Why understanding air pressure is so important to landing a plane safely

An Airbus A320 making an approach into Paris-Charles de Gaulle Airport (CDG) during bad weather in May came within 6 feet of hitting the ground well short of the runway. Now, reports in July say that the reason for the near collision was an incorrect pressure setting given by air traffic control.

The mistake was not recognized by the pilots and the aircraft made its approach well below the expected vertical path, resulting in the aircraft almost hitting the ground.

The disaster was only averted after the controller acted on an alert from their computer system and ordered the A320 to carry out a missed approach in the nick of time.

That one incorrect piece of information passed to the pilots could have caused a terrible accident. It highlights the differences in the types of approaches flown toward runways around the world depending on the facilities available at the airport and the weather on the day.

Some of these approaches are flown by simply looking out of the window, some are based on physical signals sent from the ground and some, like on that day in Paris, are flown based on a path generated by the aircraft.

Understanding exactly how these approaches differ and can be affected by air pressure is a key part of ensuring the safety of the aircraft and all those on board. So let’s break it down.

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Height vs. altitude

Though height and altitude can be interchangeable in common language, there’s actually quite a difference between the two in the aviation world.

Height refers to the vertical distance of an object above the ground. The ATC tower at Heathrow Airport has a height of 285 feet. This means that the distance an object would fall if you were to drop it off the top would be 285 feet.

Altitude refers to the vertical distance of an object above sea level, and air pressure can be incredibly important in understanding it. As weather systems move around the world, the pressure of the air changes above a certain location on the ground.

These pressure changes are like the air in an inflatable mattress. Imagine placing a model aircraft on top of your mattress and treating the floor as sea level. As you pump air into the mattress, increasing the air pressure, the aircraft rises higher than the sea. When you let the air out, lowering the pressure, the aircraft sinks closer to the sea.

As a result, pilots must be aware of the air pressure for their location in the world. To make sure they are flying at the correct altitude, they have to update their altimeter accordingly.

For example, say you’re flying over the sea at an altitude of 3,000 feet on the local pressure setting. Because you’re over the sea, this also means that your height is 3,000 feet. So there’s a safe distance between you and the water. However, what happens when you reach landfall and fly toward hills?

Assuming the air pressure stays the same, you’re still flying at an altitude of 3,000 feet, but as the ground starts to rise underneath you, your height is now decreasing. If I tell you that the tops of hills and mountains are measured in elevation — the vertical distance above sea level — you’ll understand why altitude is of far more use to pilots than height.

When flying close to the ground, terrain clearance is key. If we know the elevation of the terrain below us, we can set the correct pressure and ensure we keep a safe distance between us and the terrain.

Visual approaches

The most basic form of approach is what’s known as a visual approach and the clue to how it's flown is very much in the name.

When ATC guides the pilots close enough to the airport, they will signal the crew if they are “visual with the runway,” meaning they are able to see the runway out of the window. Depending on the weather conditions and how familiar the crew is with the airport, this could be up to 30 miles away.

With the runway in sight, ATC will clear the pilots for a visual approach. This means that they are allowed to fly in whatever way they want to get to the runway. If they want to make a beeline straight for the threshold for the runway, they can do so.

It’s exactly as if they were flying a light aircraft into a small local airfield. Importantly, at this point, the pilots are also now responsible for avoiding any other aircraft which may be in the vicinity of the airport.

This method is great for expediting an arrival as it means that ATC doesn't have to vector the aircraft around the sky to start the approach from 10-12 miles out (as is the norm for most instrument-based approaches).

Also, because the whole approach is conducted by looking out of the window, changes in the air pressure will make very little difference to the safety of the flight as the value on the altimeter isn’t so important.

Of course, the limiting factor is the weather. If the visibility isn’t great, the pilots may not be able to see the runway until they are quite close to the airfield. Indeed, on days where the visibility is really poor or the cloud base is low down to the ground, a visual approach isn’t possible.

The other problem with visual approaches, although uncommon, is the chance of landing at the wrong airport.

