i am a little confused about ATC and cold weather adjustments. I understand the concept of how your altitude is lower than indicated when in colder than ISA but... in the Instrument Flying Handbook they explain that if you apply adjustments for cold weather you should let ATC know you are doing so. So my question is wouldn’t ATC have my actual elevation based on radar? If they want me to fly at 5000 and I fly at say 6000 indicated wouldn’t ATC see me at 5000? And if I didn’t correct and I’m really flying at 4000 wouldn’t that be a hazard for many reasons? Thanks
ATC corrects the transponders pressure altitude broadcast to the current altimeter setting. They separate IFR and VFR traffic by 500 ft and IFR /IFR by 1000 ft. If you using a temperature adjustment, you need to advise. They get hinky when things are 300 ft off.
Does that mean if they want me at 5000 they will tell me to be at 6000 so that I’ll be 5000? Sorry I know I am kissing something simple here
No. The altitude they see comes from your transponder which gets it from the altitude encoder which is an altimeter set at 29.92. ATC radar compensates for local pressure if the local Pressure is other than 29.92. But it does not compensate for Temperature. You must advise ATC if you are using cold weather correction so they know you may not be complying with the altitudes on the Charts. They need to know this so they will not get another airplane to close to you. For instance, if the Chart says cross FFIXX at 3000 and you have told them you are cold weather altitude correcting, they are not going to run an airplane over you at FFiXX at 4000.
No. See my post #4. They will not try to calculate how many feet are needed to correct. You do that and tell them. They then keep other airplanes farther away to compensate for that. Here's what their rules say about it: 6. Controllers must not disapprove a pilot request to cold temperature compensate in conjunction with the issuance of an approach clearance. 10. Pilots are required to advise ATC when intending to apply cold temperature compensation to instrument approach segments. Pilots must advise ATC of the amount of compensation required for each affected segment on initial contact or as soon as possible. Pilots are not required to advise ATC when correcting on the final segment only. Controllers may delay the issuance of an approach clearance to comply with approved separation requirements when informed that a pilot will apply cold temperature compensation (CTC). Pilots will not apply altitude compensation, unless authorized, when assigned an altitude prior to an approach clearance. Consideration should be given to vectoring aircraft at or above the requested compensating altitude if possible. This eliminates pilots having to climb once on the approach.
Just to add to @luvflyin 's post... If the crossing altitude at FFIXX is 3000, you'd apply a cold temperature correction that might have you crossing at 3200 indicated altitude. ATC needs to provide 1000 feet of separation, so 4000 feet becomes an unusable altitude in your vicinity. You need to inform ATC so they don't assign another aircraft an altitude that may result in a loss of separation. (Applies only to corrections on the intermediate or missed approach segments.)
Now I see where I went wrong, I assumed they used radar to find my “true” altitude but they are using my mode c transponder which doesn’t compensate for cold. Thanks for clearing that up.
