Cold temp corrections on LNAV appr

[Mooneyboy]

Was wondering if anyone could help clarify this potential situation for me.

You are making an RNAV approach rwy 23L in Düsseldorf EDDL. The temp is -20 C. You can't use VNAV minima as you can only use this to -15 C. So you fly the LNAV minima which you can use (Airbus) NAV lateral guidance and selected vertical ( our procedures are FPA).

My companies procedures are to add temperature corrections to 1) DA/mda and 2) Non precision approach to minimum altitudes after passing the FAF.

So non temp corrected profile 23L RNAV is platform 3000ft descent at 8.8 nm (from rwy) 3 degrees with alt checks at 7nm =2420 6nm=2100 4nm 1470 and 3nm 1150.

So if I apply temp corrections to check heights after FAF according to our procedures then to be at 7nm =2740 6nm 2370 4nm 1650 and 3nm 1290.

My question is If I fly platform at 3000ft indicated as according to our procedures then the next check height from 8.8nm to 7nm is only 260ft height to lose instead of 580ft. This means if I want to start descent iaw the proc at 8.8nm then I would only need an FPA of about -1.5 instead of the usual FPA of -3 then at 7nm increase the FPA to ~-3 again.

Another option would be stay at 3000ft till 7.7nm then do -3fpa.

Am I right in my thinking or am I missing something very obvious?

Any help much appreciated.

[Johnny F@rt Pants]

You could always temperature correct the platform altitude too, then it works all the way down without having to make adjustments to the FPA.

[Skyjob]

Your angle does not change, the INDICATION on your altimeter does due temperature.
A calculated measured angle will still be flown as 3 by the aircraft.
However, be careful as all altitude readings on your altimeter which keep you safe below MSA are affected, not just those inside the FAF.
As the previous poster beat me to it, platform could be corrected to ensure correct descent point on altimeter

[galaxy flyer]

Push for a temp compensating FMS, then the entire profile will show in the altitude constraints. We have it in all Bombardier jets, it’s great.

[FlightDetent]

Mooneyboy: I agree with your thinking.

In case you do NOT adjust the intermediate approach (platform) altitude,
- in accordance with your company policy;
- to keep things simple for ATC, avoiding aircraft flying in different true altitudes and compromising clearance from whoever may be at 4000' uncorrected;
- in line with the margins already embedded in the design of the procedure;
- following the letter of regulations that all minimum altitudes must be corrected;
(4x yes from from me)

THEN: The easiest choice would be to maintain 3000 uncorrected until intercepting the desired stable descent profile. The problem here is, for a fact, that the needed calculation is very inappropriate to be done on a flightdeck, lord forbid during the approach itself!

I'd use my best judgement and fake it somehow. On a grand scale, due to the scaresity of such excercise in real life, and safety net provided by the following corrected cross-check altitudeS >Halifax, anyone, helloo??< It is only a methodical problem, not a practical issue.

CAUTION: On the Airbus, the FPV is baro-angle, for my lack of a better word.

If you would be flying down from e.g. corrected 2440' height, you need to adjust the FPA too!
- charted 2200 height and 3 deg
- corrected 2440 height 3.2 deg.

BTW thanks for the question, obviously you've done the brain-work before posting. Pleasure to discuss afterwards, hopefully my last paragraph above will be interesting for some to read.

[Superpilot]

During an LPC/OPC, I completely messed up my cold wx correction calculations. No one, not my sim partner (also a trainer) or the company TRE noticed. Just goes to show, how important this really is in over 99.9% of approaches.

In practice we fly through an air mass which is at temperature x, yet our calculations are based on y, the temp on the ground. There is zero room for a temperature inversion in the standard calculation. Anyone flying into Scandi land in the middle of winter knows the temp on the ground is often colder than what the OAT was at platform (extra cooling effect of ground snow/ice).

[Mooneyboy]

Thank you for all your replies.

Unfortunately the Airbus 320 probably not as modern as your bombardier in terms of computing power ( always thought it was very backwards when you can't display constraints and airports at the same time on the ND). Would be handy having a temp compensated profile.

