ILS Glideslope and Localizer on the Boeing PFD: Flags, Cues, and Go-Around

Boeing primary flight displays present ILS guidance differently from Airbus: instead of magenta diamonds on a vertical scale only, you typically see localizer and glideslope deviation pointers (needles or rectangles) with GS and LOC flags when signals are invalid. Simulator pilots approaching with PMDG, iFly, or MSFS default Boeings should know when cues are trustworthy, when a flashing or amber condition demands action, and how wrong QNH or capture geometry mimics a “broken ILS.”

Normal ILS indication

When tuned and receiving, the glideslope pointer centers on the scale at on-path; localizer centers laterally. Deviation dots or bars move proportionally — one dot is commonly half a degree or equivalent per model documentation. On approach with flight director and autopilot, capture occurs when aircraft enters beam width; you may see brief transient movement as filters smooth the signal.

Flags and warning states

GS flag (often amber) means glideslope information is unreliable or not received — do not descend on a flagged GS. Causes include wrong frequency, out of beam coverage, signal interference in the sim, or descending below the lower edge of the glideslope envelope. LOC flag similarly removes lateral ILS guidance.

Some models flash cues when within a few dots of full scale — that is a cue that deviation is large and autoland or continuation may be prohibited. Treat full-scale deflection on short final as go-around criteria in training unless doing deliberate raw-data practice.

Capture from above

Intercepting the glideslope from above is standard when descending on MCP altitude until GS *alive*. Until capture, do not chase the pointer below the beam — hold altitude until the pointer comes down to center, then follow. Chasing early produces a dive beneath the slope and GS flag risk. NG ROUTE cruise and T/D planning help you arrive at the STAR gate with energy for a normal descent profile.

Autopilot and autothrottle interaction

On Boeing, glideslope capture may bump A/T to retard thrust and increase rate; if you disconnect A/P low without planning thrust, the pointer may swing quickly. Practice coordinated disconnects only when proficient; otherwise fly missed approach.

Simulator troubleshooting

• Confirm NAV radio tuned to ILS — not VOR-only identifier
• Check both front panels receive — compare captain and FO scales
• Verify runway matches chart — wrong runway selection is frequent in MSFS
• Cross-check altitude with NG ROUTE METAR QNH — baro error looks like bad GS

Decision discipline

Stable approach criteria still apply: by 500 ft you should be configured, on path laterally, and with a plan for flags or windshear. A GS flag at 300 ft is not a time to switch to visual and salvage — execute missed approach, re-brief, and fly again.

ILS annunciation differs between 737 classic, NG, MAX, 777, and 787 displays. Use your add-on FCOM for exact flag colours and limits.