altimeter setting

[slr737]

Many airlines are now setting the altimeter from QNH to QNE when cleared to climb above the TA while in doc 8168 it is specified that it should be done at the transition altitude.

I was reviewing Doc 8168 and i haven't even found the allowance were you could set from QNE to QNH when cleared to an altitude in descent. Now in 8168, it says to change to QNH when passing through the transition altitude ?

As there been any change lately on this ?

thanks

[Too Few Stripes]

Are you sure you meant QNE? QNH to standard or 1013.25hPa or 29.92" makes more sense. QNE is the indicated altitude/FL with standard set.

[DaveReidUK]

Well yes, below the TA you set the altimeter sub-scale to QNH, above it you set the sub-scale (or push the button) so that the altimeter reads QNE.

But we all know what the OP meant. :O

[Too Few Stripes]

It's different in the UK, AIP ENR 1.7 Altimeter Setting Procedures.

[H Peacock]

I always thought QNE was used when landing at an airport with 1013.25 (ie SPS) set. Specifically, the QNE was the 'height' shown when the aircraft was on the threshold. I'm not sure that QNE refers to all heights read from an altimeter set to the SPS.

ie, with a sea level pressure of 1013.25 and a 'perfect' atmosphere, the QNE of a threshold 500ft above msl would be 500ft. If you were flying downwind in the circuit 1000ft above the threshold your altimeter would be reading 1500ft, but that is not the QNE.

Standing-by to be corrected!

[Too Few Stripes]

I'd agree with that definition. It goes to prove it's a pointless and potentially confusing term which imho has no place in modern aviation. It most certainly does not mean a pressure setting of standard which it's often mistakenly used as.

[slr737]

It's different in the UK, AIP ENR 1.7 Altimeter Setting Procedures.

Exactly, while in the other AIP in Europe it's the same as doc 8168. But if I remenber well it was written before in doc 8168 that you change from STD to qnh when cleared to an altitude. However it seems to have been removed.

For the other guys, QNE is the old Q code that was use before stating standard. And it has nothing to do with QFE!

[Too Few Stripes]

Nobody has mentioned QFE ? I think you misunderstand what QNE actually is. Reread what's been said or google it for further info and some source material.

[Too Few Stripes]

Further, from PANS OPS
2.4.3 References to vertical positioning after approach clearance

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

[eckhard]

In practice, every UK operator for whom I have flown has used the following SOP (or very close to it), which works worldwide, guards against forgetfulness and distraction and seems to satisfy ATC. At least, I have never heard ATC complain, nor have I reported the wrong level/altitude in the climb or descent:

On take-off, all three altimeters set to QNH.
(If cleared to a FL as part of the initial clearance, set STD on Nos 1 and 2 when above accel alt.)
When cleared to a FL, set STD on Nos 1 and 2.^
When above T. Alt and MSA, set STD on the stby.

Before top of descent, set QNH on the stby.
When cleared to an altitude, set QNH on Nos 1 and 2.*

Contingencies:
^Climbing, if required to level off at an intermediate altitude below T. Alt., set QNH on Nos 1 and 2.
If required to report passing a certain altitude, refer to the stby.

*Descent, if required to level off at an intermediate FL above T. Lvl, set STD on Nos 1 and 2.
If required to report passing a certain FL, set the stby to STD and refer to that.

For QFE operations, use a derived QNH.

[Check Airman]

Interesting SOP eckhard.

In the US, all the SOPs I've seen have us reset all 3 altimeters at TA/TL.

[Sidestick_n_Rudder]

I've done both ways, flying for different operators.

I prefer the method of setting the altimeter to QNH/STD immediately after getting the clearance to altitude/FL.

The biggest benefit is that it makes it less probable to forget to change the setting, which can easily happen when waiting for TA/TL - which is something that can kill you, or easily cause a traffic conflict (had done a circle-to-land at 400ft agl myself due to forgetting to set the QNH)

I think this outweighs any potential problems of using the other method, i.e. setting the altimeter at TA/TL. Anyway, with modern jets climbing/descending at 3-4000ft/min, accurate altitude reading during climb/descent is not practical...

[Meikleour]

Check Airman: This could be because most of the Transition Altitudes in europe are quite low (ie. 5,000ft for example ) unlike the US standard of 18,000ft.
With the low TAs and high climb rates of jet aircraft, especially at low altitudes, make this a preferred SOP.

[DaveReidUK]

ie, with a sea level pressure of 1013.25 and a 'perfect' atmosphere, the QNE of a threshold 500ft above msl would be 500ft. If you were flying downwind in the circuit 1000ft above the threshold your altimeter would be reading 1500ft, but that is not the QNE.

What would be the difference between that and sitting on the ground at a 1500 ft elevation airfield with exactly the same altimeter reading ?

