Working Principles of RPM Sensors in Aircraft

RPM SensorsFor this article, we’ve selected to explain the working principles of the RPM standard sensor for the pressurized Bendix series (manufactured by Teledyne Continental Motors):

The RPM sensor is connected to the magneto. To understand the sensor let us first begin with the “why” of things i.e. why is the magneto required in the first place:

A magneto is required to power the sparkplug. Every combustion engine (including those used in aircraft), have sparkplugs. These provide the ignition or fire required to ignite the fuel-air mixture in the combustion chamber. The sparkplug purpose and functioning are identical to the gas lighters that your mom used to have; but are now built into the gas range in your home.

To work properly, the sparkplugs in turn require an awful lot of electrical volts to create the spark. And this electrical power needs to be very precise and delivered at the right moment.

A battery would not do the job efficiently so a custom-built power generator called the “magneto” was invented.

As the output power from the magneto needed to be precise, it therefore needed to function with precision. A malfunctioning magneto will mean that everything downstream will also malfunction. So, the pilot(s) need to know the instant the magneto isn’t working as intended.

Enter the RPM sensor. To understand the working principle of the RPM sensor, it is important to know a bit about the magneto itself.

The Magneto is basically a dynamo with a rotating magnet, a built-in breaker switch, a transformer and a distribution system to channel the power to the spark plugs. For reasons mentioned above, it is important that the magnet rotates at a speed that is within the prescribed range otherwise your spark plugs (and hence the engine) will misfire. To ensure that is so, you need the aircraft RPM Sensor. The RPM standard Sensor plugs directly into the magneto and transmits data to your EDM/ RPM gauge in the cockpit.

The RPM Sensor you use will depend on the magneto brand and model inserted in your aircraft.

If you need to install a new RPM sensor, make sure:

• The new RPM sensor is precisely aligned with the correct vent plug of the magneto. Here’s how you do it: peep into the magneto hole and you should see the rotating magnet in this hole. If you see anything other than the rotating magnet e.g. the ‘gear’, then you’re on the wrong vent.

• When everything is connected and the engines fired up, the RPM Gauges in the cockpit should read 2400 rpm for a 6-cylinder engine or 1800 rpm for 8-cylinder or 1600 for a 9-cylinder engine.

If you have a 4-cylinder engine with a dual mag, the rpm should be 1800 and for a 6-cylinder engine with dual mag, it should be 2400.

• The red wire to the RPM standard sensors should be supplying 5v+.

More information on JPI manufactured RPM standard sensors here:


How Slim Line Gauges works in Aircraft?

Slim Line GaugesBack in the days of WWII, aircraft cockpits had few dials (gauges) and each was connected to any individual part of the aircraft engine. As aeroplane design improved so did the number of gauges in the aircraft cockpit.

Overtime, the aircraft cockpit got so crowded with gauges that it was impossible to keep track of all the information that was being displayed. The pilot would look at the gauges that were critical to the flight and just eye-ball the rest and hope for the best. These gauges did not have any alarm system so, more often than not the flight crew usually found out about a problem the hard way – when the engines began to ‘cough’ and sputter.

It is said that necessity is the mother of invention and clearly, when it came to the post WWII aircraft, there was an urgent necessity to improve the way information was displayed in the cockpit.

The one area that technology leap-frogged, was in electronics. The first super computer was invented and it could analyse millions of bits of information every second. So instead of continuing to use analog gauges, aircraft manufacturers and after-market component makers such as J.P. Instruments, began to see electronics and digitization as an ideal way to display engine data.

But before the humble analog gauge went digital, there was need to improve the way data itself was collected and transmitted. Enter the modern sensors.

Earlier sensors were thicker than your fingers and slow to react. Worse, some had mini moving parts (e.g. fuel sensor) inside them that would get stuck during flight. Around the time electronics leap-frogged, new space age technology gave birth to space-age materials that helped overhaul the aircraft engine sensors.

The new sensors became ultra slimline, responded to changes very fast and became more accurate. Simultaneously, the cables that transmitted the data too became ‘Slimline’ and interference-free.

Enter the new and modern slim line gauges (so called because they were less than two inches in height). The feed coming directly from the sensor was attached to each slim line gauge. So, you had a slim line gauge for OAT, Voltage, Oil Temperature, RPM, Manifold Pressure, Fuel Pressure, Oil Pressure, TIT and so forth.

How Slim Line Gauges work

The individual sensors pick up the data directly from the engine and transmit it as a voltage via wires to the individual slim line gauge for which it is meant. The electronic board and IC circuit onboard the slim line gauge interprets the incoming voltage, converts it into a meaningful number and displays it on the screen. The display was bright enough to be seen even with the sun directly behind the pilot.

