Digital watches have been around since the 1970s, along with digital calculators and even a smattering of digital indicators in cars.
And even though the digital revolution has swept the GA cockpit, steam-driven gauges persist, notably tachometers and manifold pressure gauges. Of the two, the tach lends itself most readily to a digital display.
The tachometer is the primary means of engine control, at least for airplanes with fixed pitch props. Shouldnt it be the most precise and easiest to read? The answer is yes and three companies have digital tachometer offerings that are a cut above that old junker tach that needs periodic replacement or repair.
Horizon Instruments has a 3-inch instrument called the P-1000; Electronics International offers the R-1 tach, which is a 2-inch size, and JPI sells a slim-line unit whose principle advantage is that it will fit just about anywhere on the panel.
A pivotal question is why use a digital tach at all? Analog gauges are easier to read at a glance and you already have one of those. Still, analog tachs are mechanical instruments, their pick-up systems are often mechanical and more prone to problems. Interpreting a meter between two tic marks, may be a challenge in a critical situation.
Beyond simple hours, an analog gauge has no recording or annunciating capability, thus if you want to set warning lights to squawk when youre exceeding redline in a dive or on static run-up, you cant do it. Eyeballs rule. With digital technology, you can do that and more.
The typical mechanical tachometer has a cable connected to the engine accessory case with a drive socket that spins with the crank. This spinning cable runs to the instrument panel and into the back of the instrument. A magnet on the end of the tach drive spins. Theres a meter movement inside the instrument that responds to this spinning magnet. The faster it spins, the more the meter deflects.
That mechanical motion has lots of opportunity to bind, stick or break and break mechanical tachs do. There are problems with cables, drives at the engine end and, more commonly, faults in the magnetic pick-up inside the tach head. Some owners seem to have chronic problems with failing tachs.
Electronic tachs, on the other hand, use the signals in the ignition system. The P-lead from the mags control the higher voltage spark timing. The mag leads fire the plugs with 40,000 volts, while the smaller P-lead voltage runs a couple hundred volts. This is still a strong signal that corresponds very nicely with the magneto firing and can be used to accurately measure the engine RPM.
The basic operating principle for the Horizon and Electronics International is the same. They attach to the mag switch P-lead and sense the impulses through the switch. The JPI unit uses an inductive pick-up mounted on the magneto itself. Its less invasive to the all-important ignition system, but offers a more complicated installation.
Prices of these three units seem to match their size and complexity. The lowest priced unit is the JPI tach, at a bit more than $300 retail, followed by the Electronics International R-1 at just over $400 and the Horizon Instruments P-1000 at about $500. However, the JPI may be somewhat time-consuming to install since it requires its own hole and wiring routed out to the mags, not the ignition-switch P-leads. Out the door cost will be very close for all three units, in our view.
We would estimate that the average installation would be between eight and ten hours for a single-engine airplane, counting the paperwork. Since the JPI requires access to the engine compartment and it isnt a swap out for an existing gauge, wed would add three to five more man-hours for installation. Depending on shop rate, total install costs should run in the $500 to $700 range for any of these units.
If you want a digital tach in your airplane-and we think all of these devices are good systems if not must haves-theres yet another obstacle to overcome, apart from writing the check. Is there a system approved through the TC or STC process for your airplane?
Both Electronics International and Horizon Instruments have a long list of STCs, meaning they can legally replace the tachometer your airplane had when it left the factory. The JPI tach is not certified as a replacement unit and therefore gets around the aircraft/engine-specific STC issue to some extent. Thats good if your airplane isnt covered under any STCs but bad if you want to replace a mechanical tach. All of the units are FAA-TSO or PMA-approved.
The first step in choosing a digital tach should be to contact the manufacturer and see if there is an STC for your make and model aircraft and engine combination. With STCs from American Blimp to Zenair, we think theres a good chance of finding one for your aircraft. But this alone may determine which is the best value.
All three of these units have something that a steam gauge original tach doesnt: Indicator lamps for critical engine speeds. Sure, analog tachs have red lines, but each of these digital gizmos has a warning indicator.
Green indicates normal range, red is naturally red line, or maximum RPM. The units all have yellow indications as well. The JPI, a tiny LED, comes on within 100 RPM of the red line and Horizons yellow indicates the preprogrammed transitory range some engines have, where operation should be of limited duration.
But the Horizon does much more. There are five separate programmable RPM ranges in the P-1000, which are determined by the engine manufacturer and programmed at the Horizon factory when you order the unit, according to the aircraft the unit will be installed in.
The Electronics International R-1 will have the yellow bands found in the stock analog display programmed at the factory for the specific installation requirements. The R-1 has an hour meter function that records the time the engine is running above 1300 RPM. This engine time is stored in non-volatile memory, which requires no power to maintain the time. The maximum reading is 99,999 hours. After that, it may be time for an overhaul. The left button on the R-1 is a flight timer. This timer automatically resets when the engine speed has exceeded 2000 RPM for more than 10 seconds. This time, in minutes, will continue until the engine drops below 1200 RPM.
