DME holding procedures remain one of the more misunderstood corners of instrument flying, and the confusion reflected in this pilot's question is common even among experienced instrument-rated aviators. Per AIM 5-3-8 and the guidance found in the Instrument Procedures Handbook, DME holding differs fundamentally from standard time-based holding in that the outbound leg length is defined by distance rather than time. The inbound leg is flown to the holding fix, and the outbound leg is flown until reaching the specified DME distance printed on the plate—typically something like "4 DME" or a range such as "4-8 DME" for a teardrop-style hold. Time is not used to determine when to turn outbound-to-inbound; distance is the sole governing parameter for leg length. Where confusion often arises is in blending standard entry procedures (direct, parallel, teardrop) with distance-based leg construction, since the AIM's treatment of this intersection is admittedly thin on specifics, leaving room for varied interpretations among instructors, examiners, and check airmen.
On the second part of the question—whether to lead the turn before reaching the specified DME distance to avoid overshooting the protected airspace—the answer is nuanced and depends on groundspeed, wind, and aircraft turn performance, but the general principle is yes: pilots should anticipate the turn based on rate of closure to the fix, just as they would lead a turn to intercept a radial or course. TERPS obstacle clearance and holding pattern protected areas are built with the assumption of a normal-rate turn initiated in the vicinity of the defining distance, not after blowing through it. In practice, many operators and training providers teach starting the turn a small buffer prior to the exact DME distance (often cited informally as a half-mile to one mile early, depending on groundspeed) specifically to avoid extending beyond the outer boundary of the hold, which could put the aircraft outside protected airspace, especially relevant in mountainous or obstacle-rich environments where these fixes are often published.
For working pilots, this matters because DME/RNAV holds remain prevalent in oceanic, remote, and non-radar environments, as well as in numerous domestic procedures where conventional VOR infrastructure has been retained or where GPS distance substitutes for DME. Business aviation and cargo operators flying into less-serviced airports, along with airline crews flying into secondary fields with sparse radar coverage, still encounter DME-defined holds in charted missed approach procedures and enroute structure. A misunderstanding of leg construction can lead to holding pattern excursions, ATC deviations, or in worse cases, loss of obstacle clearance margin in non-radar environments where ATC cannot immediately correct a wide swing.
This also underscores a broader trend in instrument training: as GPS/RNAV holding becomes standardized and increasingly automated via FMS-computed hold entries, some foundational manual procedures—DME distance holds, non-standard entries, and wind-corrected leg timing—are seeing less rigorous treatment in ground and flight training programs. Pilots transitioning between glass-cockpit RNAV systems that compute hold geometry automatically and older or backup systems requiring manual DME leg management should be especially deliberate about maintaining proficiency in both. The AIM's admitted vagueness here is a reminder that pilots should cross-reference the Instrument Procedures Handbook, company SOPs, and, where available, direct guidance from check airmen or DPEs familiar with the applicable ACS standards, since relying on any single source—including forum consensus—can produce inconsistent technique across an operation or training program.