QNH vs SAS

[chinny]

HI,

Would like some clarrification on the correct proceedures from an ATCO please.

When climbing out,with a high transition altitude say 17000 and the controller gives you a FL to climb to and youre at say 4000ft-what is the correct thing to do-set SAS and climb to FL or remain on QNH til passing TL? the same goes for the descent from say FL370,high T Alt ,17000 again, to 2500ft.

I know what i do but have entered into a "discussion" about it and wanted an upto date definitive.If you can could you provide a ref then that would be great.

thanks

VTSP
chinny

[Someone_Else]

Well, i'm not sure what it is you want from an ATCO point of view.

If you are referring to the altitude reporting for radar then the pressure you set in your altimeter has no bearing on the altitude information we receive from your Mode C. This is because mode C always gives us the altitude with the pressure set at 1013mb/29.92in. Our radar systems will then apply local pressure settings to each readout where it is relevant.

So maybe it's more a procedural flying question? From my point of view as an ATCO, if I had given you a decent from FL200 to A5000, I would want to be confident in the knowledge that if I stopped you at A12000 then you would do so with the correct pressure setting.

Having said all that, I seem to remember a pilot telling me that as soon as you hear climb FLXXX or climb to Altitude XXXX then you make the neccesary adjustments to the Altimeter.

[zkjaws]

I don't have the books in front of me to quote the references, but I would put my life on the following procedure (about to hop in an aeroplane -so hope it's the one that is followed)
In the climb: set QNH until passing the transition altitude (e.g. 13000ft )and then set 1013
In the descent: set 1013 until passing the transition level e.g. (FL150) and then set the QNH.
Any other procedure used is likely to result in an aluminum shower - if the QNH is in the region of 30 hPa different from 1013.
The two thousand feet in the tranistion layer will always be a no-mans land - so don't trust pilot level reports, unless you confirm the datum to be used - QNH or 1013.
Some_Else is right in saying that this has no affect on the Mode C readout and is only crucial when getting pilot reports of levels passed/occupied.

Jaws

[JAR]

UK AIP (10 Apr 08) ENR 1-7-1
ENR 1.7 — ALTIMETER SETTING PROCEDURES

5.1.4 Within Controlled Airspace a pilot should set one altimeter to the latest Aerodrome QNH prior to take-off. While flying at, or
below, the Transition Altitude vertical position will be expressed in terms of altitude based upon the Aerodrome QNH. When cleared for
climb to a Flight Level, vertical position will be expressed in terms of Flight Level, unless intermediate altitude reports have been
specifically requested by Air Traffic Control.

5.3.1 When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will
pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight
Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level
vacating report.

[Glamdring]

Ideally you would leave it set to QNH until passing transition altitude. That way when asked for a passing level the controller can cross reference what you report with the mode C. But this does the leave the pilot open to forgetting. Probably better to go with the procedure above.

[LEGAL TENDER]

What a load of bollocks

Climb FLIGHT LEVEL = set 1013
Descend to ALTITUDE = set QNH
any other practice, is a level bust waiting to happen.

Modern aircraft have a switch that changes the altimeter instantly from whichever QNH is set to Standard Pressure Setting(and vice versa)
But in case of an analog altimeter, would you expect the pilot to wind the dial say from QNH 985 to 1013 on passing transition altitude ??? = Level bust guaranteed !!!

[Kiltie]

Think Level Bust Causal Factors

Waiting for Transition Altitudes / Levels before applying pressure settings is an ancient practice that has historically led to many level busts. The AIP entry one poster kindly listed above is the safest method to follow. Waiting until transition leaves the pilot open to distraction. Years of dialogue between the CAA / JAWG / NATS and airlines providing level bust risk management strategies have eradicated this practice hence the AIP method.

You will invariably have a standby altimeter to monitor MSA in the climb out when early 1013 is selected if you have any terrain awareness issues to satisfy.

Not from an ATCO as you asked, but ignore at your peril.....

[Tail-take-off]

In the dim & distant past it was a requirement to remain on QNH until within 2000' of TA. Obviously you would need to remember to set 1013 prior to reaching TA to avoid potential level busts (with a low QNH).

However this requirement was abandoned to cut down the number of level busts.

[Somewhere In Time]

The change from local QNH and SAS 1013 and vice versa should take place in the transition layer, between transition altitude and TL. This is to ensure that all AC will be on same altimeter settings.
Therefore if you are just airborne and the ATCO gives you climb to say FL300, DO NOT set SAS 1013 until you are passing transition layer.
Vice versa, if you are at FL300 and ATCO gives you descend to 3000ft, DO NOT set local QNH until passing TL.
Otherwise we risk having AC on different altimetersettings in the same altitude- or flightlevel-areas which again might result in loss of separation!

Ie, if local QNH is 983 and you set SAS 1013 too early on climb you will actually be 810ft lower that what you should be!
Your altimeter will indicate say 5000ft on the climb but you are actually at 4090ft in relation to all other traffic flying on QNH 983.

So when ATC gives you trafficinfo on traffic at 4500ft, you think you are in the clear but you're not!

[ferris]

Your TCAS will also be fooled!

Wrong. Safely ignore anything this guy says.

