Bleed air basics

Bleed air originates from ports in an engine’s compressor section. As you might imagine, this air is hot. Really hot, like around 500 degrees Fahrenheit. To cool it to a usable temperature, the system’s tubes route this hot air past ram air or fan-driven cooling ducts, then on to heat exchangers for further cooling. A pressure-regulating valve controls the amount of bleed air sent to the heat exchangers; pilots can control this flow as well, using cockpit controls for selecting cockpit, cabin—and in some airplanes, baggage compartment—temperature levels. This cooling method—which is really a glorified radiator—is called an air cycle machine, or ACM. There may be as many as three ACMs on larger airplanes.

Another cooling method is via vapor cycle technology, which uses liquid, R-134a Freon refrigerant that’s compressed and condensed, then passed through evaporator coils. It’s similar to systems used to air condition homes and cars. This is a supplemental subsystem used in case the airplane’s heat exchangers become overwhelmed.

Cockpit and cabin heating and cooling is just one use for bleed air. By far, its role in pressurizing the cabin is of paramount importance. In a typical setup, a constant flow of conditioned bleed air is sent to the cabin and is “trapped” and held there, allowing the cabin altitude to be lower than that of the airplane’s actual altitude once aloft.

This process begins after engine start, when the pilot ensures that the pressurization controller is properly set (usually Auto, Norm, or Both). When a flight plan is entered in the flight management system, the controller’s software automatically learns the field elevations of both the departure and destination airports. Now it’s ready to do its magic: maintain prescribed normal pressurization levels and ensure that cabin altitude limits aren’t exceeded. It does this by opening and closing a pair of spring-loaded outflow valves set in the aft cabin bulkhead. Pressure differential becoming too high? Then the outflow valves open slightly, creating a calibrated “leak” that relieves excess pressure. Getting too low? The outflow valves close, raising the cabin pressure.

Once takeoff power is applied, the flow of bleed air increases and the cabin begins to pressurize. On some airplanes, you can sense a pressure jump as the airplane lifts off and the outflow valves close. Recent-model turbine airplanes do away with this sometimes-annoying pressure “bump” by having a prepressurization mode, which automatically closes the outflow valves while still on the takeoff run. As power is applied and the thrust lever angle(s) go past 85 percent or so while still on the ground, the outflow valves slowly close and the pressurization controller brings the cabin pressure differential to around 200 feet below field elevation. Result: no bump. After the landing gear squat switches open after liftoff, the system goes into flight mode. In flight mode, solenoids open and close the outflow valves to hold cabin altitude during a climb and reduce it during a descent.

All this digital automation is quite a contrast with older, analog turbine airplanes, which require the pilot to manually select a cruise altitude and cabin rates of climb and descent. Today, automation rules. What could possibly go wrong?

A left- or right-side air duct could overheat, in which case you’ll see an overheat annunciation. What now? The emergency/abnormal checklist will probably tell you to select a cooler temperature. If that doesn’t work, then you’ll probably be told to check the cabin temp circuit breaker or select the bleed air source from the other engine. Still have, say, a left duct overheat message? Then select the right air source, pull the left side’s circuit breaker, and control cabin temperature using the left thrust lever.

Cabin overpressurizing? Reduce power. Still overpressurizing? Pull the pressurization control circuit breaker and descend. Smoke or fumes in the cockpit or cabin? Use the Dump switch to dump the cabin pressure, then descend to a breathable altitude.

Now let’s say you have a complete depressurization—and a red crew advisory message to go with it. The next steps should come easily; they’re all memory items. First, put on oxygen masks and select 100 percent oxygen. Set microphone select switches to the oxygen mask position. Then make an emergency descent.

Emergency descents are often featured as the last entry in checklists for bleed air overheats, leaks, overpresssures, valve failures, and control faults. Other types of descents or diversions are almost always called for when bleed air driven engine-inlet, wing, stabilizer, and tailfin anti- or deice protection malfunctions or fails. In these cases, the advice is usually to shut off the flow of bleed air and exit icing conditions. Emergency descents because of bleed air problems may be more likely in airplanes with a single source. Twins have redundant bleed air sources—one from each engine. (If you’re lucky enough to have an auxiliary power unit, you have three bleed air sources.) But with a single-engine turboprop or jet you’re limited to a sole-source bleed air system.

Bleed air systems are usually trouble-free, but all those misfortunes, and more, can happen to any pressurized airplane. Just remember that systems and procedures can vary greatly by airplane type, so the information set out in this article should be considered very general. To learn the gospel for the airplane you fly, scan your abnormal and emergency checklists, and consult the airplane flight manual.

Thomas A. Horne is the former editor of AOPA Pilot, Turbine Edition.