View Full Version : Low OAT and Altimeter Corrections

20th Dec 2009, 08:12
Hi Folks,

1. Is it necessary to do corrections when flying under radar vectors for an ILS in Europe?

2. Should I do corrections to comply with the altitude constraints (QNH based) during a RNAV app?

Thanks and Merry Christmas,


20th Dec 2009, 10:37
As far as i'm aware, if you're under vectors then ATC will apply suitable adjustments if deemed necessary.

On the Rnav, as long as there is a minimum temperature published on the plate then I don't think adjustments are necessary.

20th Dec 2009, 14:15
1) This is for Norway, check your country's AIP and NOTAMs for the relevant information; except in the Farris and Oslo TMA, the controller is responsible for correcting for low temps. To be on the safe side, don't accept an altitude you are not comfortable with.

If you do need to correct, you do need to request a new altitude from the controller, unless you are cleared for an approach. You do need to correct your minimas for low temp.

2) You should indeed correct minimum altitudes for low temp on a RNAV (like any non precision) approach. However be aware that for mandatory altitudes (specified to cross AT that altitude) you do need to either obtain a specific clearance for crossing at a different (higher) altitude, or stick with it. Compare to MSA to decide if it's safe.

If in doubt, ask ATC! But don't accept an altitude below what you're happy with :)

20th Dec 2009, 14:36
In very cold weather you are much closer to the ground than you think. ATC should adjust according when vectoring you. (SHOULD).

On approach you must correct all altitudes especially minimums. At very cold temps you could be well over 100 feet closer to the ground than indicated.

Have a look at the cold temp. correction chart. Imagine a Cat 1 approach at -25C. Imagine yourself thinking there's plenty of wiggle room here, I'll start my MAP in 20 ft. or so. Suddenly you've returned to earth before your time. Maybe permanently.

To draw you a picture imagine taking some tropical air in a balloon and then chilling it to well below freezing. The balloon will constrict or get smaller. If this is the air mass which you're in, a given indicated altitiude will be closer to the ground than it would be in a warmer air mass. You've heard of shrinkage. All men know about it. Same but different.

Your actual altitude (measured with a string or some such device) will always be closer to the ground when it's below ISA temperatures (and conversly, the opposite). When we're all motoring along at our flight levels we're all effected the same so it doesn't matter. It only matters when close to the hard stuff.

20th Dec 2009, 14:50
Bfisk. I'd argue that crossing altitudes on an approach should be corrected too. How else will you get a good G/S check. To paint the picture again, imagine you are at a FAF which should indicate 1500 on that particular approach. On a cold day your altimeter may indicate 1600 there. If you've already done your math you expected this, but if you haven,t you may be confused and even suspect that your G/P indications are wrong.

20th Dec 2009, 15:01
@5LY: yes, thank you, I suppose we are all aware of the physics behind it. I interpreted the original question as a procedural/operational one.

Do also note that a glideslope check altitude is not a mandatory crossing altitude. Obviously if you are on the glideslope and there's a check, say passing OM at 1500' AGL on the plate, and the temp is -10, then you should expect to see some 1650 or so on the altimeter. This is different from a STAR/long approach which may instruct you to cross fix XXX at a given altitude, say 5000'; not above, not below. That may be for procedural purposes, and in such a case it would not be acceptable to deviate from that altitude without an amended clearance.. You do not need such an amended clearance for adjusting minimum altitudes.

The difference on the plates:
5000 and simply 5000.

First one you need to request reclearance for, but not for the latter. Agree? :)

20th Dec 2009, 15:11
Cold weather operations can be very tricky. things to consider are de-icing procedures along with a good understanding of Hold Over Times. The old saying when cold and low lookout below is true for reasons already mentioned in the thread. Adjust DA on a CAT 1 app. Also O/M altitude and sector altitudes(not FL'S). Don't forget acceleration alt. after T/O in the event of an engine failure. And all non-precision minima including circling minima. This is not only good airmanship but mandatory.

20th Dec 2009, 15:15
Bfisk. Yes we're on the same page.

21st Dec 2009, 03:29

tie domi
21st Dec 2009, 03:48
all procedural alt. need correction radar atc alt do not see jep vol 1,Canada regulations etc.,