In US/Canada (at least), the chart will tell us at which temperature to start compensating (like my first examples), and we just activate compensation in the MCDU, then we can use VNAV minima.

Link to post
Share on other sites
5 minutes ago, DaveCT2003 said:

It's truly a cold day in hell when you have to us the tables for Midland Texas (chart example provided above)!

 

 

Haha! That's the example I almost used for hot. My airline flies into there and I did it once in the summer, all you need is the temp to be above like... 74*F? and need to temp comp. I remember briefing the approach and saying to my captain, "what the heck". Then I looked at the elevation haha.

Link to post
Share on other sites

all this is very interesting, but as far as I know, we will not be able to test this in sim because it is not implementedūüėĒ

 

but nevertheless it's interesting:

 REQUIREMENTS FOR COLD TEMPERATURE COMPENSATION

  1. Following Commission Regulation (EU) 965/2012, EASA states in documents AMC2 CAT.OP.MPA.126 and AMC2 NCC/NCO/SPO.OP.116, section (d) item (1):
    1. For RNP APCH operations to LNAV/VNAV minima using Baro VNAV:
      1. 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;
      2. when the temperature is within promulgated limits, the flight crew should not make compensation to the altitude at the FAF and DA/H; and
      3. 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.
    2. For RNP APCH operations to LNAV minima, the flight crew should consider the effect of temperature on terrain and obstacle clearance in all phases of flight, in particular on any step-down fix.
  2. In addition, ICAO states in Document 8168 "PANS-OPS" Volume III Aircraft Operating Procedures, chapter 4:

    4.3 TEMPERATURE CORRECTION

    4.3.1 Requirement for temperature correction
    The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere.¬†In such conditions, an approximate correction is 4 per cent height increase for every 10¬įC below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above ‚Äď15¬įC.

Link to post
Share on other sites
  • Deputy Sheriffs
3 hours ago, atav757 said:

This is how we do it in the US"

 

As I also fly in the US, could you give me an international airport approach chart example please? Just tell me the airport and which approach and I will look it up myself. Thanks

 

Now with regard to the AS Airbus. It does not have a compensated baro vnav system installed so it is defacto a "uncompensated Baro VNAV system". I just read FAA AIM 5-4-5 and it states:

 

Screenshot 2021-01-08 at 19.24.55.png

 

I do not know what you mean with VNAV minimum as I only know a LNAV/VNAV or a LNAV minimum in this case. At my airline, equipped with uncompensated Baro vnav systems, we may only fly to the LNAV minimum and I also interpret the FAA AIM that way. 

 

 

Link to post
Share on other sites
2 hours ago, The Dude said:

 

As I also fly in the US, could you give me an international airport approach chart example please? Just tell me the airport and which approach and I will look it up myself. Thanks

 

Now with regard to the AS Airbus. It does not have a compensated baro vnav system installed so it is defacto a "uncompensated Baro VNAV system". I just read FAA AIM 5-4-5 and it states:

 

Screenshot 2021-01-08 at 19.24.55.png

 

I do not know what you mean with VNAV minimum as I only know a LNAV/VNAV or a LNAV minimum in this case. At my airline, equipped with uncompensated Baro vnav systems, we may only fly to the LNAV minimum and I also interpret the FAA AIM that way. 

 

 

 

Look at jepp charts for BIKF RNP 01. It will say point blank: "Baro-VNAV not authorized" for example.

 

I suppose then this comes down to an aircraft optional equipment thing.  Remember that companies are authorized to do different things and aircraft have different optional equipment. According to our SOPs, as long as the compensation is activated in the FMS via the MCDU, the aircraft is considered to have a compensated Baro-VNAV. 

 

From the horse's mouth in my attachments.
 

You also didn't display the rest of the NOTE section after that paragraph in the AIM which states:

 

Quote

NOTE‚ąíMany systems which apply Baro‚ąíVNAV temperature compensation only correct for cold temperature. In this
case, the high temperature limitation still applies. Also,
temperature compensation may require activation by
maintenance personnel during installation in order to be
functional, even though the system has the feature.

 

This paragraph in the AIM tells me that compensated baro-vnav exists.

