Let me start with the introduction — Page 501, Lesson 591

I want to walk you through the Flight Data Recorder chapter now. This is the system that preserves the aircraft's data so that, after an accident, investigators can determine exactly what happened. Let me start with the introduction. Commercial aircraft carry a flight recorder that records various aircraft parameters during the entire duration of the flight. The main function of the flight data recorder, which we abbreviate as FDR, is to preserve the aircraft data in order to determine the cause of any aircraft accident. But it's not only for accident investigation — it's also used to gather information for trend analysis and for trouble shooting. Trend analysis means looking at how parameters change over time to spot developing problems before they become failures. In smaller aircraft, the FDR may be combined with a cockpit voice recorder into a single unit. Now let's look at the FDR designs. The FDR records the last 10 or 25 hours of aircraft data on a digital storage device. That storage device is housed in a shock resistant box — it has to survive a crash — and that box is painted red so it's easy to find among wreckage. It's located at the rear of the aircraft, normally under the fin. On the front of the unit there's an underwater locating device, abbreviated ULD, which helps find the recorder if the aircraft goes down in water. Let me show you the physical layout of the recorder so you can picture it. Now the FDR components. The FDR consists of four components. First, a recording system. Second, a control unit on the overhead panel. Third, a control unit on the pedestal — that's the centre console between the pilots. And fourth, the Data Interface and Acquisition Unit, abbreviated DIAU. The DIAU is the heart of the data collection. Information from the DIAU is sent to the Aeroplane Condition Monitoring System, abbreviated ACMS, together with inputs from aircraft systems. Those inputs come from air conditioning, auto-flight, flight controls, fuel, landing gear, navigation systems, pneumatic systems, the APU — that's the auxiliary power unit — and the engines. This information is used for system monitoring and trend analysis, and it can be downloaded via the aircraft printer, or via ACARS, or via the ATSU. ACARS is the aircraft communications addressing and reporting system, and ATSU is the air traffic services unit — these are the data links that let you pull the recorded information off the aircraft without physically removing the recorder. Now the control unit on the overhead panel. This unit also controls the cockpit voice recorder, the CVR. It has a spring-loaded switch labelled GND CTL, which stands for ground control. That switch can be selected to ON or AUTO. Let me give you the exact behaviour of each position. In the ON position, the CVR and the DFDR are energized and the ON light is lit. DFDR is the digital flight data recorder — that's the modern digital version of the FDR. So ON simply powers both recorders up and shows you a lit ON light. In the AUTO position, the CVR and the DFDR are energized under two conditions. First, on the ground with one engine running. Second, in flight, whether the engine is running or stopped. So AUTO is the normal operating mode — it ensures the recorders are running whenever the aircraft is in a condition where something meaningful could happen, but it doesn't run them continuously on the ground with all engines shut down. Now the control on the pedestal. This is much simpler — it consists of a push-button labelled EVENT. When you press it, it sets an event mark on the DFDR recording. Think of it as a bookmark. It marks a specific moment on the recording so that, at a subsequent analysis, that event can be found rapidly on the recording. So if something unusual happens during a flight, the crew presses EVENT, and later the investigators can jump straight to that point instead of searching through hours of data. Let me show you the full system layout so you can see how the DIAU, the ACMS, and the recorder all connect together. So to summarise the key points: the FDR preserves data for accident investigation, trend analysis, and trouble shooting. It records the last 10 or 25 hours on a digital storage device in a red, shock resistant box under the fin, with an underwater locating device on the front. The four components are the recording system, the overhead control unit, the pedestal control unit, and the DIAU. The DIAU feeds the ACMS along with inputs from the aircraft systems, and that data can be downloaded via the printer, ACARS, or ATSU. The overhead GND CTL switch has ON and AUTO positions with specific energising conditions, and the pedestal EVENT button places a bookmark on the recording for rapid later analysis.