Glossary of Terms

	Title	Definition
	MSA (Minimum Safe Altitude)	Altitude below which IFR aircraft may not descend unless specifically authorised to do so by ATC. Takes into account high terrain underlying an air route.
	MTBUR	MTBUR (Mean Time Between Unit Replacement, or Mean Time Between Unscheduled Removal)= (Number of flight hours x units installed per aircraft) /(Number of unscheduled removals during that period) MTBUR is not the same as MTBF. An LRU can undergo removal without actually having failed. So, great for measuring rates of removals or failures,but do they actually indicate reliability. Common assumption: MTBUR indicates the average (mean) time a component spends in service.So it is often used to set maintenance program intervals-If the MTBUR is 30,000 hours, the program is set for 28,000 hours Fact: MTBUR has nothing to do with the amount of time a component spends in service
	N1	RPM of Stage 1 fan of turbofan jet engine, expressed as a percentage of normal maximum fan speed.
	N2	RPM of gas turbines of turbofan jet engine, expressed as a percentage of normal maximum turbine speed.
	Nautical mile (nm)	Measure of distance used for navigation in the air and at sea. Equal to one minute of an arc of latitude on the earth's surface. Is 800 feet longer than a statute mile and equivalent to 1.853km.
	Nav	Navigation.
	Navaid	Radio navigation aid.
	ND	Navigation Display.
	NDB	Non directional beacon. Ground based medium frequency radio transmitter sending continuous signals in all directions for use by aircraft fitted with ADF (radio compass).
	NOTAM (Notice to Airmen)	Message concerning changes to serviceability of aerodromes, radio and navigation facilities.
	NTSB	National Transportation Safety Board.
	Objective Evidence	Information, which can be proved true, based on facts obtained through observation, measurement, test or other means.
	Octas ("eighths")	Expression of cloud amount. Eight octas (or eighths) represents a completely overcast sky; four octas a half clouded sky.
	Optimum Safety	The associated risks that have been identified have been accepted provided that all identified controls are implemented and enforced.
	PAAST	Pan American Aviation Safety Team

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