By Mark R. Twombly
Most turbine-powered airplanes are designed to cruise in the flight levels—18,000 feet msl and above—where the air is thin, true airspeeds are high, fuel flows are relatively low, and the weather is down below. To fly high comfortably requires that the cockpit and passenger cabin be pressurized, and the key to effective pressurization is a “tight” pressure vessel, meaning that there are few places where higher-pressure air inside the cabin can leak out into the lower-pressure atmosphere. One area that demands special treatment because of the high potential for leaks is the cabin door.
The basic method for sealing the cabin door is to use engine bleed air to inflate a flexible tube affixed to the outside perimeter of the door. Bleed air is tapped from the compressor section of the engine and routed through lines and valves to the door seal, which inflates and seats against the fuselage door frame to completely fill the gap between the door and the door frame. This keeps pressurized air inside the cabin from escaping around the door. Depending on the aircraft, the door-seal system may also incorporate a pressure regulator to reduce raw bleed air pressure to a lower pressure appropriate for the door seal.
Closing the door handle extends a series of locking pins or bayonets (or latches, depending on the airplane and size of the door) from the door into the doorframe. The number of pins varies by aircraft, and can be as many as 12 or more. When a crewmember closes the cabin door—and it should only be a crewmember—he checks a series of mechanical flags or indicators positioned at various points on the inside of the door to confirm that the pins are fully extended and the door is locked in place. Once the pins are locked in place, the door becomes an integral part of the pressure vessel. Some aircraft also have an electromechanical lock that must be activated before the door will seal. Closing the door handle and extending the locking pins opens a valve that allows pressurized, regulated air to flow into the door seal. The system may also incorporate a check valve that prevents the seal from suddenly deflating if bleed air pressure is lost.
Problems with the door seal usually show up in the cockpit annunciator system. An illuminated door-lock light means just that—the door is not locked. If the door is locked and the door seal inflates but air manages to leak past the seal as the airplane climbs to altitude, a cabin altitude warning light may illuminate on the annunciator panel. This will force a descent to a lower altitude where cabin pressurization is not critical.
A leaking door seal usually can be heard. One possible culprit is ice. Moisture can collect in the door seal pressurization line and freeze at altitude, compromising the seal. The procedure for handling a leaking door seal is the same as for any non-critical emergency: Go to the checklist.
The transition from piston aircraft pilot to commander of a turboprop or even a pure jet often demands a variety of new skills based on concepts first learned during private pilot training. Consider the efforts to sort out the differences between a glass cockpit and one with steam gauges. A little old-fashioned book learning combined with a few hours of computer and in-flight practice pushing buttons and entering data and an aviator is well on his or her way. No Herculean schooling needed; the tasks are just more complex.
Another element of flying larger airplanes that harkens back to those private pilot days is a healthy respect for weight and balance calculations. In a Cessna 150, or a business jet, the location of the aircraft’s center of gravity (CG) has a significant effect on the performance of the airplane.
When the rules of science are abused, the results can be disastrous, as the crew of a Challenger business jet learned when their aircraft crashed at Teterboro Airport in early 2005. Because of improper loading of people, fuel, and baggage, the Challenger’s CG was far outside the forward limits of the envelope. At takeoff speed, physics took over and the aircraft refused to leave the ground, sailing off the end of the runway, across a road, and into the side of an adjacent building. Four people on board suffered serious injuries.
Early commuter airliners were also famous for challenging first officers to quickly run weight-and-balance numbers in their heads to decide whether passengers needed to be moved around the cabin before takeoff. And the calculation would be completely different on the next leg. Professional pilots all understand that an airplane loaded outside of the standard performance envelope can quickly transform them into test pilots since manufacturers don’t guarantee aircraft performance when weight-and-balance calculations are mismanaged. Changes to the aircraft’s CG based upon fuel burn during a long flight must also be considered.
Figuring out whether a Cessna 150 is safely snuggled inside the performance envelope before takeoff requires simple math. Weight multiplied by arm (distance from the center of gravity) equals moment. Total moments divided by total weight equals the center of gravity.
If the weight is less than maximum takeoff weight and the CG is located within the safe portion of the envelope, the airplane should perform normally. In a turbine aircraft, the calculation is precisely the same, although there are a few more variables that can affect the outcome. Many of the current generation of turbine aircraft are equipped with flight management systems that simplify the weight-and-balance calculations.
If a pilot understands the concept of properly balancing the airplane before flight, and the need to restrict weight when necessary, the only tough part is learning a few new terms and precisely where they fit in this old math problem.
On small aircraft, the greatest weight an aircraft can carry is expressed as maximum gross weight. On a larger aircraft, the term is often called maximum gross takeoff weight (MTOW). The reason for adding the term takeoff is that some aircraft are certified to leave the gate with more fuel than they are allowed to use at throttles-up. This figure—maximum ramp weight—accounts for fuel burn on the ground during long taxi routes. The aircraft must never be allowed to exceed MTOW, however, at the time takeoff begins.
At the other end of the spectrum is the standard empty weight. This translates into the actual weight of the aircraft plus the standard items normally aboard when the aircraft is delivered. Standard items include unusable fuel, engine oil, toilet fluid, all emergency equipment, and all brake and hydraulic fluid. Next up is basic operating weight (BOW), the total of the aircraft’s standard empty weight plus standard items and any installed options such as galley or video equipment, or even extra fuel tanks. It’s the BOW that also includes mundane items such as the weight of the pilots and the bags of charts needed to fly IFR.
The next calculation is maximum zero fuel weight (ZFW) or the maximum amount of people and bags that can be carried, period. Any weight beyond this maximum can only be fuel. This structural restriction exists because fuel in the wings does not contribute to the twisting motion of the wing—especially at the tips—as much as does fuel in the fuselage. No math games are allowed on this one, so the pilot may never exceed the maximum ZFW by taking on less fuel, for example. To determine an aircraft’s maximum allowed payload for crew, passengers, bags, and fuel, simply subtract the standard empty weight from the maximum ZFW. Zero fuel weight can vary, but it normally also includes a maximum figure in the aircraft paperwork.
Finally, a weight seldom found in airplanes weighing in at less than 12,500 pounds is maximum landing weight. Often the landing gear on an aircraft is strong enough to support the machine for taxi and takeoff, but not strong enough to support a heavily loaded aircraft during the structural shocks of a landing. This explains why a jet that encounters an emergency right after takeoff will normally fly around to burn off fuel before returning for landing. Older jets used to be capable of dumping fuel in an emergency, but few aircraft are equipped for that option any longer.
Robert P. Mark, an ATP-rated corporate pilot and newly minted Cirrus standardized instructor pilot, is based in Evanston, Illinois. He is the author of McGraw-Hill’s new Professional Pilot Career Guide and editor of the syndicated aviation blog Jetwhine.com.