Altimeter

[ElNull]

What does it mean by Altimeter is calibrated to ISA conditions?

Does it mean at a sea level with 15C deg and pressure of 1013 hPa the altimeter will read zero?

[speedrestriction]

It means that in the theoretical ISA atmosphere the for an altimeter calibrated to ISA, indicated altitude = true altitude.

[ElNull]

So what I said is theoretical correct?

[fireflybob]

Does it mean at a sea level with 15C deg and pressure of 1013 hPa the altimeter will read zero?

Assuming 1013 hPa is set on the sub scale and no instrument error then yes.

[mrmum]

Yes, pretty much

[ElNull]

Thanks for replies!

[ElNull]

Another question came to mind. I'm a little bit confused about on which pressure exactly the QNH are based on. I know the QNH is a pressure setting which when set on altimeter will read altitude above sea level. My question is the QNH is an adjusted sea level pressure or airfield pressure?

[H Peacock]

What does it mean by Altimeter is calibrated to ISA conditions?

Does it mean at a sea level with 15C deg and pressure of 1013 hPa the altimeter will read zero?

Vitally, it means more than just that. As speedrestriction correctly stated it means that the altimeter will read correctly throughout an ISA. Quite possible to have an altimeter which could still read zero with 1013 set, but if not calibrated to ISA it would then under/over-read as it was moved away from sea level. Ie, when indicating FL400 it could well have significant errors despite correctly reading zero at sea level.

Aviation altimetry relies on accurate calibration throughout an altimeter's operating range and not just at one point.

[fireflybob]

QNH is atmospheric pressure at aerodrome level reduced to sea level according to an ISA lapse rate of temperature (1.98 deg C per thousand). This is because the thickness of the layer between two pressure levels depends on the mean temperature of the layer and you want the altimeter to read field elevation at the datum point.

As a matter of interest QFF is atmospheric pressure at aerodrome level reduced to sea level assuming no temperature change (isothermal) in the layer.

Isobars are lines joining placing of equal QFF and what you see on a synoptic chart.

Just for the sake of completeness QFE is atmospheric pressure at aerodrome level (or runway threshold in the case of "threshold" QFE).

Hope this helps.

[ElNull]

Ok now to end it I have another question. A high QNH value (higher than 1013) such as 1020 means it's a high pressure out there and a low QNH indicates a low pressure (be ready for some bad weather). And since pressure decreases with height then why at an airport with a 7700 feet elevation have a QNH higher as 1022?

[Skyjob]

I think you are confusing QNH and QFE.

The actual pressure at the airport located at 7700' will read 0' when QFE is selected, this is the actual pressure at the station.
Any reading of the altimeter taken are then Above Aerodrome Level (AAL).

Most airlines and companies do not fly QFE but QNH.

When flying QNH procedures, the airport altitude will be displayed on the altimeter tape when on the ground at the station. So if the airport is at 7700' then the altimeter will display 7700'. The pressure setting you are required to set to achieve this reading of 7700' is the QNH.

The QNH value is essential the value that reads 0' when at sea level in actual weather conditions.
The QNH only varies with the high or low pressure of passing weather fronts.

The QFE varies from station to station as the altitude of each station is different and there may be a correction for high or low pressure to be taken into account.

When flying QNH, all obstacles around an airfield which are referred to an elevation above sea level, are referenced to through the altimeter.

When flying QFE, all obstacles around an airfield are referred to in relation to height above/below airfield elevation. This requires continuous changes to the usable QFE when transitioning to areas away from the airport used for reference and new charts need to be referred to using a different reference station.

Airline operators predominantly use QNH. When flying higher in the sky in the airways, flight levels are used, all of which are based on all aircraft flying the same Standard setting, or 1013hPa, the worldwide acclaimed average standard pressure. This then results in all aircraft maintaining required separation.

[ElNull]

SkyJob thanks for the explanation, However I was confused about the QNH value which usually a big number on a high elevation airport. And as far as I understand the higher the value the higher the pressure.

[fireflybob]

However I was confused about the QNH value which usually a big number on a high elevation airport. And as far as I understand the higher the value the higher the pressure.

