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THE TURBINE PILOT’S FLIGHT MANUAL
While many pilots are leery of flying aircraft without mechanical control linkages, fly-by-wire systems have been used by the military for years, and the benefits are such that pilot acceptance continues to grow. It’s important that all of us pilots get used to the idea of alternate control systems, since manufacturer interest in them is very high right now. Another recent development, for example, is fly-bylight technology, where pilots are connected to their precious control surfaces via optical interfaces.
Pressurization Pressurization is one of those aircraft systems that is very simple in concept but surprisingly complicated in execution. The principle, of course, is to seal up the airplane’s cabin into a pressure vessel. Air is then pumped in to maintain internal pressure as close as possible to that at sea level. The complexity, however, begins with the aircraft fuselage as a pressure container. The pressure vessel does not occupy the entire fuselage but rather uses pressure bulkheads, plus the outer skin, to contain the passenger cabin and some or all cargo
areas. Control cables, wiring, and plumbing must pass through the pressure vessel, with further perforation by exits, windows, and emergency exits. To make matters worse, the aircraft fuselage changes dimensionally with every pressurization cycle. Obviously, sophisticated engineering and maintenance is required for such an aircraft. (See Figure 5.8.) In turbine aircraft, a steady supply of engine bleed air is used to pressurize the cabin. Cabin pressure is then controlled by modulating the exhaust of cabin air via outflow valves. Outflow valves are manipulated via a pressurization controller operated by the pilot. The main measure of a pressurization system’s efficiency is known as its maximum differential (or max diff). This is simply the maximum ratio of cabin pressure to outside air pressure that the pressurization system and vessel can sustain. Max diff varies significantly by aircraft type. This is due to many factors, including pressure vessel design, engine bleed air capacity, and aircraft weight and power considerations. For many pressurized aircraft, certified maximum operating altitude is determined not by the airplane’s service ceiling but by the ability of the pressurization to meet supplemental oxygen
high-pressure bleed air from engine compressor section aircraft altitude: 24,000 ft.
environmental system cools and conditions air
cabin altitude: 3500 ft.
Air is pumped into an aircraft’s “pressure vessel” in order to reproduce the atmospheric pressures found at lower altitudes. Note that the pressure vessel does not envelop the entire fuselage. Some baggage compartments, for example, are usually located outside of the pressure vessel. Don’t put any animals or temperature-sensitive materials there!
FIGURE 5.8 | Cabin pressurization.
