High-Tech Engine Monitor


PC technology for cockpit assistance

By Jack Kane

In too many instances, it seems that products available for light aircraft are stuck in a several-decades-ago time warp. One of the few areas id which newer technology is available is on the instrument panel. A lot of modern technology has been applied to navigation and communication devices. But with certain notable exceptions such as the Insight GEM engine management for tight aircraft remains mired in the ancient technology of rotating magnet tachometers, bourdon tube pressure gauges, and hordes of special-purpose galvanometers.

The two major problems with the conventional array of engine instruments are (l) they consume far too much panel space, resulting in the fact that (2) they require: far too much pilot attention to monitor them well enough to detect latent problems before they become serious ones.

The typical plethora of disjointed engine instruments adds significantly to pilot workload and contributes to the problem of "heads-down" flying. Engine instruments tend to be located in places on the panel that are difficult to see, therefore less likely to he included in the regular scan. All too often they are scattered about the instrument panel in seemingly random order rendering them nearly immune to regular, frequent scanning. This can allow an engine problem to seem to be sudden, when in actuality the instruments may have been warning about it for hours.

Solution to gauge growth

There is, however, a modern, affordable engine management system known as the MicroMONITOR, which uses digital computer technology to replace all the conventional engine instrumentation. This system is an elcgantly simple alternative to the conventional vast (or half-vast) array of engine instrumentation, and it conserves a huge amount of both panel space and cash. This system integrates the measurement, display and monitoring of all the important engine operating parameters. It simultaneously displays 13 different parameters on an easy-to-read 5.5-inch by 1.5-inch, backlit Liquid Crystal Display (LCD).

More important, the MicroMONITOR reduces pilot workload while dramatically increasing safety by watching all the engine parameters all the time, and by alerting the pilot when it detects something potentially troublesome If the system detect~ a parameter out of limits, it sounds an alarm into the pilot's headset and blinks the display of the offending parameter.

The MicroMONITOR system is only 6.25 inches wide by 3.26 inches, and weighs about 1.6 pounds.

Five Functions

The functions of the MicroMONITOR system can be grouped in five major categories:

  1. Alarm System:
  2. Powerplant Management;
  3. Fuel Totalizer;
  4. Timers and Clocks;
  5. User-Specific Inputs.

The following sections describe each of those categories in detail. Although the following description of the capabilities of the MicroMONITOR may at first reading appear complex, keep in mind that it is a well thought-out system providing a dense set of capabilities.

The unit is remarkably simple to operate add quite user-friendly. Figure 2 shows a full scale drawing of the front panel of the instrument with all the display segments on. It might be useful reference as you read through the description of the system. The monitoring of powerplant parameters is an important part of this system's overall functionality. Integral to that monitoring is the alarm system by which the system alerts the pilot of an event (an out-of-limit excursion, a timer expiration, etc.).

There are 14 MicroMONITOR events that have alarms associated with them: 10 engine parameters, 3 switch closures and a timer expiration. During system calibration (and anytime thereafter), the user defines and the system stores the alarm-limit values for the engine parameters. The discrete alarms (on/off/time-out) are pre-defined. Whenever any alarmed parameter goes outside the defined limit or a discrete event occurs, the MicroMONITOR activates an external (user-optional) 90 dB audio transducer and an alarm signal (user-optional) into the aircraft's audio (headset) system. To identify the source of the alarm, the MicroMONITOR blinks the display of the parameter or event on and off.

The pilot acknowledges an alarm by momentarily pressing the SIL/VOLT button (Figure 2, lower right). That silences the audio, but the display of the parameter continues to blink until the value goes back into limits.

Powerplant management

The system continuously measures and displays 13 powerplant parameters, monitors the 10 that have alarms, and alerts the pilot if any of them goes outside the user-defined operational limits.

