POS INIT — Aligning the IRS

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📍 Route: POS INIT · EGLL⏱ ~3 minutes📊 4 steps🎓 Foundations

Before the FMC will do anything useful, the inertial reference system has to know where it is. POS INIT is where you tell it.

What this scenario covers

The IRS is a self-contained navigation system: once aligned, it knows where it is by measuring how it has moved since. But it cannot find its own starting point, so the crew supplies it — the reference airport, and then the position, on the POS INIT page.

Alignment takes several minutes and cannot be rushed or done while the aircraft is moving, which is why it is one of the first actions of the flow rather than one of the last.

How it applies in flight ops

This is why crews board an aircraft and immediately power the IRS: everything downstream, including the FMC's position and therefore its LNAV, waits on the alignment finishing. A late alignment is a late pushback.

The entered position is cross-checked against the gate coordinates. An error of a degree here — a transposed digit in latitude, say — produces an aircraft that is confidently wrong about where it is, and the FMC has no independent way to notice.

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Step-by-step: POS INIT — Aligning the IRS

  1. 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).

  2. 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.

  3. 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.

  4. 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.

What to take away

  • The IRS measures movement; it cannot find its own start point.
  • POS INIT takes the reference airport and position to begin alignment.
  • Alignment takes minutes and needs the aircraft stationary — do it first.
  • Cross-check the entered position; the FMC cannot catch a plausible wrong one.

The rest of Foundations

Background reading

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