When to dial in QNH from std after starting descent ?

[teropa]

Hi,

I have a very simple question, that seems to have many answers. I was hoping the pros here could give a definate answer to this, with some backing up as well.

So, when should I dial in the QNH setting in the altimeters after starting the descent from cruise altitude (ie. with std setting initially)?

Do I dial it immediately after I get the clearance to descend below transition level ? i.e. "finnair 302, descend to 3000 ft, qnh 1013",
OR
Do I dial it once PASSING the transition level. ?

My common sense tells me that the best way would be to change from std to QNH once passing the TL (because of many reasons), but there seems to be pilots who think otherwise. I also know that the aviation law states that one should do it after passing the TL, but it's also allowed to dial it immediately after the clearance, IF an approach clearance has been given AND level offs ABOVE TLs are not expected.

What do you think, and how do you do it yourself ??

regards
Tero

[ran]

hi tero

the normal or standart procedure for your ? is : when your flying in a FL the controller can see it and will say something like " desent to 3000 feet qnh 1007" when you are req. desent or when you are told. Then you start your desent and then dialing in the qnh.
The reson for is that if you are desenting and waiting to dial in the qnh you need to remember the qnh and the TL and maybe you will forget it and then there is a problem and a lot of papers to be filled out.

best regards
Ran:ok:

[Tinstaafl]

Depends where you're flying. Different countries have different rules.

In Oz everyone changes the subscale passing the transition altitude/level, CTA & OCTA. At least everyone is on the same setting if they're in the same area.

In the UK it can be changed as soon as you're cleared to climb or descend to above or below the TA/TL unless you've been asked to report passing a altitude or level on the way up or down, in which case you have to leave the change until after the report. It's even more complicated because the UK uses a myriad of TA/TLs. :rolleyes:

Guess which system is simpler....

[RomeoTangoFoxtrotMike]

You might want to check out this thread (http://www.pprune.org/forums/showthread.php?s=&threadid=89867)

[Miserlou]

Hi RAN,

What's a nice boy like you doing in a place like this?

[fireflybob]

The UK Air Information Publication (AIP) states that when cleared to descend to an altitude you set QNH, unless further flight level vacating reports are required. There is nothing complicated about this and it is not necessary to even be aware of the TA/TL to apply this system. It's also much safer in terms of terrain clearance since if you omit to set QNH with pressure below STD (1013.25 Hpa/Mb) you run the risk of descendng below MSA/SSA.

It is a while since I checked but the ICAO Pan-Ops used to say set QNH as soon as cleared to an altitude unless further level flight above the Transition Level is anticipated.

Individual States publish differences in which case one should be aware/compliant etc and one hopes that Company Operations Manual clearly define how to operate the altimeters.

Finally, despite the fact that it is important to have a standard procedure for management of altimeters, it's equally important to have an understanding of WHY you are doing what you are doing with the altimeters at the time - basically, this boils down to good training.

[Tinstaafl]

"...nothing complicated about (it)..."???


Lets see...

IF you have been told to report then wait until the last report, which wiill be at any random level that meets that controller's requirements on that particular flight for that particular instant.
IF you haven't been told to report passing a level, then set immediately
IF the controller has a change of mind and subsequently requires a report then change setting (When? Immediately? Approaching the the guestimated level?), change back after.
IF OCTA then change at the TL. That is, first figure out just what the TL might be where you are since it varies all over the geographic place.
IF other cruising traffic reports at a level that's probably near the one you're passing, how do you assess the conflict?

OR/

Everyone above a single, common TA for the country is on FLs, those below on a QNH/area (regional) QNH.


Which is simpler?

[Willit Run]

Doesn't seem to really matter where you are in the world, when you are in the FL envelope, stay at 1013, when you reach the transition level/altitude, switch to local altimeter, it will just save a whole of trouble if you are are requested to level off before you reach the altitude, and then the altimeter has been preset before it should be! VOILA, you are now at the wrong altitude!!!!!! How many times have we all forgot/been distracted and that has happened!!!!!?????????? For gods sake, we are all human!
Keep it simple! ain't worth fussing over!