When to select standard pressure

[Bob Scott 7,716]

The ICAO definition of transition altitude (per ICAO Annex 15) is: "The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes."   So flight at the transition altitude under ICAO rules should be done with QNH set.

The US FAA AIP says that the altimeter should be set to QNE (29.92in/1013HPa) at or above 18,000 ft (AIP para 3.3.2), so flight at the US standard transition altitude (FL180) is done with QNE set.

The key is in how the altitude is specified by ATC.  US ATC will never clear you to "18,000 ft", but only to FL180.  If an EU controller clears you to "FL50" that signifies a flight level at QNE...if they clear you to 5,000 ft, that means an altitude with QNH set.  Similarly, restrictions in published procedures will be specified as a QNH-based altitude (e.g. at/above 5000 is depicted as 5000) or as a QNE-based flight level (e.g. at/above FL50 is depicted as FL50).

When in doubt, as always, query the controller.

 

[Overload 91]

Been a long time since I flew commercially, but when I did (small turboprops) the  company procedure was climbing, once cleared to a flight level the pilot flying would  set standard straight away, no matter what altitude he was at,  and in descent, once cleared to an altitude,  the pilot flying would set the QNH straight away. 

The logic being, if the pilot flying waited until the transition altitude, sooner or later he'll miss the exact moment, and could cause a loss of separation, in descent, even more important, very low barometric pressure could have the aircraft a thousand feet lower than the equivalent flight level, with terrain separation implications. 

Eugene

[Ray Proudfoot 8,930]

Sorry for the late post. As a long time user and supporter of Radar Contact we often had complaints about the program telling the user they weren’t at the correct altitude / flight level.

It’s a very simple rule. If you’re instructed to climb / descend to an altitude you should be on QNH.

If instructed to climb / descend to a flight level you should be on Std Pressure.

So to answer @sunnydaze’s question you would remain on QNH until cleared higher. I live right under the LISTO departure from 05. 😁