 

 

IMG-0041.PNG

IMG-0042.PNG

Link to post
Share on other sites
  • Deputy Sheriffs

Thanks for the additional info!

 

I did not mean to discredit you or your RW operations here. I just pointed out that, this being the AS Airbus forum, you may not fly the LNAV/VNAV minimum if the temperature is out of the limits if the aircraft has an uncompensated vnav system (like the AS Airbusses). Just read my original post again:

 

6 hours ago, The Dude said:

This is party incorrect. Not authorized means just that. You may not fly the LNAV/VNAV if you have an uncompensated Baro Vnav system and you fall outside of the temperature restrictions. If there are other minima listed you may/might fly those when you compensate but not the approach with LNAV/VNAV minima.

 

Once again, thanks for discussion. It is always nice to hear about different SOP's and equipment.

Link to post
Share on other sites
10 minutes ago, The Dude said:

Thanks for the additional info!

 

I did not mean to discredit you or your RW operations here. I just pointed out that, this being the AS Airbus forum, you may not fly the LNAV/VNAV minimum if the temperature is out of the limits if the aircraft has an uncompensated vnav system (like the AS Airbusses). Just read my original post again:

 

 

Once again, thanks for discussion. It is always nice to hear about different SOP's and equipment.


Absolutely my friend! Didn't feel discredited, I'm interested that I now see AS doesn't model, and actually most airlines don't have this cability on the 320 series. So I was the odd one out! I love learning new things about other ways to fly these machines.

  • Upvote 1
Link to post
Share on other sites
  • Deputy Sheriffs
16 hours ago, radeon29 said:

it turned out to be an interesting discussion, but still wanted to summarize for the airbus, what do we do in 1 case:

1.thumb.png.2c68c243e465b520aa4e7061116d87cf.png.12d5770b807b16b887c082a96f14a8f8.png

and what do we do in 2 case:

1.png.7a69ff93cea84c13f2d7f90edf8778a8.png.63d88e6d118016b2c0e97c6546e86614.png

?

 

The Airbus Pro FMGS/MCDU is more or less a combination of two different systems and though I haven't really compared it too much over the past few years I believe it's more Thales like.  Due partially to it being a combination, you won't find a Temp Comp selection on the Approach page, and the reading on this says if you have a Temp Comp that's where you enter the compensation and if you don't have a Temp Comp then it's the compensation is computed automatically (which is very easy to do).  That said, I believe Frank (The Dude) the correct, Airbus documentation (up to 2017 anyway) says not to perform such uncompensated non-precision Baro approaches regardless of whether or not the certifying agency (FAA/CAA/ICAO) allows it and there is available information on the approach plate.  This is Airbus, and I've been away from Boeing ops (way over 10 years) so I have no idea what Boeing says on this subject.

 

Can you do it in the sim?  Absoltuely.  Look, here is the deal (and I used to say this all the time in my Navy days)... we get so used to automated systems that we sometimes forget how to do things without them.  So, in a cold oir hot weather situation as we're talking about, all you need to do is get the aircraft systems to see the aircraft at a different altitude.  How can we do that if the system doesn't do it automatically or if there is no Temp Comp entry on the Approach Page?  Easy!  Adjust your altimeter AFTER you are below Transition Altitude (Level).  You can do the calculation or better yet use the Tables easily enough, so you know what the addition or subtracton of the altitude is... so simply adjust that on your altimeter when you're below the Transition Altitude and in level flight for a few seconds.  Ta-da!  Now, is that cool real world?  I don't know, it may be depending on the aircraft type, but since Airbus says it's a no-no anyway (again, up to 2017 which is the latest documation I have for this) it's a mute point.  But you casn do it in the sim.

 

If we were still heavily developing the Bus I'd add the automated Temp Comp to the list of additions to consider, but we are only wrapping up things for the Airbus Pro series.  Maybe for MSFS though, as it's simple enough to add.

 

Frank is a VERY experienced Airbus pilot and I always defer to him, so hopefully he'll wrap this up by weighing in on last time.