If the pilots misidentify the airport at which they are expecting to land, there is a chance that they may continue to land at the wrong airport.

This happened to the crew of Boeing’s Dreamlifter 747 aircraft in 2013. The crew started an approach to the correct runway at the intended destination. However, when looking out of the window the crew misidentified the runway and instead turned the aircraft toward the runway at another airport a few miles away — on which they landed several minutes later.

If the crew had continued to follow the original instrument approach, they would have avoided this mistake.

What is an instrument approach and how do pilots use them to ensure they land safely at the correct airport?

Related: The 10 most spectacular airport approaches from the cockpit

Instrument approaches

The majority of approaches flown into major international airports around the world are instrument approaches. What do I mean by an "instrument approach"?

When flying visually, like in the approach discussed above, our primary source of determining what is up and what is down is looking at the horizon.

It's very much like how we stop ourselves from falling over when we walk. However, if you were to try and walk with your eyes closed, very quickly you’d become unsteady on your feet and start to wobble.

It’s the same when flying through a cloud. When we no longer have any visual references available to us to determine where we are in space, we need help from our flight instruments. On the screens in front of us, they tell us where the nose of the aircraft is in relation to the horizon, if we are turning, our speed, our altitude and our heading.

Not only do they give us these basic flight parameters, but they can also guide us toward the runway when the cloud is low to the ground. For this, we have two types of approaches: precision approaches and non-precision approaches.

Precision approaches

As the name suggests, precision approaches give us a great deal of accuracy when flying in poor weather conditions. So accurate, in fact, that we can trust them to guide us down to the exact touchdown point on the runway even in zero visibility.

The reason they are so precise is that the system relies on signals sent up from transmitters by the runway which are then detected by systems on the aircraft. The most common type of precision approach is the Instrument Landing System.

An ILS uses two sets of radio beams. One is at the side of the runway to indicate the correct touchdown spot, known as the glideslope. The other is at the far end of the runway to indicate the centerline, known as the localizer.

These two transmitters send up a cone of radio signals that systems on the aircraft can detect. They then display the location of the beams in relation to the aircraft on the screens in the flight deck. The pilots use these to maneuver the aircraft so that it flies down the correct approach path to the runway.

Benefits

The main benefit of the ILS is that it's incredibly accurate. Pilots can use these signals to land the aircraft automatically in absolute zero visibility. The wheels will touch down on the runway exactly where they are meant to and the autopilot will keep the aircraft on the centerline until the aircraft slows enough for the pilots to take over.

In fact, the only reason why there is minimum visibility required for these kinds of approaches is so the pilots can find their way from the runway to the gate.

If there is a small hill or other obstruction such as a building on the extended centerline, it's possible to change the angle of the glideslope. Most glideslopes bring aircraft down on a 3-degree slope, allowing the pilots to fly at a steady speed.

Related: 8 of the most challenging airport approaches for pilots

However, if there is an obstruction, this can be changed to make the approach steeper. This is usually due to terrain around the airport where a steeper glideslope will keep approaching aircraft clear of the hills.

One of the most famous steeper approaches is at London City Airport (LCY), where pilots must fly a 5.5-degree approach to keep clear of buildings. This doesn’t sound like much but it requires special training to ensure that the aircraft doesn’t accelerate too much before landing on a relatively short runway.

When it comes to the pressure setting, because the beam sent up from the ground is constant, an incorrect pressure setting in the flight deck will not cause much of an issue.

When flying down the approach, the altimeter may appear to show an erroneous altitude for what the pilots were expecting at that distance from the runway, but the glideslope will still guide the pilots to the correct touchdown spot.

Downsides

The main problem with an ILS is that the glideslope and localizer beams can only be sent out in a straight line. As a result, the only way to fly an ILS approach is by flying straight toward the runway. There's no way of bending it around a hill or other large obstruction.

Likewise, there is a limit as to how steep the glideslope can be. If it's too steep, the pilots will have no way of keeping the speed under control. This can result in touching down at too fast a speed and risking running off the end of the runway.

