Postition Pressure Switches
Installation, Care, and Diagnostics
By Mike Weaver
Electromechanical switches — from the common toggle and push button switches to the more advanced position (limit) and pressure switches — are a mission-critical component that every aircraft relies upon to maintain its airworthiness. Toggle or button switches are used to activate or deactivate systems within an aircraft and are usually found in the relatively benign environment of the cockpit or cabin. Pressure and position switches are used to sense various operating conditions in order to provide feedback to the cockpit or an aircraft control system. Due to their locations, these switches must endure some of the toughest environmental conditions on the aircraft such as high and low temperature extremes, high vibration, and exposure to FOD, as well as dust, salt spray, and de-icing and hydraulic fluids.
Consider this typical switch application. Icing conditions can be a dangerous situation for aircraft. In this case, an electromechanical switch, or in some cases, several switches, work behind the scenes to ensure the flawless performance and initial activation of the aircraft’s de-icing system. Pressure switches sense the pressure drops caused by the build-up of ice and send a signal to the pilot and on-board computers to activate the anti-ice system, thus reducing the danger posed by the ice build-up. Additional uses of electromechanical pressure and position switches include, but are not limited to; position indication of slats and flaps, landing gear, bleed air valves, and thrust-reverser systems.They are also used to indicate fuel pressure and oil level.
For purposes of this article, only the operation of pressure and position switches will be reviewed. However, other types, like temperature and level switches, also exist. These switch types have similar internal switching mechanisms but differ in the way they are externally actuated.
There are distinct differences between switches that react to pressure and those designed to give position feedback. Pressure switches are designed to react to an external pressure applied to an actuator surface (like a plunger, snap-disc, bellows, or diaphragm) by a medium such as air, oil, water, hydraulic fluid, fuel, oxygen or other gas. Pressure switches are physically connected to the operating medium by ports. Position switches (also known as limit switches) utilize an external actuation mechanism, such as an arm or plunger, to activate the internal switch. The switch arm or plunger in the application is activated by a cam, plate, roller, or plunger located on the aircraft or aircraft subsystem.
Snap acting switches
In both the pressure switch and position switch, the external activation mechanism then interfaces with some type of internal switch mechanism. The internal action of the switch can differ depending upon the needs of the application and the switch manu-
facturer. Most precision switches used for aircraft applications utilize a self-energized, snap acting blade to open and close circuits. When the actuator reaches a pre-determined operating point, the common contact accelerates away from the normally closed contact and comes to rest on the normally open contact. Because this all happens within two milliseconds, snap acting switches are well-suited to applications using inductive loads. The snap action ensures that the blade is always loaded against the opposing contacts. These loading forces are high enough to provide vibration resistance and minimize contact bounce. Since they activate immediately upon reaching the operating point, they are not sensitive to the rate of actuator movement. A snap acting switch works well for applications in which activation is needed after a slow building change reaches a certain point — such as a pressure drop in a filter bypass or anti-ice system.
A less complex direct acting switch can be used in applications where the rate of change in the medium is very rapid – as is the case with a solenoid valve plunger. Direct acting switches use blades or wires driven directly by the actuation system. Since they do not "snap," the actuator must move rapidly to ensure contact transfer within the required time.
Preventative care and maintenance
Generally speaking, electromechanical precision switches are robust mechanisms designed to operate effectively and efficiently, even in the harshest of environments. Provided they are manufactured under controlled conditions, they are more likely to succumb to old age than infant mortality. It is not uncommon for switches to operate flawlessly for hundreds of thousands of cycles. Problems can arise, however, if the specified electrical load, thermal, pressure, or vibration conditions are exceeded. When this happens, circuit resistance anomalies and calibration drift can cause hard or intermittent failures. While many switches for extreme aircraft applications are hermetically-sealed, contamination of critical exposed parts (such as a pressure port) can spell trouble. In addition, if the enclosure of a hermetic switch is damaged, the loss of the seal can lead to internal contamination. Internal contamination causes high circuit resistance, which can give a false open circuit indication. In some of the most modern applications flying today, current levels through precision switches can be as low as two milliamps, thus absolutely clean contact components, gold-plated contacts, and truly hermetic, back-filled enclosures are essential.
Very little preventive maintenance is needed to keep electromechanical switches operating at peak performance. However, any damage to a switch enclosure can hinder the switch’s performance, therefore it’s important to keep it out of harm’s way. Avoid dropping the component or using it as a ladder step or handhold, and protect all external harnesses that are part of the switch assembly. Always use the proper tools to remove and install a switch to avoid damaging the case. Always be certain that the mounting bolts are torqued correctly in order to reduce damaging vibration and to maintain proper calibration and/or actuation and release. Finally, if the switch is a pressure switch, make sure the port is kept clean and free of foreign material to prevent calibration errors and false readings. Also, remember that some pressure switches have components inside their pressure port. These components are designed to ensure the proper actuation and release over the life of the product and in some cases, act as a pressure snubber. These components should not be adjusted or removed unless the manufacturer’s Component Maintenance Manual (CMM) instructs otherwise.
