GPS Approaches /Temperature compensation

[Cbf]

Is there a procedure to compensate altitude variation due to temperature, especially in cold condition, during GPS LNAV/VNAV approaches?

[MFarhadi]

If the temperature is below the minimum charted cold temperature limitation, then you cannot shoot the RNP approach to LNAV/VNAV minima (uncompensated Baro-VNAV). Remeber the temperature compensation *must* be automatically done (not available on 747-400 as far as I know), as crew are prohibited from manipulating altitudes inside final approach segment.

[Cbf]

If no compensation is applied during approach (cold condition), how to proceed when runway in sight before DA?
Follow F/D (or A/P) with a shallower slope or disconnect F/D (A/P) and follow PAPI?

[United744]

The temperature limit is a LIMIT for UNCOMPENSATED baro altitude.

If the chart says "not below -30 deg. C" and OAT is -25 deg. C, then you are legal to commence the approach.

LNAV/VNAV approach is a non-precision approach, so you must have visual contact with the runway before descending below the MDA and crossing the MAP.

If you do not have, or you lose sight of, the runway; you must go-around.

It is a manual landing, the same as any other non-precision approach. The only time the aircraft can auto-land is with suitable ILS.

[MFarhadi]

Just to add, don't forget to "manually" compensate all the "hard" altitudes; meaning, you should still increase your minimum when flying at colder temperatures than ISA.

The "uncompensated Baro-VNAV" is just for the final approach segment. You shouldn't touch anything inside FAF, but still adhere to the increased minima for your DA/MDA.

[andrej]

FAA has a pretty good website on this:

https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.8.html

Maybe you are aware of it, but I find it useful.

Cheers,

[Cbf]

Thanks all for your comments and tips. If I understand, I just have to compensate (increase) DA or MDA during cold condition. Is there a rule of thumb?
I suppose it must be done for all approaches.
What about MSA?

[MFarhadi]

MSA is also a "hard" altitude (hard: when you go below that, you'll hit something hard). So, yes; you should temperature-compensate that too. The only altitude you shouldn't correct for cold temp is the radar vectoring altitudes ATC would give you.

For all other approaches other than RNP to LNAV/VNAV minima, you still need to correct all minimum altitudes. The only difference is, for Baro LNAV/VNAV, if you modify the approach altitudes, you legally cannot fly that.


*edit* I used ICAO jargon as much as I could. For US and Canada, it's still RNAV approach. Confusing.

[simonijs]

Skybrary (Eurocontrol) published an Excel tool to calculate the correction to apply.

https://skybrary.aero/bookshelf/cold-temperature-correction-tool

Regards,
Simon

[Mawea]

If the temperature is below the minimum charted cold temperature limitation, then you cannot shoot the RNP approach to LNAV/VNAV minima (uncompensated Baro-VNAV). Remeber the temperature compensation *must* be automatically done (not available on 747-400 as far as I know), as crew are prohibited from manipulating altitudes inside final approach segment.

Hi,
our B744 and B748 are both Baro VNAV approved and we add the cold temperature correction (4% height increase for each 10deg deviation below ISA) and enter these values into the FMS - we are allowed to do so for temp corr, in any other case you are right. Dunno if it applies only to the NG FMS.

Until the charted temp limit you could fly down to the VNAV minimum.

Below the chart temp you have to compensate the temp and use the LNAV minimum but could still fly the approach using LNAV and VNAV.

But talking about these kind of altitude mishaps and or traps. Keep in mind that a wrongly higher QNH setting would result in a worse situation - descending parallel below the intended path and into the GND before the RWY.

Hopefully not of course, so keep me and you digging in our SOPs

Best regards
Jay

[Cbf]

Hi Jay,
You mean that you modify altitude constraints in FMS (NG) LEG pages?

[MFarhadi]

Until the charted temp limit you could fly down to the VNAV minimum.

Below the chart temp you have to compensate the temp and use the LNAV minimum but could still fly the approach using LNAV and VNAV.

Nice! I didn't know you could modify the altitudes inside IF for some operators.

The note usually mentions: "For uncompensated Baro-VNAV systems, LNAV/VNAV not authorized below -xxºC (-xxºF) and above xxºC (xxxºF)."

My understanding is, if your system does not have cold/hot temperature compensation, then and only then you are not authorized to use LNAV/VNAV minima. If your SOP is to modify for cold temp compensation, then you should be alright to use the LNAV/VNAV minima...? If you are using temp comp, there is no need to revert back to LNAV minima, and that's what the note says (as far as I understand).

[Cbf]

Hi Jay,
I just have an example:
2 weeks ago,I performed Great Falls KGTF RNAV (GPS) Y RWY 21, LNAV/VNAV, with PSX.
OAT on airport was - 20°C. LNAV/VNAV approach may be performed if T° is above -24°C
FMS LEGS page shows:
SOTOW...6200A
DEELM...5300A
WAGAN...4380A
RW21....3720
TUCKB...8000
MSA is 8500Ft
GP3.0°/TCH 59Ft/Airport elevation 3680Ft/TDZE 3669Ft
Approach plate: KGTF RNAV Y 21
How to procceed for the Temp corrections, if approved of course?

[Mawea]

Hi Cbf,

you could fly the approach in LNAV and VNAV to the LNAV/VNAV minimum. But it doesn't hurt to calculate the cold temperature correction and decide if you want them inserted or not.

It is required if temperatures are below -24 deg and it is recommended if below ISA -20.

MSA 8500' -> 9040'
6200' -> 6480'
5300' -> 5480'
4380' -> 4460'
Minimum:
3930' -> 3960'

In case you want them inserted just type the new altitudes in respective LSK /4460 eg and select the new minimum on PFD.

By the way LNAV minimum would be 4000' -> corr. 4040'

Take care

[Cbf]

Thanx Jay.

I just have few more questions:
- Which formula do you use?

Do you apply correction:
- Always or you start at STD+/- 10 for example?
- when a NDB, VOR, VOR/D or LOC approach is selected in FMS (FAF,FCA,MDA...)?
- When ILS with GS approach is selected in FMS (GS interception altitude, FCA,DA...)?

I suppose it requires an ATC clearance

[Mawea]

Sorry no formula used on my side. We have a tool on our EFB to calculate the desired ALTs directly in the charts. As mentioned above, add 4% of height for 10 deg deviation below ISA.

To stay with your KGTF example,

DeltaISA = 15- 1,98 *ALT/1000 = ISA at 3669' =7,7deg -> temperature deviation is ISA -27,7 deg ->

MSA 8500 - Elev 3669'= 4831ft height

-> 0,04*27,7/10= 0,1108  so 11,08% to add on the height:
4831'*1,1108= 5366 + 3669 = 9035' is your temperature corrected MSA. 9040' is what my tool is suggesting, propably my rounding error somewhere, or the algorithm has margins.

You add these corrections to minimas on all approaches and MSAs, intermediate but not to radar vectored alitudes, they are compensated already. But yes, if you have your corrected MSA and ATC clears you for a standard approach you would tell ATC you would temp corr. the 6200' and start your RNAV Y 21 out of 6500'

For ILS approaches you don't need to edit any altitudes in the FMS since the vertical guidance is not effected by the cold temperature.

Also the GoAround altitude needs to be compensated, here in Great Falls
8000' -> corr 8480' is below MSA
I see you listed TUCKB, very good!
Cheers

[Cbf]

Hi Jay.
Thank you for your explanations. I have found a table on the web that gives correction according to temp/Height. I will try to generate a xl file.
Concerning ILS, I just thought about the GS capture point (Sometimes named FAP). In cold condition, the capture will occur later.