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Chapter 15 | Operational Information
IFR Operations in Turbine Aircraft In most respects, operations under IFR (Instrument Flight Rules) are similar for all civil aircraft. However, there are a few IFR procedures specific to flight at high altitudes (the realm of most turbine aircraft) and to turbine aircraft themselves. Note that in some cases, regulations and procedures specify turbine-powered aircraft; in other cases, turboprops are differentiated from turbojet aircraft. For purposes of IFR operating regulations, the term “turbojet” includes both turbojet and turbofan-powered aircraft and means any “pure jet” or nonpropellerdriven turbine aircraft.
Profile Descents A profile descent is simply a published transition procedure from cruise f light to an instrument approach. Courses, headings, and altitudes are prescribed, usually from a high-altitude cruise structure intersection or navaid down to interception of an ILS (instrument landing system) or nonprecision approach. Profile descents may begin as far out as 100 miles or more from the destination airport, depending on the aircraft’s cruising altitude. A profile descent normally allows a continuous descent from cruise, interrupted only by a brief level-off to slow to 250 kias (knots indicated airspeed) by 10,000 feet MSL (mean sea level).
Jet Routes Turbine aircraft routinely travel at high altitudes above 18,000 feet. Altitudes at and above 18,000 feet are known as flight levels. Twenty-one thousand feet, for example, is known as “FL 210” and pronounced as “flight level two-one-zero.” At these altitudes, aircraft no longer travel on victor airways but instead use jet routes, or J-routes. The jet route system is a VOR-based route structure, similar to the victor airway system. J-routes are established from FL 180 up to and including FL 450. On high-altitude enroute charts jet routes are depicted in black on NOS charts and blue on Jeppesen charts. They are identified on the charts by the letter “J” and the J-route number. When examining a high-altitude chart, the first thing you’ll notice
is the length of airway legs between VOR stations. Most legs are over 100 NM in length, due to the high speeds at which turbine aircraft travel. If J-routes had short legs like victor airways, flight crews would be continually changing frequencies and adjusting the course knob. Because of the high altitudes involved with J-routes, VOR reception at long ranges is possible. Above FL 450, aircraft typically navigate directly from point to point, often with some type of RNAV system. (See Chapter 13, Navigation, Communication, and Electronic Flight Control Systems.) ATC approval and radar monitoring are required on these types of routings.
Altimetry and IFR Cruising Altitudes at Flight Levels A few more words are in order regarding operations up in the flight levels. All operations at or above 18,000 feet must be under IFR flight rules, and all aircraft must set their altimeters to 29.92 inches Hg. One reason for the standard altimeter setting is that high-altitude weather, at any given location, doesn’t necessarily correspond to the surface weather below where altimeter settings are established; it would be dangerous to project surface altimeter settings all the way up to very high altitudes. Another reason is that it would be impractical to require flight crews to reset altimeters to local reporting stations within 100 NM of the aircraft, as is required below 18,000 feet. At jet speeds, that would require an altimeter change roughly every ten minutes. Requiring a standard altimeter setting above 18,000 feet does cause a few complications, however. If the surface altimeter setting is very low, aircraft f lying up at 17,000 or 17,500 feet could start bumping into those flying at FL 180 with 29.92 altimeters. Therefore, the FAA has defined limitations as to what minimum flight levels may be used under given situations. For example, FL 180 may not be used when the local altimeter setting falls below 29.92. As the altimeter continues to fall, more flight levels are restricted. (See 14 CFR 91.121 for a full breakdown of minimum flight levels.) IFR cruising altitudes between FL 180 and FL 410 are set up similarly to IFR cruising altitudes below FL 180. Above FL 180, we continue the use of odd altitude assignments for eastbound magnetic
