The discrepancy between ForeFlight's frequency notation and the FAA Chart Supplement (formerly AF/D) at Galveston's Scholes International Airport (KGLS) illustrates a persistent challenge in IFR operations at part-time towered facilities: EFB data annotations do not always mirror official source documents, and pilots must understand why those differences exist rather than simply trusting one source over the other. At KGLS, 135.35 is the clearance delivery frequency, and the pilot's instinct to use it is correct. ForeFlight has added a conditional qualifier — "when tower is closed" — that the Chart Supplement does not include, likely because ForeFlight's data team interpreted the operational context differently or pulled from a supplemental source that highlighted the frequency's role during after-hours operations.
At Class D airports with part-time towers, clearance delivery operations take on a different character depending on whether the ATCT is staffed. When the tower is open, CD may be handled on a discrete frequency or combined with ground; when the tower is closed, the airport typically reverts to Class E or Class G airspace, and IFR clearances are obtained via phone to the overlying TRACON or Center, or via remote clearance delivery frequencies assigned to that facility. The Chart Supplement's instruction to "call the number when ATCT is closed" points to this standard protocol — meaning the 135.35 frequency is intended for use during tower operating hours, not as a remote clearance delivery channel after hours. ForeFlight's annotation, while technically useful, introduces ambiguity by implying the frequency is exclusively a closed-tower resource rather than the standard CD frequency.
The comparison with KDWH (David Wayne Hooks Memorial Airport) is instructive. Hooks is a high-activity general aviation airport in the Houston area where CD and Ground are separated, and both ForeFlight and the Chart Supplement agree on the frequency assignments without qualification. That consistency is the norm at busier facilities with well-defined, stable frequency structures. KGLS, by contrast, is a smaller operation where frequency functions can overlap or shift depending on tower status, creating exactly the kind of ambiguity that produces conflicting notations across different data sources.
For working pilots — particularly those operating under Part 91, 91K, or 135 at smaller regional airports — this scenario underscores why the Chart Supplement remains the authoritative source for frequency and communication data, with EFB platforms serving as a convenience layer that may add editorial interpretation. When ForeFlight and the Chart Supplement diverge, the correct procedure is to resolve the conflict before flight, either by contacting the facility directly, checking NOTAMs, or calling the relevant TRACON. Assuming the EFB annotation is more accurate than the government source is an operational habit that can lead to errors, particularly at airports where frequency assignments change based on tower hours or where recent procedural changes have not yet propagated uniformly across all databases.
The broader takeaway for IFR operators is that data currency and source hierarchy matter in ways that good situational awareness must account for. EFB platforms aggregate data from multiple sources and apply their own formatting logic, which means annotations like "when tower is closed" may reflect an editorial decision rather than a charted procedure. Pilots flying into unfamiliar Class D airports — especially in high-density metro areas like Houston where multiple fields operate in proximity — should verify clearance delivery procedures during preflight planning rather than at the run-up pad, ensuring that frequency confusion does not become a workload issue at a moment when attention should be on departure coordination.