5G signals are having an unexpected side effect on aircraft operations – here’s how pilots are dealing with it

Dec 11, 2021

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Last week, the Federal Aviation Administration released an Airworthiness Directive (AD) which in effect banned aircraft from carrying out certain types of approaches when there could be possible interference from 5G telecoms signals. The AD covers all transport and commuter category aircraft in the USA, meaning that 6,834 aircraft on the US registry are affected. This includes the smallest private jets up to large airliners such as the Boeing 777.

The FAA is concerned that the 5G telecommunication signals, the next generation of cellular broadband internet, could interfere with equipment onboard the aircraft which determines the aircraft’s height.

As a result, the FAA has mandated that a NOTAM (NOtice To AirMen) be released at airports where 5G interference could be a problem.

So why exactly could 5G signals be a problem and how does this affect systems onboard the aircraft?

What goes up, must come down

For pilots, knowing how high they are is an important detail that they must know at all times.  Ensuring that we stay clear of any terrain is critical to flight safety, especially when flying in zero visibility through clouds. The only way to do this is to know our altitude.

But it’s not just keeping a safe distance from terrain we need to concern ourselves with. Even when flying in clear conditions with good visibility, we need to know what level we’re at in order to stay safely separated from other aircraft. Additionally, when flying an approach to land, particularly in foggy conditions, it’s key to know our height above the ground as we get closer to the runway if we’re to ensure a safe and smooth landing.

You may have noticed that I used 3 different words just then to describe the aircraft’s position relative to the ground. This wasn’t just me trying to be a good writer by using similes, in fact, there is quite a difference between the terms altitude, level and height.

Knowing our vertical distance from the ground and other aircraft is key to a safe flight. (Photo by Robert Smith/MI News/NurPhoto via Getty Images)

Altitude

Altitude refers to the vertical distance of an object above sea level and is measured and displayed in the flight deck by the altimeter. The important factor here is that this is very much dependent on air pressure. 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.

To ensure that the altimeter is displaying the correct altitude, we must change the pressure setting, known as the QNH, to the correct value for the location at that time. This is particularly important during take-off and landing as it gives us accurate clearance from any mountainous terrain.

(The video below shows the altitude on the right-hand side and the radio altimeter counting in the lower-middle)

Flight Level

As aircraft fly around the world, the air pressure is always changing. Constantly updating our QNH for the duration of a 14-hour flight would be a tedious exercise and can also leave you open to human error. If the crew of one aircraft forgot to keep theirs updated, a loss of vertical separation between aircraft could occur. As a result, once above a certain altitude, pilots set a standard pressure setting, 1013 Hectopascal’s (hPa) — the unit of pressure measurement — and fly at what is called a flight level.

The altitude at which this happens depends on the country. In the UK it is 6,000ft, in the USA it is 18,000ft. Any flight below this cut off is flown at an altitude and any flight above it is at a Flight Level. So, when your IFE screen shows that you are flying at 38,000, that’s not strictly true. What it actually means is that you’re flying at FL380. Depending on the air pressure at that moment, the actual altitude could be anything on either side of the 38,000ft value.

For every 1  hectopascal (hPa) — the unit of measurement we use for pressure — change, the altitude differs by 30ft. This is fine when climbing but can be potentially dangerous on the way down. If flying at FL70, the QNH is in effect 1013hPa. If the local QNH was also 1013, then the altitude would also be 7000ft.

However, if the QNH is 973hPa, a difference of 40hPa, the actual altitude will be 6,800ft (7000ft – (40hPa x 30ft) ). When flying close to mountains, this could prove fatal if not taken into consideration. It’s why pilots use the phrase “High to low (QNH), look out below”

Height

What is really useful when flying close to the ground, is knowing the height of the aircraft. The height refers to the vertical distance of an object above the ground. So when making an approach to land, particularly in the latter stages of an approach in thick fog, knowing the aircraft’s height above the ground is key.

As the height of the aircraft will change second by second as it passes over buildings, small hills and trees, a much more advanced instrument is required. To do this we have the Radio Altimeter, more commonly known to pilots as the Rad Alt.

Radio Signals for accurate readings

A radio altimeter uses radio signals to accurately measure the distance between the aircraft and the ground immediately below it. The first radio altimeters used a transmitting antenna to send out a signal and a receiving antenna to pick up the signal as it bounced back off the ground. Knowing the speed that the radio wave travelled, the system could then work out the distance between the aircraft and the ground.

