Quick pilot question from a ZJX Controller

[airkiwi]

Hopefully you guys can help me out with a question my fellow controllers couldn't find an answer for.

If you're assigned a block altitude of 170-FL190, what do you use as an altimeter setting? 29.92 or the local?

[Jason]

In the US, local altimeter below 18,000, 29.92" at or above 18,000. If you dip down below 18,000, we switch back to local altimeter whether we're cleared in a block altitude or just assigned a normal climb or descent.

14 CFR 91.121:

(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—

(1) Below 18,000 feet MSL, to—

(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;

(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or

(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or

(2) At or above 18,000 feet MSL, to 29.92" Hg.

(My emphasis in bold)

Hope this helps.

Best,
Jason

[airkiwi]

So you're saying a flight established in a block will be switching back and forth between 29.92 when above FL180 and whatever the local is depending on where they are in the actual block?

[Jason]

So you're saying a flight established in a block will be switching back and forth between 29.92 when above FL180 and whatever the local is depending on where they are in the actual block?

At or above FL180, yes.   FAR 91.121 requires us to.  If we left 29.92" set below 18,000ft. and the local altimeter setting was even just a few tenths different than 29.92, then below 18,000 ft. we might end up descending below 17,000 ft.  When we pass through (or level off at) 18,000 ft. we reset the altimeter to 29.92" since any altitude at or above 18,000 is a pressure altitude whether assigned a block altitude or not.

[Greg Calderin]

Remind me not to fly in ZJX. Especially since they don't know the altimeter rules. haha

[ridejumpfly]

Don't issue a block that will be half in and half out of the FL's..  Problem solved..

[captray]

I don't think you would ever get that block of altitudes issued in the States.
In other countries, where the transition level is lower it would be no problem.

[airkiwi]

I don't randomly issue the block altitudes, military pilots frequently request them - but it's great that you guys would want to deny them. Particularly on Veterans Day.

If you're a pilot then you're the one responsible for knowing the altimeter rules, and if you don't then you shouldn't fly through ZJX.

A block altitude of 160-FL180 is also commonly issued to pilots across the US. Obviously, as Jason mentioned, you change the altimeter when descending or climbing through FL180, yet no one's answered the actual question of what the setting is when maintaining the block.

[Jason]

...yet no one's answered the actual question of what the setting is when maintaining the block.

It depends where you are in the block.  There is no regulation that specifies what altimeter setting to set in a block altitude which begins at an altitude below the transition altitude and ends above the transition altitude, therefor 91.129 applies and you would switch depending on where you are in the block.