View Full Version : Temp correction for ALL approaches??

King on a Wing
30th Dec 2009, 21:55
Say you are doing an Rnav approach with appropriate minimums set.The approach is built on a temp of say +15 C. But OAT is -30 C. I'm pretty sure we would need to apply the cold temperature correction to the chart and thereafter use it with the corrected heights.My question is would you need to do this temp correction for a conventional approach...say an ILS.
Thanks in advance ..

30th Dec 2009, 22:37
Yes, any approach, precision or non-precision, with a temp at or below 0C at ground level needs to have a temp correcton applied. Some GPS Apps are cleared to a given temp such as -10C with no correction (because it is built in and approved by the governing body,and stated on the charts) below which temp a managed approach may not be made and the relevant temp correction must be applied.

30th Dec 2009, 22:55
Interesting question.

Firstly, if you are going to apply temperature correction to procedural altitudes, you must get agreement from ATC. You can accept or reject a clearance, but you can't adjust the cleared altitudes without telling anyone.

Once you are on final approach, you can add corrections if you wish, but bear in mind that these will be progressively smaller as you approach the airport. Even in your very chilly example, the correction would mean adding 36ft to a DH of 200ft. Step-down fixes further out from the field might require adjustment but once you are cleared for the approach you can cross these points at a higher-than-charted altitude. Holding and procedure turn 'platform' altitudes should be adjusted as well, but you had better tell ATC in case other traffic is departing/arriving/holding above you.

RNAV approaches normally have a lower OAT limit, below which temperature the use of baro VNAV is not approved. This is to ensure obstacle clearance on the 'GP' calculated by the VNAV element of the approach. In colder than standard conditions, the 'GP' will be shallower than published and vice-versa.

On an ILS, the glideslope signal is fixed in space, so you will always follow the published descent path, whatever the temperature. Of course, the altimeter may overread as you cross markers, DME checkpoints, etc, but you will be at the correct true altitude for your position on the glideslope.

The real problems with cold temperature correction occur before the final approach, when you are at significant altitudes above the airport. In your example (ISA dev -45) the error will be 180ft per 1000ft. Given an MSA or minimum holding altitude of 5000ft, I would want to add 1000ft to that in order to ensure obstacle clearance. ATC may apply temperature corrections to charted altitudes. If they don't, you are quite entitled to reject the cleared level and suggest an alternative higher one with which you would be happy. What you can't do is apply a correction without telling ATC, as this has obvious dangers vis-a-vis separation from other traffic.

Hope this helps.


King on a Wing
31st Dec 2009, 08:33
Certainly helps Eck,
I completely agree with the fact that an ILS should be flown at uncorrected heights once established.But would'nt u need to apply a correction to the other platform heights like say the Faf holding altitude.And should'nt Atc be aware of this corrected altitude since everyone would be 'correcting'.
In the Rnav approach I am quite sure that u need to use the 'corrected' altitudes in order to be able to use the Vnav function of the approach.Once again....wondering why Atc should not be aware of the correction.Most Rnav precision approach charts clearly depict the temperature at which they were 'caliberated'.Which is also the minimum temp at which an uncorrected Lnav/Vnav approach can be flown.Below this temperature(OAT),a temperature correction MUST be applied to ALL approach altitudes in order to satisfy obstacle clearance requirments and meet legal requirments for a coupled Lnav/Vnav approach.As you so corrrectly pointed out.However,still wondering if one would need to apply the same corrections for the same conditions for a more conventional precision approach.....say a ILS.And once again would one need to inform Atc....
Thanks again

31st Dec 2009, 09:25
And don't forget to crosscheck your documentation too. For example if you use the IAN approaches on a 737 you are not allowed to apply temp correction in the FMC for the approach waypoints, however you have to fill in the descent forecast page which does include an ISA correction and actual QNH, both are used to fly the correct glidepath on non precision approaches. Makes it easy as you do not have to fiddle around with the temp correction table much as the FMC takes care of that on its own.

31st Dec 2009, 09:37
There are many threads on this subject if you use the search function.

It appears that you have forgotten what the FAF altitude is for.....to check that you are on the correct glideslope? If you choose to disregard the temperature correction then you have not done so and may become another statistic.

