QNH Subscale Errors

[Riles4365]

Hi, quick question. Im a student ra Aus pilot studying for my bak exam.

 

I know that atmospheric pressure decreases 1 hPa for 30 feet of height gained. So if the actual msl pressure is 1028 hPa but you set your altimeter to 1026 hPA, what is the height error and why? I know it’s 60ft but why is the height error 60ft lower than actual? Shouldn’t the lower qnh mean a decreased atmospheric pressure resulting in a reading higher than actual ?

 

would appreciate all answers as really want to fully understand the reason.

 

thanks very much

[old man emu]

Have a look at these videos. The instructor presents the topics simply. Just be careful because it's American and she refers to using inches of mercury for pressure.

https://www.youtube.com/watch?v=XPNn7YCA9GQ:49

 

[Garfly]

Riles, thinking these issues through can get tricky with the apparent double negation involved.

 

But I think of it this way, if I set my altimeter 2hPa lower than where it should be set (in order to indicate the aircraft's height above the sea) I'm effectively telling the instrument that it is 60 ft. higher than it really is. So, in response, it dutifully reports that it is 60 ft lower than it really is.

 

So when I land on the beach the instrument will swear blind that I'm actually 60 ft below the waves. (Until I wind the sub-scale up 2 hPa again.)

 

Another way to think it through is by way of the other technique: that is, before take off, to wind the hands of the altimeter until they show the airfield's (known) elevation in feet. This is close to reverse-engineering the QNH for that day and that place, but the official area QNH might be a bit different because its designated - and regularly updated - as a standard for everyone to use over a wide area.

 

You probably know that most modern phones have an accurate barometer built in. Also, apps are available that turn that pressure instrument into a familiar looking altimeter (employing the same confusing logic ;-) These are interesting to play with. You can try out your theories about altimetry at home, away from the aeroplane.

 

A good one is this "Precision Altimeter" from Radiant Technology.

 

http://www.beliteaircraftstore.com/ios-apps-precision-altimeter/

 

Edited July 21, 2020 by Garfly

[Yenn]

If MSL pressure is 1028 you will read 0 feet at sea level if 1028 is set on your altimeter.

If you have set 1026 on your altimeter or to put it another way, you have told it MSL is 1026, it is going to read minus 60 feet at sea level because it is 2 hPa higher then you have set.

[Flightrite]

I've got a headache after watching those vids?

[old man emu]

Im a student ra Aus pilot studying for my bak exam.

 

Like many students, aiming for a full understanding of a topic, you are reading too much into why we have sub-scales on an altimeter.

 

There are two "Q" codes we use, QFE and QNH.

QFE is the air pressure at the airfield reference point. You find the QFE by turning the sub-scale knob until the altimeter reads Zero Ft. Assuming that the airfield is as flat as a billiard table, and altimeter set at QFE will be the same as the airfield reference point. In reality, if you are sitting on a hump that's higher than the reference point, you'll be out be a couple of feet - no worries.

 

QNH is an arbitrary value set by the airspace controlling authority for a defined bit of airspace for a certain period of time. When the sub-scale is set to QNH (note it's Area QNH), then the altimeter will show the altitude above Mean Sea Level, based on an average atmospheric air pressure for the Area.

 

USES:

We use QNH virtually all the time. Because the QNH value has been provided by AirServices, it is the current reference value for all aircraft operating below a nominal 10,000 feet. By everyone using the same sub-scale value, vertical separation can be maintained, especially if the Quadrantal Rule is followed in cruise. As pressure systems move across the Local Area, Air Services will amend the Area QNH as atmospheric pressure changes.

 

You can use QFE if you want to fly at a constant height above the airport reference point. You just have to keep an eye on the lowest safe altitude of the country you are flying over. I suppose it would be useful to a helicopter pilot doing lifting work to know how high above ground the copter was. You can also use QFE to check the serviceability of the altimeter by obtaining actual airfield air pressure from an accurate barometer, then setting the sub-scale to that value and seeing if the altimeter reads zero.

 

Does it matter in practice if there is a minor error of a couple of hPa? Firstly, a couple of hPa is less than 100 feet. You won't fly into Cummulo-granitis if you are maintaining 500 feet clearance. You might get a parallax error as you read the sub-scale, but that will still be only a few hPa - well within safe clearance. Finally, the altimeter internal gearing might be worn and the needle or the sub-scale gearing movement might be a bit sloppy. That's why altimeters have to be re-calibrated on a regular schedule.

 

Here's another video which might help you understand QFE and QNH. It's English, so ignore the bit about where the transition level is.

 

[Flightrite]

Then there's QNE?

[antonts]

if the actual msl pressure is 1028 hPa but you set your altimeter to 1026 hPA, what is the height error and why? I know it’s 60ft but why is the height error 60ft lower than actual? Shouldn’t the lower qnh mean a decreased atmospheric pressure resulting in a reading higher than actual ?

 

QNH is pressure on sea level, or, in opposite words, "sea level is on this pressure level". If we set QNH less then actual (bigger pressure height) it means we said to barometer that it got sea somewhere HIGHER than in reality, so it shows LESS difference between current position (measured pressure height) and sea.

