Cold temperature corrections on LNAV/VNAV minima
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As far as I know, when an approach procedure designer makes an approach, the make the minimums as low as possible without violating any of the requirements for the approach design with the most important one being minimum terrain clearance.
If the temperature at Frankfurt is at -15 C, the lowest allowable temperature for uncompensated FMC's, the aircraft will be a full 50 feet lower than the 400' height above the runway compared to a standard day.
The only way it makes sense to me that no temperature correction is allowed for the LNAV/VNAV minimums of 720' indicated altitude(400 above the runway) is if the designer of the approach took this alititude error at -15 C into account and raised the minimums the appropriate amount so that the required terrain clearance is always ensured for any temperature down to -15 C.
Is this what happens at the design stage?
Last edited by JammedStab; 20th Dec 2015 at 13:50.
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Well, up to -15 I would fly it using LNAV/VNAV minima correcting the 720 to 760, possible correcting the intermediate altitude of 4000 as well. Below -15, say -20, I would use the LNAV minima of 840 corrected to 910 and would modify the alt at lompo to 4570 in the FMS. It would then calculate a new path from LOMPO at 4570 terminating at 50 over the threshold. I would make a little table in which I would correct the distance to alt table and check on my way down to see If I'm on the correct altitude.
This is how we train to do it, other companies may have different procedures.
This is how we train to do it, other companies may have different procedures.
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The only way it makes sense to me that no temperature correction is allowed for the LNAV/VNAV minimums of 720' indicated altitude(400 above the runway) is if the designer of the approach took this alititude error at -15 C into account and raised the minimums the appropriate amount so that the required terrain clearance is always ensured for any temperature down to -15 C.
Is this what happens at the design stage?
Is this what happens at the design stage?
Only aircraft flying the approach using a vertical GPS input are not required to correct for cold weather as they fly the vertical profile based on satellite input not a barometric one.
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As far as I know, when an approach procedure designer makes an approach, the make the minimums as low as possible without violating any of the requirements for the approach design with the most important one being minimum terrain clearance.
The obstacle clearance surface for the final approach is 500 feet at the FAF and tapers to 200 feet at threshold, the slope being basically 2.5%. This is where LNAV/VNAV gets the 250 HAT, 200 + 50 foot momentary descent.
Typically, the obstacles tend to be in the missed, which raises the DA/MDA. Obstacles in the approach phase, that is a whole different world...that is where NOTAMS come from! In design, its called find a different glideslope (based on above) or find a different path. When I was designing the RNP procedures with FAA approval, they did not allow obstacles in the approach.
I remember the test RNP procedure into Bradley, couldnt get it to work with the approach because of the ridge, as the FAA would not allow obstacles in the approach, but to match their design for the test, you had to. Found out that the FAA exempted themselves from that requirement (but no one else)
Sky...ac on LPV or GBAS are not required to compensate for temperature on final approach segment only. Other segements, temp comp is required.
Last edited by underfire; 22nd Dec 2015 at 09:52.
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For both RNP APCH APV (LNAV/VNAV minima) and RNP APCH NPA (LNAV minima) approaches, the initial, intermediate and missed approach segment flight plan fixes need to be corrected for cold temperature.
Note: DA/MDA must also be corrected for low temperatures
Note: DA/MDA must also be corrected for low temperatures
The above section is applicable when the temperature is below (or above) what is shown on the plate....not all the time..
the FCOM blends the conditions a little too much.
Up to what is shown on the plate, say -15, you do not correct the final approach segment. You still must correct the initial and intermediate segments.
Only half a speed-brake
You still must correct the initial and intermediate segments.
regards, FD.
addendum: picture hint
Last edited by FlightDetent; 22nd Dec 2015 at 23:07.
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Yes, however MDA needs to be corrected.
Care must be taken to see if the cold temperature correction has been taken into account for a tailored procedure.
As an example, for certain tailored procedures, the low temp listed on the plate was taken into account for the obstacle surfaces, aircraft, and turn performance.
As noted with PASC RNP, the public RNP is good from -18C to 54C (??? place on fire?) Using -18C, the procedure is good for about 25% of the year.