By the nature of the question, you are in the US, chinny? Follow whatever your company SOP states. In the absence of an SOP, follow the advice taken from the AIP (above). You should change at the transition layer/level- however, human factors have dictated some common sense be applied (hence the AIP procedure).

This is a forum with international following, so rules will naturally vary from location to location. The mechanics of TCAS doesn't.

[Somewhere In Time]

Ferris, you're right about the TCAS. The transponder always uses SAS 1013. My mistake. TCAS will give accurate readings regardless of local QNH settings

However, I dont understand why you say disregard anything I say when you later in your post agree with me in saying pilots should change altimetersetting passing transitionlayer/level.
Make up your mind

http://www.eurocontrol.int/airspace/...nce_Manual.pdf
Take a look at page 10..

[chinny]

hi all

Hi eveyone,

Thanks for your inputs-greatly apreciated-it appears that there are still different ways to skin the cat and still a variety of procedures/rules being followed.

I wanted an ATCO as thought they would have the latest access to a definative answer.

The question was raised- as a "new broom sweeping clean" and a new regime is trying to impliment changes and i just wanted to have the correct answer before i shoot my feet off.

What i dont follow is....JAR refs to an AIP which says one thing, and S-I-T provides a pdf manual that states completely the opposite

The procedure i have always used is to have both pilots on the SAS / QNH as appropriate to stage of flt-ie set FL when told to go to one and set QNH when told to go to alt-regardless of your current alt and leave the stby on the pressure your are leaving. I am being told that this is wrong-regardless of where in the world you are.

Always issues with big pressure changes but hopefully that is an airmanship thing that is practiced and dealt with.


Thanks again.

VTSP
chinny

[mm_flynn]

The only time the difference in procedure will make a difference is if you have an 'interaction' with ATC or another aircraft.

If it is aircraft-aircraft and identified by TCAS, these are always generated vs 1013.2

If it is aircraft-ground and identified by EGPWS, this has nothing to do with the pressure setting.

If it is an interaction with ATC, it will either be
a) and instruction to expedite (nothing to do with setting) or

b) a request to report passing level/altitude (which clearly needs the right setting)
c) an unplanned instruction to stop climb/descent and an altitude/level (same issue)


These last two are the only cases where the setting matters and I would be interested in the ATC view.

If you have cleared someone from an altitude to a FL and need to ask them their current level or to stop climb, do you expect the pilot to be climbing with reference to a QNH or SAS?


----
note - I have ignored the somewhat special case of UK ATSOCAS calling traffic

[Glamdring]

At my unit we require the aircraft to confirm passing altitude or level on first contact so that we can verify the mode C. The departures are all cleared to a flight level but when below the transition layer we require the report as an altitude. This is because the radar processor automatically converts the mode c to altitude when below the transition layer.

[CommandB]

In my airline our SOP is:
Above 3000ft AGL AND cleared to climb to a Flight Level, set STD (1013).
In the descent when cleared to an Altitude set the QNH given.
This is our SOP and having talked to some controllers once ATC have cleared you to a FL/Altitude, they expect you to be on the relevant pressure setting! Plus as someone has already said, if you start doing your own thing and waiting for transition level/altitude = level bust waiting to happen.

[411A]

All very interesting in Euro/UK land, however...the original poster asked about a high transition layer situation, so...
The following is done in the USA where the transition layer is at FL180.

Climbing....leave altimeter on QNH until passing 17,500 feet, then change to 1013.2/29.92

Descending...leave altimeter on 1013.2/29.92 until passing FL180, then change to local QNH.

Japan is about the same, except the transition layer is FL140...as I recall.

Saudi Arabia....transition altitude 13,000 feet, transition level FL150.
Make altimeter setting changes at the specified point indicated climbing/descending.

Note.
In the above listed locations, this what ATC is expecting you to be doing..airline SOP's or not.

Also, some might be wondering about the altitudes/flight levels assigned by ATC in these higher transition layer areas.

USA....normally, the lowest FL assigned is FL190, the highest altitude assigned is 17,000....however, in cases of very low pressures, these altitudes/Flight Level assignments (as indicated above) are changed to cater for the low pressure conditions...all done automatically by ATC.

Japan...same as USA, except with reference to the transition layer at FL140.

Saudi Arabia....anything goes, beware

[clipped_wings]

ICAO Doc 8168 Volume 1

2.1.4 References to vertical position
2.1.4.1 The vertical position of aircraft operating at or below the transition altitude shall be expressed in terms of altitude. Vertical position at or above the transition level shall be expressed in terms of flight levels. This terminology applies during:
a) climb;
b) en-route flight; and
c) approach and landing (except as provided for in 2.4.3, “References to vertical positioning after approach clearance”).
Note.— This does not preclude a pilot using a QFE setting for terrain clearance purposes during the final approach to the runway.

2.1.4.2 Passing through the transition layer
While passing through the transition layer, vertical position shall be expressed in terms of:
a) flight levels when climbing; and
b) altitude when descending.

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.
Note.— This applies primarily to turbine engine aircraft for which an uninterrupted descent from a high altitude is desirable and to aerodromes equipped to control such aircraft by reference to altitudes throughout the descent.