Can you hear me now: How pilots communicate with ATC while 35,000 feet in the air

May 30, 2020

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When you’re in a sealed, pressurized tube five miles above the ground, being able to communicate effectively is essential. In the early days of aviation, flags and light signals were used before designers were able to fit basic radio equipment into aircraft.

Modern aircraft now have an array of communication devices from the rudimentary HF radios of old to sophisticated satellite-based systems that enable us to talk almost as if we were on a mobile phone.

VHF radios

How it works

The most common form of communication in aviation, very high frequency (VHF) radio calls are what we use for around 95% of our communications with ATC. In simplified terms, the transmitting station sends a signal that travels in a straight line and is picked up by the receiving station.

VHF comms provide clear voice communications. However, as the radio signals travel in straight lines, they are limited by the curvature of the earth and objects that they may come into contact with, such as hills and mountains.

Image - Charlie Page/The Points Guys
(Image by Charlie Page/The Points Guy)

The distance which a VHF signal can travel depends on both the height from which the signal is sent and the height of the receiving station. If both the sender and the receiver are on the ground, the distance will be relatively small. If both stations are in the air, the distance the signals can travel is much further.

When communicating between a ground-based station like Air Traffic Control and an aircraft, the distance is somewhere in the middle.

To work out the exact distance a VHF signal will travel, you can use the following equation:

VHF signal distance in nautical miles = 1.23 x (√height of transmitting object in feet + √ height of receiving station in feet)

So, with an aircraft at 36,000 feet and the ATC radio tower at 100 feet, communication will be possible up to 250 nautical miles away. However, this is the theoretical optimum and will be much less in practice due to transmitter power and receiver inefficiencies.

Read more: The challenges for pilots when landing at hot and high airports

How pilots use it

Most commercial aircraft will have three separate VHF radios. Not only does this give us backups in case of a failure, but it also allows us to use to a number of different frequencies at the same time.

On the 787, the three VHF radios are identified as “VHF left,” “VHF center” and “VHF right.” The tuning and control panel (TCP) is — unsurprisingly — used to tune and control all three VHF radios.

VHF-L is used primarily for communications with the active ATC frequency, VHF-C for our datalink and VHF-R to keep a listening watch on the emergency frequency, 12.5.

The TCP gives us two options for each radio — “active” for the frequency we wish to receive and transmit on and “standby” for a frequency that we would like to have ready to use at the push of a single button.

The tuning and control panel on the 787 Dreamliner. (Image by Charlie Page/The Points Guy)

To tune one of the radios to the desired frequency, we simply type in the numerical part of the frequency using the number keys and hit the key-select button adjacent to where we want the frequency to go. For example, if ATC (on VHF-L) asks us to change to another ATC frequency on 133.65 MHz, we’d type “13365” on the number keys and press the first key-select button on the left-hand side.

When we do this, two things happen.

Firstly, the active VHF-L frequency changes to 133.65 as requested. However, instead of the previous frequency (123.45 in the photo) disappearing from view, it flips across to the SBY column. This is particularly useful in the event that we are unable to make contact with the new ATC unit — a fairly common event.

Instead of trying to remember the frequency on which we were talking to the previous ATC unit, the TCP has already saved it for us. All we need to do is hit the XFR button and we will be back on the previous frequency. When ready to talk on the frequency, we press one of the three push-to-talk buttons in our seat area and this transmits what we say.

Read more: How pilots operate cargo flights on passenger aircraft

HF radios

How it works

VHF radios are great when flying over areas where there are regular radio towers to send and receive signals. However, when flying over the oceans, VHF comms are not really an option. In these situations where the curvature of the earth prevents the use of VHF, we have to revert to something a little more basic.

Whilst high frequency (HF) radio signals aren’t as strong as VHF signals, they are actually able to travel much farther. How?

Image - Charlie Page/The Points Guy
(Image by Charlie Page/The Points Guy)

In the outer edges of the earth’s atmosphere, between 37 to 630 miles altitude, is the ionosphere. Here, a shell of electrons and electrically charged atoms and molecules are ionized by energy from the sun. HF radio waves bounce off the ionosphere and back down to earth, enabling them to travel great distances and get around even the biggest of mountains.

The layers of the Ionosphere (Image Naval Postgraduate School / Public domain)
The layers of the ionosphere. (Image courtesy of the Naval Postgraduate School Public domain)

During the day, energy from the sun causes the D, E and F layers to become heavily ionized, making the layer more active. At night, with less energy from the sun, only the E and F layers are active. As a result, lower frequencies are better quality during the night and higher frequencies are better during the day.

However, what HF gains in distance, it loses in quality. Quite often, the quality of the signal is so poor that it’s impossible for either station to hear each other.

How pilots use it

By pressing the HF button at the top of the TCP, we are able to bring up the menu of HF radios. As these are not used as often as VHF radios, there are only 2 HF radios — left and right.

Tuning the HF radio is exactly the same as the VHF radio. Type in the number, select where you want it to go and it’s ready to use. Once again, there is a standby frequency to give us greater flexibility. This is particularly useful when crossing the Atlantic.

