Pressure altitude and altimeter settings
An altimeter does not measure height. It measures pressure, and converts it using a standard atmosphere that today's air almost certainly is not. The subscale moves the zero of that scale onto a pressure surface of your choosing — sea level, the airfield, or the 1013.25 datum the whole flight-level system hangs from. Every figure on this page is a consequence of that one fact, including the two ways it can quietly leave you a few hundred feet lower than the needle claims.
What you have set, and what it reads
Switching these does not move you. The page re-derives the subscale value and the reading for the same spot in the air, which is exactly what happens when you reset in the climb — the needle jumps, you have not.
From the plate, not the GPS. QFE cannot exist without it. Below sea level is a legitimate answer in a few places and the arithmetic copes.
A flight level tells you nothing about your height above the sea. Without a QNH from somewhere, the question has no answer at all.
Altitude above mean sea level
The same spot in the air, on all three settings
| Setting | Subscale | Needle reads | Which means |
|---|---|---|---|
| QNH | Altitude above the sea | ||
| QFE | Height above the airfield | ||
| Standard | Pressure altitude, so |
An airspace base published as a flight level
A flight level is not a height. It is a pressure surface, labelled with the altitude it would have in standard air. On a low-pressure day the whole 1013.25 scale sinks toward the ground and takes every flight level down with it, so a base published as FL65 arrives somewhere below 6,500 ft on the QNH you are flying. Climbing a thermal towards it, you have less room than the number says, and nothing in the cockpit tells you so.
Off your own chart. This page ships no airspace and never will.
On your altimeter, that base is at
You are at ft on the altimeter. Read the base as the number it is published with and you would think you had ft of climb left. You have ft. The difference is the datum offset and nothing else — it is the same ft that separates your QNH from 1013.25, and it is there all day whether or not anybody works it out.
You are at ft on the altimeter, which on this QNH is already ft above that base. Compare your reading with the published number alone and you would make it ft it instead. The whole of that disagreement is the datum offset — the same ft that separates your QNH from 1013.25.
The habit that makes this safe is dull and effective: on a day when the base that matters is a flight level, put 1013.25 on the subscale once, look at what the needle says, and know that number. A glider climbing at four knots covers a gap that size in a minute or two.
Flying with a setting that is no longer right
You set the QNH at the launch point and go cross-country. Three hours later you are eighty kilometres downwind, inside a different pressure, and the subscale still says what it said this morning. The altimeter is not broken and nothing on it looks wrong. It is simply referencing a datum that has been left behind.
Pressure difference
Exact error
Rule of thumb says
Needle says / you are at
From high to low, look out below. You carried a datum that is higher than the local one, so the needle is generous and the ground is closer than it says. This is the direction that hurts, because every decision made on that reading — a field selection, a ridge crossing, a final glide — is made with a cushion that is not actually there.
The needle is being pessimistic about terrain, which sounds harmless and is not the whole story. You are genuinely higher than it says, so the number you are keeping below an airspace base is the wrong one and you can be inside controlled airspace while the altimeter says you are clear of it.
Nothing to see: the datum you have set is the datum you are flying in. This is what checking the setting buys you, and it costs one radio call.
Notice how the rule of thumb and the exact figure differ here. Thirty feet per hectopascal gave ft where the true answer is ft. The next section says where the crossover falls and why the sign of that error is not fixed.
Thirty feet per hectopascal, and where it stops being true
Differentiate the altimeter's own formula at 1013.25 and the answer falls straight out: feet per hectopascal, or feet per inch of mercury. The familiar 30 and 1,000 are those two numbers rounded up, by different amounts: near the ground the hectopascal rule overstates the correction by and the inch rule by .
The gradient is not a constant, because air thins as you climb: a hectopascal buys more feet up high than it does on the ground. The 30 ft figure is exactly right at about ft, and 1,000 ft per inHg at about ft. At your present pressure altitude of ft the true local gradient is ft per hectopascal.
So where does it matter? For subscale arithmetic — anything comparing one setting with another, which is all of the work above — the relevant figure is the one at the datum, and 30 is a tenth too big in the conservative direction. On a twenty hectopascal error near 1013 that is about fifty feet, and nobody has ever been hurt by fifty feet of pessimism.
The generosity is not unconditional, though, and that is the part which gets taught as though it were. The crossover above is a height, but it is also a pressure: ft is the same surface as a subscale reading of about hPa. Work in QFE at a site that high and both of the settings you are comparing sit below the crossover, so the true gradient is already more than 30 ft per hectopascal and the rule understates the error instead. Carry it higher still and it gets worse: at FL100 the real figure is nearer 37 ft per hectopascal and 30 is then close to a fifth optimistic. Below the crossover the rule is on your side; above it, it is not, and nothing about the rule tells you which side you are on.
The other way the needle lies: cold air
Everything above is about the datum. Get the datum perfectly right and there is a second, independent error waiting, and it is not a subscale problem at all. The thickness of a layer of air goes with its temperature. Cold air is thin, so the column between the airfield and you is shorter than the standard atmosphere says — and the altimeter, which knows only the standard atmosphere, reads the standard thickness. You are lower than it says, and this one gets worse the higher you climb.
Standard for this field would be .
Over , the needle is out by
ISA deviation . The 4 ft per 1,000 ft per degree rule would say ft.
Colder than standard, so the column of air between the airfield and you is thinner than the instrument assumes. Your true height above the airfield is about ft where the instrument says . Above the field that puts you below the needle, which is the direction that matters. For thermalling in open country it is academic. For a ridge, a wave flight over high ground, or a winch launch failure decision in freezing air, it is not — and it adds to whatever the setting error is doing rather than being cured by it. Nothing arranges for the two to cancel, and when they happen to agree in sign that is how people arrive at the ground earlier than planned.