With only three exceptions, each of these parameters has its own dedicated display position and is displayed continuously. The display positions for the parameters, as well as the various switches, can be seen in the diagram. The Fuel Remaining and Fuel Flow parameters share the GAL display position. The user selects which is displayed by the position of the GPH button. Parameters 11 and 13 are only displayed when the user presses the SIL/VOLT button, at which point the GAL position displays Bus Voltage and the MAP position displays Outside Air Temperature. When the user releases the button, those two positions revert to normal.

microMonitor Functions & Alarms





Engine RPM

High Limit


Manifold Absolute Pressure (INCHES Hg)

High Limit


Oil Pressure (PSI)

Low Limit


Oil Temperature (DEG C)

High Limit


Fuel Pressure (PSI)

Low Limit


Fuel Flow (GPH)



Fuel Remaining (GAL)

Low Limit



High Limit



High Limit


Carb (or Injector) Temp

Low Limit


Outside Air Temperature



Alternator (or Battery) (AMPS)

Low Limit


Bus Voltage (VOLTS)


The EGT and CHT positions display the readings from one thermocouple each. To implement a multi-cylinder display capability, an optional multiplexing switch is available to select the cylinder being displayed (and monitored).

Fuel system management

If the Fuel Remaining drops below the user-defined Low Fuel limit, the system blinks the GAL display position and sounds the audio alarms. When the fuel tanks are refilled, the user moves the MODE knob to TACH/FUEL and presses the PRE button (simultaneous depression of the 1 and .1 buttons). That loads the user-defined Full Tanks value into the Fuel Remaining parameter. IF the tanks are not topped, the pilot updates the Fuel Remaining parameter to the new value with the 10, 1 Bad .1 buttons.

The Endurance parameter is shown in the CLOCK display position whenever the SIL/VOLT button is pressed.

Timers and clocks

The MicroMONITOR contains five different time functions: two time-of-day clocks, an event-timer, a flight-timer, and the Engine Hours counter. The user selects the required time function with the MODE knob on the lower left corner of the instrument. The time function selected appears in the CLOCK display position (bottom row, center, between the AMP and CRB positions.

The 24-hour format time-of-day clocks keep time in two user-selected time tones: the primary time zone: (usually GMT) and a second time (usually local). The local zone is simply a user-defined number of hours offset from the primary zone. The user selects the appropriate clock by moving the MODE knob to GMT or LMT. The time (HOURS/TENTHS) appears in the CLOCK display position.

Moving the MODE knob to TIMER selects the system's dual-function event timer. It operates in either count-down (to zero) or count-up (from zero) mode, and it provides an alarm whenever it reaches zero (in either direction).

The selection of count-up or count-down operation occurs based on the value in the CLOCK display when the user presses the START button. If the display is zero, the timer operates in count-up mode; non-zero produces count-down mode. The user has three ways to set the initial value of the timer: with the 10, 1. and .1 buttons, with the PRE button which loads a user defined preset value, or the RST button which stops the timer and sets it to zero. The timer begins running in either mode when the user presses the START button. (Note that the RST, PRE and START ''buttons" are actually paired combinations of the 10, 1, and .1 buttons. The combinations are clearly defined by labels on the front panel.)

Count-up mode has a maximum value of 59.9 minutes, after which the time rolls over to zero, the alarm occurs, and the timer runs on until stopped by the user. This mode can be used as a reminder to switch fuel tanks (in case you are so hamfisted that you can't feel an hour's worth of fuel imbalance in the stick).

If the value in the CLOCK display is non-zero when the user presses the START button, the timer operates in countdown mode. It alarms (audio and visual) at zero, and if not stopped by the user, continues to run, now in count-up mode. This function satisfies a longtime wish of mine: an approach timer that sounds an audio signal into the headsets when the FAF-ro-MAP count-down expires.

Moving the MODE knob to FLT TIM selects the system's Flight Timer function. It operates like a typical Hobbs meter, accumulating time only when the system detects oil pressure (i.e.. engine running). It has a maximum value of 2.5.6 hours.

