PNG ATC questions

[A Squared]

Hi all,

Hoping there are some PNG pilots who can shed some light. Looks like I'll be operating in PNG for the next couple of years and I'm trying to get a better familiarity with the rules and procedures.

OK what's the deal with "Area QNH"? Seems like an oxymoron. I understand that it's convenient for separation purposes to have everyone on the same altimeter setting. But, presumably, you wouldn't be using it on an approach or other times when terrain clearance is an issue, so why not just use QNE?

Any guidelines on when to switch to "area QNH" ? I've been switching when leaving controlled airspace, or when clear of terrain on departure aerodromes in uncontrolled airspace. (and obviously, when below transition altitude/level, which is where we operate, most of the time.)

Also, where does "area QNH come from? (Yeah, I know it comes out of the radio) is it based on Moresby QNH? Seems like its close to that most of the time.

[chimbu warrior]

Area QNH is used when established in cruise flight below the transition level (20,000 ft in PNG).

For climb or descent below the transition level, you use departure airport or destination QNH, as applicable.

"Area" can be interpreted as regional.

All will be revealed when you refer to the PNG ATC section of Jeppesen, or the PNG AIP. This might be worth a read, as I am assuming you will need to do the Air Law exam to convert your licence.

[A Squared]

Area QNH is used when established in cruise flight below the transition level (20,000 ft in PNG).

For climb or descent below the transition level, you use departure airport or destination QNH, as applicable.

Right, so pretty much what we've been doing.

"Area" can be interpreted as regional.

All will be revealed when you refer to the PNG ATC section of Jeppesen, or the PNG AIP. This might be worth a read, as I am assuming you will need to do the Air Law exam to convert your licence.

Already passed the Air Law exams although still waiting to receive my ATPL Am operating on a Certificate of Validation for the present.

I have read the Jepp PNG ATC section and the entire AIP, I don't recall anything addressing the "Area QNH" in either. Maybe I missed it.

I guess that I'm trying to understand why everyone doesn't use QNE in cruise. An "area QNH" is arbitrary and doesn't give you an accurate altitude, so why use 1013. Seems like I'm missing something.

[MACH082]

Area QNH gives you the average QNH in the area of concern. When departing an aerodrome and operating below transition you switch to area. Prior to descent point you would switch to local QNH of the aerodrome you are operating to. If this is unavailable or forecast as per the TAF, you would use the higher minima on your approach plates. If you are using area QNH at an aerodrome you must add an additional 50 feet to the minima.

[morno]

I guess that I'm trying to understand why everyone doesn't use QNE in cruise. An "area QNH" is arbitrary and doesn't give you an accurate altitude, so why use 1013. Seems like I'm missing something.

Because if the QNH was actually 1000 and you were using 1013, there's an error of 390ft. Area QNH has a maximum inaccuracy of 5 hectopascals (150ft), because the area QNH can differ only up to a maximum of 5 hectopascals from the actual QNH in the area, before a division is required.

Take a look at the maximum height of terrain in the country and then take a look at the transition altitude. Notice something? Now do a bit of thinking and you might work out why they use QNE above that and QNH below it.

morno

[MACH082]

Using QNE in cruise would be pretty dopey......

Yep I'm cruising at 8000 feet, but hang on, the field elevation was 7000 feet so I'm really cruising at 15000 feet, hmmmmmmm. I can imagine the separation and radio calls in class g :D

If you're thinking of QFE, then yes it is acceptable to use for departure provided you switch to area QNH after departure. If another aircraft or approved observer can give you QFE even better.

Even the poms only use QNE when in the circuit. Something about not being able to add the elevation to the circuit height and the complex mental arithmatic it entails ;).

[A Squared]

Area QNH has a maximum inaccuracy of 5 hectopascals (150ft), because the area QNH can differ only up to a maximum of 5 hectopascals

Perhaps that is intended to be true, but I can assure you I frequently see differences in excess of 10 Hpa between "Area QNH" and local QNH


Take a look at the maximum height of terrain in the country and then take a look at the transition altitude. Notice something? Now do a bit of thinking and you might work out why they use QNE above that and QNH below it.

Right, I understand what you're trying to suggest here, and you'd have a point if the "area QNH" was close enough to the actual local QNH. It isn't. The "area QNH" is frequently further from the local QNH than is QNE.

[A Squared]

Using QNE in cruise would be pretty dopey......

Yep I'm cruising at 8000 feet, but hang on, the field elevation was 7000 feet so I'm really cruising at 15000 feet, hmmmmmmm. I can imagine the separation and radio calls in class g

You're quite obviously confusing QNE and QFE. You might wish to review before calling others dopey.

QNE is standard atmospheric pressure, 1013, or 29.92" hg.

QFE is the altimeter setting used to get an altitude above the field elevation.

[A Squared]

Using QNE in cruise would be pretty dopey......