Any system component can be damaged if the entire system is not operating as specified. If it appears that the system has experienced excessive heat, pressure, vibration, or voltage spikes, it is critical to check the installed switch or switches to rule out any damage that may have occurred to this component as well.
When troubleshooting an aircraft’s sub-systems, if a switch is identified as a contributor to a system failure, begin the repair procedure by consulting the CMM or Engine Manual for specific switch part numbers. Additionally, key parameters of the failed switch, required equipment, testing procedures and suggested repair procedures are provided in the CMM. Though most switch testing procedures are not complicated, it is important to adhere to the specified procedures and equipment suggested by the switch manufacturer. Occasionally, the manufacturer will call out special test equipment, such as electrical loads, transient protection, or calibration fixtures, to reduce the risk of damaging a switch under test.
Testing and troubleshooting procedures generally fall into two categories: electrical or mechanical. Electrical testing includes contact resistance and varistor voltage checks, and the testing of dielectric and insulation resistance. Mechanical testing involves the inspection and operation of the actuation mechanism to determine calibration status. Measurements of pre-load, pre-travel, overtravel, and actuation force are key to a proper inspection of the switch. Of course, there’s no substitute for a simple visual check for damage, whether external or internal after teardown.
While it’s important to follow the CMM instructions carefully, there are a few points that should be emphasized to ensure proper testing. First, make sure that the proper test loads are used. Loads in excess of the switch rating can easily cause contact damage. Similarly, overpressure can damage a pressure switch. It’s also important to pay attention to the rate at which voltage or pressure is applied. This is especially true with pressure input, as the port is physically limited and too much pressure too fast will often yield a false reading. Finally, make sure to follow the sequence and procedures provided in the CMM. Failure to do so can yield inaccurate results and cause damage to the switch.
Another option in the case of a switch failure or for preventative maintenance is to work on a switch in the field (if possible). These types of repairs are usually limited to repairing the external actuation/de-actuation mechanisms, enclosures/housings, wire leads, and other electrical connections. In general, the internal environmentally- or hermetically- sealed switch element is not usually repairable in the field but there are exceptions.
Before starting any work, there are a few things to keep in mind. First, and as mentioned before, follow the instructions provided in the CMM — it is the best insurance policy. CMMs are written by experts to ensure the proper operation of their product. So use the recommended equipment and follow the procedures described.
As for more general tips, first do no harm to the switch. Avoid damaging any connector threads during disassembly. When cleaning parts, make sure not to get solvent on parts such as O-rings that become damaged. Be sure to avoid kinking or pinching any connecting wires inside the cover. In the case of a pressure switch, don’t try to repair the internal threads or sealing surfaces on the pressure port. That is a sure way to get chips inside the switch and really cause problems. In repairing a switch, cleanliness is your top priority. Keep the switch free of contamination, such as solder flux and particulate matter, which can cause resistance problems or even cut off current flow. Be especially careful to avoid contamination with any silicone-based compounds. If silicone is present, the electrical energy used in the switch will change it to silica over time and cause a failure. Since silicone vapor can cause such contamination, no silicone-based materials should be present in the same room with an open switch. Also, external actuation mechanisms can be altered by contamination. For instance, the operating points of a "plunger" type position switch can be changed by foreign matter working its way into the cavity between the plunger and the switch shell/housing.
Problem? Call a Specialist!
In the event a system problem is confirmed to be caused by a switch, the best procedure would be to send the switch back to the manufacturer for analysis (if the switch is in warranty, displays a unique failure mode, or failed at very low cycles) and/or overhaul. Switch manufacturers have the knowledge, equipment, and personnel resources to evaluate and service a switch. Due to the myriad of switch designs in the field, it is almost impossible for every field maintenance repair facility to carry all the necessary equipment and possess all the know-how to service all the switches flying today. Since a manufacturer designed and built the switch, they can more easily pinpoint and deal with a problem. Another option might be to send the switch to a manufacturer’s authorized repair center. Also, keep in mind not all switches are reparable, either for economic reasons or due to their design. In this case, a failed switch should be properly disposed of and replaced with a new switch.
In the event a switch needs to be returned to the manufacturer, the more information provided to the manufacturer on the nature of the failure and the operating characteristics of the switch before failure, the better. Key points to include are the reason for removal, specific problems observed, hours in operation, and any operating conditions worth noting (high temp, over voltage, etc.). This information will cut down on the detective work, but more importantly, will give the manufacturer insight into performance trends and possible improvements.
Today’s aircraft use a wide array of switches in various applications. Although they have a variety of functions and constructions, they all share a common thread: with proper installation, regular inspection, and preventive maintenance, they will provide many hours of reliable service.