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Mode C Veil
The airspace within 30 nautical miles of the Class B primary airports listed in 14 CFR 91 Appendix D, from the surface up to 10,000 feet MSL, where an operable Mode C (altitude-reporting) transponder and ADS-B Out are generally required. The Mode C veil appears on sectional charts and affects equipment planning even for flights that never enter Class B.
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