QNH when?

[catocontrol]

When would you expect an aircraft to set local QNH when cleared from FL200 to e.g 5000 ft? (Transitionlevel is 80 in this example)

I have discussed it with a pilot. He says that he will set local QNH when leaving FL200. I on the other hand, expect them to wait until passing T-level.

From DOC4444

12. Altimeter setting procedures
12.1 Expression of vertical position of aircraft
12.1.1
For flights in the vicinity of aerodromes the
vertical position of aircraft shall, except as provided for in 12.1.1.1 and 12.1.1.2, be expressed in terms of altitudes at or below the transition altitude and in terms of flight levels at or above the transition level. While passing through the transition layer, vertical position shall be expressed in terms of flight levels when ascending and in terms of altitudes when descending.

12.1.1.1
After approach clearance has been issued and
the descent to land is commenced, the vertical position of an aircraft above the transition level may be expressed by
reference to altitudes (QNH) provided that level flight above the transition altitude is not indicated or anticipated

[Spitoon]

In the UK there are some more explicit rules - but they only expand on the PANS-ATM text, and only really apply in the UK. But make sure you are asking the right question. In a modern transport aircraft there is more than one altimeter and each airline has its own SOPs for altimeter setting. The ICAO text refers to reporting of levels - not altimeter setting. I believe that many airline SOPs will mean that one altimeter is set on 1013 and another on QNH until the aircraft is below TL.

[Standard Noise]

One altimeter - at or passing T level.
Two altimeters - whenever the pilot wants to, as long as he/she refers to altitude only once past the T level.

[tescoapp]

Well if its any help.

I have worked with 3 different SOP's and they all have you setting QNH as soon as you leave your current level when cleared to a Alt when arriving at destination, be it from FL200 or FL60, on all altimeters.

I think 12.1.1.1 covers it.

The only reason why you wouldn't set it before leaving the level is because when you wind in the QNH the auto pilot thinks you have suddenly changed level and does some not very pleasant pitch changes to correct it. So as soon as you knock out Alt hold and begin your decent you wind it in. And its usually the trigger for the Approach checks.

[AirNoServicesAustralia]

I think you will find that in reality most pilots change to QNH when cleared to an altitude due to the workload issues that quite often occur during the phase of the flight that corresponds with passing through the TL. As controllers we obviously would prefer that they stay on 1013 until passing through the TL but from what I have been told and what I have seen this isn't usually the case.

Only becomes a really big problem when you subsequently have to reclear them at a FL, and they forget to switch back to 1013, which has happened to me before.

[fireflybob]

Several years since I researched this one so it might have changed (but I doubt it!):

The UK AIP said you set QNH as soon as you are cleared to an altitude unless further flight level vacating reports are required and then set QNH after the last FL vacating report. The ICAO Pan-Ops said set QNH unless level flight above the TL is anticipated.

Obviously follow company SOPS but the short answer subject to those is set QNH as soon as cleared to an altitude. The hazards of not setting QNH (especially with QNH less than 1013 mb) from the pilot's point of view is that you might bust the MSA and/or have a CFIT!!

[Spitoon]

I think you will find that in reality most pilots change to QNH when cleared to an altitude due to the workload issues that quite often occur during the phase of the flight that corresponds with passing through the TL. As controllers we obviously would prefer that they stay on 1013 until passing through the TL but from what I have been told and what I have seen this isn't usually the case.
Only becomes a really big problem when you subsequently have to reclear them at a FL, and they forget to switch back to 1013, which has happened to me before.Not everyhere is like Australia. Speaking as a simple controller, I can understand why crews want the QNH set as they descend towards a variable TL with a TA that is often little more than 1000 or 2000 ft above nearby terrain (which is the situation over large parts of Europe).

[tori chelli]

Fireflybob has it right for the UK
you need to differentiate between reporting vertical position (flight level or altitude) and altimeter setting.
The UK rules state that if you are cleared to an altitude, you set the QNH straight away, the only caveat being that if you have been asked to report a passing flight level then you keep 1013 until such time. This works in reverse when climbing from altitude to flight level(s).
To avoid the error described by ANSAus, if I need a flight level report having cleared down to an altitude and the A/c is above the TA, I ask for "report your passing level on 1013mb
Lecture over
Tori