Warmer than standard, so the column is thicker than the instrument assumes. Your true height above the airfield is about ft where the instrument says . Above the field that leaves you a little higher than the needle: comfortable for terrain, unhelpful for staying under a base.
Standard temperature for this field, so there is no temperature error to correct. This is the only condition in which the altimeter is telling the exact truth, and it is a coincidence rather than a normal state of affairs.
What this assumes, and what it can't know
The pressure arithmetic is exact, not approximate. An altimeter genuinely is a standard-atmosphere machine, so modelling it as one is not a simplification: the conversions between QNH, QFE and 1013.25 are the instrument's own maths, run forwards and backwards. Where a rule of thumb appears on this page it is shown beside the exact figure, never instead of it.
It assumes the instrument is right. Real altimeters have position error from the static source, hysteresis, friction and a calibration tolerance. The usual pre-flight check — set the QNH and see whether the instrument reads the field elevation — passes an altimeter that is out by a few tens of feet, because the published tolerance is a few tens of feet. The exact figure differs between authorities and I am not going to quote one at you; look up the one your own regulator publishes. None of that is modelled here. If the altimeter in front of you and this page disagree by a few tens of feet, the page is not the authority.
The QFE it derives is a conversion, not a report. QNH is defined as the standard-atmosphere reduction of station pressure to sea level, so converting back to station pressure is exact — but only for the elevation you typed. A QFE passed to you by an airfield comes off their barometer for their reference point, and a hectopascal of difference from rounding at either end is entirely normal. Use theirs.
Settings are quoted to a resolution, and so is your subscale. A QNH given as a whole hectopascal already carries about 27 ft of granularity before anybody makes a mistake, which is why the last few feet of any of these numbers are not worth arguing about — and why the hundreds of feet are.
No airspace, no transition altitudes, no airfield elevations, no published settings. All of those are somebody else's publication and all of them get revised. A stale copy on a web page is worth less than nothing to the pilot reading it, so you type what your chart, your plate and the radio told you.
Not yet checked by anyone but me. If you instruct, or one of these assumptions is wrong, I would genuinely rather hear it than not — tell me and I will credit you here. This is ground school, not a flight computer, and not an authority on your aircraft. Fly the numbers in your own flight manual and the instruments in front of you.
Why British gliding clubs fly QFE
Set QFE and the altimeter reads zero on the ground. Everything it then shows is height above the airfield, which is the quantity almost every decision in the circuit is actually made on. A cable break at three hundred feet is a different problem from a cable break at eight hundred; the briefing, the checks and the instructor's questions are all phrased in height above the ground, and QFE puts that number on the instrument directly with no arithmetic to do while the aeroplane is doing something.
It also makes the teaching portable in the one way that matters. A thousand feet means the same thing on the downwind leg at every club in the country, whether the field sits at fifty feet or eight hundred. A student on QNH would have to learn a different pattern of numbers at every site they visit, and would have to do a subtraction on the base leg. Practice is not universal — some clubs fly QNH, and there are good airspace arguments for it — but QFE remains ordinary at British gliding clubs and there is nothing quaint about the reason.
What changes when you leave the circuit is that the world outside stops sharing your datum. Airspace bases are published as altitudes or as flight levels; controllers, other aircraft and every chart in the cockpit are on QNH or on 1013.25; and terrain elevations are heights above the sea, not above your club. On QFE, a hill marked at 2,100 ft and an altimeter reading 2,400 ft are simply not comparable numbers. The moment you commit to a cross-country you have taken on the job of holding two datums in your head, or of resetting — and the field you actually land in will have an elevation of its own that your QFE knows nothing about.
The practical consequence for a pilot moving up to cross-country flying is a habit rather than a rule: know which datum every number in front of you belongs to, and say it out loud. "Two thousand four hundred, QNH" is a complete statement. "Two thousand four hundred" is not, and it is the incomplete one that gets written in a field-landing report.
Transition altitude, and what happens when you reset
Below a published transition altitude, vertical position is expressed as an altitude on QNH. Above the corresponding transition level, it is expressed as a flight level on 1013.25. The point of the arrangement is that everybody operating up high shares one datum, so two aircraft on the same level are genuinely at the same level, no matter how the pressure varies along the route between them. Down low, where terrain matters more than mutual separation, the local sea-level datum is the more useful one.
The value of the transition altitude is published for the country and often for the individual airfield, and it is on your chart rather than on this page, because it is exactly the sort of number that gets amended. The mechanical part is what is worth internalising: when you wind the subscale from QNH to 1013.25, the needle jumps by the datum offset — the figure the calculator above calls the QNH level, currently ft — and you have not moved a single foot. Nothing about the aircraft changed. The scale changed underneath it. Press the standard button at the top of this page and watch the reading move while everything physical about the situation stays put.
There is a related subtlety worth knowing. Between the transition altitude and the transition level lies a transition layer, and on a low-pressure day that layer would be squeezed thin — because the flight levels have sunk toward the fixed transition altitude. So the transition level is raised on those days to keep the layer usable. That is the same effect as the airspace calculation above, seen from the other end: low pressure pushes the whole flight-level system downwards relative to the ground.
Most gliding happens below the transition altitude, which is precisely why the reset is unfamiliar and why a pilot climbing unusually well on a good wave day is the one most likely to meet it unprepared. If a base that matters to you is published as a flight level, work the number out on the ground, before the climb, when it costs nothing. The density altitude calculator takes the pressure altitude computed here and adds the temperature, if the question you actually have is about launch performance rather than airspace.