Business & GA, Commercial, Military


By Capt. Terry Hanson | November 1, 2000
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People flick them, push, punch, twist and pull them. But few give much thought to them until they don’t work. Then we are more likely to assume that the light, gauge, motor or pump is busted. It can’t be the switch. Switches are so reliable that we don’t give them much thought.

The design engineer, the pilot, the human factors specialists–they all have different requirements for the simple switch. But first and foremost, we all want it to work.

I fly commercial heavy transport aircraft. I also fly light single engine and twin prop planes for recreation, and I have flown military aircraft of varying capabilities. A common element in all aircraft is the use of switches. An aircraft switch must be reliable, easy to use, durable, and often unique.

Reliability is easily measured. Maintenance tracks everything done to an aircraft. It is no problem to mine the data to determine the mean time between failure (MTBF) for any aircraft component. Most switches have MTBF’s measured in decades. When they break, though, the entire flight operation shuts down.

The pressurization system "landing altitude set" knob on the Boeing 757 is a sturdily made rotary switch that is exercised on every leg a commercial airliner flies. I had never seen one break. Yet, while attempting to set the field elevation of my destination recently, the knob spun freely in my hand. I examined it closely. It looked like an allen set screw may have backed off. Maintenance told a different story–internal separation. We were grounded until the pressurization controller could be replaced. A single rotary switch, not a half-inch long, used all of the time with never a hitch, stopped me from flying my multimillion dollar aircraft.

Texture, Shape, Position

Switches come in all sizes and shapes. Ever wonder why? For example, the switches that operate the "flight guidance system" must be uniquely different from all others. The idea here is that each autoflight function must be controlled by a uniquely textured, shaped and positioned switch.

The B767 is a case in point. Pilots are taught how to stabilize the aircraft even if the cockpit is full of smoke or condensation fog from a rapid decompression. The objective is to safely descend the aircraft to a more breathable altitude even if the pilot instruments are obscured. Here switch design comes to the rescue.

Spin, Push, Spin, Pull

First, the pilot reaches up to the glareshield and touches the knurled knob for the mode control panel’s (MCP) altitude capture window. This knob is on the right side of the MCP. The pilot feels the knob; he knows it’s the correct one because he has been trained to recognize its position, texture and shape. Spin this switch to the left, counter clockwise. This produces a lower altitude in the altitude capture window.

Next, the pilot feels along the left side of the mode control panel until he feels the "aircraft speed" knob, which is between the two lowest square push switches on the panel. Feeling down to the right from the speed knob, the pilot touches the bottom most push switch on the right side of the knob. This is the "flight level change" button. Push this switch and the throttles move to Idle. The aircraft "speed window" opens.

Reach back to the speed knob. Spin it to the right, clockwise. This action produces an increased speed in the indicated airspeed (IAS)/ Mach display window. Now the aircraft will depart its previous altitude for the lower one set in the altitude capture window, doing so with the power at idle and using the lowered nose to achieve the higher airspeed set in the IAS/Mach window.

Finally, the pilot reaches to the forward left side of the "center pedestal." This contains the throttle quadrant. He feels for the uniquely shaped and uniquely positioned speedbrake handle. This lever can extend flight spoilers when deployed while airborne. Pull the speedbrake lever aft. This increases the rate of descent.

You are now in a "blind" emergency descent. The crew can fight the problem, clear the air, while the automation and "switchology" helps them.

Expensive Switches

First-generation jet transports like the Boeing 707, DC-8, and KC-135 all contained "lever-lock" switches for the important stuff. You could bump the switch getting in or out of a pilot seat and be reasonably certain that it would remain in the set position. A case-in-point are fuel lever switches.

On many aircraft, fuel lever switches are pulled out, moved forward and spring loaded to a seated "on" position. The design is sound. Still, a proficient aviator makes certain that the fuel lever switches are properly seated. We don’t want the levers to slip out of the "run" position during takeoff roll.

Lever lock switches are inexpensive compared to the switches found in "glass" cockpits. These "switchlight" designs are easy to break if an inexperienced person tries to replace a burned-out internal bulb. Pilots often are taught to never try to replace a bulb in a "switchlight." Airlines feel that fewer "swichlights" are broken if mechanics do the job. A single switchlight cost upwards of $450 dollars on the shelf.

Some switches are beautiful. There is no other way to describe them. Our 737-800 aircraft have a unique switch on their MCP’s that clearly reveals that its designer is an artist. The switch has a finned, flared base that curves into a fluted top. There is no doubt that this switch is different from all the others on the MCP. It can be easily recognized by touch even in a severely obscured environment. It reminds me of a finely carved piece of ivory. This switch is a design example of "form and function" conspiring together to produce fine art.

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