Supersmart squat switches
You just hopped into the cockpit of your idling brand-new bizjet, and the sleeve of your shirt snagged the Fuel Dump switch and flipped it to the Dump position. Yow! In a few seconds, you think, several hundred gallons of Jet-A will be fire-hosed onto the ramp. You'll have to shut down, of course. Then the linemen will run to the fire extinguishers, the fire department will be on its way, and so will the nearest branch of the Environmental Protection Agency. Television crews and police will come. Your career as a jet-man is seriously in question.
But a little-known system has intervened on your behalf, sparing you disgrace. It's the weight-on-wheels (WOW, for short) system—a sort of goof-proofing nerve network that lives in near anonymity. It's activated whenever the weight of the airplane is on the wheels, hence the name. Most general aviation pilots are already familiar with one type of single-mission weight-on-wheels device: the landing gear squat switch on retractable-gear airplanes. When the airplane's weight compresses the squat switch, the gear retraction sequence is designed to be disabled—even if you make the mistake of moving the landing gear control switch to the Up position. So if you're on the ground and inadvertently raise the gear lever, the gear should stay in the Down position. I say should because there can be problems with squat switches, and an airplane's landing gear geometry has a lot to say about its tendency to fold up under stress. If you're rolling down a bumpy runway, the switch may open, allowing the gear to come up anyway. Same thing if there are large side loads on some retractables. Anyway, you get the idea: Landing gear squat switches aren't infallible—but they mean well, and they've saved many a pilot from humiliation.
In turbine-powered airplanes, WOWs take the squat switch concept much, much further. Instead of a single switch, there can be load sensors on each main gear assembly, plus logic cards that send the appropriate weight-bearing information to computers, which in turn command the appropriate systems. As our earlier example demonstrated, if there's weight on the wheels, fuel dumping (if the airplane is equipped with a fuel dumping system) can be disabled. Depending on the airplane type, WOW disables other functions and components. These can include heat to total air temperature probes, power to yaw dampers, and rerouting bleed air to prevent the cabin from pressurizing on the ground. Other functions of a WOW (sometimes called an air/ground/logic system) turn off the ship's TCAS (to prevent false alarms to arriving airplanes), dial down the windshield or other anti-ice heating levels (to prevent overheating), and turn off any engine synchronization features so that asymmetric power for taxiing won't tax this equipment.
WOW systems also can save your bacon prior to, and during, takeoff. For example, weight on wheels helps to prevent antiskid from operating at taxi speeds below 12 mph or so, ensuring smooth braking action at slow taxi speeds. Takeoff warnings can also be generated on annunciator panels or EICAS (engine information and crew alerting systems) displays when there's weight on the wheels and the airplane isn't configured properly for takeoff—if the flaps and/or slats are not in the takeoff position, say, and you advance the thrust levers past a certain value. Until reaching a certain climbout altitude, WOW systems also help to suppress distracting ground-proximity warnings.
In the air, WOW-air/ground/logic systems help in a major way by defeating thrust reverser activation. Once the wheels sense that the airplane has taken off, there's no way a pilot can inadvertently open a thrust reverser—a disastrous condition that could cause an immediate loss of control.
As soon as some turbine-powered airplanes touch down on their main gear, WOW systems can lessen pilot workload by automatically deploying ground spoilers—large flat panels at the inboard portions of the wing upper surfaces—which help to improve braking effectiveness.
WOW systems also can act as counters to help maintenance technicians keep track of airplane cycles and diagnose the cause of an in-flight squawk on a specific route of flight.
You won't find much written about WOW-air/ground/logic systems in pilot training or operating handbooks. They're the silent helpers that pull our fat out of the fire when we make stupid or inadvertent mistakes. Just don't put all your trust in a WOW system. They can, and do, fail in many strange ways, leaving us an opening for error. That's why, if pilots don't really deal with these systems, mechanics certainly do. Tell a mechanic about a sudden fluky squawk—say, an inability to synch up the fans—and the first chapter he'll turn to in the maintenance manual will be tabbed WOW. If a thrust reverser pops open in flight, the weight-on-wheels system will certainly be a candidate for major exploratory surgery.
Additional information on the weight-on-wheels system can be found on AOPA Online ( www.aopa.org/pilot/links/links0008.shtml). E-mail the author at [email protected].