In other words, why would one be QNE and not the other ?

[eckhard]

AFAIK, because QNE refers to the reading on the altimeter when it is on the threshold of the runway in question. So: it is not an altimeter setting like QNH or QFE; nor is it the reading of the altimeter with 1013.2hPa/29.92inHg set (that is 'Pressure Altitude'); rather, it is specific to a runway threshold and enables a pilot to add the desired circuit or procedural height, thus yielding the desired altitude at the time.

I think?

[Too Few Stripes]

It's a complete waste of time and of no use whatsoever. Delete it from your aviation vocabulary and you'll be better off! My initial comment regarding it was to highlight the misunderstanding of what it is when the OP used QNE instead of standard or 1013.25 or whatever else you'd like to call it. As an industry we love to over complicate things and have a ridiculous amount of abbreviations and acronyms, in most cases we're our own worst enemy!

[eckhard]

Yes, I agree but it's interesting from a historical viewpoint. With the modern digital altimeters that we have now, the QNE procedure should never be required.

I think it has its origins in that cases of abnormally low or high atmospheric pressure prevents one being able to set the QNH on the older mechanical altimeters as they would run out of settings below about 950mb and above about 1030mb.

So, the QNE would be transmitted to the aircraft, the pilot would set 1013/29.92 and then add appropriate amounts to the runway elevation to derive the procedural altitudes to be flown.

[Check Airman]

This could be because most of the Transition Altitudes in europe are quite low (ie. 5,000ft for example ) unlike the US standard of 18,000ft.
With the low TAs and high climb rates of jet aircraft, especially at low altitudes, make this a preferred SOP.

That makes good sense now. I know that you guys use different altitudes for each airport (or is it group of airports?), but forgot that they can be quite low. Is the same procedure used on Airbus airplanes? I'm asking because they'll flash the altimeter setting at you when you pass TA/TL.

[Derfred]

Out of interest, for those of you who change the altimeters prior to TA/TL, how do you deal with "at or above" or "at or below" requirements on charted arrivals and departures?

I'm just interested as this is completely foreign to me, but obviously SOP in other parts of the world.

[Goldenrivett]

how do you deal with "at or above" or "at or below" requirements on charted arrivals and departures?

We have the luxury of a 3rd Altimeter (standby).
On departure it is left on QNH until above TA / MSA (which ever is higher) and is set to QNH before descent on arrival. The AP Altimeter reference is set early to the controlling setting (QNH or STD) (to avoid potential ALT BUST).

[Derfred]

Yes, but your FMC is basing it's altitude restrictions on your primary altimeters (assuming you have an FMC).

So if on departure, say TA is 10,000', you have a SID with an "at or above 8000" at a waypoint, and your clearance is to climb to FL140.

I have trouble with how the FMC (or you for that matter) complies with the "at or above 8000" if you have already selected the primary altimeter reference to STD prior to passing 8000'.

There is in fact a STAR in YBBN that has a limiting* descent restriction of "at or below FL120" with a TL of FL110. Your typical clearance at this point is to descend to 9000'. This can really catch you out if you go to QNH early.

*I use the term "limiting" here to indicate that (depending on runway) this waypoint is below the normal descent profile so is the "controlling" waypoint for the descent, meaning correct altimetry at this waypoint is vital.

[eckhard]

Well as far as I remember, the Airbus FMGC is clever enough to comply with altitude restrictions as entered, despite having a different setting on the altimeter. Don't know about Boeings, although I should!

The STAR at YBBN is an example of poor ATM, whether by design or subsequent modification of the procedure. I think it is unacceptable in today's busy environment to 'design in' potential traps for unwary pilots and controllers.

The low transition altitudes in the U.K. are another example, especially if the SID clears you to a Flight Level.

Procedures and airspace design should be as simple as possible and subject to several 'test flights' in the simulator before being released. Perhaps they already are but sometimes one is left wondering, 'who on earth came up with this idea?'

The proposed raising of the Trans Alt in the U.K. to 18,000ft is a welcome move. Let's hope that the rest of the world follows suite. That will still leave the CIS, Mongolia and China (and N Korea) to get with the programme and ditch Meters and QFE (and mmHg)!

[portmanteau]

Derfred: follow the rules and you cant go wrong.
" On departure change to std setting when passing TA and on arrival change to QNH on passing TL".
By all means stick std into a spare altimeter on departure but dont refer to that altimeter until passing TA. Ditto on descent, stick qnh into the spare but dont refer to it until passing TL.

[eckhard]

Well as far as I remember, the Airbus FMGC is clever enough to comply with altitude restrictions as entered, despite having a different setting on the altimeter. Don't know about Boeings, although I should!


Tried it yesterday and the 787 doesn't allow for different settings, so you need to have the correct setting when using VNAV until you've finished complying with any altitude restrictions.