Just because it may be common practice does not make it legal. Part 95.1303(b) is pretty clear about MURS use aboard aircraft in flight. It is illegal and punishable to the full extent of the law if caught. FCC fines can be quite hefty like upwards of $10,000 to $25,000 depending on the charge(s). If you want to mock the law go ahead. Sooner or later you will probably get caught. I've seen such cases first-hand...
§ 95.1303 Authorized locations.
(a) MURS operation is authorized:
(1) Anywhere CB station operation is permitted under § 95.405; and
(2) Aboard any vessel of the United States, with the permission of the captain, while the vessel is travelling either domestically or in international waters.
(b) MURS operation is not authorized aboard aircraft in flight.
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73, Dave K4EET