ROUTE Page Basics

Origin and destination are 4-letter ICAO codes from your Operational Flight Plan (OFP) — the paperwork dispatch provides before every flight. KJFK = New York JFK, EGLL = London Heathrow. Getting this wrong means the FMS calculates the wrong route, fuel, and winds.

Origin and destination are 4-letter ICAO codes from your Operational Flight Plan (OFP) — the paperwork dispatch provides before every flight. KJFK = New York JFK, EGLL = London Heathrow. Getting this wrong means the FMS calculates the wrong route, fuel, and winds.

Origin and destination are 4-letter ICAO codes from your Operational Flight Plan (OFP) — the paperwork dispatch provides before every flight. KJFK = New York JFK, EGLL = London Heathrow. Getting this wrong means the FMS calculates the wrong route, fuel, and winds.

Company routes are pre-built flight plans stored in the airline's FMS database. Instead of entering every waypoint manually, you type a route code from the OFP. The dispatcher has already verified this route for airspace restrictions and NOTAMs.

The flight number comes from the OFP and matches your ATC callsign. BAW = British Airways ICAO code, 178 = flight number. It's how controllers identify you on radar and how ACARS associates this FMS with the correct flight.

A 737 route is not flown the moment you finish typing it. ACTIVATE stages it, and the lit EXEC key commits it. That two-step is the FMC safety net: you can review a staged change before it becomes the active route. Every Boeing tutorial you watch passes through this sequence.

Every prediction the FMC makes is built on this page. The FCOM puts it plainly: "The performance initialization page allows the entry of airplane and route data to initialize performance calculations. This information is required for VNAV calculations." The prompt you just pressed is the FMC saying it still needs something: it is "Displayed only on the ground when required entries on the PERF INIT page are incomplete." The next four steps are the four entries a crew makes here before every flight. The right-hand column also takes a forecast CRZ WIND and temperature, but those are optional: "If no entry made, the FMC assumes zero wind for preflight predictions."

Zero fuel weight is the aircraft with its passengers, bags and cargo on board but no usable fuel. It comes from the load sheet, and the FCOM makes it the normal entry: "Normally the ZFW is entered from the airplane dispatch papers and the FMC calculates the airplane gross weight." That is why you type nothing at 1L (GW/CRZ CG). On the aircraft the FMC adds the fuel on board to your ZFW and shows the gross weight there itself. This trainer has no fuel gauges to add, so 1L stays blank here. 55.0 tonnes is well inside the 737-800's 61,688 kg maximum zero fuel weight.

RESERVES is the fuel you plan to still have in the tanks when you land at the destination: fuel the flight plan does not let you spend on the trip, kept for a hold or a diversion. It comes from the OFP, and the FCOM makes it compulsory: "Enter fuel reserves for the route. Entry is required to complete the preflight." The FMC checks against it for the rest of the flight: "USING RSV FUEL is displayed if the estimate is less than the fuel reserve value entered on the PERF INIT page."

The cost index comes from the OFP. The airline sets it by weighing the price of fuel against the cost of flying time. The FCOM: "The cost index is used to calculate ECON climb and cruise speeds." The two ends of the range show what it does: "0 causes the ECON speed to be MAX RANGE; 500 results in a minimum time flight." And it is not optional: "Entry is required to enable use of VNAV mode."

CRZ ALT is the level you plan to cruise at, and the FMC plans the climb to it: "The T/C point is where the airplane reaches the cruise altitude entered on the PERF INIT page." The FCOM makes it required: "Cruise altitude is required. Enter the cruise altitude for the route." The right level depends on weight. JFK to Heathrow is almost 3,000 nm, a long way for a 737-800, so it leaves heavy with fuel, at around 70 tonnes. At that weight the FCOM's cruise tables put its optimum altitude near 36,000 ft. FL350 sits just below that, and it is an odd level, which the semicircular rule gives to eastbound flights.