QNH Confusion

[lazerin]

Need a little help to clarify pressures and QNH. If the QNH is increased from 1010 to 1015, then is the air more or less dense than it was at 1010?

 

In my head, a lower QNH value means the air is more dense, i.e. 1010 is more dense than 1015. I came to this conclusion by looking at the weather logs. At night, the QNH seems to go down, and I assume that with the night being colder, the air would be denser...

 

But this seems wrong according to the books. They say that there is a drop of 1hPa per 30ft of altitude. Doesn't that mean that as the QNH falls, then the air is getting less dense?

 

Argh!

 

 

[Student Pilot]

It's as it reads, the lower the QNH the lower the pressure. Don't go trying to overthink things, makes it too hard.

 

 

[lazerin]

Haha, ok, I like that.

 

 

[Mazda]

Yes, you are correct on both counts. The air is more dense when the QNH number is higher, and less dense when the QNH number is lower.

 

So when you hear an ATIS or see in the forecast that the QNH will be 991 (as it was a couple of weeks ago), and it is a hot, humid day, in practical terms the aircraft will not perform as well as it does if the QNH is 1022 and the temperature is lower.

 

Yes, it is also true that the density (not QNH) decreases with altitude as (in simple terms) there are fewer air molecules in the same volume of space as height increases. (Think in terms of a climber needing oxygen when climbing Everest). That's one of the reasons why the aircraft does not perform as well at altitude, and why it is important to look at take off/landing performance figures when operating from a higher elevation airstrip.

 

Remember the ISA standard atmosphere at sea level is 15 degrees C and 1013 hectopascals. If it is hotter and/or less dense, the aircraft won't perform as well. You might see in performance tables indications for ISA, ISA +10 degrees etc.

 

 

[Guest Cloudsuck]

Imagine you have a clear plastic bicycle pump. You put your finger over the end and compress the air inside by pushing the handle end of the pump (increase pressure). The air inside the pump is now more dense as you now have the same amount of air particles in much less space. Therefore the higher the pressure, the more dense the air.

 

 

You need to remember that the atmosphere has weight, and the amount of weight pushing down causes pressure which changes the density.

 

 

To relate atmospheric pressure change through altitude (i.e. atmospheric pressure decreasing with altitude), think of the atmosphere as an ocean. The pressure at 1 metre below the water is not much at all whereas the pressure at 1000 metres below is huge because you have the weight of all that water above you and water is heavy (1 litre = 1 Kg). The atmosphere is the same, it has weight.

 

 

 

[Guest basscheffers]

QNH is simply the pressure at sea level for where you are, that is really all you have to think about.

 

Now for the 30ft/hPa thing: the simple rule of thumb is that if you wind the QNH number up, the altimeter reads higher, and vice versa. If the QNH is 1010 and your altimeter is set to 1015, imagine you must have wound it from 1010 (the actual QNH) to 1015 and this would make it 5*30 = 150 feet higher. The numbers went up, so the hands of the altimeter went up. There will be a question like this in the pre-solos and/or BAK and this is all you need to know to answer it correctly.

 

Where everyone gets confused is that for the altimeter to read higher, the outside air pressure must be lower. So if you are at 300 feet AMSL and the real QNH is 1000, then the pressure required to read 300 feet must be 1010. But if you set the QNH to 1005, the pressure required to read 300 would have to be 1015. But it is not, the real pressure where you are is still 1010 and is lower than the altimeter "expects" it to be for the set QNH. And like I said at the start of this paragraph, the lower the pressure, the higher the altimeter reads. So in this case the altimeter will read 450, rather than 300.

 

Hope I explained this right! :)

 

I saw you are booked in to fly right after me on Friday, hope to see you there!

 

 

[lazerin]

Haha excellent, it all makes sense now. I was just having a silly moment.

 

So just for my own sake: at night, temperature reduces, thus air has less kinetic energy and the pressure is less. Hence, QNH is lower at night with reduced temperature.

 

All good

 

Bas, I hope the wind isn't too bad tomorrow, I'm still struggling on the landings and if it's bad, might have to call it off. Good news is I've already done the solo exams and that wasn't too bad, just gotta get these landings!! Have a good flight tomorrow :thumb_up:

 

 

[Guest basscheffers]

Having a look at the Edinburgh forecast, which is usually close enough:

 

FM020300 21014KT CAVOK

 

14 knots straight down the strip on 21! And if you had not noticed yet, the south/south-west winds are usually the calm ones. (northerlies are turbulent at YPPF)

 

I was very lucky with a fair few "light and varied" wind days when I started circuits, but this shouldn't be too bad for training.