Read more: What’s happening in the flight deck during a rejected takeoff?

The HF menu on the TCP. (Image by Charlie Page/The Points Guy)

When flying to North America, leaving Irish/Scottish airspace, the controller in Shanwick will give us a primary HF frequency to use and also a backup frequency should the quality of signal be too poor on the primary. They will also give us a primary and backup to contact Gander in Canada when halfway across the Atlantic.

We normally put the primary frequency as the active on HF-L and the backup as the active on HF-R. We then put the primary Gander frequency in the standby for HF-L and the backup in the standby for HF-R, ready to use when needed.

By setting them up in this configuration, we are able to try and contact Shanwick using both the L and R radios at much the same.

CPDLC

How it works

With the lack of VHF comms over oceans and the unreliability of HF, what we really need is a way of communicating with ATC without the need to physically speak to them. CPDLC — the controller pilot data link communications — is a form of text messaging that enables us to communicate with ATC without the need for voice communications.

When out of VHF coverage, the CPDLC system uses satellites to connect with ATC units on the ground. ATC can then use this to send us instructions relating to changes in our altitude, heading, speed and radio frequencies.

We can also send requests to ATC should we wish to change altitude or change our course to deviate around thunderstorms.

Flying over the Atlantic, CPDLC is used as a primary means of communication with HF as a backup. When flying over land, it is used in conjunction with normal VHF communications.

How pilots use it

The CPDLC system is designed for simplicity. To set the system up, we must first log on to the ATC unit we wish to connect to. Each unit has its own four-letter code, which we enter into the login page. For example, London is EGTT.

Once logged on, ATC is able to send us instructions via CPDLC messages instead of making a radio call. This reduces the volume of calls on the radio frequency and also reduces the chance of a misunderstanding.

The CPDLC response buttons on the 787. (Image by Charlie Page/The Points Guy)

When receiving a CPDLC message from ATC, for example, a clearance to climb to a higher altitude, it pops up on a screen to the right of the PFD. To reply to the message, we simply press one of the three buttons in the flight deck — accept, cancel or reject.

To send a message to ATC, we use one of the preset pages in the CPDLC menu.

When crossing the Atlantic, we must get clearance from the controllers on the ground. In days gone by, this exchange would have been done by voice over HF radio. However, with a lack in clarity of transmissions, this process was time-consuming and errors were easily made.

By using CPDLC, the exchange with ATC is much quicker and far less prone to mistakes. We simply fill in the boxes with the required information and hit the send button — very much like filling out a form on a web browser.

Read more: Flying the length of Africa: London to Johannesburg from the pilot’s perspective

With the press of a button, we can inform ATC of a range of information. (Image by Charlie Page/The Points Guy)

The simplicity of CPDLC is also of great benefit in emergency situations. Should the need to divert arise mid-Atlantic, getting hold of ATC on HF could be a laborious process. Instead, in seconds, we can send an emergency report to ATC with all the details they will need to know.

SATCOM

How it works

SATCOM, or satellite communications, enable us to make voice calls similar to using a mobile telephone. Using the INMARSAT system, a network of 13 geostationary satellites, we are able to make and receive calls from almost any location in the world.

SATCOM coverage over the earth. (Image courtesy of inmarsat.com)

Calls are initiated by either using pre-assigned short code numbers or using direct dial numbers using a country code and phone number. The call is beamed up to the INMARSAT network and then routed to a ground base station. From here, it is then connected to the intended recipient.

How pilots use it

The SAT button on the TCP brings up the SATCOM menu as seen in the photo below. From here we have access to a directory of numbers just like in the contacts on your phone.

To make a call, we simply select the number we wish to dial and hit the “make call’ button. We then hear a ringing tone before the person at the other end picks up. There is often a small time delay when making a SATCOM call. As a result, it’s essential that we speak slowly and allow time for the other person to hear all of what we’ve said.

Immediate medical assistance is just a phone call away. (Image by Charlie Page/The Points Guy)

SATCOM is useful when it comes to operational calls but invaluable when a passenger becomes ill during the flight.

Most airlines have access to a service called Medlink. Wherever we are in the world, we are able to call directly to a hospital in Tucson, Arizona. Here, specially trained doctors are able to assess, diagnose and prescribe treatment for a sick passenger. If the situation is serious, they can advise us that a diversion to land at a nearby airport would be in the best interests of the passenger.

The clarity and speed of the SATCOM call mean that a sick passenger can get quality treatment even when five miles above the Pacific Ocean.

Bottom line

Depending on the scenario and geographic location, we have a number of communication channels available to us. VHF is the method we use most of the time, enabling rapid interactions with ATC at times when speed is of the essence like on takeoff and landing.

However, as VHF only works with “line of sight,” satellites have enabled us to communicate almost as effectively when in remotes areas via CPDLC and SATCOM. The use of text-based messages also reduces the risk of misunderstandings between ATC and pilots, leading to an increase in in-flight safety.

Featured photo by Charlie Page/The Points Guy

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