The radio altimeter measures the height of the aircraft above the ground (Image Charlie Page/TPG)

The radio altimeters on modern aircraft use the same principle but with a slight twist. Instead of sending out a single frequency radio signal, the beam emitted by a modern rad alt constantly changes its frequency in the 4.2-4.4 GHz band. When the receiving antenna detects the returning signal, it measures the frequency.

As the rad alt system knows the rate at which the frequency of the wave changes, by comparing the frequency of the emitted signal to that of the received signal it is able to determine the height above the ground.

However, because the antennae on the aircraft are quite small and the accuracy of the system is so good, what the rad alt is actually measuring is the distance from the ground to the aircraft fuselage. This is fine, but the first part of the aircraft to hit the runway is the landing gear, not the fuselage (on a good landing, at least!). The difference between the two is known as the residual height.

What we as pilots really need to know is the height of the gear above the ground. To do this, knowing the distance between the antenna and the lowest part of the landing gear, the rad alt system is able to calculate this height and display it on our screens.

The rad alt system displays the height between the ground and the main gear to the pilots (Image Charlie Page/TPG)

 

The Problem with 5G signals

According to Ericsson, a global leader in mobile telecoms infrastructure, “5G is the fifth generation of cellular networks. Up to 100 times faster than 4G, 5G is creating never-before-seen opportunities for people and businesses.” In a nutshell, this means that your Flight Radar 24 app will load a lot quicker and you’ll be able to stream Ultra HD Netflix whilst on a train as if you were at home.

However, 5G signals operate in the 3.7-3.98 GHz band, just 220 megahertz difference from that of aircraft radio altimeters.

As safety is the primary concern in all things aviation, in April 2020 RTCA formed a working group to find out if the proximity of the two bands would cause any problems for aircraft and if this would have an impact on safety.

They found that there is a chance that 5G signals could cause interference on all types of radio altimeters fitted on commercial aircraft and that these signals could cause radio altimeters to fail or give erroneous readings.

While this issue undergoes further investigation, the safest thing to do is to warn aircraft operators and pilots about the threat. NOTAMs will be issued at airports where 5G signals may cause a problem to aircraft rad alts.

How will this affect my flight?

The first thing to say here is that this issue will not affect flight safety in any way. Most approaches to land are carried out in good visibility and do not require the use of the radio altimeter. Where the rad alt is needed, though, is when it gets really foggy and we are required to carry out an autoland.

Most approaches to land at major international airports use the Instrument Landing System (ILS). This comprises ground-based transmitters which send signals up to the aircraft to guide us down to the correct touchdown point on the runway. On a good visibility day, we will fly this manually all the way to touchdown with the autopilot disconnected. This can be done with visibility as low as 550m and is known as a Category 1 (CAT1) ILS approach.

However, when the visibility drops below 550m visibility, most airlines require their crews to perform an autoland.

Landing in foggy conditions requires an autoland (Image Shutterstock)

In these situations, the aircraft uses the same ground-based radio signals to fly down towards the runway but this time the crew let the autopilot fly the aircraft all the way to touchdown. This is not because the autopilot does a better job (conversely, if your landing is quite firm, the likelihood is that it was an autoland) but it enables the pilots to take a step back and monitor the instruments more intently, just in case something untoward happens. This gives them more time to react and deal with the problem.

However, in order to perform an autoland, we require an operative and reliable radio altimeter. This is because the aircraft takes its cues on when to start the flare (the slight raising of the nose just before touchdown) and when to reduce the engine power to idle in time for the touchdown from the rad alt indications.

In the event of possible 5G interference at a particular airport, it most likely means that crews will be forbidden from carrying out autolands.

However, as we only ever perform autolands on days where the visibility is really low, the chances of this affecting your flight is pretty low.

Bottom Line

As with all things aviation, safety is at the forefront of everything that we do. As a result, until such time that there is more information and evidence that 5G signals do not cause interference with radio altimeters, the safest option is to take steps to ensure that there will be no adverse effects on flight safety should the rad alt system fail.

Most approaches are flown manually by the pilots. Only very few are autolands. On average, I reckon I do just one autoland in the aircraft a year (even though we must do several in the simulator every 6 months to ensure that we are proficient). As a result, this Airworthiness Directive by the FAA will have very little impact on the vast majority of flights.

Featured Image – Getty Images

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