When the user moves the MODE knob to TACH/FUEL, the system displays Engine Hours in the CLOCK position. The Engine Hours function accumulates hours and tenths that the engine spends at a pre-defined 100% RPM value, to a maximum of 6553.5 hours. It works the same as the "hours" function on a mechanical tech. That is, it simply counts engine revolutions multiplied by a fraction (determined by the 100% value: 1/150000 for 100% = 2500 RPM, for example). That is, if the 100% value is 2500, then with the engine spending one hour at 2250, the hour meter accumulates only 0.9 hours (2250/ 2500). Likewise, for one hour at 2700, the meter accumulates 1.08 hours (2700/2500). This timer function has two additional features: the user defines the 100% RPM value at which the meter operates, as well as the initial value for the Engine Hours parameter.

If a user chose to define his 100% point as 2700 and if he operated predominately at 2400 RPM, then his TBO, as defined by tach time, would be extended 12%.

All the time functions are concurrent. For example, the user can start the count-up timer, and it continues to operate, even when the user switches the display to some other time mode.

Discrete inputs

The system provides input connections for three user-specific events, each signaled by an external switch closure. Each event has its own display position (the 123 display in the middle of the second line, Figure 2) and its own alarm. When any of those closures occurs, the display blinks the number (l, 2, or 3) which corresponds to the closure, and the audio alarms activate.

As an example of their usefulness, I have adapted these three inputs on my aircraft to provide alarms for Low Gyro vacuum (1), Carbon Monoxide in the cockpit (2). and an Over voltage Trip (3). A switch detects Low Gyro Vacuum. It is installed upstream of the vacuum regulator, and is adjusted to open at vacuum values above 3.5". Carbon Monoxide is detected by a CO sensor. The alternator regulator signals an Over voltage trip.


The system has a backup mode of operation in case of aircraft electrical system failure, in which power for several hours of operation is provided by the (optional) backup battery. The recharging system for the battery is integral with the design, and fully automatic, When operating in backup battery mode (selected by the POWER knob, lower left corner), the BAT display (underneath RPM) is on. When the battery has discharged to a level at which less than one hour of operation remains, the LO letters are added, producing LOBAT. The electroluminescent backlighting of the LCD makes it quite clear and easy to read, even in bright light. The level of back lighting can be adjusted at will.


The functionality of the system is achieved by a combination of specialized hardware and firmware. The hardware is implemented in a motherboard/daughterboard PC layout. The motherboard contains the I/O signal interfaces, power supplies, switches, and the like. The daughterboard contains the digital electronics, and is based around an 8040 micro-controller CPU chip, a 2732 EPROM, an HI-3506 input MUX and a fast 10-bit A/D converter. It is a clean design, with no patchwork or trace-cutting required. It has adequate processor resource for the required tasks, and is built on mature, table technology. All the ICs are socketed for easy maintenance if needed. The instrument has a professional-looking face, with a substantial bezel, solid switches, and an excellent LCD. The all-metal case fits neatly into the quick disconnect mounting tray, and all connections to the airframe occur through one edge connector mounted to the tray.

The system has a simple calibration mode in which the user calibrates each sensor to correct for tolerances in the interface circuitry and for the sensors themselves. At first I was concerned that the calibration only accounts for position shift (zero point) of the sensor curve. Academically, I would have preferred bath zero-point and rate compensation. However, the extensive checkout testing I performed on my units satisfied me that the existing calibration capability is adequate, largely because the sensors are essentially linear and reasonably correct in rate.

Building the kit

The fact that this instrument comes as a kit may be thought of by some as a disadvantage. However, the kit is very well thought out, the documentation is exceptional, and the build time is low.

It comes with three very well-written and clearly illustrated manuals; (1) Assembly, Calibration and Test; (2) Installation; and (3), User Operation Instructions. The Assembly manual is so clear and the kit so complete and well organized that anyone who can follow cookbook directions and has average manual dexterity can successfully build this kit in 20 hours. Knowledge of electronics is not a prerequisite; the manual even teaches soldering methods. The small components (resistors, capacitors, diodes, etc.) are organized by number on cards, and the manual identifies each by color bands, by the printed coding on the component, and/or by pictures.