The altimeter is measuring the thickness of the layer between two pressures - one you have selected on the sub scale, the other the actual atmospheric pressure sensed by the altimeter.

For the altimeter to indicate altitude above mean sea level the sub scale needs to be set to pressure at sea level.

[BOAC]

QNH value which usually a big number on a high elevation airport.- it sounds as if you haven't quite got it? The QNH for a 'high' airfield will ALWAYS be higher than the QFE for that airfield - it should be obvious? Is that what you mean by 'a big number'?as far as I understand the higher the value the higher the pressure. - again - yes - and the actual atmospheric pressure at that airfield will always be LESS than the pressure at sea level - no?

[ElNull]

Ok, QFE is the actual pressure at airfield. QNH is derived from the actual pressure at airfield reduced to sea level using ISA. Now how do you anticipate a high pressure and low pressure air at the airfield using QNH? Should I compare the actual pressure at airfield (QFE) to the ISA pressure and if it is a high value then it's high pressure and vice versa? Or is there a simplest way?

[Skyjob]

If a higher pressure area approaches, the QNH value will rise to read correct airfield altitude... Simple
If you would not increase the QNH an altimeter error would start to develop.
Similarly, the QFE value would need to change to correct for the same altimeter error.

[fireflybob]

Ok, QFE is the actual pressure at airfield. QNH is derived from the actual pressure at airfield reduced to sea level using ISA. Now how do you anticipate a high pressure and low pressure air at the airfield using QNH? Should I compare the actual pressure at airfield (QFE) to the ISA pressure and if it is a high value then it's high pressure and vice versa? Or is there a simplest way?

Not quite sure what question you are asking here.

Why would you want to anticipate a "high pressure and low pressure at the airfield"?

At a high elevation airport (for example Mexico which is circa 7,000 feet above sea level) the QFE would be about 256 millibars less than the QNH. From a weather perspective the lines on a weather chart (isobars) join places of equal barometric pressure at sea level (let's say QNH to keep it simple) - so if you want to know whether the pressure (from a weather perspective) is "high" or "low" best to use QNH.

Hope this helps

[ElNull]

Thanks again for replies - firefly I was just wondering if there is a way to predict some bad weather which is usually associated with low pressure using an altimeter setting rather than weather charts.

[fireflybob]

Ah I did wonder that!

Bad weather is not always associated with low pressure (necessarily).

What is more significant is whether the pressure is rising or falling and at what rate.

Rapidly falling pressure is more often than not a sign that the weather is going to deteriorate (rapidly!).

But the pressure could be (relatively) low with pressure rising quite quickly which would suggest a general weather improvement.

[ElNull]

Firefly I appreciate your answers, but can you give me an example of how actual pressure is reduced to sea level to get the QNH? Or even lead me where I can find some calculation about it? Thanks in advance

[fireflybob]

Firefly I appreciate your answers, but can you give me an example of how actual pressure is reduced to sea level to get the QNH? Or even lead me where I can find some calculation about it? Thanks in advance

Ok you asked!

The formula for the altimeter is

h = 221.1 Tm (log p1 - log p2)

Where h is height in feet, Tm is mean temperature in degrees absolute and p1 is pressure at bottom of the layer and p2 pressure at the top of the layer (in Hectopascals).

So let's say you want the QNH at an airfield which is 5,000 feet amsl where the temperature is 20 degrees Celsius and the pressure at airfield level is 843 Hpa.

We have to compute the mean temperature of the layer using a lapse rate of 1.98 deg C per 1,000 feet. (Lets call that 2 deg C to keep the math simple!).

The mid point of the layer would be at 2,500 feet - so the temperature at that point would be 2.5 X 2 degrees less than 20 deg C which = 15 deg C = 288 deg Absolute.

So substituting in the formula:-

5,000 = 221.1 X 288 (log QNH - log 843)

0.07852153 = log QNH - log 843

log QNH = 0.07852153 + log 843

log QNH = 0.07852153 + 2.92582757462

log QNH = 3.00434910462

Antilog 3.00434910462 = 1010.0644932

Therefore QNH = 1010 hpa to nearest hpa

Next question?

PS Hope the math is right!