The question of how to correctly pronounce "IMC" during radio transmissions reflects a broader and ongoing tension between formal ICAO radiotelephony standards and the informal conventions that develop organically across real-world aviation operations. ICAO Doc 9432, the Manual of Radiotelephony, establishes that abbreviations which cannot be pronounced as natural words should be transmitted by spelling each letter individually using the ICAO phonetic alphabet — meaning the technically correct formal pronunciation of IMC in a radio call is "India Mike Charlie." However, in practical day-to-day operations across high-density airspace, the compressed spoken form "eye-em-see" has become so universally understood by controllers and pilots alike that it functions as a de facto standard, particularly in the United States under FAA jurisdiction.
The distinction carries more operational weight than it might initially appear. Radiotelephony standardization exists primarily to eliminate ambiguity and reduce the potential for miscommunication in environments where accent, frequency congestion, and background noise already introduce significant degradation of intelligibility. When pilots blend informal pronunciation habits across international operations — transitioning, for example, from domestic Part 91 flying to transatlantic or oceanic operations governed more strictly by ICAO procedures — inconsistency in phonetic adherence can create momentary confusion, particularly with non-native English-speaking controllers who rely more heavily on the standardized phonetic framework Doc 9432 prescribes. For Part 135 and business aviation operators conducting international trips, the phonetic alphabet rendering "India Mike Charlie" is the safer and more universally defensible choice.
It is also worth noting that IMC as a term in radio transmissions appears most frequently in pilot reports (PIREPs), position reports, and requests for instrument approaches or deviations, contexts in which precise mutual understanding between crew and ATC is essential. The FAA's Aeronautical Information Manual, while not as prescriptive as Doc 9432 on this specific point, generally encourages plain-language efficiency where clarity is preserved — which is why "eye-em-see" has survived in American airspace without meaningful pushback from controllers or examiners. Instrument check rides and IPC scenarios rarely penalize the abbreviated spoken form, reinforcing its persistence in the training pipeline.
The broader trend this question surfaces is the continued divergence between ICAO-codified standards and national operational culture, a gap that becomes most visible during international flying, type rating training conducted under EASA or Transport Canada frameworks, or operations into airspace with stricter phraseology enforcement. Corporate flight departments operating Part 91K or scheduling international charter operations under Part 135 would be well-served to train crews toward the Doc 9432-compliant phonetic rendering as a baseline, then permit abbreviation to common usage where appropriate. The cost of over-standardizing toward "India Mike Charlie" is negligible; the cost of a read-back error in IMC at a congested international airport is not.