In addition, the PC cards are silk-screened with the outline and numbered identity of each component (R12, U49, etc.). The Assembly manual is so thorough, it even includes complete schematics for those who can use them. The other two manuals (Installation and Operation) are of the same high quality as the assembly manual.

If a customer prefers not to build the kit himself, the manufacturer has subcontractors who will build and test the system for the customer.


The cost of this system is the surprising part. The basic kit costs $969, which includes most of the sensors (exceptions listed below), audio alarm annunciator, cable connectors and manuals.

The additional components which must be purchased to complete the system, and their approximate costs are:

Fuel Flow transducer:         Floscan 201                   $150
MAP transducer:               Data Instruments SA015PSIA.   $225
Carb Temp sensor:             Richter B5                    $125
EGT Thermocouple:             Type K                        $25 ea.
CHT Thermocouple:             Type J                        $35 ea.
CHT/EGT Multiplex Switch:                                   $25
Backup Battery:                                             $20
Shielded cable for sensors:                                 $0.75/foot

The documentation provides all the above information, with part numbers and suggested sources for each additional component.

As an example of total cost, a complete system for a 4-cylinder engine with CHT and EGT probes for all cylinders, with all the sensors, switches, backup battery, and necessary shielded cable, ready to install, would be approximately $1775. Compared to the cost of conventional instruments to display the same parameters, plus a fuel totalizer, plus the timers, that cost is quite low. That comparison of course, does not take into account the unique full-time monitoring capability the system provides.

I installed the first MicroMONITOR I built on my engine dynamometer, and after using it, I wonder how I ever got along without it. In the aircraft, the system is, to my mind, truly indispensable. In both VFR and IFR flying, my workload is reduced considerably. I know that the engine systems are being watched far more closely than they can be by any human.


As with anything in the real world, this system has a few, albeit minor, disadvantages.

Aside from the arguable disadvantage of a kit, the only other significant disadvantage I can see is that the system does not have any STC's. However, that is only a concern for owners of certificated aircraft. And the guy who is smart enough to recognize the value of this system will also be smart enough to find an A&P who will install it with a Form 337.

One might invoke the standard objection to digital instrumentation, specifically, that a digital presentation requires more mental effort to assimilate both instantaneous and trend information than does an analog presentation. However. digitized engine parameters are easier to digest than are navigational data, for example. The alarm function of the system overcomes a large amount of the "digital'' objection". The greater apparent precision suggested by a digital display requires a bit of user adaptation in order to discount insignificant variations in a displayed parameter. (For example, a PSI fluctuation in a 70PSI oil pressure reading is insignificant.)

Until I Used this system I was very negative toward digital instrumentation in aircraft. But after using it, I discarded that attitude quickly.

This is a picky nit, but in the ideal world, the system would be really complete if it included the gyro vacuum parameter.

For those who prefer the capabilities of the Insight GEM for full capability multi-cylinder CHT/EGT monitoring, an optional at modest cost ROM revision is available for the MicroMONITOR.

This firmware version substitutes a continuous presentation of Bus voltage and Fuel Remaining for the EGT and CHT displays. That makes all the interesting engine parameters in view at a glance, without button pushing.

For the truly industrious builder, the manufacturer can also provide a serial output port which feeds all the engine data to some other entity for further processing, recording, or whatever.

The MicroMONITOR is produced by a small but established company, Rocky Mountain Instrument in Thermopolis, Wyoming. It is a mature product, having been marketed since 1984, with over 300 units currently installed.

(Note: RMI also produces the MicroENCODER which is, in actuality, an Air Data Computer, available in both kit and completed form. It was described in the January, 1991 issue of Avionics Review.)


RMI NOTE: The above unedited article (except for layout) appeared in Avionics Review, April 1993