With the tailored procedures, there is another procedure, using the same waypoints, that is good from -50C to -19C. Same waypoints, different altitudes making certain that obstacle clearance in maintained though the missed, and even OEI at those temperatures.
Negative, but no problems if you do provided you tell ATC and they are happy with the decision.
I have a question, since you are aware that the DA considers a 50 momentary descent, and the MDA is a glass basement, do you adjust your DA given your actual momentary descent? Do you adjust your decision altitude, based on temp correction and your actual momentary descent, given the MDA?
Last edited by underfire; 23rd Dec 2015 at 03:21.
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SBAS-ABAS
You need to correct the temperature on a RNP APCH based on ABAS, that is down to LNAV/VNAV or LNAV minimums and
you don´t need to correct the temperature on a RNP APCH based on SBAS,that is down o LPV or LP minimuns.
SBAS based RNP APCHes use a geographical altitude instead of barometical altitude. A kind of radioaltimeter.
you don´t need to correct the temperature on a RNP APCH based on SBAS,that is down o LPV or LP minimuns.
SBAS based RNP APCHes use a geographical altitude instead of barometical altitude. A kind of radioaltimeter.
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This is what Transport Canada have to say:
Advisory Circular 700-023
Regardless of whether the FMS provides temperature compensation of the vertical path or not, all altitudes on the approach, including DA, should still be temperature corrected.
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Superpilot....
Note on the approach plate if it states non-compensated baro vnav...
if it does, then that is your answer.
not sure where the (if below zero comes from) but look at the NA below/above
If it does, then that means that the minima has been designed down to the NA below/above temperature WITHOUT compensation.
Note on the approach plate if it states non-compensated baro vnav...
if it does, then that is your answer.
not sure where the (if below zero comes from) but look at the NA below/above
If it does, then that means that the minima has been designed down to the NA below/above temperature WITHOUT compensation.
Last edited by underfire; 14th Nov 2016 at 01:11.
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A certain airline in Hong Kong does not even give all information above AND the info we do get is not even correct. The person in charge of it comes to the office on the little bus wearing a helmet.
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Hi Superpilot, I take it what you have there is company guidance?
That much is certainly correct. Zero no doubt because that is the warmest temp in the ICAO correction table for a sea level airport. Although somewhere in the ICAO verbiage it says something suitably vague like 'if it is very much colder than ISA'.
This one would seem to refer to 'uncompensated baro-VNAV'. According to the FAA AC 90-105A:
I assume that is a company or type specific thing.
I'm not sure what the definition of gate is there. Anyway, as far as altitudes, according to the FAA AIM:
ie if your airport is on the FAA 'Cold Temperature Restricted Airport' list, and it is cold enough, you will correct the requisite segment altitudes. Meanwhile, according to Eurocontrol guidelines:
...which sounds a little like 'don't bother correcting your segment altitudes if doing a baro-VNAV procedure'. Whilst according to Transport Canada:
The various references don't exactly dovetail. And I would not second guess company rules; there may be a good reason. But if the airport temp is cold enough for a correction then to my mind it makes sense to correct all segments like the Canadians say (and the FAA now also have a 'correct all segments option'). You would for an ILS so I don't see why you wouldn't for baro-VNAV. Discuss.
Would it be back on profile at minima or at TDZE? It strikes me that an uncompensated baro-VNAV would still be slightly below profile at minima.
Is VNAV minima available?
- If yes you may fly down to VNAV minima provided that:
- The VNAV minima has been corrected for cold temperature (if below zero) thereby increasing
- The actual temperature is not below the published minimum temperature
"Uncompensated baro-VNAV systems may not operate to the LNAV/VNAV DA when the actual temperature is below or above the temperature limitations. The temperature limitation will be shown as a note on the procedure. If the aircraft contains a temperature compensation capability, manufacturer instructions should be followed for use of the baro-VNAV function."
And according to the FAA AIM:Aircraft using baro VNAV with temperature compensation or aircraft using an alternate means for vertical guidance (e.g., SBAS) may disregard the temperature restrictions.