As a matter of course, having checked your flight manuals, you may find that managed (LNAV/VNAV) non-precision coupled approaches may not be used in temperatures less than 0C.

It appears that you say you can apply the corrections only if you wish to? I'm sure your Ops manual will not give you that wording.

King on a Wing
31st Dec 2009, 09:52
I thought an Rnav approach was a precision approach...and not otherwise.Thus the correction requred.Because u CAN do the approach below 0 C.
And Denti.....yes I remember the 737NG Fmc.It did have a very useful function on the descent forecast page.Used it all the time for Rnav approaches.Unfortunately never at such low temperatures.

Piltdown Man
31st Dec 2009, 10:50
Eckhard has provided a superb explanation. I would only add the obvious in so much that Rad Alt minima are the only heights that do not need correction.


31st Dec 2009, 15:54
ALL CAT I altitudes must be corrected down to and including the DH. It is true that the glideslope is 'fixed', however; on a CAT I approach, all altitude and fix crossing references are based on the pressure altimeter hence, all fixes must be corrected as well. These may seem minuscule corrections but for the sake of being 'right', they must be done. The most important correction on a CAT I is the DH, where the DH/DA is predicated on the use of the pressure altimeter. The correction must be added to the altimeter otherwise, for an uncorrected altimeter, you will reach your true DH at a lower true altitude. In essence, an uncorrected altimeter will always lead you to a lower DH in cold temperatures which has the affect of 'busting limits'.
On the other hand, CAT II and CAT III limits are all based on RA in which case no correction is required at the DA, BUT the intermediate and final fix crossings must be corrected because these altitudes are all based on pressure altimeter readings hence, you will have a correct altimeter reading as you pass by the fix.

Under radar control, maneuvering altitudes are not required to be corrected as ATC is supposed to apply temperature corrections to the vectoring altitudes. As a good situational awareness procedure, you will want to correct the MSA and procedure turn altitudes just to make sure that ATC isn't vectoring you below safe altitudes. If in doubt, you can always ask ATC if they have made the corrections.

Missed approach altitudes must be corrected as well as these altitudes are based on pressure altimeter. If you do carry out a MAP under radar, the controller will provide you with the correct altitude in most cases but again for situational awareness, the MAP altitude should be corrected.

If you are carrying out a VNAV approach, the crossing altitudes shown on the legs page must be corrected, however; do NOT change the fix itself.

From the TC AIP:

With respect to altitude corrections, the following procedures apply:
1. IFR assigned altitudes may be either accepted or refused. Refusal in this case is based upon the pilot’s assessment of
temperature effect on obstruction clearance.
2. IFR assigned altitudes accepted by a pilot shall not be adjusted to compensate for cold temperatures, i.e., if a pilot
accepts “maintain 3 000”, an altitude correction shall not be applied to 3 000 ft.
3. Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots.
4. When altitude corrections are applied to a published final approach fix crossing altitude, procedure turn or missed
approach altitude, pilots should advise ATC how much of a correction is to be applied.

My two cents as a check airman and having flown the arctic for many years.

31st Dec 2009, 17:20
Wise words from 777AV8R! The only point I would argue with is regarding the correction to baro VNAV altitudes (see below).

In my experience ATC don't broadcast temperature corrections as such. They may have minimum vectoring altitudes which are increased in cold conditions, but they wouldn't necessarily tell you that the cleared level had been increased - they would just issue the clearance (maybe an ATC expert can clarify?)

The problem may be worse in a non-radar environment when procedural service is offered. I have heard some European ATIS contain advisories such as, "due to cold temperatures, pilots should add 200ft to all minimum altitudes" (or words to that effect). This 200ft correction may be appropriate for the procedure turn or base turn 'platform' and also for the IAF and FAF step-down altitudes. It would be a bit excessive for a DH of 200-400ft. It would be insufficient for a minimum holding altitude of 5000ft.

The point is, you can't have a single-figure correction, because the temperature error is proportional to the vertical displacement from the altimeter setting reference (i.e. proportional to your height above the airport).

