The question raised in this forum post touches on a scenario that every instrument-rated pilot eventually confronts: what happens procedurally when a glideslope fails after the aircraft is already established inside the final approach fix on an ILS. The regulatory and procedural answer is straightforward but easy to overlook in the heat of an approach. When the glideslope becomes unusable, the ILS reverts to a localizer-only approach, and the missed approach point changes from a DA (decision altitude, a point in space defined by altitude) to a MAP defined either by DME/RNAV distance or, absent that, by time from the FAF at the appropriate groundspeed. If the only means of identifying that localizer MAP is timing, then yes, starting a clock at the FAF is not just a "good practice" — it is the only legal way to identify the missed approach point in the absence of DME or a stepped-down fix. This is precisely why the timing table exists on the approach plate's minimums section, keyed to groundspeed.
For working pilots, this scenario is a good reminder that briefing an ILS should never be a single-line affair. A complete approach brief includes contingency planning for backup navigation and missed approach identification, not just the primary DA and course. Professional crews are trained to treat every precision approach as if it might degrade to a non-precision backup, briefing the localizer minimums, the timed or DME-defined MAP, and the required actions if the glideslope flag appears. In part 121 and 135 operations, this is standard SOP: PF and PM brief "glideslope out" contingencies before the approach, including what altitude to level at, what the new MAP look like, and how it will be identified. In light GA and instrument training, this discipline is often glossed over because glideslope failures are rare in practice — but the FAA's Instrument Procedures Handbook and AIM both make clear that when a precision approach becomes non-precision, the pilot must fly the entire non-precision procedure, including using time as backup MAP identification if no other means (DME, cross-radial, RNAV waypoint) exists.
This matters operationally because relying on "eyeballing" a rough distance or altitude cue instead of timing can lead to descending or continuing beyond a legal MAP, which is both a regulatory violation and a genuine safety risk in low IMC, particularly with terrain or obstacles near minimums. Timing is especially critical at localizer-only minimums where the safety margin between MDA and obstacles can be tighter than the ILS glideslope's protected corridor. It's also a reminder that MDA is a "no lower than" altitude while a MAP by time is a hard limit — if the timer expires and the runway environment isn't in sight, a missed approach must be executed immediately, regardless of altitude.
More broadly, this question reflects a recurring theme in instrument training: modern glass cockpits and GPS overlays have reduced the day-to-day reliance on old-school timing techniques, and many newer instrument pilots have limited real-world practice with them. Yet timing remains a required skill under 14 CFR 61.65 and remains tested on the instrument checkride precisely because backup navigation methods must be second nature when automation or ground-based navaids fail. As the National Airspace System modernizes with more RNAV and less reliance on ILS/localizer infrastructure, instructors and check airmen alike continue to emphasize that pilots must be fluent in both the primary and degraded modes of every approach they fly — a principle just as relevant to a Part 135 charter crew shooting a backup localizer approach into a mountainous strip as it is to a student pilot on an early cross-country IFR flight.