ACARS - How Texting Works

Okey, texting with airplanes happens all the time. It is part of the whole process. The pilot needs to know stuff, and without tying up the air with a bunch of information the pilot may mis-interpret, or need to read later, the pilot and folks on the ground can communicate with a medium most of use use, in text.

In most aircraft, there is a keyboard and a screen up near both pilots. They have the ability to use this device to send questions to the ground, and the ground has the ability to send messages up to the pilots. Information can include weather, or flight plan changes, gate assignment. Almost anything can be sent to the pilots on the screen.

For the most part, this system works similar to a cell phone. There are ground stations all over the country. These ground stations listen on certain frequencies for a signal on a certain frequency. when these ground stations hear a message, they forward it to the assigned receiver. Each airline has assigned address(es). Delta doesn't want United hearing their messages, as much as Jet Blue doesn't want Southwest hearing their messages. Each aircraft has its' own address as well.

These ground stations are owned by various carriers, similar to cell phones. ARINC and SITA are the two major players in the world. There are some smaller carriers as well, and they are limited to certain regions in the world. The carriers don't typically inter-connect messages. If your airline is using ARINC all messages will be on ARINC equipment once they leave the operations center, until they get to the aircraft.

Different stations in the world use different frequencies so the aircraft don't overcrowd a single ground station. The ground station frequencies are similar to the VHF navigation and communication frequencies already used on the aircraft. Most of the ACARS frequencies are in the 129 to 137MHz range. Each ground station can cover about 200 miles on these frequencies.

If a pilot wants to send a message to a dispatcher in the pilot's airline operation center the pilot would tune to the nearest frequency that is on their chart, and enter the message on the keyboard. The message would get transmitted to the ground station and the carrier would forward the message to the operations center for that airline. When the dispatcher receives the message, they can enter a response. The dispatchers response will be forwarded to the carrier, and based on the last known location, the carrier will forward the message to the nearest ground station. The ground station will send the message to the aircraft.

There are a couple 'if's above. The communications protocol is quite robust, allowing for queued messages to stay queued until the ground station receives the message, and acknowledges it. If a ground station is out of service, or the aircraft is tuned to the wrong frequency, the message will sit on the aircraft, until the situation improves. If nothing else, the messages will be cleared when the aircraft power cycles itself (IE shutdown, and brought back up), no one wants to hear about something that happened yesterday.

There are automatic messages sent over ACARS as well. When the aircraft is first powered up, and the pilot initializes the computers a message will typically be sent to the operation center. This message will go into a database, and allow the airline to look at when things got started, what flight the aircraft is assigned to, and other such information. When the doors are shut, and the brakes are released an out gate time message will be sent to the operations center, and when the aircraft squat switches are showing no weight on wheels, an off ground message time is sent. The time messages that the operations center knows about and uses are called the OOOI (ooey) times, Out gate, Off ground, On ground, and In gate. There are other times, like in range that the gate wants to know about as well.

The pilots will use ACARS for many operational items. If ATC needs to divert and aircraft, the ACARS will be a way the dispatcher and the pilot can determine if there will be operational impacts to ATCs request. Will there be enough fuel to take the new route, or will the new route cause people to be delayed are all considered. If the pilot needs to know about weather ahead, some airlines have the capability to send messages to the aircraft if there are significant changes to the weather.

The ACARS unit will ding when a new message comes in. This ding is handy should the pilot be working a situation in the air, and need to know when the resources on the ground have more information. The ding can be a distraction when the pilots workload is high. Most airlines limit the ding to when the aircraft is above 10000ft. Messages can still happen when the aircraft is below 10000ft, but the ding will not distract them.

Next time you are flying, and you wonder where the pilot got all the up to date information, it probably came over the ARARS unit on the airplane.

CPDLC - Texting For Pilots

Texting and driving is against the law. Texting and flying, no problem. 

Ok, we aren't talking about your family  asking you to stop by the grocery store pickup some milk on the way home. On most commercial aircraft there is a text based communications system. This is usually the ACARS system, the Aircraft Communications And Reporting System. This display and keyboard is right there usually in the panel, and encouraged to be used in flight.

The ACARS communications start early in the flight. Most airlines participate in the Pre-Departure Clearance (PDC) program where the pilots get the clearance right from the tower on the ACARS screen. The pilot is required to request the clearance before the flight, and the tower will automatically deliver the clearance to the screen.

The ACARS system is also connected to the aircraft maintenance department. The engine and other parts of the aircraft are connected to the computers that are connected to the ACARS system. When the aircraft is in a certain state, an automated message will be delivered to the ground. Engine status will be delivered when the aircraft is in cruise state, and not accelerating (stable cruise report). On ground time will be delivered when the aircraft has main gear down.

Recently, ICAO has standardized the phraseology and the FAA have started delivering ATC messages to the cockpit. The messaging is called Controller Pilot Data Link Communications (CPDLC).  There are some limitations to these messages, generally they will be standard communications. Things like "turn left heading 240 degrees", or "climb and maintain 360". These are normal mundane type messages that the controllers say everyday over and over. The ATC screen has templates of these standard messages, where the controller only need to enter the heading and altitude.

The messages must be acknowledged, or they will be assumed to not be received. The pilot can acknowledge the message or say unable. The controller has the option of using the voice to find out more, or offer a better different message.

There is a huge misnomer, Air Traffic Controllers don't actually control aircraft. The controllers offer suggestions to pilots. Pilots can always do what is needed to operate the aircraft safely, regardless of what the controllers are telling them to do. Normally following the controllers directions will be the safest thing to do, but there is always the option.

The ICAO 4444 document Procedures for Air Navigation Services Air Traffic Management has a chapter on the CPDLC messaging.

    CHAPTER 14. CONTROLLER-PILOT DATA LINK COMMUNICATIONS (CPDLC).

There is another document 9694 Manual of Air Traffic Services Data Link
Applications that has additional guidance. These CPDLC messages have various levels of urgency, and alerts, and are outlined better in these two manuals.

There is a fear that using CPDLC will prevent pilots from eavesdropping on other pilots. There is a possibility that may occur. CPDLC message are addressed to a specific aircraft, and to a specific ATC center. Normally, when ATC is talking (using voice communications) to the aircraft in an area, they talk on the one frequency that all aircraft can hear. The benefit to that is that if aircraft are near each other, and a command will make another pilot question the intention, the eaves dropping pilot can ask for clarity. Sometimes controllers make mistakes, and pilots can ask. If someone is put on the same altitude and opposite course as another aircraft, the pilot not getting the command make question the controller. With CPDLC addressed messages, other aircraft cannot "hear" those commands.

Mostly the CPDLC systems are in use in the oceanic realm. There is quite a bit of separation going on in that area, and communications has been poor over the ocean. In the past, the oceanic communications, has been over HF voice channel. CPDLC has actually improved the performance of the communication over the ocean.

Long term, some CPDLC messaging will be added to the enroute area. Perhaps to a limited extent, the TRACON will start to get some CPDLC messaging.