The higher you are above the airport, the greater the error. The colder it is, the greater the error. (Technically, the average temperature deviation of the air mass beneath the aircraft has to be taken into account. In practice, most guys take the deviation from ISA at the airport and assume a standard lapse rate for the air mass between the airport and the aircraft.)

Having flown frequently in Greenland, Canada and Russia, I have routinely applied cold temperature correction (and strong wind correction) to MSAs. These then become MOAs (Minimum Operating Altitudes). The MOA can be 2000ft or 3000ft higher than the charted MSA.

When it comes to descent and approach, I calculate a temperature correction in the form of 'xxxft per 1000' of which I make a mental note. In the case of complicated procedures, I may even make hand amendments to the charted altitudes.

The formula I use is that the error will be 4ft per 1000ft for every degree Celsius deviation from ISA.

If the approach is radar vectored, I assess each cleared altitude against terrain/obstacles and either accept the cleared altitude or tell ATC the minimum altitude that I can accept at the present time. Once I'm on an ILS, I will follow the procedure but be prepared for an over-reading altimeter (or under-reading DME) when I check the glideslope for validity. For example, with a deviation from ISA of -25C, on the platform approaching the glideslope (assuming a sea-level airport) I will say to myself something like, "OK, I'm 2000ft above the airport, the error is 100ft per 1000, so my altimeter will over-read by 200ft at the glideslope intercept, which means that if I fly the charted altitude, I will actually be at a true altitude of 1800ft and we will intercept the glide about 0.6nm later than expected." If I am concerned that the platform altitude is adjacent to terrain, I will tell ATC that I am maintaining 2200ft indicated and then I will expect the glideslope intercept to be at the charted DME value (because I will then be at a true altitude of 2000ft).

At the OM (or equivalent), I will be on the GS and at a 'fixed point in space', so my true altitude will be correct, but my indicated altitude will be somewhat higher (say 150ft in this example).

At DA, the error will only be about 20ft so in reality I would probably ignore it, but I agree that one should apply a correction to be accurate. In the event of a missed approach, I would be aware of the increasing error and might tell ATC that I was applying a correction if I felt that terrain clearance was an issue.

I try to fly non-precision approaches in the same way as an ILS (as far as practical) and so most of the above would apply.

In the case of a GNSS/GPS/RNAV approach with baro VNAV guidance, my understanding is that we are not allowed to modify the altitudes that are encoded into the FMS in association with the waypoints on the approach. Under cold conditions, the aircraft will therefore be at a true altitude that is lower than charted. At the FAF (a fixed geographical position), the VNAV glidepath (VGP) will be intercepted at this lower true altitude. This VGP will intersect the runway threshold at a nominal 50ft. The angle of this VGP will therefore be shallower than the nominal 3°.

The minimum OAT published on the chart is the lowest temperature at which the encoded altitudes meet obstacle clearance criteria without correction. You may wish to adjust the MSA and minimum holding altitudes but once you are cleared for the RNAV approach, if you are using baro VNAV, you must not adjust any encoded altitudes. If the temperature is below the stated minimum, you cannot fly a baro VNAV approach. You will have to fly an ‘LNAV only’ approach. Then you can apply temperature corrections if you wish to, as you are no longer using VNAV. (There may be different rules for VNAV systems with temperature compensation, but I am unfamiliar with this.)

So, to summarise, you can apply temperature correction for a platform altitude on an ILS or non-precision approach if you feel that terrain is an issue. You can’t on a baro VNAV approach. If the temperature is outside the limits, you cannot fly a baro VNAV approach. Generally, tell ATC if you are applying a correction, especially to level procedural altitudes. They need to know what your vertical position is compared to other traffic, which may not be applying a correction at all, or may have worked out a different correction to yours.

Similar considerations should apply to SIDs.

An altimeter error is also apparent in ‘hotter-than-standard’ temperatures, but as this is in a safe direction (you are higher than you think) most guys ignore it. It can make itself known if you check the DME against the glideslope. Have a look next time you’re in PHX or DXB!

Here is a chart I developed as a handy reference (sorry about the spelling mistake in the Title):

Imageshack - altimetertemperatureerr - Uploaded by eckhard747 (http://img52.imageshack.us/i/altimetertemperatureerr.gif/)

A Happy New Year to all my readers (yes, I mean both of you!)