 

 

[Ewen McPhee]

QNH why bother

 

Just a question. If you know the height of the airfield is 760 feet AMSL. Can you disregard the QNH and just twiddle the knob to that height? . Why bother with QNH at all?

 

 

[Guest brentc]

Just a question. If you know the height of the airfield is 760 feet AMSL. Can you disregard the QNH and just twiddle the knob to that height? . Why bother with QNH at all?

Nope, because the QNH adjustment only goes so far and you won't be able to select all altitudes with this method.

 

 

[Guest basscheffers]

But it sounds like a reasonable method if you do not know the local QNH at all. (i.e.: the strip in your back yard out in the sticks)

 

 

[facthunter]

QNH Relevance.

 

If you are at an airport where you know the elevation then you just set that elevation on your altimeter and everything is OK. When you are going X-country, you have the forecast QNH for your area and you set that in the subscale. This is good if your altimeter is within tolerance. In practice, I continously set the altitude to correspond with the GPS altitude , and that is so accurate that (when everything is working) you couldn't do better. It is just a matter of using the data available to you at your fingertips. YOU have to determine the validity of it. The legality of what you are doing is way behind the technology. but the non- TSO'd GNSS data cannot be used, officially. Be aware that the reference that you should use is the area QNH. Legally, and that If your altimeter is outside tolerance, then it should be rectified. Hmmm. Nev..

 

 

[Guest basscheffers]

In practice, I continously set the altitude to correspond with the GPS altitude , and that is so accurate that (when everything is working) you couldn't do better.

But could that theoretically not become a problem with vertical separation? I thought the whole idea of area QHN was that everyone in that area shows the same on their altimeters.

Also, be very, very careful of GPS altitude: unless you have a very good unit with very good reception, altitude error can be much greater than horizontal errors. Here's an explanation of why this is the case. I would not trust the hand-held one sitting on my knee!

 

I wonder, in GPS precision approaches, do they use GPS or steam gauge altitude?

 

 

[facthunter]

Legality.

 

I think that I have stessed the importance of using the forecast area QNH. But requiring that that altimeter is calibrated within tolerences. The GPS is a back-up which is very accurate which you can use to relate to the legal requirements. ie set the area QNH. as forecast. I basically suggest that you use ALL sources of information. Nev..

 

 

[Mazda]

Set the QNH to airfield elevation when operating at a field with no aerodrome QNH available. The disadvantage is that unless you are right at the aerodrome reference point for elevation (or you use the threshold elevation from DAP) you might not actually be at a place on the airport that is at that elevation. Sometimes the aerodrome reference point could be at the windsock on the hill for instance.

 

That's fine if there is nothing else available though, and it is normal practice. Using an aerodrome QNH means everyone is on the same QNH, so when the altimeter reads 1000 feet, you'll be at the same level (and everyone will know at which levels to find everyone else).

 

In terms of IFR, you can even reduce the minima IF you can get actual aerodrome QNH, and it must be "actual" - within the last 15 minutes. Note in all of this I'm talking about aerodrome QNH, not area QNH.

 

Enroute the rules changed a while back. You used to have to set area QNH (from the forecast), then set aerodrome QNH when descending to that aerodrome. Now we can use the QNH from a station within 100 miles, so you could get the ATIS or AWIS to get the QNH from a nearby aerodrome.

 

 

[Guest pelorus32]

[snip]I wonder, in GPS precision approaches, do they use GPS or steam gauge altitude?

They need an augmentation system that uses ground based beacons - such as WAAS, EGNOS etc. In addition the system can be augmented by barometric input giving rise to so-called Baro-VNAV approaches.

 

Interestingly a survey by either CASA or ATSB or ASA found that pilots believe that their workload was more intense when conducting a GNSS approach than say an ILS, VOR, DME etc.

 

Regards

 

Mike

 

 

[Student Pilot]

No doubt about casa, why GNSS? because you can use four letters instead of three, it's more of the "This page is intentionally blank" type stuff.

 

 

[facthunter]

GNSS.