 

Its confusing, for right logic and no errors you have to spend time and mind efforts, so just memorise "subscale goes the same direction". If we turn the knob and rise QNH - main needle goes also UP.

[old man emu]

Then there's QNE?

 

QNE: Indicated altitude at the landing runway threshold when 1013.25 hPa or (atmospheric pressure at sea level in the International Standard Atmosphere) is set as altimeter setting.

 

QFF: Atmospheric pressure at a place, reduced to MSL using the actual temperature at the time of observation as the mean temperature

 

 

Does that mean that flight above 10,000 feet is on QNE?

 

 

With an accurate QNH set, a VFR altimeter(s) should read site elevation to within 100 ft (110 ft at test sites above 3300 ft) to be accepted as serviceable by the pilot. If an aircraft fitted with two VFR altimeters continues to fly with one altimeter reading 100 ft (110 ft) or more in error, the faulty altimeter must be placarded unserviceable and the error noted in the maintenance release.

[Flightrite]

Now I have got a headache!??

[old man emu]

Now I have got a headache

 

[ClintonB]

[ATTACH type=full" alt="1595403701488.png]54876[/ATTACH]

Speaking of Bex, does anyone know if he is OK and where his build is up too.

[cscotthendry]

 

But I think of it this way, if I set my altimeter 2hPa lower than where it should be set (in order to indicate the aircraft's height above the sea) I'm effectively telling the instrument that it is 60 ft. higher than it really is. So, in response, it dutifully reports that it is 60 ft lower than it really is.

 

Gary: I still don't het that. If I tell my altimeter that it is 60 feet higher than it really is, shouldn't it report that I am 60 feet higher than I really am?

[cscotthendry]

If MSL pressure is 1028 you will read 0 feet at sea level if 1028 is set on your altimeter.

If you have set 1026 on your altimeter or to put it another way, you have told it MSL is 1026, it is going to read minus 60 feet at sea level because it is 2 hPa higher then you have set.

Yenn: Something looks screwy with your math. Are you saying that 1026hPa is 2 higher than 1028 hPa? That doesn't seem to add up to me!

[Edit]

OK, I think the penny just dropped for me so I'll add my 2 cents worth for an explanation:

1) Air pressure rises as we descend and decreases as we go up.

2) As Yenn said, if the current pressure at MSL is 1028 and we are sitting at sea level and the altimeter is set at 1028, it will show 0 feet.

3) If we set the subscale for 1026, we are telling the altimeter that the current atmospheric pressure at MSL is 1026, but it is seeing 1028.

4) Since the altimeter 'knows' that pressure increases as you go lower, it is going to see the current 1028 (higher pressure) as being below sea level.

 

Riles: Thanks for posting that question. It has helped get my brain in gear on this.

Edited July 23, 2020 by cscotthendry

[Flightrite]

Wind off hPa wing off Alt ?

[Garfly]

Yes, Scott, me too. It's that tricky double negative thought process that multiple-choice examiners love to use to catch out the unwary, or, more likely, the over-thinkers. [Not, necessarily, the ignorant.] I got the impression that the OP was pretty savvy, in no real need of help with the basics of altimetry, just hung up on one of those maddening apparent contradictions; as we were. LOL

 

Well, we can't know for sure, since Riles hasn't got back.

Edited July 24, 2020 by Garfly

[Dick Gower]

It gets easier if you remember that

(a) the altimeter indicates the height in feet from whatever pressure level is set on the sub-scale on the basis of 30 FT per hPa (roughly).

(b) a pressure of 1013.2 is only the pressure at seal level in the standard atmosphere and in real atmospheres this pressure will be something else.

[ClintonB]

Set it at what is published for airport or area and we should all be ok, without the headache And at worst case look at your iPad or phone and go with it.

[Tony van Vliet]

All excellent and informative....but not always easy to remember.

 

However you can remember this :

 

HIGH TO LOW - CAREFUL GO!

 

Simply put If you set 1013 in your altimeter subscale and your local QNH is 1003

Your Altimeter will OVERREAD by =/-300ft

So if you join the circuit at 1000ft indicated on the altimeter your actual altitude will be 700ft.

 

If you have a HIGH Setting on the Altimeter and the actual pressure is lower, you will be LOW so carefull GO

[old man emu]

I have an app on my phone that reports the altitude of the phone. I wonder if it does this by using the GPS system, which not only gives Lat and Long position, but also height ASL.

[Flightrite]

The vertical aspect of any GPS (outside of high quality aviation units) is very inaccurate.

[onetrack]

OME - Not sure about your app, but this altitude app (link below) says ....

 

"This altimeter app, to measure altitude, uses:

- GPS satellites triangulation - works without internet connection,

- barometer pressure sensor (if available in your device) - high accurate data; if internet connection is available it calibrates itself to improve accuracy,

- online networks location services (wifi and other) - need internet connection.

 

You can use each sensor separate or all together."

 

https://play.google.com/store/apps/details?id=com.exatools.altimeter&hl=en_AU