Altimetry in 1 – 2 – 3

Altimetry: love it or hate it, but it’s been linked to some serious BARO VNAV incidents recently.

From overall awareness to pressure setting errors, it could go from “everything’s fine” to a big ball of fire pretty quickly 🔥 

It doesn’t help that it’s also one of the most disliked topics in the ATPL exams. There are some crucial elements to understand, like:

🔸How do you apply a temperature error correction? 

🔸Why can true altitude change with a constant altimeter reading?

 🔸And what do RNP approaches even have to do with any of this?

Let’s answer all of these, while keeping things short, snappy, and easy to follow! ✅ 

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How Does an Altimeter Work?

It all starts with the altimeter, and the flaws that it annoyingly comes with.

An altimeter doesn’t REALLY measure altitude, it’s just a barometer with some fancy linkages that ‘translate’ it into altitude.

So how does it do this? 💡

Let’s start with the static port. The static port collects…well, static pressure!

It directly feeds this to the main chamber of the altimeter. This means that the pressure inside the chamber is the same as the atmospheric pressure outside the aircraft.

Then, we have something called an aneroid capsule. This is an air tight capsule, which has a calibrated pressure inside of it that never changes.

Usually this pressure is the standard pressure of 1013 HPa (or 29.92 Hg for our American friends), although some are calibrated differently. It sits inside the main chamber of the altimeter.

This aneroid capsule is able to expand or compress, based on the difference in pressure between the chamber and the inside of the capsule.

If we fly really high, the static pressure is lower than standard pressure, so the capsule expands ⬆️

On the other hand, if we fly really low, the static pressure is higher than standard pressure, so the capsule compresses ⬇️

The final step then is to convert the movement of the capsule into a reading on the altimeter display via some gears and linkages, which are calibrated based on the International Standard Atmosphere (ISA) model.

Pressure settings

So what about the different pressure settings? 💡

From our perspective as pilots, you set 1 of 3 different pressures:

1️⃣ QFE (Pressure at Aerodrome Level)

You can set an altimeter to 0 ft with the aircraft on the ground, this results in it giving you your height above aerodrome level in flight. The pressure this gives you on the reading is the QFE: the pressure at aerodrome level.

2️⃣ QNH (Pressure at Mean Sea Level)

If you set the altimeter to read the aerodrome elevation while on the ground, then it will show the approximate altitude above mean sea level.

Your altimeter will now show “altitude” throughout your flight, calibrated to mean sea level (MSL).

3️⃣ Standard Pressure (1013.25 hPa)

Above a certain altitude called the transition altitude, aircraft can be instructed to set the standard pressure in their altimeter.

This results in the altimeter showing flight levels from that point onwards, and aims to have all traffic refer to the same reference to avoid separation issues.

Will the Helicopter Clear the Mountain?

So, the regional pressure setting is 1002. However, you still have your altimeter set at a previous QNH of 1022 hPa, which indicates 5000 feet. The upcoming high ground is 4500 feet, and you’re flying straight and level (indicated).

Assume 1 hPa = 30 feet, as per the International Standard Atmosphere (ISA).

Will you clear the high ground? ⛰️

Let’s take a look at this:

1️⃣ The difference in pressure (1022 – 1002) gives us an altitude difference of 20 hPa x 30 feet = 600 feet.

2️⃣ The corrected altitude therefore becomes 5000 feet – 600 feet = 4400 feet.

3️⃣ The high ground was 4500 feet, so this would not end well… 💥

The same applies going from flight levels (based on 1013) to local QNH, where the local QNH is lower. If the QNH or pressure setting increases, then your true altitude also increases (which is less of an issue).

Altimeter Errors

Ok, so let’s talk errors. There are quite a few that could ruin our day, and have caused accidents and incidents.

Temperature Error

From high to low, look out below! ⬇️

Colder air is more dense right? Therefore, you will get a smaller volume to occupy the same amount of physical air within a vertical air column.

This volume is your true altitude. In other words, if the air gets colder, your true altitude will be lower than your indicated one. You will descend if you keep your altimeter reading the same.

So how much difference does this make? Well, ICAO PANS OPS (Doc 8168) states:

So, let’s look at an example ⤵️

• Our decision altitude is 500 feet MSL
• Our temperature at the aerodrome is 0 degrees Celsius, which is at MSL 🌊

In ISA conditions, MSL is 15 degrees celsius, but we have 0 degrees celsius. A difference of -15.

So in this case, we need a 6% increase in decision height, which gives us a correction of 30 feet.

Do we need to add or subtract this? 💡

Well think about it this way, we are flying LOWER than we think because the temperature is lower than what ISA specifies for MSL!

So, to correct for this, we need to fly higher, i.e add the correction to our indicated altitude. So we add the 30 feet to the decision height, which gives us a new decision height of 530 feet ✅

For larger temperature differences with ISA than -15, see this table:

Barometric Error

Again: From high to low, look out below! ⬇️

Imagine you have 1022 hPa on the altimeter, the altimeter now shows you the vertical distance between the level of 1022, and the pressure around your aircraft (Let’s say it’s 855 hPa). This would mean an altitude of roughly (1022-855) x 30 = 5000 feet.

As you fly along though, the pressure at sea level no longer is 1022, but has reduced to 1002.

What does this mean? 💡

As you can see in the image, if we keep indicated altitude the same, we will actually reduce our true altitude, i.e we are descending by 600 feet based on a 20 hPa reduction! ⬇️

To avoid this, we need to update our pressure setting, or be aware that our true altitude will reduce because of the reduction in pressure at our new location!

This is especially relevant during BARO VNAV approaches, or any other approach that uses a barometric reference for the vertical guidance.

Instrument Error

Life isn’t perfect, neither are instruments! Linkages, friction, manufacturing, imperfections, etc.

All of these combined, result in an altimeter that doesn’t show the EXACT altitude above a certain pressure level, it will always be ever-so-slightly inaccurate ❌

Pressure Setting Error

The most common one: human error! Setting the wrong pressure setting can cause anything from collisions to Controlled Flight Into Terrain (CFIT) ⛰️

This one still comes back every now and then, for instance during this BARO VNAV incident.

Position Error

Measuring static pressure accurately while flying through the air sounds easier than it is sometimes. The goal is to collect ‘undisturbed’ air, but we all know that air around an aircraft is anything but undisturbed 💨

This is the reason you often see test aircraft with those large sticks on the nose. They’re aimed at collecting air that has not interacted with the aircraft yet in any shape or form.

Lag Error

The indicated altitude can only change so quickly. If you were to reduce or increase altitude extremely quickly, the altimeter could lag behind a little. This is especially true for fully analogue ones, but can be partly corrected for.

Conclusion

Yea, altimetry might not be the most loved topic, but understanding it properly, as well as the ways in which it can ruin our day is pretty important as a pilot.

From altitude miscalculations to more serious risks like CFIT, the consequences of neglecting altimetry basics are quite obvious.

Keep refining your knowledge, and always stay mindful of your pressure settings, especially when pressure or temperature reduces along your flight!

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️