Yes I had considered increasing the platform alt to 3390ft and starting the final descent from the 8.8nm however my companies procedures don't mention this. I'm sure nearly a decade ago that our procedures did mention about increasing platform alt but this seems to have been taken out. I have certainly not discounted this method and as Skyjob mentions your platform will also be affected by cold temp so it seems to give you more safety margin relation to terrain to do this. Also I've noticed a lot of the VNAV approaches ( in normal temp conditions) we fly the FMGC seems to compute a 3 degree profile from a higher alt than the platform indicated on the chart. Some as much as 1300ft above platform and ATC have never questioned us yet so increasing platform for temp is not dissimilar.

FlightDetent. You bring up a very good point about keeping things simple for ATC. Could imagine a tricky situation especially for busy parallel approaches although saying that the chance of having to two non ILS approaches in a busy international airport at -20 would be practically zero. Also hadn't considered the FPV to be baro angle and increasing FPA. I'm sure we used to have a table for that but that's been removed ( not sure why).

I think the chances of it happening for real are pretty slim ( in the sim anything is possible) but if it did happen hopefully you would have enough time to plan for it. Initially looking at this scenario it looks relatively straight forward at first but actually going through the cold temperature tables and correcting each step down height without making any errors takes a fair bit of time and capacity. As Flightdetent says I certainly would not want to be doing this calculation on the approach. There is a handy quick to use App which calculates the profile temperature corrected however not sure I would be too confident whipping out my phone mid sim/ check flight without a raised eyebrow from the trainer.

I bring this scenario up as I've heard it been given in the sim where you are given a diversion with not much time to perform an LNAV approach needing temperature correction. I think certainly a few potential gotchas if not considered before hand.

[Dracarys]

As FlightDetent suggests, you also need to correct the FPA.

What you want to actually fly doesn't change. You still want to commence the descent from the same true platform altitude and distance and arrive at the TDZ. But in order to do so your indicated platform altitude must be appropriately higher (temperature corrected), and to descend from that indicated altitude to zero over the same distance (because the distance doesn't change) requires a steeper indicated FPA (note - the true FPA will still be 3 degrees because the true platform hasn't actually changed). So, despite it appearing like a steeper approach, it's only steeper in an "indicated" sense. The aircraft will actually fly the same physical profile as on an ISA day (if the temperature corrections you made were accurate).

Personally, I wouldn't feel comfortable flying an uncorrected platform altitude in order to intercept the vertical profile later (especially in IMC) because terrain clearance could be substantially reduced. But I don't know what the law is regarding this.

Hope it helps.

[EMIT]

Platform

Quote:
Personally, I wouldn't feel comfortable flying an uncorrected platform altitude in order to intercept the vertical profile later (especially in IMC) because terrain clearance could be substantially reduced. But I don't know what the law is regarding this.
Unquote

According to the law, you need TO CONSIDER the effects of temperature on the approach.

Take a situation where the MSA in the area of the approach is 1.700 ft and the platform altitude is 2.000 ft.
I would consider the effect of temperature such that, as long as the required correction is less than 300 ft, the (indicated) platform altitude of 2.000 ft is still above the (truely required) Minimum safe Sector Altitude of 1.700 ft.
In that case, keeping it simple and starting from 2.000 ft would be safe.

For adjustment of descent point DME, the simple one in three rule is applicable - a 3 degree descent is approximately 300 ft per nautical mile, so if your correction would be 200 ft, that would equate to two thirds of a nautial mile, in other words, start descent 0.7 nm later than published distance.

Hope this helps.

[PT6Driver]

My understanding was that all altitudes, msa and below should be corrected. Which would include the platform altitude.
If the whole approach is being done procedurally then this is straightforward, however if under radar control one needs to establish whether or not the controller is taking into account the low temp. In the uk for example they don't. If you are concerned about terrain clearances then you should correct and tell atc of you intentions.

[FlightDetent]

The understanding in the last two posts, while most likely honest and driven by elementary logic and feeling of self preservation, is not the same as regulatory wording.

