Setting QNH on descent

[valhalla634]

As a general survey, what are your various airlines out there doing when on descent? Do your SOP's allow you to set QNH as soon as cleared to an altitude or must you wait until passing the transition level?

[eckhard]

Major British Airline: set QNH as soon as you are cleared to an altitude.

[safelife]

Major German Airline: set QNH as soon as you are cleared to an altitude.

[valhalla634]

Thanks for the replies so far. Interested to see some more.

Our Boeing Fleet (not Airbus) has just decided to change SPO's to changing to QNH "AT and NOT BEFORE Transition level on descent.

The claim is this is standard ICAO ( even though the "other" fleet doesn't) and everyone else in the world does this.

[compressor stall]

The STBY alt when cleared to an altitude, the others passing TL.


Unadulterated Airbus procedures say to set QNH when approaching TL and cleared for an altitude.

[Clandestino]

SE Europe minor flag carrier; ATR, A319, Q400: set QNH when cleared to altitude.

ME LoCo, B737: set QNH when passing TL.

[john_tullamarine]

The potential problem with resetting the subscale when leaving a high FL is the possibility that you are going to be held at some intermediate level prior to transition ..

For high descent rate aircraft, leaving it until transition is a recipe for high workload entering the lower levels. However, at some convenient level approaching transition makes sense.

Going back some years, AN's practice was to set the QNH as part of the FL 150 descent checks and that seemed to work out pretty fine.

[FlightDetent]

Whilst descending, I (EU) had never heard of any flight operation that would keep STD until the TL, all re-set immediately with clearance to an altitude.

Plus, you can add all of the airlines operating in the old East block.

Going up, different story.

Deleted this part ...

[Chesty Morgan]

TL would suggest down as they'd still be on STD until then.

[FlightDetent]

AAAAHhh. Indeed, sorry.

[fireflybob]

It's a while since I researched this but the UK AIP stated set QNH as soon as cleared to an altitude (on descent) unless further flight level vacating reports required, in which case set QNH after last required report.

ICAO stated set QNH when cleared to an altitude unless level flight about the Transitional Level is anticipated.

Of course the threat of setting QNH late is the risk of busting Safety Altitude especially with low atmospheric pressure notwithstanding that most operators require QNH to be previously set on the Standby Altimeter which on some types may be the reference for pressurisation.

[winterOPS]

Old gigg: TP operation as soon as cleared to an altitude
New gigg: Jet operation as soon as cleared to an altitude

[BleedingAir]

Interesting responses, I had no idea setting QNH immediately when cleared below the TL was so common.

Around here (Aus/NZ/Pacific), it's standard to set QNH passing the transition level, no earlier. Not sure why I'd want to do otherwise.

[FlightDetent]

And in many places, accordingly to the above techniques, ATC will not transmit TL to the pilot anyways.

[Johnny F@rt Pants]

Not sure why I'd want to do otherwise. Too easy to forget not to reset the altimeter.

I have never done anything other than reset QNH as soon as I've been cleared to an altitude. This includes working on a turbo-prop for 2 different operators, one UK and one Swedish, and now working on B737 for a large UK leisure airline.

[Information Unicorn]

U.K. loco...

STD soon as cleared to an altitude, and standby altimeter passing FL200 latest.

[BleedingAir]

Too easy to forget not to reset the altimeter.

I have never done anything other than reset QNH as soon as I've been cleared to an altitude. This includes working on a turbo-prop for 2 different operators, one UK and one Swedish, and now working on B737 for a large UK leisure airline.

I suppose it's what we've "grown up" with professionally, I know we're regarded as a bit different here down south. Multiple turboprop operators and 2 x 737 operators in this part of the world, all operated the same, and forgetting the QNH at transition never happened, it's a totally standard procedure. And in the 737 you'll get a nice amber flag if you're a few seconds late with it. And this operating into some very terrain rich environment (e.g. Queenstown).

[FlightDetent]

Is AUS TL110, all the times? Is your flag on 737 set up correctly all the times?

Not trying to poke you, but rather thinking out loud what is different in places where the best practice crystalized differently. In line with ICAO's to set QNH with the receipt of altitude clearance, (where intermediate level-off above TL is expected the TL will need to be specified in that clearance by ATC)

Text is parenthesis is mine.

[valhalla634]

Interesting replies. Thanks everyone for keeping it subjective.
I think the US/NAM has it about right. 18,000/FL180. Done!
Fine for Oz/NZ to have a standard of FL110, but this varies I think with low pressure situations.
And for those who fly the world over, I think setting QNH on descent when cleared to an altitude covers it all.
Pilots don't generally like waiting to set things when they know they need setting. Plus the verbal set, X-checked etc that goes with it is better done higher up than lower down when getting more and more vectoring, speed control etc. in my opinion.

[Capn Bloggs]

and forgetting the QNH at transition never happened
Brave...

The Aus regs are clear, set QNH through FL110.
Incorrect. Prior to Transition. ENR 1.7 section 2.1.4.

[valhalla634]

Capn Bloggs

Could you paste in ENR 1.7 section 2.1.4.?

Thanks

[vilas]

This is not an issue of SOP but AIP. If it doesn't say so it shouldn't be done. In India it is not done. In Europe yes.

[BleedingAir]

Capn Bloggs

Could you paste in ENR 1.7 section 2.1.4.?

Thanks

From the NZ AIP:

2.2.2 The pilot of an aircraft must:
(a) when climbing above 13,000 ft, set the altimeter to 1013.2 hPa;
and
(b) when descending through FL150, set the altimeter to the
appropriate zone area or aerodrome QNH.

[BleedingAir]

To the OP, I withdraw my previous comment about the transition level not changing in Aus. It does with QNH, my bad, you're correct. Been a while since I studied the Aus regs.

[mustafagander]

It needs to be kept in mind that a lot of European T/Ls are very low - 5000ft or so. Hence set QNH as soon as cleared to an altitude, you just run out of time otherwise.

[Capn Bloggs]

2.1.4 The position to change between QNH and 1013.2HPA shall always be in the Standard Pressure Region on climb after passing 10,000FT and prior to levelling off, or on descent to a level in the Altimeter Setting Region prior to entering the Transition Layer and is shown in Figure 1.

http://s26.postimg.org/9tj45tqdl/image.jpg (http://postimage.org/)

[FlightDetent]

True mustafagander. Even less, and they do change for QNH less than 1013, and cold temperatures. Some modern ATIS are just METAR announciators and do not have TL info on them. Other places are 7000ish. Elsewhere you could be crusing at FL70 and enter TMA from a side and set QNH for this, as in the TMA TL is FL120. Weird stuff.

Hence the other approach to switch ASAP works the best. To support the risk assesment case, descent to altitude from above, let's say, FL180 is seldom the clearance. Typically last assigned FL is around 120 (whether or not you get to maintain it), mostly because of ATC sector fragmentation which is due to national borders and capacity demands.

[valhalla634]

The above diagram states that PRIOR to Transition "Layer" set QNH. It does not state AT.

[Skyjob]

Good airmanship:

Set QNH on SBY Altimeter when received in cruise prior to descent.
When first cleared to an altitude set QNH on main altimeters and crosscheck the value set to be near enough the one set on Stnadby Alreceived during flight to avoid a blunder error.

[AerocatS2A]

The above diagram states that PRIOR to Transition "Layer" set QNH. It does not state AT.

Correct. BleedingAir's comments are not in accordance with the Australian rules.