Pressure settings: why and what use?

Question

I've just a basic question that deeply revolves around aviation, but have never understood? Why does certain air pressure change in different weather conditions? One day it may be sunny and you may have an air pressure of 1008 mPa (~29.76 in), next it may be 1019 mPa. What good does it do to the airplane in providing it with a different air pressure, which when you increase or decrease it, the altitude goes higher or lower, what use does this have?

Also, why, above transition altitude (commonly 6,000ft in the UK, 18,000ft in the US), does the pressure setting change to standard (1013 mPa = 29.92 in)? Again, what use is this and why is it good to think we're at a different altitude than what we are? My only guess is because airplanes perform differently at certain pressures/altitudes, so by thinking we're at 100ft, when infact we're at 300ft, makes us believe the airplane will perform like what it should do at 100ft STANDARD(!) ... ???

Please clarify.

Thanks

Answer 1

It's all to do with the aircraft's instruments... specifically the Altimeter.

The Altimeter tells you how high you are... but in actual fact, it's not actually reading the physical height between you and the ground, it's measuring air pressure. The lower the pressure, the higher the needles read.

If you can imagine a nice, hot, sunny day... chances are the air normal pressure will be higher in general. If you get in the cockpit and call air traffic, they will give you one of 2 (or perhaps both) values... QFE (pressure setting for the airfield) and QNH (pressure setting for sea level).

The altimeter has a small k**b which lets you dial in these figures... if you set the QFE figure, your altimeter needles will read dead zero when you are on the runway... so it's always good to set this if landing or taking off. If you set the QNH value, you're neddles will probably read higher than what you perceive... you're actually being told how high you are above sea level (even if you're on the ground, you will be higher than sea level... unless you're in a few very special places on the earth!).

If the next day the general air pressure is lower, then this will be reflected in the altimeter... it will appear to read higher than it really is.

As for the transition altitude, there is actually a "layer" (called the transition layer odlly enough!) which exists between the Transition altitude (as indicated with QNH set) and the nearest usable flight level above that as measured by the standard pressure setting (different countries define thier "useable" flight levels differently, but it's usually every 5 or 10 - eg FL15, FL20, FL25 etc)... This gives you a layer in which there is a margin for error... but as most small planes fly below transition altitude, and large planes above, it's not usually a problem. Basically, if you're below the transition layer, you must have QNH set... if you're above, you must have the standard setting set.

Now... If you're not confused already, here's why they use a standard pressure setting above the TL... If every aircraft above the TL has the same standard setting, they will ALL have altimeters that read the same... so if ATC clear one plane to FL75 (which is 7500ft according to standard pressure) and they clear another to FL80 (8000ft), if they get close or thier paths cross, they will be 500ft different vertically... if one of them had a non-standard pressure setting, they could potentially be at the same level... even though the altimeters read differently. So, Standard Pressure is a safety thing.

Interestingly, you'd think using that same logic that the QNH setting would also be standard... but as a pilot can move form one pressure to another different pressure in the course of a flight, the world is split into what's called Altimeter Setting Regions (ASRs)... and these regions have different QNH settings that always ensure you are seeing height above sea level in that area.

In summary... imagine a flight from airport A to airport B...

Plane takes off from Airport A using QFE (Airport setting) as this then tells the pilot exactly how high he is relative to the ground. As he climbs away from the airport, he sets QNH... and now his altimeter is reading the same as all other aircraft in that region... the height above sea level. When the pilot reaches and passes the Transition altitude into the transition layer, he sets standard pressure then proceeds to his assigned flight level.

Eventually, he decends and passes into the transition layer again... he now sets the local QNH, then as he approaches the airfield, he sets QFE again.

Hope that all makes sense!

Answer 2

There's a few reasons we use a common altimeter setting above FL180. One is because all airspace above that is Class A airspace, so unless you're descending out of that airspace, it's just easier to use the same setting. FL180 is also a safe arbitrary altitude in the contiguous US because it's well above the highest terrain in this area.  

Answer 3

The altimeter works off barometric pressure. When you set the Kohlsman to the current altimeter setting(barometer reading) your altimeter reads field altitude, not 0, as altitude is measured above sea level.  There are radar altimeters that read your distance above the ground.(agl) The altimeter setting is very important if you are planning to land. When the proper setting is applied and you know the field elevation, you can fly the right altitude in the pattern and know how far above the ground you are, more useful at night when you can't see the ground until you are close to it.

Since there is no method to discover the actual barometric pressure above 18,000 feet all are set to 29.92" If all aircraft use the same altimeter setting their altimeters will read the same.  

Answer 4

Aircraft altitude is measured based on outside static air pressure.   Air from outside the plane will push on a sensor (the static port) and this pressure will translate into an altitude on the altimeter.  This static pressure also comes in to play when calculating airspeed using the dynamic air pressure read by the pitot tube.

When the outside air pressure changes based on weather conditions, the pressure exerted on the static port changes as well.  Without adjustment, the plane's instruments would read this as a change in altitude (higher altitude when the pressure decreases, or lower when it increases.)  Since the physical altitude of any given airport doesn't change when the air pressure does, aircraft must calibrate their instruments so that they know their actual altitude above the ground.  

When Air Traffic Controllers give an instruction such as "Altimeter 29.99" it is telling pilots that the air pressure at sea level is 29.92in.  Adjusting the altimeter to this setting allows the instruments to display the correct altitude.   This comes in handy when navigating near terrain, or when flying in IFR conditions, when knowing your correct altitude above the ground may be important for avoiding obstacles.

The standard "29.92" setting when at cruising altitude is to help with traffic separation.   Since aircraft from different origins will have different altimeter settings, there is no assurance that one plane flying at FL250 (flight level 250, or 25,000ft) will be at the same height as another aircraft at FL250.   By setting all altimeters at the same 29.92, air traffic controllers can know for certain that planes passing with as little as 1000 feet of vertical separation will actually be 1000 feet away from each other.  Any deviation based on different altimeter settings could be disastrous, as two planes may believe they are at separate altitudes, when in fact they are dangerously close.

When it comes to aircraft performance, altitude and pressure don't have such a dramatic effect.   While some planes may have trouble taking off in very low pressure conditions (at high altitude airports when it gets very hot out) the main reason for the air pressure settings is to calibrate the pilot's instruments to read the actual outside conditions.