POS INIT — Aligning the IRS
beginnerBefore the FMC will work, the inertial reference system (IRS) must be aligned with a known position. Learn where POS INIT lives, how to enter the reference airport, and how to set the IRS position to start alignment.
Start this scenario
Open the SimTuts 737 CDU trainer and select “POS INIT — Aligning the IRS” from the scenarios list to walk through it step-by-step on the simulated keypad.
Open trainer →Step-by-step: POS INIT — Aligning the IRS
- Open the INIT REF index. Press the INIT REF key.
INIT REF is the gateway to the cold-and-dark setup pages: position, performance, takeoff, approach. We start with position (POS INIT).
- Open POS INIT. Press 2L (<POS INIT).
POS INIT is where you tell the FMC where the aircraft is, so the IRUs can align. Until they do, the FMC has no position and much of it is inhibited.
- Enter the reference airport. Type EGLL and press 2L (REF AIRPORT).
The reference airport gives the FMC a known position (the airport reference point). On the real aircraft you would normally refine this to the actual gate, but the airport is enough to start the alignment.
- Set the IRS position to start alignment. Press 5R (SET IRS POS).
SET IRS POS down-selects the reference position into the inertial reference units, which begin aligning (about 10 minutes on the real aircraft — do not move the aircraft during this). Once aligned, the PFD/ND come alive and the rest of the FMC setup behaves normally.