View Full Version : low temp effect on minimums

9th Aug 2011, 10:33
airbus sops state that if ni minimum OAT is published on the approch chart,the effect of low OAT on obstacle clearance needs to be evaluated.Could someone explain the impact of low temperatures on obstacle clearance.i guess it has to do with density altitude,however if someone can throw some light on this clearly

compressor stall
9th Aug 2011, 12:26
Method 1.

Google "Getting to grips with cold weather operations" by Airbus and download it, and read the last chapters.

Method 2.

Use your search button in PPRuNe. There was a thread in this very forum in the last three days. :rolleyes:

10th Aug 2011, 02:12
thanks stall,you could have given me a hint though:O

10th Aug 2011, 06:45
It's a correction on your true altitude, as the lower temperature deviates the indicated ALT from your true ALT in a unsafe direction (remember that your altimeter is calibrated to ISA conditions only).

It's about 4% for every 10 degrees of isa deviation.

It's important to apply the correction (if not done by the ATC) from at least MSA down to the minima.

check this: http://www.smartcockpit.com/data/pdfs/flightops/meteorology/Getting_To_Grips_With_Cold_Weather_Operations.pdf

All the best.

10th Aug 2011, 06:52
Another interesting read: SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/flightops/aerodynamics/3)


10th Aug 2011, 14:34

The first document has what you are looking for in Section E. Note that the issue is not really effect on minimums so much as it is on procedural segments that are higher. The intermediate segment may have as little as 500 feet of obstacle clearance. Yet if it is 2,000 feet above field elevation and the temperatures are very low, you may impact terrain while flying at the proper indicated altitude in the intermediate segment.

Also, where initial approach segments are on the order of 5,000 to 6,000 feet, or so, higher than field elevation, even the minimum initial segment obstacle clearance altitude of 1,000 feet can no longer exist at all.

10th Aug 2011, 15:10
There's also a section in the Meteorology section in the Jepps charts that explains the corrections.

10th Aug 2011, 15:25
This procedure is an excellent example of the cold weather traps (some say "terps are traps") in an IAP that arrives over terrain considerably higher than the airport, particularly in the intermediate segment.

Note that the intermediate segment begins at JANUD fix, with descent to 4,800' authorized after passing JANUD. Note the step-down altitude is 4,800' with clearance of approximately 700' over a mountain peak (4,088' high point) that is almost on the localizer centerline.

Last winter, during a severe cold spell, an aircraft got a EGPWS terrain warning descending on barometrics in this segment.

A cold weather additive would have been most appropriate but the FAA has refused to get into that business. The US Air Force parted company with the FAA 15 years ago on this issue, thus they do cold temp additives. Some smart pilots do it on their own but, alas, those are few and far between.


10th Aug 2011, 20:04
When we are dealing with baro-vnav, we are very aware of the cold temp corrections, and I have always wondered why this has not been an issue before.
Within the approach procedures, the FAA has a very slim margin for cold temperatures, 0.917, and this coupled with the ISA of the airport for the effective glidepath and the ROC, doesnt leave much room for colder temperatures. (it doesnt take temperatures a whole lot to get a 3 degree GPA to an effective 2.5 degree GPA)
Given all of the recent NPA constant descent conversions, it seems to me that the temp limitations should be shown on the plate...

Additionally, the different boxes handle to prox warn differently, especially important in the latency transition areas from initial to intermediate 1000' ROC to 500' ROC.

One trick we use in Alaska, is the 3 degree GPA is typical down to -20C, then the same procedure, with a 3.5 GPA is used from -45C to -21C....

11th Aug 2011, 02:06

When we are dealing with baro-vnav, we are very aware of the cold temp corrections, and I have always wondered why this has not been an issue before.

With baro-vnav, it is dealt with only in the final approach segment (FAS). It has to be because criteria require that the baro-vnav final approach segment be accorded quasi-precision vertical path obstacle clearance surfaces; i.e., the way to escape having to have FAS step-down fixes.

11th Aug 2011, 02:29
‘Low temp effect on minimums’: not only minimums or an initial procedure – also think about departure.
During a low temp departure from uncontrolled airport, using the local pressure setting, an aircraft received an EGPWS warning shortly after flap retraction altitude. The aircraft was quite close to terrain.

11th Aug 2011, 11:24
Check the new format FCOMs, it expplains why they have eliminated a table that corrected FPA depending on temp

Ian W
11th Aug 2011, 12:32
There is no logical justification for not issuing temperature corrected pressure settings (i.e. QFF instead of QNH). The difference in vertical clearance from terrain can be over a 1000ft.

If several aircraft have to be separated by altitude in the terminal area it would be unsafe if some of them used (different) altitude corrections for temperature.

11th Aug 2011, 13:37
Very bad plan, Ian W:
Selecting a different pressure setting produces an almost linear shift of the indicated altitudes (30ft/hpa).
On the other hand the cold temperature effect is increasing with elevation above the source - e.g. at -20°C at the airport the altimeter at 200ft shows 30ft less, at 2000ft the error is 280ft.
Folowing your suggestion: If you correct for 280ft (about QNH minus 10hpa) then you will be clear of your obstacle at 2000ft, but you will end up about 270ft above a charted 200ft-minimum . Not desired.
When you correct for the 30ft error at the 200ft minumum, you will strike the obstacle at 2000ft. Also not desired.
Changing the pressure setting during the descend? Unthinkable!
Your proposal sounds like an instruction for CFIT...

If you are not happy with a cleared altitude you get from ATC (due to temperature) just tell the controller! In affected areas ATC are usually already adding some hundred feet to keep you away from terrain.

11th Aug 2011, 14:19
I'm not sure that using a different altimeter setting is the answer since the necessary correction becomes greater as altitude above the ground station is increased. Aren't approved weather stations altimeter settings already temperature compensated?

However when blended with the ground station pressure and temperature, using DADC output to calculate and display true altitude would be almost as accurate as differential GPS. It's just taking temperature compensated baro VNAV one step further. During extreme temperature ops, ATC might find it necessary to increase separation minima between aircraft, but it's better than the present system. In any case, the problem will likely solve itself eventually when GPS becomes the standard IFR altitude reference for terminal ops. That's a ways down the road though.

So in the meantime, we'd be well advised to use the correction charts when the temperatures start trending towards the extreme.

That's just my take on it though...