A significant rite of passage for a pilot arrives when he or she logs those first hours of retractable-gear time. With a checkout in a disappearing-wheel airplane, however, comes the responsibility to clearly understand how that gear operates in case something mechanical coughs along the way.
One of the primary tools needed to ensure the proper landing-gear operation on a light aircraft is the logic switch on the gear—often called a squat switch. In a turbine aircraft, the squat switch, called here a proximity device or a weight on wheels (WOW) switch, is often tied in to a variety of complex systems such as brakes, aircraft pressurization, and spoilers. Dassault Falcon’s Woody Saland, manager of technical programs at the company’s Teterboro, New Jersey, facility, says the weight on wheels switch is designed to tell the aircraft’s systems the status of the aircraft at any given time. “Literally, is the aircraft on the ground or in the air?” Saland says. “The WOW switch and the system choices it helps make keep the pilots aware of what they need to know for the particular phase of flight.”
In a light aircraft, failure of the weight on wheels switch could translate into the pilot being unable to retract the gear on takeoff because, for some reason, the switch still thinks the aircraft is on the ground. This is what prevents someone who might be climbing around the cockpit when the aircraft is powered up from inadvertently retracting the gear on the ground. In a turbine airplane the WOW switch keeps the pilot from pressurizing the cabin on the ground because the computer realizes that could be a dangerous option. Of course, if the switch is faulty, the aircraft is actually airborne, and the cabin cannot be pressurized, then the flight must return to the shop for repairs.
In a light aircraft, there might be a single WOW switch, while an aircraft as sophisticated as Dassault’s Falcon 7X has three to provide redundancy. Computer systems on board aircraft like the 7X are so complex—there are seven computers for the flight control system alone—that, thanks to the logic information from the weight on wheels switches, it knows what kinds of messages are important to the pilot at various times during the flight and which are not. The key is to present only the correct ones when the pilot is in a position to do something about them. For instance, an oil filter that is nearing the end of its life will trigger a message on the engine crew alerting system (ECAS) if the aircraft is on the ground because the computer realizes the pilots can do nothing about the situation while cruising along at FL 430. So why bother the pilots?
Other examples of sophisticated actions in a turbine aircraft from the rather unsophisticated weight on wheels switch include lighting up the pitot and windshield heaters, notorious for frying on the ground if left running too long. Thanks to the weight on wheels switch, the Falcon’s computers are smart enough to turn these systems on just before takeoff, but only if another piece of logic falls into place. The aircraft must have weight on wheels and a throttle position set for takeoff power, as the airplane is about to commence its ground roll.
Suppose the jet pilots have just touched down and must for some reason reject the landing. On a normal touchdown, the WOW switch told the aircraft to deploy ground spoilers to slow the machine. On the go, however, extended spoilers could be fatal. The weight on wheels switch senses takeoff power and tells the computers it is OK to retract the spoilers so the aircraft may again take off. The WOW switch logic also controls thrust reversers. No pilot wants those extended while still airborne. The WOW logic only allows deployment when the wheels are firmly planted on terra firma.
What’s one of the coolest features controlled by the weight on wheels switch aboard Dassault’s 7X, according to Saland? “When a system fails, the WOW switch prioritizes system computer messages. It cues up the correct checklist for the pilot without any action on the part of the pilot.” The days of “find me the emergency checklist” are over.
Robert P. Mark is a 6,300-hour ATP who was named the Airbus Aerospace Journalist of the Year in 2004.
You know that office-supply commercial in which an office worker turns a stressful situation into a no-brainer simply by pressing a big red button marked with an “Easy” label? That same red button could apply to a single-point pressure fueling system, which turns a sometimes-difficult chore into a no-brainer—almost.
Refueling is a critical function on a turbine-powered airplane because of the large overall capacity of the tanks compared to piston-engine aircraft. That generous capacity raises issues of accuracy in terms of how much fuel is uploaded, balancing the fuel load between the various tanks, and the time it takes to pump all of that fuel. Single-point fueling addresses all of those concerns.
On most turbines, topping off all the tanks can severely limit payload—the number of passengers and weight of cargo that can be loaded without exceeding maximum takeoff weight and center-of-gravity limits. Thus, before each leg of a flight the pilot in command (PIC) takes a look at expected payload and the fuel that remains in the tanks, then computes the additional fuel needed to complete the next leg, plus reserves, and instructs the FBO to add precisely that amount, rather than ordering the tanks topped.
It’s much easier for the fueler to ensure accuracy in the amount of fuel uploaded, and how that fuel is distributed among the tanks, if the line technician can use a single filler port rather than move around to individual wing tanks. The single-point fuel port typically is located low on the aft fuselage, where it is easy to reach. The fuel is delivered under pressure, about six psi minimum per tank. The convenient one-stop fuel-port location and pressure delivery makes it a much faster fueling process than over-the-wing fueling—an important consideration if the crew has asked for a quick turnaround or there are several aircraft waiting to fuel up.
Single-point fueling also reduces the potential for fuel imbalance problems. It is not unusual to hear of an incident in which a line technician conducting over-the-wing fueling attempts to fill one wing tank before moving to the other wing, only to have the aircraft tip toward the heavy wing. That makes for a bad day for everyone.
Another advantage of single-point fueling is that it makes it much easier to distribute fuel among primary and auxiliary tanks. For example, some jets have an auxiliary fuselage tank for long-range trips, and for ballast needed in certain loading configurations. Single-point fueling makes it possible to partially fill the fuselage tank, top it off completely, or leave it empty and just add fuel to the wing tanks.
It’s important that a crewmember monitor the single-point fueling process to ensure that the line technician completes the prefueling testing procedure, which verifies that the fuel hose is connected properly to the filler port, and that the fuel truck’s (or fuel station’s) automatic cutoff system is functioning. It’s also important to verify that the correct tanks are being filled, and no others. For example, it is possible to inadvertently select one wing tank open to fueling and the other closed, or select the fuselage tank open when the intent is to leave it empty.
-Mark R. Twombly