Right, let's pick this up on the FMC start-up sequence — Page 279, Lesson 325

Right, let's pick this up on the FMC start-up sequence. We've just completed the power-on self-test, and upon its successful completion, the system displays the IDENT page. This IDENT page is our first real interaction with the Flight Management Computer. Its primary job is to allow the crew to confirm the aircraft details on start-up. Think of it as the system introducing itself and asking us to verify its identity. Crucially, this page confirms the navigation database in operation. The navigation database is the stored information about airways, waypoints, and airports that the FMC uses to build our route. Importantly, it also gives us an indication of the date of changeover to the next programme. The database is updated on a cycle, and this page tells us when the next update is due. If the database is out of date, it will tell us to change to the in-date programme. So we're checking that the FMC is running the correct, current information. Once we've confirmed the IDENT page, the system initiates a series of prompts to direct us through the route procedures needed for the flight. This is a guided sequence, and it directs us to the POS INIT page. On the POS INIT page, we must check the FMS clock time against the aircraft clock to ensure synchronization between the systems. This is critical for two reasons. First, data is saved on the Flight Data Recorder, the FDR, against time. So if the clocks disagree, the recorded data is misaligned with the actual time of the event. Second, the FMC generates Estimated Times of Arrival, the ETAs, and these need to be in line with the times indicated by the aircraft clock. As we complete this task, we must also ensure that the airfield datum or gate position, if available, is entered accurately. This position is used to allow for IRS alignment. The IRS is the Inertial Reference System, which needs a known starting position to align its gyros and accelerometers. This entered position will be suitable for alignment, but it's often updated at the take-off point to obtain the best possible initial position for use in flight. So we set a good position now, and refine it later. After completing our tasks on the POS INIT page, we're directed to the RTE page. Here we enter our starting and destination airport ICAO designators. ICAO designators are the four-letter codes for airports, like EGLL for London Heathrow. We may then expect to enter our flight number details and identify a "standard" company route to take us to our destination. This is a pre-defined route that the company has set up for that particular destination. If a standard route is not available, other actions will have to be taken to input the information into the system manually. We may now move on to the PERF INIT page. This page updates the aircraft to its current performance and weight configuration for the route to be flown. On this page we may input details of fuel weight, fuel reserves required, and cruise altitude. And in the case of the B747, we can even input if we are carrying a fifth engine to our destination upon its suspension point on the wing. That's a specific configuration for that aircraft type. At this stage we may also enter Cost Index requirements related to our flight. The Cost Index is a value that balances fuel cost against time cost, and we'll discuss that in more detail later in the chapter. So the sequence is: IDENT to confirm the database, POS INIT to synchronise time and set the initial position, RTE to define the route, and PERF INIT to set the performance parameters. Each page builds on the last to prepare the FMC for the flight.