I have stumbled upon a situation, one that seldom comes up for me but may be a common occurrence for super short hauls in some jets and most turboprops. I flew a 1 hour flight between KBOS and KEWR, and the cruising altitude was 14000 feet, as it was in the real world flight from flightaware, with a 737-700.

 

I updated the altimeter setting shortly before acquiring the cruise altitude, and left it in place until after I started descent, updating to the destination airfield setting. How often is the altimeter updated when flying below the flight levels in the U.S., 18000 feet? And was I correct to wait to adjust the altimeter to the destination airfield setting when I was established in the descent. My thought is if I had done it before the TOD in level flight, I would have upset the cruise attitude as the aircraft would have suddenly descended or climbed to adjust to the new baro setting, therefore causing potential vertical separation issues with surrounding traffic

Hi, I would think as ATC gives all traffic in their sector the correct altimeter setting when below transitionlevel, they are eliminating any separation errors that could occur because of different settings.

How often is the altimeter updated when flying below the flight levels in the U.S., 18000 feet?

 

Whenever you switch to a new center (Albuquerque Center to St. Louis (I think) or from center to approach (Memphis Center to Nashville approach).

 

And was I correct to wait to adjust the altimeter to the destination airfield setting when I was established in the descent.

 

I think you change it whenever you receive the new altimeter setting.

Sometimes the controller will have you flying at the area QNH, usually center, while approach will probably have you flying on local QNH. It's more important that all the planes in the same sector are flying on the same QNH rather than having your plane cruising at exactly 14000 feet. That's why you should always use the altimeter setting your current controller gave you. If youre in doubt ask (that is if you fly online, I dont think its possible to query the default ATC about the QNH setting for your location, or anything else for that matter)

This is an oversimplification of the real deal, but you will receive updates:

When listening to the ATIS prior to dep

Ground will often issue it with taxi instructions

Departure will issue it on first contact

Controllers will then issue it after every handoff (en route/center - under lowest usable FL only*)

Approach on initial contact

 

*Despite what you might think, there are days when you can't use FL180 because the local pressure 18,000 is higher than standard pressure 18,000 (FL180). Search "lowest usable FL table" and you'll find a quick graphic to show you when it is avail or unavail, based on pressure. For this reason, you'll often see language of "lowest usable FL" instead of "FL180."