Communications Management Unit — Page 330, Lesson 389

This is the start of a new chapter — the Communications Management Unit, which sits right at the heart of the aircraft's data-link and communications architecture. Before we get into the hardware itself, I want to walk you through the glossary that opens this chapter, because every one of these acronyms is going to come back again and again. Let's take them in the order they appear. We start with CNS — that's Communication, Navigation and Surveillance. These are the three pillars of the air traffic system: communicating with the aircraft, navigating it, and surveilling it — that is, knowing where it is. Then we have CNS/ATM — that's Communication, Navigation and Surveillance / Air Traffic Management, specifically the CNS/ATM programme for North America and Europe. So CNS is the technology, and ATM is the management framework that uses it. Next, CPDLC — Controller Pilot Data Link Communications. This is the system that lets the controller and the pilot exchange messages digitally, over a data link, instead of by voice on the radio. Then FANS — Future Air Navigation Systems. That's the umbrella concept that these data-link systems fall under. Now, the hardware and software components. CVR is the Cockpit Voice Recorder — the device that records the audio in the cockpit. DCDU is the Data Link Control and Display Unit — that's the cockpit display and control panel the crew uses to interact with data-link messages. DCL is Departure Clearance — the digital version of the clearance you get before pushback. FANS A is the Data Link Package for Oceanic/Remote airspace/Accommodation — note that's the package designed for the long-haul, over-ocean environment. FANS B is the Data Link Package for High Density airspace — that's the one for busy, congested airspace like Europe or the US. FDAMS is the Flight Data Acquisition and Management System — this gathers and manages flight data. FMS is the Flight Management System — the central computer that manages the flight plan and navigation. FWS is the Flight Warning System — the system that generates the warnings and alerts in the cockpit. Now the radio and data-link side. HF is High Frequency — and here's the exact definition: a portion of the RF spectrum, defined as 3 MHz to 30 MHz. HFDL is the High Frequency Data Link — and note this is described as an ACARS communications media. So HFDL is a way of carrying ACARS messages over HF radio. VHF is Very High Frequency — defined as 30 MHz to 300 MHz. VDL is VHF Data Link, and VDL2 is VHF Data Link Mode 2 — that's the specific mode used. VDR is the VHF Data Radio — the actual radio hardware. Then we have the interface and management units. HMI is the Human Machine Interface — the point where the human interacts with the machine, the display and controls. LRU is the Line Replaceable Unit — that's a component that can be swapped out on the line, at the aircraft, without sending it to a workshop. MCDU is the Multi-function Control and Display Unit — the main CDU in the cockpit. MIDU is the Multi-input Interactive Display Unit — and note it's often used as a third cockpit CDU. MU is the Management Unit — and here's the key: it's often referred to as the ACARS MU. So the Management Unit is the brain that handles ACARS. OCL is Oceanic Clearance — the clearance for oceanic crossings. OOOI — this is a nice one — stands for Out of the gate, Off the ground, On the ground, In the gate. Those are the four key events in a flight that the system tracks: leaving the gate, taking off, landing, and arriving back at the gate. POA is Plain Old ACARS — and the definition is the ACARS communications protocols in effect before the introduction of VDL. So POA is the legacy ACARS, before VDL came in. RMP is the Radio Management Panel — the panel used to manage the radios. SDU is the Satellite Data Unit — the unit that handles satellite communications. SITA is Société Internationale de Télécommunications Aéronautiques — that's a French organisation, and it's described as an Airline Telecommunications and Information Service. So SITA is a service provider for airline communications. Finally, STDMA is Self-organizing Time Division Multiple Access — that's a channel access method where the system organises itself to share the time slots. Now, one thing I want you to notice: this glossary is the vocabulary for the whole chapter, and it's dense. But the key thread running through it is the distinction between the voice world — HF, VHF, the RMP — and the data-link world — VDL, HFDL, CPDLC, ACARS, and the MU that manages it. When we get into the Communications Management Unit itself, you'll see how these all tie together. Also, I should mention — the chapter after this one is Flight Director Systems, which starts on page 332. That's a separate topic, so we'll leave that for later. For now, let's make sure you're comfortable with this glossary, because every one of these terms is going to appear in the systems we're about to study.