 

I think that GNSS is a general term for navigating by geostationary satellites. GPS is ONE of 3 potential systems, (at the moment).

 

Mike's comment about using DME, that is a homing procedure which permits a let-down in the vicinity of the airfield, in the event of non-availability of other aids. You have no tracking information. The only time that you know where the station is, is when you are abeam of it, because the rate of closure at that point becomes zero. You have solved the ambiguity, (left or right) earlier. You can read the distance when the rate of closure is zero.

 

By contrast, a DME arrival is when you are tracking to a major airport from a certain direction or sector, and you are cleared to make a DME arrival. This is where DME steps are published for that airport and by not going below the steps, you remain in control area and have height above terrain, ( and you don't run into us guys who are stooging around under the steps).. Nev..

 

 

[Mazda]

The DME/GPS arrival requires the use of another navaid for directional information. The DME/GPS is only providing distance information and the navaid it is based on (eg NDB) is shown on the plate. You must use that aid (i.e. the NDB) for tracking information, and the DME or GPS for distance information. You descend to the prescribed altitude at the published distance.

 

Yes, GNSS is the generic term.

 

Remember we don't have approved WAAS in Australia, and Airservices has now scrapped its ground based augmentation system.

 

 

[Guest TOSGcentral]

Gawd help us! With all due respects to the previous posters what the hell has this to do with simple ultralight VFR flying where you get a met forecast (if you want to be technical), make a flight plan (if you are that way inclined). Call up your local gossip group on radio (if you have one) set an area QNH and then press on ....

 

Spending most of the time looking out of the window and and enjoying the view!

 

I really must be getting old - flying essentials seem to have changed yet I did not notice the last time I flew. It really does get worrying

 

 

[Ultralights]

dont forget the simple option of calling up Mel or Bris centre and asking for the area QNH? or when enroute, look up the AWIS or ATIS of nearby airports and get the QNH that way. its the best way to ensure you will be on the same QNH subscale as similar IFR and VFR GA traffic in the area

 

 

[Student Pilot]

Spending most of the time looking out of the window and and enjoying the view!

I really must be getting old - flying essentials seem to have changed yet I did not notice the last time I flew. It really does get worrying

The old KISS principal, seems like people are just reinventing the wheel. With more performance and freedoms there is more responsability. It does all seem to be getting away from the original movement, how long before the first pressurized JET ultralight? It seems the latest machines have to have full IFR panel with three axis auto pilot and costs what a house used to.

 

That's where the likes of the Thruster/Tyro/Drifter still have a lot of attraction, their original first generation ultralights that are proper stick and rudder machines.

 

 

[Guest Cloudsuck]

Frankly, I applaud LAZERIN for learning the theory and putting the question. Perhaps he has aspirations of getting a PPL/CPL or higher. Either way, I think the theory is good and it is interesting. At our club, we have some pilots who, "Have 900 hours in Drifters" and as a result can't possibly learn any more because they know everything. Sometimes I get almost as much enjoyment out of the theory as I do out of flying. Keep up the good work LAZ...

 

 

[Guest basscheffers]

Is it enough to know to set you QNH and look at the dial to see how high you are? Of course.

 

But a thorough understanding of how and why the thing works and how it can fool if abused or broken you might save your life one day. (though probably not in CAVOK, no.)

 

But some people do enjoy actually understanding the exact workings of things, even if you don't require that knowledge to operate it.

 

So I like these discussions!

 

 

[facthunter]

Altimetry.

 

One small comment. Were I live and fly, (around Melbourne NW.) you are bumping up against the bottom of control area steps constantly. IF you don"t get your altimetry, tracking, and height keeping right, you end up infringing controlled airspace. This concentrates the mind, if you intend to keep flying.

 

Just a little "watch it" here,also. Give the lower level of the CTA steps a 200' avoidance, as the IFR types can fly right on that height.

 

Getting back to the altimetry. QNH is the figure which when set in the subscale, makes the altimeter read the elevation of the aerodrome. You HAVE to remember the height of the aerodrome, because all your circuit heights relate to it.

 

Conversely, IF you set the altimeter to read aerodrome elevation, the subscale will read the aerodrome QNH. ( IF THE ALTIMETER IS WORKING PROPERLY). Sorry to shout this, but no-one has picked it up, and I did want to make that point.

 

You don't have a TSO'd altimeter, and it is not checked. Nev...