"all minimum altitudes"

[PT6Driver]

Flight detent.
If you are on a procedural approach and descend to a platform altitude then that is a minimum altitude for that proportion of the approach.
The ultimate responsibility for terrain clearance rests with the commander of the aircraft and all the documentation that I've read has been quite specific on the requirement to request a higher altitude if required to ensure terrain separation.
Further the documentation indicates that commanders should request a higher platform altitude in order to meet the profile requirements on a npa

[lederhosen]

In northern Scandinavia, which is the only place I have regularly come across this the tower gave temperature corrected altitudes to fly so it was all fairly obvious.

[Old Safewing]

In an ideal world

I have no argument regarding all previous posters.

Correcting altimeters for cold temperature is necessary.

So why don’t all airport approaches that experience ISA -15°C and below publish a corrected MSA?

Is there a chance that the “fat” in many procedures cater and cover many low temperature scenarios?

What about altimetry system error?

What about temperature inversions?

It does not absolve your responsibility to check but would everyone agree that when you correct every crossing altitude homogeneously in a non homogeneous environment it is a little like spinning the wheel of fortune.

Say you’re shooting an NPA to minima and you observe you’re getting high. What is the necessary correction and how long will it take to correct? Suck it and see right?

Also most platforms must contain a certain level of fat ref obstacles because we fly level into to an approach path ie it is not normal practice to intercept a glide path from above?

I’m curious how many correct platform and all subsequent altitudes and how many would delay final descent point from the published platform if terrain wasn’t the issue?

[Ian W]

Perhaps it is time to move to GLS approaches with GBAS where the vertical distance is based in corrected GPS and not based on a barometer (altimeter) that has errors, is incorrectly set many times (see ASRS) and needs correction for temperatures.

[galaxy flyer]

why don’t all airport approaches that experience ISA -15°C and below publish a corrected MSA?

Because every temp below ISA requires it Ken correction. The correction, SL airport, will be different at 0 than at -20 or -40. Now, temps above 0C have minor corrections, but they still exist. It shouldn’t be a surprise the Canadian- based OEM includes Temp Comp on all its FMS installations.

[EMIT]

Arithmatic

The Original Poster had a question about the RNAV (GPS) 23L approach in EDDL at low temperature conditions.
Since a picture is worth a thousand words, let’s have a look at the situation.

[EMIT]

In the subsequent postings, there is some disagreement as to whether the approach altitude of 3.000 ft should be corrected for low temperature or not. Some posters are emphatic that all (minimum) altitudes should be increased, others state that altitudes should be increased, when necessary. So what is true?
Let’s have a look at the (European) law (quotes from Regulation (EU) 965/2012 on air operations ).

CAT.OP.MPA.125 Instrument departure and approach procedures
(a) The operator shall ensure that instrument departure and approach procedures established by the State of the aerodrome are used.
(b) Notwithstanding (a), the commander may accept an ATC clearance to deviate from a published departure or arrival route, provided obstacle clearance criteria are observed and full account is taken of the operating conditions. In any case, the final approach shall be flown visually or in accordance with the established instrument approach procedures.
(c) Notwithstanding (a), the operator may use procedures other than those referred to in (a) provided they have been approved by the State in which the aerodrome is located and are specified in the operations manual.

AMC2 CAT.OP.MPA.126 Performance-based navigation
…
(2) Temperature compensation

(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.

(ii) 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.

Another relevant document is ICAO Doc 8168, Aircraft Operations (relevant, but not the law).
From there. The relevant paragraph.

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°C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above –15°C.
4.3.2 Tabulated corrections
For colder temperatures, a more accurate correction should be obtained from Tables III-1-4-1 a) and III-1-4-1 b). These tables are calculated for a sea level aerodrome. They are therefore conservative when applied at higher aerodromes.

[EMIT]

The legal text seems to contain a contradiction: for an approach using VNAV minimum, the FAF altitude must not be changed, but outside the final approach segment, the effect of temperature should be considered – where would you have to apply the higher altitude, if needed, without touching the FAF altitude (remember, you are not allowed to add waypoints to the string of waypoints that came out of the database?).

