during declared emergencies such as VFR traffic attempting to avoid severe weather formations. In the United States, class A airspace is measured using flight levels, and begins at FL180 up to FL600, which is about 18,000 to 60,000 feet as measured using an altimeter at standard pressure (29.92 inHg, 1013 mbar). An exception to IFR-only flying in this airspace is the occasional allowance of sailplanes within designated wave windows which may be opened by air traffic control when high altitude flights are to be conducted into mountain lee waves. Traffic advisories In the United States, Canada, and Australia, a pilot operating under VFR outside Class B, C, D airspace can request "flight following" from ATC, to receive continuous verbal updates on air traffic. This service is provided by ATC if workload permits, but it is an advisory service only. The responsibility for maintaining separation with other aircraft and proper navigation still remains with the pilot in command (PIC). In the United Kingdom, this is known as a "Traffic Service". In other countries it is known as "Flight Information Service". Pilot certifications In the United States and Canada, any certified pilot who meets specific recency of experience criteria may operate an airworthy aircraft under VFR. VFR cruising altitude rules in the US and Canada In the US, there are specific VFR cruising altitudes, based on the aircraft's course, to assist pilots in separating their aircraft while operating under visual flight above 3,000 ft above the surface (AGL) but below 18,000 ft Mean Sea Level (MSL). Unofficially, most pilots use these rules at all levels of cruise flight. FAR 91.159 states that any aircraft: On a magnetic course of 0-179 degrees shall fly at an odd thousand ft MSL altitude +500 feet (e.g., 3,500, 5,500, or 7,500 ft); or On a magnetic course of 180-359 degrees shall fly at an even thousand ft MSL altitude +500 feet (e.g., 4,500, 6,500, or 8,500 ft). Low flying rules in the US In the US, Part 91 (specifically 91.119) of the Federal Aviation Regulations controls the minimum safe altitudes by which aircraft can be operated in the National Airspace System. "Notwithstanding" rule Though specific altitudes are called as noted below - there is an overreaching general requirement to maintain sufficient altitude that if a power unit fails, an emergency landing without undue hazard to persons or property on the surface can be made. This may