31st Dec 2009, 21:46
Happy New Years!

Eckhard...from my B777 FCTM:

Quote: "A vertical path suitable for use of VNAV is one that approximates 3 degrees and crosses the runway threshold at 50 feet. To obtain a VNAV path, maximum use of the RNAV database is recommended. For approaches where an RNP is specified, or approaches where a DA(H) is used, the waypoints from the FAF onwards may not be modified except to add a temperature correction where appropriate, to the waypoint altitude constraints."

No argument there. It is required. Don't correct on my sim/linecheck=BUST

31st Dec 2009, 22:49
I stand corrected!

Our SOPs do not allow such modifications, as I described earlier.

It's interesting to learn that others do!

Thanks for the info.


31st Dec 2009, 22:53
This is actually straight from the Boeing FCTM, Eckhard. All the altitudes in the database are referenced to pressure altitude and the mathematics are computed backwards from that, to created a slope. If the corrections aren't made, the VNAV path will be shallow.

1st Jan 2010, 00:51
Thanks for a good discussion guys.
My companys SOPs are similar to Eckhards: We cannot use VNAV (ie instead we use use V/S or FPA) if any altitude modifications are made to the LEGS page at or beyond the FAF.
I wish we only had the the Boeing 777 FCTM to consult but our FOM lays out our VNAV approach validation procedure and prohibits ANY lateral or vertical modification at/after the FAF. A cynic might question whether the person who wrote it really knew their stuff but hey...
Our current (though constantly changing!) fashion is to only adjust DA/MDA and eng out accel alts from 0 down to -15C degrees OAT at airfield (if no published min temp) and then all altitudes when colder than that.

Conveniently we can still use LNAV/VNAV most of the time as it is unusual (for most of our destinations) to get below - 15C.

galaxy flyer
1st Jan 2010, 01:34

In my Canadian-built biz jet, I can enter the airport and temp data into the FMS and it will calculate the temp correction, apply it to all the appropriate altitudes and also the VNAV angle. Pretty clever, but I am surprised that the B-777 doesn't do it. The FMSs are both Honeywell and Rockwell-Collins, btw.


1st Jan 2010, 02:35
Hi Gang..THAT it is. This is a little understood part of our operations. I've spent a lot of my life operating in extreme environments.

Galaxy: That is a nice feature to have. What type of Challenger aircraft? The 777 certification never had that type of software loaded and I would believe that this wouldn't be upgradable. There are certain software upgrades that can be done but this sounds like a new format which wouldn't meet the criteria.

1st Jan 2010, 02:39
No LAND 3...are you saying that you aren't allowed to adjust CAT 1 DH?

galaxy flyer
1st Jan 2010, 02:42

Both Global and Challengers (300 and 605) have temp compensation built in to the software. Enter the data, the altitudes show as "inverse video", if the pilot's accept the temp comps, all the VNAV will be based on compensated altitudes.

Here's on I knew but never thought of--temp comp'ing the accel altitude on departure in the OEI case.


PS Another great Canadian feature--cockpit oiling of the engines and APU. No mucking with ladders on cold, icy ramps. Excellent safety idea!

1st Jan 2010, 02:45
Ooooooo...good one! Who said this forum isn't useful? I'll remember that one. In fact, I'll have to look that one up and see what the books say about it.

1st Jan 2010, 02:48
That would have been handy in my C-130 days...standing on a ladder in -55C up in Resolute Bay while balancing against a 50 K wind!

1st Jan 2010, 06:30
are you saying that you aren't allowed to adjust CAT 1 DH?

Hi, we do apply cold temp corrections to DA's and MDA's when OAT is 0 or below. But since this only involves setting it on the PFD rather than making any changes to the FMC it doesn't affect the validation of a VNAV approach (until -15C when we have to correct all the other alts as well).
That is my take on it anyway!

1st Jan 2010, 15:59
Yes, then that would be the correct procedure.

3rd Jan 2010, 14:52
Thanks for the discussion guys, it removed some doubts I had.

3rd Jan 2010, 15:21
Sometimes this forum does some real good!

Safe flying....