The peak engine speed is also stored. The peak is the maximum RPM that the tach has seen for more than three seconds. The information is stored even when the power is off. These features make this unit a neat gadget for rental use. You can see a precise amount of engine time and know what the maximum RPM the engine experienced during the last flight, if you record the information before the next trip.
Horizon Instruments also has similar features in the P-1000. The engine hours can be pre-loaded to the time shown on the mechanical tach, if desired, thus picking up where the old steam gauge left off. The P-1000 includes an engine overspeed trap that records the highest RPM, but this one doesnt retain the information through power cycles.
Another nice feature of the Horizon P-1000 is the mag drop function. The unit is actually two tachs in one box, one connected to each magneto. When you shut off one mag, the unit records the last RPM from the missing side and then displays the difference between that RPM and the current RPM, simplifying run up for those of us who have trouble squinting at and making sense of an analog tach. Besides the digital readout of the difference in RPM, the indicator for a dead mag comes on, as a further check of the ignition system integrity. The JPI doesnt have any of these recording features, but merely displays the overspeed warning and current RPM in bright red LEDs.
Two of the units, Horizon and Electronics International, use liquid crystal display, while JPI is an LED display. The warning indicator lamps in all are LEDs.
The Electronics International readout is a mere 3/8 inch tall by 3/4 inch long and this is a problem if the tach is mounted too far from a direct line of sight. It simply requires concentration to find the display and interpret it. Pilots will probably come to depend on the analog arc portion for the routine scans and digital for fine- tuning power. This consists of a circular row of green LEDs calibrated from 1200 to 2700 RPM, with painted tick marks at critical RPMs for easier reference at a glance.
The LED numbers in the JPI are almost as large as the entire LCD in the Horizon display and their bright red color makes them all the more visible. An older version of this unit-which we have installed in our company Mooney-has a manual dimming switch, the newer version has automatic dimming via photocell. Either way, day or night, those big LEDs are impossible to miss. Best display award goes to JPI.
A two-step button dims the Horizon unit, while the Electronics International is connected to the aircraft dimmer controls its display. The Horizon display is nicely illuminated at night and we would rank it second in ease of viewing, with the EI as third.
The simplest tach is the JPI, with no controls at all. The Electronics International unit has two small buttons, labeled FLT time and TACH time. There are sub modes that require the user to essentially double click the buttons for peak RPM and to view the engine hours down to the tenths.
There are three buttons on the Horizon Instruments tach. Holding the button changes the display. For instance, the left button displays the engine hours. The first number displayed is the hours and the hundredths of hour shows when you let go. Just pushing the left and right buttons masks, or disables that magnetos tachometer display. According to Horizon, this mode is useful in diagnosing magneto maladies.
The center button controls the display dimmer, push once for dim, push again for bright. In all of these units, the red line RPM indications are at full intensity.
Installation of the Electronics International and Horizon tachs is easy; they fit in standard openings and all of the connections are inside the aircraft cockpit, since each uses the P-leads for RPM info.
The JPI Slimline blends nicely into the companys gauge package. Its panel cutout is non-standard, as a stand-alone tach and the installer will have to route the wiring out to the mags and install the pick-ups.
These units come with pre-wired cables and theyre easy to connect, if you can make crimp terminal connections and install a circuit breaker. They are easy installations for the aircraft owner, but they should be inspected and signed off by a mechanic just to be certain that you havent crossed any wires. We found that the instructions for the Electronics International and Horizon Instruments units were quite clear, JPI didnt provide us with documentation to evaluate, although we asked.
For precision and reliability, nothing beats a digital tach, in our view. Replacing an analog unit, which may be erroneous, can save maintenance dollars in the long run, as well as providing accurate performance information. Although we dont consider these instruments must haves, they should be considered during periodic upgrades or if youre having chronic problems with a stock tach. Frankly, theyre simply better instruments.
If replacement of a stock instrument is driving the decision, the Horizon is the best value, in our estimation. In many cases, it can replace the stock tach and has the most useful features. Its in the middle as far as viewability is concerned and the installation is relatively easy.
The JPI is our second choice as an excellent back-up or supplemental gauge. In our Mooney, its the primary source of RPM info and we would replace it with another if it failed. (In fact, it recently did fail and JPI repaired and upgraded the tach for $30, laudable support for an older product.)
Electronics International also makes a complete set of these 2-inch gauges and alone, or combined, we like the combination of analog and digital display but overall, we find the Horizon and JPI easier to read. If your panel is tight, however, and you cant find room for the Horizons 3-inch bulk, you wont go wrong with the EI, even though its more difficult to read than the other two.
-by Gary Picou
Gary Picou is Aviation Consumers avionics editor.