Precision approaches also require extensive equipment to be installed at the airport and regularly maintained. This is both time-consuming and expensive.

Non-precision approaches

With the improvements in onboard navigation systems, particularly GPS, new types of approaches are becoming common at airports around the world.

Required Navigation Performance approaches also guide pilots toward the touchdown point on the runway. However, instead of using ground-based signals, they use a path generated by the aircraft’s own navigation system.

The approaches are constructed by the governing authority of the airport and will detail the inbound lateral course to the runway and also the vertical descent profile known as the glide path (note the subtle difference in terminology from an ILS).

As part of the process, the designers of the approach will consider factors such as terrain and obstacles to ensure that the aircraft remains safe at all times when flying the procedure.

These approaches are then published and loaded into the aircraft’s computer system by the airline, ready for the pilots to use when landing at the airport.

Benefits

The beauty of RNP approaches is that they can be created for any runway around the world, without the need for complicated and expensive ground-based equipment.

Once the assessment of the surrounding area has been made, the approach can be designed to suit the exact topography of the area. This means that the approach does not have to be straight-in like an ILS requires.

A great example of this is the airport in Innsbruck, Austria. Because it's on the valley floor between mountain ridges thousands of feet high, traditionally there was only one way in and out of it. The terrain to the west meant that a straight-in approach wasn’t possible, so a form of ILS was created from the east, allowing aircraft to make an approach in bad weather.

The problem came when the wind was blowing from the east. It required pilots to fly down the approach, then break off and fly a tight visual approach as they circled around to land on the other end. This required the weather to be good enough to allow for such a visual approach.

With the advent of RNP approaches, instead of flying the approach from the east, pilots are now able to fly down the twisting valley from the west, using accurate onboard systems to navigate from one GPS waypoint to the next.

This lines them up perfectly with the runway without the need for a tight visual maneuver at the end, resulting in the ability to land in weather conditions less favorable than before.

Downsides

What RNP approaches offer in flexibility, they lack slightly in accuracy.

As we're all familiar with from using mapping apps on our phones, the accuracy of GPS data isn’t always 100% reliable. Even though the systems on aircraft are much more accurate than on a mobile phone, there's still always the chance that the position of the aircraft calculated by the aircraft systems is slightly different from reality. When you’re flying down a mountainous valley in clouds, this is really important.

As a result, the minimum weather conditions allowed for an RNP approach are more restrictive than for an ILS. The minimum visibility required to start the approach is greater than with an ILS and if they want to continue to land, the pilots must make visual contact with the runway at a greater height above the ground.

How the pilots set up for the approach matters, too. The approach is based on data generated by the aircraft, so it's imperative that the pilots use the correct information. As with anything in the computer world, bad data creates poor results.

Before flying the approach, the crew must ensure that the lateral and vertical paths in the flight computer match up with what is on their approach charts.

Any errors here should have been detected back up the chain when the approaches were created and loaded into the system. However, sometimes mistakes do occur. An incorrect waypoint or erroneous approach angle could result in the aircraft colliding with the ground.

The pressure setting the pilots use is also really important, as the crew of the Paris aircraft found out.

If the wrong pressure setting is used, the aircraft will create a vertical profile based on an incorrect height above the ground. This could result in the aircraft flying toward a touchdown point well short of the runway. The added threat here is that everything in the flight computer will match up with what is on the pilot’s approach charts, lulling them into a false sense of security that what they are doing is correct.

As a result, strictly complying with flight procedures is imperative to ensuring the safety of the aircraft on an approach.

Related: How aircraft are designed to keep you comfortable at 37,000 feet

Bottom line

The advances in GPS navigation on aircraft have opened new possibilities in runway approaches.

In theory, an accurate approach can be created to a remote strip in the middle of a mountain range. This allows pilots to land when the weather is less than perfect, opening up opportunities for those in the community around the airport.

However, like with all computer-based applications, the end result is only as good as the system’s operator — the pilot. As a result, it’s key that they understand how the approach they are flying works and what parameters it is based upon.