Let’s look at the minimum certified temperature for this chart (-15).
The Minimum Sector Altitude, which is the controlling altitude with regards to obstacles, is 2.800 ft. Purely by coincidence, the reason for this particular MSA is situated right underneath the approach transition from BAM VOR-DME, an obstacle with elevation 1782 ft.
Let us apply temperature correction for -15 degrees:
(1782 – 138) * 0,004 * 30 = 198 ft.
Temperature corrected obstacle elevation then is 1782 + 198 = 1.980 ft
Apply 1.000 ft margin above corrected elevation and round up to next hundred feet gives temperature corrected MSA of 3.000 ft, which is exactly the specified minimum altitude from both Initial Approach Fixes to the FAF.
So, a certain logic can be discerned in the design of the approach chart. Approach designers on the forum can probably confirm that such is intentional, per design specifications.
For an approach using LNAV minima, you can see that the use of 3.000 ft on the published tracks satisfies the required obstacle clearance down to -15 degrees, just as it does for the VNAV minima.
Similarly, the missed approach altitude of 4.000 ft does not run into obstacle problems at that temperature.
For colder temperatures, VNAV minima cannot be used, and when using LNAV minima, MSA must be corrected, for that use the table. When using the table, you probably end up with a bit more fat in the calculations, because you wil probably wind up with the round up to next 100 ft in the original MSA, plus round up of figure to choose in the table plus round up to the next 100 ft in your end result.
For example, at -20 degrees, accurate calculation
(1782 – 138) * 0,004 * 35 = 245 gives corrected obstacle 2.026 ft gives corrected MSA 3.100 ft.
Table 2.000 and -20 gives correction 280 ft, added to MSA 2.800 ft gives corrected MSA 3.100 ft.
Table 3.000 and -20 gives correction 420 ft, added to MSA 2.800 ft gives corrected MSA 3.300 ft.

If you choose to not correct the MSA, and base your minimum flyable altitudes on that all important figure, but apply the correction directly to the published altitudes for the different approach segments, then you end up with even more fat on the bones, because then you would add 420 ft to 3.000 ft and round up, so your result would be 3.500 ft until the FAF.

Question of OP was, at what altitude would you be legal and safe at -20 degrees?
Answer is, at 3.100 ft you are legal and safe. If you choose 3.500 ft based on more conservative methods, you are also legal and safe, whichever altitude you choose, keep ATC informed.
OP stated the corrected crossing altitudes for 7, 6, 4 and 3 mile fixes – don’t forget the one at 5 miles, that one even has an obstacle clearance altitude associated with it!
If you choose 3.100 as corrected platform altitude, then you will have to delay final descent until about 8 miles. If you want to start final descent at 8.8 miles, then you have to increase platform altitude to 3.400 ft. I won’t display the calculation here.

Another interesting item about the LNAV or VNAV choice is the minima – in this plate, the LNAV minimum is substantially higher than the VNAV minimum and at -15 the difference would be even larger: VNAV as published, LNAV corrected upwards to 750 ft.
The reason for that difference lies in the design method of obstacle criteria: for the LNAV minima, the obstacle clearance is determined under the assumption that directly after passing a controlling fix, the aircraft drops like a brick to the next published obstacle clearance altitude (even though in actual operations, big aircraft are required to be flown via a Continuous Descent Final Approach). For VNAV minima, the obstacle clearance plane (through which no obstacle may penetrate) is connected to the sloping descent path. Especially obstacles that are somewhat farther away from the runway may hamper the LNAV minima more than they do the VNAV minima.

Finally, as reply to post # 11 – as the quoted paragraphs show, in post # 9, fairly accurate “regulatory wording” was used.

[JammedStab]

So bottom line for approaches without vectoring onto final......Correct all altitudes prior to the FAF.

When using non-temp corrected FMS units.....

No VNAV allowed when the airport temp is below what is published on the chart.

VNAV is allowed with no corrections to be made to the FAF altitude or later on the approachif the airport temp is at or above the published temp on the chart.

LNAV is allowed at any temp and must always be temp corrected when the airport temp is in the "standard" range requiring cold altitude corrections.

Is this correct?