A new amazing feature was added into the slim line gauge – thanks to its IC circuit, it could compare the incoming data with pre-entered data coded into the IC chip. Two or three LED bulbs were added to the gauge. So, for example, if the RPM Sensor count was normal, the green LED bulb would light up. If the RPM went up the yellow LED bulb would like up and if the count exceeded specified limits, the red LED bulb would light up. This was a huge relief for the pilot as he only needed to look at the gauge if any LED other than green lite up.

For options on modern slim line gauges, please visit:

Installation of RPM Sensor for Bendix Mag in Aircraft

RPM Sensor for Bendix MagJust so there is no confusion, the RPM sensors we refer to, are the ones used for measuring the RPM of the Bendix Magneto or the Slick Magnetos in aircraft. Some aircraft use Dual Magnetos but Bendix Magnetos and the Slick Magnetos are more widely used.

One of the main reasons the Bendix Magnetos are popular is because they are easily overhauled and for any aircraft; this is a winning feature as it saves time and money. Amongst the most prominent companies that manufacturer highly accurate aircraft RPM sensors, is J.P. Instrument – they manufacture sensors for Bendix Magnetos as well as Slick Magnetos.

Just in case you are unaware, spark plugs in any aircraft engine are powered by the magnetos – these act like mini power generators and even have a built-in transformer, breaker switch and a distributor – all used to provide high voltage to the spark plugs.

In a standard magneto (or dynamo), there are rotating magnets and electrical coils inside the housing. As the magnets rotate, the electrical coils generate power. The rotation has to be kept within pre-prescribed limits as the output voltage has to be constant and uniform. Any variations will lead to instant problems as the spark plugs will either not fire at all or go out of sync and the engines will misfire. The pilots therefore need to be aware the Bendix magneto is working properly and this is enabled via the RPM Sensor – a small-ish device that is attached to the magneto and provides the RPM readout in the cockpit.

Installation or replacing your RPM Sensor for Bendix Magneto is easy

Once the housing is open, just remove the vent plug from the port on the magneto (the vent plug covers the portion of the magneto that contains the rotating magnet). Remove the existing RPM sensor if any and insert the new RPM sensor (Dual, Bendix and Slick Magnetos have different RPM sensors).

Gently route the wiring bundle towards the firewall – while doing so, make sure you do not attach the bundle directly to the ignition, harness or magneto p-leads. Maintain a bit of slack in wiring so nothing gets unplugged during flight.

Technical data to remember while installing your new RPM sensor:

• Align the sensor with the correct vent plug of your Bendix magneto – when the position is correct, you should be able to see the rotating magnet through the vent – not the ‘gear’.

• The RPM for a 4-cylinder engine would be 3600 rpm, 2400 for a 6-cylinder engine, 1800 for 8-cylinder and 1600 RPM for a 9-cylinder engine. If using a dual mag, a 4-cylinder engine should have rpm of 1800 while that of a 6-cylinder engine should read 2400.

• The red wire of the sensor should be connected to the ‘+ve’ supply rated at 5 volts.

When replacing your old RPM Sensor, make sure you buy a good quality RPM standard sensor – you really don’t want false or inaccurate RPM reading.

More information on JPI manufactured RPM standard sensors here:

Working & Features of Oil Temp Probe in Aircraft

Oil Temp ProbeFor the uninitiated, an oil temperature probe is required keep a check on the oil temperature. The oil temperature is vital to maintain its viscosity. A low temperature will make the oil useless whereas a higher than normal temperature will risk a whole lot more than just engine damage.

An oil temperature sensor is inserted into the aircraft engine (please see latter part of this article on how to install the oil temperature probe), and the wires are connected to the oil temperature indicator or EDM if your aircraft cockpit sports an EDM. If the EDM permits it, the pilot can pre-set an alarm for the oil temperature so an audio-visual alarm sounds if the temperature drops to below a certain level or raises above a certain level (usually 180F or thereabouts).

Since oil plays a vital role in any aircraft, the pilots need to ensure the oil temperature probe and gauge are not only in good working condition, they are also accurate. This is done by an expert mechanic who will spot oil temp probe / gauge calibration errors. All such errors need instant attention – either recalibration or a replacement.

Once the mechanic completes his task, it is best if the gauge, sensor, and interconnect wiring be calibrated by a qualified technician or agency before flight. This is deemed prudent as even a small 5% or 10% make a huge difference to the engine. The error could result in the pilot take unnecessary measures or not taking any measures. The latter especially could prove fatal. From experience, we can state that even new oil temperature probes need calibration – or at least testing to ensure the reading they provide is accurate.