Yep I'm cruising at 8000 feet, but hang on, the field elevation was 7000 feet so I'm really cruising at 15000 feet, hmmmmmmm. I can imagine the separation and radio calls in class g.......

....Even the poms only use QNE when in the circuit. Something about not being able to add the elevation to the circuit height and the complex mental arithmatic it entails .

You're quite obviously confusing QNE and QFE. You really ought to review the terms and understand them before calling others dopey.

Review, for your benefit:

QNE is standard atmospheric pressure, 1013 Hpa or 29.92"hg. You set it above transition.

QFE is the setting you use to get a height above the surface of a specific aerodrome.

Nobody but you has suggested using the latter, and then only because you were confused on the meaning of the terms.

[A Squared]

Morno,

Lets say that QNH for an aerodrome is 1007 and the reported area QNH is 1018 (yes, I have seen 21 hpa difference between QNH and Area QNH, the difference here is 10 Hpa or more, more often than not )

In this case, which gives a more accurate altitude for that location: an altimeter set to "Area QNH" or an altimeter set to QNE?

[NCD]

A squared.

You obviously work in PNG.

If you can't work it out, just talk to the guys that have employed you. One would imagine that they would have SOP's that cover such THINGS.

Btw, please learn quickly, because if you do fly in PNG, i would appreciate you knowing what you are doing!!

[Captain Nomad]

A Squared, I know what you are getting at as I have operated there myself. The 5 hectopascal difference morno refers to is an Australian rule and I don't think you will find it in PNG. There are often large differences between area QNH and terminal QNH especially at highland ports.

A prime example of your question would be encountered when departing one of the highlands airports (eg Hagan) in the morning. Having received a relatively accurate terminal QNH corresponding with elevation, when leaving the terminal area you change to the area QNH. Sometimes this can change the altimeter by 500ft or so and this may not be very accurate when operating to a nearby strip but will invariably get more accurate the lower the elevation destination you go to.

The importance of being on area QNH is relevant as other aircraft within those regions will be seperating on the basis of this datum and with the huge number of airstrips and variation of airstrip elevations along with the lack of many terminal QNHs outside of the main ports this is the best compromise.

I once remained on a highlands terminal QNH when going to a nearby highlands strip in the morning and when leaving a terminal area via a gap I found myself almost head-to-head with an inbound Twotter on area QNH that I should have had better separation with. Lesson learned - don't make the same mistake!

Local operators are used to the inaccuracies of area QNH (especially in the highlands in the morning) and will, in addition to altimeter, use other points of reference to ensure an accurate circuit height.

[morno]

Captain Nomad, yes, I was referring to the Australian rule. I assumed it would be an ICAO thing and therefore apply. But silly me, it is PNG we're talking about and since when did Australia strictly follow ICAO rules, :E.

morno

[Captain Nomad]

No worries! :ok: I've pulled up the odd ATC every now and then in Australia 'cos I noticed a big discrepancy (then they found the 'correct' area QNH)! Probably wouldn't have raised an eyebrow if I was back in PNG... :E

[A Squared]

Thank you, Captain Nomad, yes that's what I'm seeing, large differences between Area QNH and local QNH at highlands airports. For what it's worth, and to address NCD's concerns, I am familiar with the altimetry procedures in PNG air law and in my company's FOM, and I follow them. I was more looking for a better understanding of why the system is the way it is

[...still single]

A Squared,
The large difference between Highland ports and area QNH is because of the large difference between ISA conditions and actual PNG atmospheric conditions -PNG being much warmer, the altitude vs pressure scale is 'stretched'.

In PNG, 10 000ft pressure altitude is not even close to 'true' 10 000ft, so when a port at say, 5 330ft (actual elevation) calculates the QNH required to give a pressure altitude of 5 330ft there is usually a difference of 10-13 HPA compared to a sea-level port, even if the atmospheric conditions are the same for both.

So, in reality, there is not a huge variation of QNH across the country. The variation is in the actual elevation the QNH is being calculated for.

[Ozzie Mozzie]

Just back to the 5HPa rule for a moment, I thought that applied to different regions of area QNH, not the difference between area and local QNH. Leaving aside the fact that you would expect local QNH and area QNH to be pretty similar

[A Squared]

....still single. Thanks. I was wondering why the difference was consistently large, even though the geographic distance was not. That makes sense that the altitude/pressure gradient is "stretched" if you will, by the higher overall temps.

[Captain Nomad]

Ozzie Mozzie, it can effectively apply to both. Check out Jepp ATC page AU-501:

"1.2.2.2 Area QNH Zones will be subdivided, if necessary, to meet the following standards of accuracy:

a. Area QNH forecasts are to be within +/-5 hPa of the actual QNH at any low-level point (below 1000ft MSL) within or on, the boundary of the appropriate area during the period of validity of the forecasts;
b. Area QNH must not differ from an adjoining Area QNH by more than 5 hPa."

Note the reference to elevation accuracy constraints in this quote which Still Single has well explained for the PNG situation.