- The approach must be flown with the autopilot on and fully managed
- There is no requirement to increase altitude gates as terrain separation is guaranteed up to the minimum published temperature
The charted temperature limits are evaluated for the final approach segment only. Regardless of charted temperature limits or temperature compensation by the FMS, the pilot may need to manually compensate for cold temperature on minimum altitudes and the decision altitude.
ie if your airport is on the FAA 'Cold Temperature Restricted Airport' list, and it is cold enough, you will correct the requisite segment altitudes. Meanwhile, according to Eurocontrol guidelines:
The flight crew is responsible for any necessary cold temperature corrections to all published minimum altitudes/heights including the altitudes/heights for the initial and intermediate segment(s); the DA/H; and subsequent missed approach altitudes/heights, except for APV/BARO-VNAV approach procedures. In accordance with ICAO Doc 8168, PANS-OPS, Volume I, Part II, Section 4, Chapter 1, § 1.4.1, the final approach path vertical path angle (VPA) is safeguarded against the effects of low temperature by the design of the procedure.
...which sounds a little like 'don't bother correcting your segment altitudes if doing a baro-VNAV procedure'. Whilst according to Transport Canada:
Regardless of whether the FMS provides temperature compensation of the vertical path or not, all altitudes on the approach, including DA, should still be temperature corrected.
The various references don't exactly dovetail. And I would not second guess company rules; there may be a good reason. But if the airport temp is cold enough for a correction then to my mind it makes sense to correct all segments like the Canadians say (and the FAA now also have a 'correct all segments option'). You would for an ILS so I don't see why you wouldn't for baro-VNAV. Discuss.- A typical autopilot coupled approach will fly below the profile up to minima where it will be back on profile.
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Wow. You guys don't half know how to make things difficult.
APV Baro (Lnav/Vnav)
- within charted temperature limits - no correction required to the platform heights but correction required to minimums.
- outside charted temperature limits - correction required to platform heights, some aircraft have a function certified to do this automatically in which case use it. If you don't have the function then calculate it yourself. Correction to minimums is required.
APV SBAS (LPV)
No correction to platform heights required as GPS height isn't effected by cold weather.
Minimums must still be corrected because they are referenced to barometric altimeter.
APV Baro (Lnav/Vnav)
- within charted temperature limits - no correction required to the platform heights but correction required to minimums.
- outside charted temperature limits - correction required to platform heights, some aircraft have a function certified to do this automatically in which case use it. If you don't have the function then calculate it yourself. Correction to minimums is required.
APV SBAS (LPV)
No correction to platform heights required as GPS height isn't effected by cold weather.
Minimums must still be corrected because they are referenced to barometric altimeter.
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APV Baro (Lnav/Vnav)
- within charted temperature limits - no correction required to the platform heights but correction required to minimums.
- outside charted temperature limits - correction required to platform heights, some aircraft have a function certified to do this automatically in which case use it. If you don't have the function then calculate it yourself. Correction to minimums is required.
- within charted temperature limits - no correction required to the platform heights but correction required to minimums.
- outside charted temperature limits - correction required to platform heights, some aircraft have a function certified to do this automatically in which case use it. If you don't have the function then calculate it yourself. Correction to minimums is required.
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oggers,
Many thanks. Nope, this is my own understanding of things. No consolidated company info exists on this. Just a myriad of texts and documents. Gates is me referring to the platform altitudes at various distances. I was hesitant to call them platform altitudes as this might imply some kind of intermediate level off altitude.
Many thanks. Nope, this is my own understanding of things. No consolidated company info exists on this. Just a myriad of texts and documents. Gates is me referring to the platform altitudes at various distances. I was hesitant to call them platform altitudes as this might imply some kind of intermediate level off altitude.
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Oggers,
You are of course correct. I should have left the "If you don't have the function then calculate it yourself" bit out and said if you don't have the system you can't do the approach.
It's been a long day.
You are of course correct. I should have left the "If you don't have the function then calculate it yourself" bit out and said if you don't have the system you can't do the approach.
It's been a long day.