 

Link to post
Share on other sites
3 hours ago, DaveCT2003 said:

Can you do it in the sim?  Absoltuely. 

no, so this(the effect of cold or hot weather on the altimeter) is not realized in sim, or am I wrong?

 

3 hours ago, DaveCT2003 said:

Frank is a VERY experienced Airbus pilot

understood, thanks, I hope he clears up my last question;)

Link to post
Share on other sites
  • Deputy Sheriffs
7 minutes ago, radeon29 said:

no, so this(the effect of cold or hot weather on the altimeter) is not realized in sim, or am I wrong?

 

understood, thanks, I hope he clears up my last question;)


So far as I know, the barometric pressure provided in METAR/ATIS is uncompensated.  This I believe is necessary for the many different types of aircraft.  If my presumption is correct, then would indeed be in the sim, though you would certainly need a weather engine.

 

Link to post
Share on other sites
13 hours ago, DaveCT2003 said:

the barometric pressure provided in METAR/ATIS is uncompensated.

I think we are talking about different things ...
pressure is not compensated, we just make a altitude correction by adding extra, moreover, in the case of the airbus, we shouldn't to altitude correctionin any of the cases when we use VNAV minimums and you no fall outside of the temperature restrictions:

Spoiler

1.thumb.png.2c68c243e465b520aa4e7061116d87cf.png.12d5770b807b16b887c082a96f14a8f8.png.5fd208aad585188c3f84efde40743c87.png

(in this case - 16 or +54).?

13 hours ago, DaveCT2003 said:

If my presumption is correct, then would indeed be in the sim, though you would certainly need a weather engine.

This was also discussed here, in none of the simulators this is implemented:

 

(even if it is - 60 or +60 in the sim, the altitude will not depend on the temperature)

Link to post
Share on other sites
  • Deputy Sheriffs
9 hours ago, radeon29 said:

I think we are talking about different things ...
pressure is not compensated

 

Reported pressure my friend.  We just need to keep things in context to keep the blue side up.  You stated that this was not modelled in the sim.  Well, the only way to know is to first find out if temp comp was taken into consideration by airports (that's where you'd see this in the sim) and the way that would be reported would be pressure on the ATIS or METAR.  I checked real world and I was correct, compensated pressure is neither calculated or reported real world and also not reported in the sim, so there is nothing to model therefore it is correctly modelled in the sim.  That's step one.

 

So, since we have a weather engine which provides accurate barometric pressure and tempreture, and we have an airport altitude, we have all we need not only to experience barometric temp comp, but adjust for it as well. 

 

So it's far from a mute conversation, it's accurately modelled in the sim and can be used by sim pilots. Few do, but it's modelled.

 

 

Link to post
Share on other sites
  • Deputy Sheriffs
1 hour ago, radeon29 said:

Ok, we will wait for an exampleūü§ó

 

1. When in the correct paramters for a hot/cold temp compensation:

 

a. Use the provided tables (CAA/FAA/ICAO) or tables specific for aircraft type, to calcuate the Temp Comp (provided in altitude).

 

b. For an aircraft without a Temp Comp in the FMS/FMGS/GPS, while at level flight just before or at the FAF and with airport baro pressure set, adjust altimeter to add the altitiude specified by the table in Step A.

 

c. Done.

 

 

Now again, whether it's advisable by the aircraft/FMS/FMGS manufacturer or certifying activity to do this is another matter. But that procedure will work.

 

Link to post
Share on other sites
On 1/12/2021 at 2:07 AM, DaveCT2003 said:

But that procedure will work.

this is not implemented in sim Q.E.D:

METAR:

 

Spoiler

UERR 151800Z 25002MPS 4000 BR NSC M41/M45 Q1018 R25/390234 NOSIG RMK QFE732/0976

Latest Jepp:

 

Spoiler

23.thumb.png.98862a49f9b2a6779d3244e7cdb79cf5.png

 

FAF alt 2470 ft (D5.1 DEM):

Spoiler

22.thumb.png.1be962bd9befd1054d6a9f42cc9b94e5.png

 

If this were implemented in the sim, then we would be about 320 ft lower(if the effect of temperature were to be implemented, the correct altitude with corrections should be approx.  2790 ft).

 

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...