These days however, with the arrival of IC based integrated avionics and LCD displays, engine data is generally more reliable than what they once were just a few years back.

J.P.I. manufactured EDM’s allow for the display of several different engine gauges on a small but clear to read LCD screen.

Installation of the oil temperature probe in aircraft

The Oil Temp Probes in aircraft is installed by removing the pipe plug located on the front of the engine inline with the push rods. This oil galley feeds the valve lifters. Insert the probe supplied with the kit and check for leaks after installation. The probe leads are routed back to the cockpit along with the EGT wires. Attach these to the appropriate gauge or EDM.

More information on oil temperature probes can be found here:

Aircraft Engine Monitoring Systems – How it Works

Aircraft Engine Monitoring SystemsCalled by various names including EDM (Engine Data Monitor), ECM (Engine Condition Monitoring) etc. these are essentially aircraft engine monitoring systems. These electronic monitoring cum display units are at heart, a very simple device and this article aims to explain in simple language, how these devices work.

There are two parts to any aircraft engine monitoring system:

1. The EDM / ECM itself and,
2. The probes

The probes:

If you have ever been inside the ICU of any hospital, you might have notice patients hocked up to a monitoring system. On the patient are attached various sensors all leading to the monitoring unit.

The sensor probes in any aircraft work in exactly the same way. These probes have a sensor housed within an appropriate metallic body that enables the mechanic to clamp it onto the aircraft engine. The sensor itself is usually inserted into the engine. The reading is transferred via electrical wires back to the cockpit and is then connected to the aircraft engine monitoring system.

Any aircraft – even the single engine ones have numerous probes. In days gone by, each probe used to be connected to its specific dial in the cockpit. The cockpit therefore used to have numerous dials and indicators. These days however, the probe converts the reading into an electrical pulse which is transmitted via electrical wires to the aircraft engine monitoring system.

The aircraft engine monitoring system:

In absolute simple terms, the aircraft engine monitoring system has three parts to it – display, intelligent data comparison and, data storage.

Display: Every EDM or ECM aims to display the data in a way that helps the pilot(s) to get the gist of what is happening inside the engines in a single glance! The data is presented graphically and in figures. Most EDM’s of today also permit the pilot to choose what is displayed and how.

Standard data comparison: Most EDM / ECMs permit the pilot(s) to key in upper and lower limits for every probe. The aircraft engine monitoring system compares the incoming values to the pre-entered upper and lower limits. So, if the EGT (Exhaust Gas Temperature) is approaching or is higher than the upper limit (or lesser than the lower limit), that was keyed in, the aircraft engine monitoring system will trigger an audio-visual signal thereby gaining the pilots attention.

Intelligent data comparison and analysis: Aircraft Engine Monitoring Systems can not only accept data from the probes inserted into various parts of the engine, it can also accept inputs from the GPS navigation and compare the navigation output with level of fuel in the tank. It can analyse and deduce whether there is enough fuel to complete the trip and provide this information to the pilots. If the pilot(s) change the destination the Aircraft engine monitoring system will recompute fuel availability in seconds.

Data storage: Aircraft engine monitoring system have built-in memory modules to store data. This stored data can be downloaded for analysis via the USB or other ports located on the aircraft engine monitoring system itself.

Engine Data Monitor 730 System Vs 760 System

Engine Data Monitor 730 System and 760 SystemWe present a comparison between EDM 730 and EDM 760 – both Engine Data Monitors are manufactured by JP Instruments; World leader in onboard flight instruments and aircraft engine data management systems.

JP Instrument’s aircraft engine data monitors provide the pilots with real time information for aircraft. These systems constantly monitor the health of the aircraft engine and any anomalies are instantly brought to the pilot’s attention via audio-video alarms. This leaves the pilots to concentrate on the flight experience rather than constantly monitoring the engines.

This article provides you with relevant information to compare two great engine data monitoring systems manufactured by J.P. Instruments; the EDM 730 and EDM 760.

EDM 730
From $1,557.00 to $3,965.00)

1. Easy to install.
2. Clear, Full-Colour Graphics
3. Easy-to-read data display
4. Easy to programme
5. Annunciation of exceedances.
6. More data per page – less page swapping.
7. Fully approved.

Package Form Factor:

1. Requires only 3-1/8″ space
2. Easy to adjust location.
3. 4 mounting options.
4. Just 65mm depth
5. Easily upgradable from 700 series JPI EDM’s.
6. Data download via USB port.
7. Pilot programmable parameters.
8. Horsepower display in percentage.
9. Just press button for Rich of peak or lean of peak operation.
10. Graphically display of RPM and manifold pressure.
11. Computerized fuel-flow system.
12. Fuel management can be linked to GPS.
13. EDM scanner function.
14. Display in portrait or landscape.
15. Ideal for 4/6/7/8/9-cylinder engines,
16. Turbocharged engines

1. JPI Carb Temp Option 10-27103
2. JPI RPM Option 10-01720
3. JPI Oil Temp Option 10-27100
4. JPI Oil Pressure Option 10-04075
5. JPI OAT Option 10-27095
6. JPI Manifold Pressure 10-04512
7. JPI TIT Option 10-27090

More information here:

EDM 760
From $3,675.00 to $8,1000.00)

Using the latest microprocessor technology, the EDM will monitor up to twenty-four critical parameters in your engine, four times a second, with a linearized thermocouple accuracy of better than 0.1 percent or 2 F° which has been verified and tested by the FAA and thus TSO’d (Technical Standard Order). The EDM is constantly “red-line” checking all 29 critical parameters (not 16) that are automatically checked FOUR TIMES A SECOND, regardless of current display status.

1. Latest microprocessor technology
2. Monitors 24-critical aircraft engine parameters – 4 times every second.
3. Linearized thermocouple for better accuracy – verified and tested by the FAA.
4. Uses just two buttons for all programming functions of the EDM.
5. Computer Assisted Diagnostics for the entire engine.
6. 29 Alarms, including Shock Cooling, EGT Differential, Fuel flow and Alternator Voltage.
7. Excellent variable scaling of EGT Bar Graph.
8. Patented LeanFind Mode automatically identifies the first and last cylinder to peak and while patented PeakFind system has quick responding probes to automatically capture EGT or TIT peak value.
9. USB Data Port.
10. FAA approved Fuel-flow monitoring system.
11. Alphanumeric scanning display of 29 functions or channels

1. EGT Probe PN- M111
2. PC Interface Cable for EDM 700, EDM 800 and EDM 760
3. ALternate USB Download Box
4. CHT Bayonet Probe

More information here:

Features and Specifications of CDT Probe in Aircraft

CDT ProbeA CDT probe is used in aircraft as well as in ships. The ones used in ships are known as Conductivity, Temperature and Depth probe. It provides data on water salinity or lack thereof, it’s temperature and water depth at current location.

The CDT probe used in aircraft on the other hand, is used to measure compressor discharge temperature of the aircraft compressor and is therefore installed in the engine compartment just ahead of the inter-cooler. The CDT probe kit for the aircraft usually comes accompanied with a stainless-steel clamp to be fitted around the air-port leaving the inter-cooler.

The CDT probe kit as supplied by US-based J.P Instruments features a thermocouple type ‘K’ CDT Probe along with a stainless-steel clamp thimble and also includes one stainless steel exhaust seal washer and one stainless steel screw type clamp.

The purpose of wanting to know the discharge temperature of the aircraft compressor is to prevent the possibility of detonation brought about by excessive heat in the compressor of the aircraft.

What usually happens is that, the aircraft intercooler cools the air so much that the upper limit of temperature is usually never reached. Basically, pilots need to know the inlet temperature limits for the aircraft engine and the temperature of the air at this point. So long as the CDT indicates a temperature reading that is below what the maximum should be, the aircraft is fine. If CDT temperature reading is above the upper limit, the pilot has to take a call on whether the conditions justify the reading.

As we are aware, CDT can be critical under certain circumstances e.g. staging a dual annular combustor for a high bypass turbofan commercial jet engine where fuel-to-air ration can be critical.

Specifications and working of the CDT in aircraft: To obtain the correct compressor discharge temperature reading, the probe is mounted externally but directly in the flow path of the air discharged from the compressor. As the air flows, it touches at least one thermocouple and data is returned to the indicator (or EDM) in the cockpit.

Basically, a hole bored into the engine case and the CDT is fitted into that. The exhaust air gets channelled into this hole and into the housing containing the CDT probe which in turn measures the air temperature. The CDT probe returns n electrical signal to the connected display unit or EDM Monitors mounted in the cockpit. Modern EDM’s can compare the current temperature with the pre-set max temperature and in the event of an anomaly, sound the appropriate audio-visual alarm.

What matters is the response speed of the CDT. Any temperature spikes should be brought to the pilot’s attention immediately – and this is where the JPI CDT scores. JPI’s grounded CDT Probes are manufactured using a space-age material, Hastaloy-X, capable of withstanding harsh sulphur atmosphere of temperature exhaust gas. Also, it’s 1/16″ in diameter cable is less susceptible to temperature loss and therefore are more accurate than the fatter temperature probes manufactured by the rivals. You can select and buy CDT probes here: