Black Box Basics

Advanced graphics and sophisticated data-gathering at your fingertips

Consider this scenario. You have departed into instrument conditions in your capable, turbocharged piston twin. After a short ride through the bumps, you pop out on top of the clouds into brilliant sunshine. Leveled off in cruise, you're cleared direct; and upon fiddling with your GPS to get there, you discover a juicy, better-than-forecast tailwind. Because you've been flying with these engines for years now, you feel comfortable that they won't let you down.

You carefully manage them in the transition to cruise — closing the cowl flaps, tweaking the power and prop settings, and performing an approximate cruise-power leaning. The stock gauges, including those for single-point cylinder-head temperature (CHT) and turbine-inlet temperature (TIT), all show "good green." You go on to lean carefully to peak TIT and then back to 50 degrees Fahrenheit rich, just as your type-specific training has taught.

As your passengers begin the in-flight meal service, you hear and feel a slight hesitation. First a small bump, then another. A quick scan of the instruments reveals nothing amiss — fuel flows are where they should be, and the gauge pointers show no change. You tweak the mixture slightly, but the occasional miss persists. Now you think you've got it pinned down to the right engine...probably, um, most likely. Sure, that's it.

The passengers continue to eat, blissfully unaware, but your stomach is churning. Just what's going on here, anyway?

Is there a doctor in the house?

Like kids playing with that silly board game, "Operation," you are stuck with making a diagnosis and possibly attempting a treatment with tools little more useful than electrified tweezers. And while the usual factory-standard instrumentation may be fine for simple airplanes and those times when everything works as planned, they are inadequate otherwise. Kind of like asking the cast of ER to check out your aching pancreas.

Stock instrumentation gives you only part of the picture — and sometimes the information is little more than a blurry representation of the facts. Take, for instance, the typical CHT indicator. Usually, this instrument monitors the cylinder that in certification testing was the hottest in the worst-case condition; usually this is during a full-power, max-angle climb. Great. Well, not exactly. There's no guarantee that the chosen cylinder will be the hottest in cruise — in many cases it is not, because of changes in under-cowl airflow. For example, look at the flurry of service bulletins issued by Beech on the IO-550-powered Bonanza; several times in the past few years, owners have been asked to relocate the CHT probe.

How, then, do you manage temperatures in cruise, particularly in highly stressed turbocharged applications? With a single-point CHT, you could be setting up for cruise with one or more (unmonitored) cylinders running significantly hotter than you think. In addition, the certification testing invariably took place on a production-new airplane, with straight and true baffles and proper sealing. Now that your mount's 20 years old, do the cylinders cool the same?

Exhaust-probe placement is just as much a black art. For normally aspirated installations, the probe is usually located at the cylinder that peaks first during leaning. It is far from a sure thing that this jug peaks first in all flight regimes and for all power settings. This is especially true in carbureted airplanes. (Cessna 182 owners know well the routine of slightly closing the throttle from full in cruise to improve the cylinder-to-cylinder mixture distribution.)

In other installations, notably the Bonanzas and Cessnas with big-bore Continentals, the stock EGT probe may reside in the exhaust system at the junction of three cylinders. This probe location gives you a general idea of what the engine's doing, exhaust-gas wise, but it's also possible to see one reading with all three cylinders working fine, and much the same reading with one of them in distress. Turbocharged installations typically use TIT in lieu of per-channel EGT, picked off just upstream of the turbocharger's turbine inlet.

Natural selection explained

Darwin would have loved watching the progression of engine- monitoring instrumentation. From the basic, single-probe EGT systems of the 1950s, the field over time came to include multiple- probe setups with user-manipulated switches. Crank the knob around to see what's happening in the exhaust stream — when you have a spare moment, that is. Given the luxury of time, this kind of multiprobe arrangement allows you to peek into each cylinder for its overall health, but it's up to you to conjure the big picture.

Then scanning instruments arrived, which sampled each probe sequentially and flashed a number at the pilot. These systems grew into more sophisticated iterations, with warning lights and user- definable alarms.

But the real revolution came from Insight Instruments by way of the GEM, for graphic engine monitor. Debuting in 1981, the GEM combined all-cylinder EGT and CHT with a purely graphic display; only the models designed for turbo installations had numeric readouts. The concept of graphic representation of numeric data brings many advantages, such as the ability to discern powerplant troubles at a glance. Take a peek at the instrument on takeoff; in seconds you'll know if all cylinders have good EGT readings.

At the beginning of the 1990s, the GEM was joined in the marketplace by J. P. Instruments' EDM-700, an outgrowth of the firm's EDM-500, a numbers-only monitor. The 700 had a GEM-like look but went beyond the Insight product with numeric sampling of the channels. In this way, you could see the big picture and also home in on the details. Seems cylinder number two's EGT is running about 1,400 degrees — flip through the channels — oh, make that 1,389. JPI also introduced data logging with the first EDM-500 and carried it through to the 700. With this option you must buy the remote processor and entrust the data to a removable PC card.

Leaders of the pack

Now the current generation is well-established, with a revised GEM and two versions of the EDM-700, as well as exciting new models for twins, the Gemini 1200 and the EDM-760. We recently spent time with the Gemini and the EDM-760 in a Piper 602P Aerostar and a Beech B55 Baron, respectively. Both the Gemini and EDM-700 fit in 3.125-inch instrument holes, and the JPI supplants the stock EGT gauges; the Gemini is not currently approved for primary indication, so the standard needles must stay. Neither is approved to replace the original CHT. Worry not, singles drivers, because the GEM 610 is almost entirely the equivalent of the Gemini in features and capabilities, just as the EDM-700 is to the -760.

In practice, these monitors take much of the guesswork out of managing a pair of engines and contribute greatly to combustion awareness without overloading your interpretive skills. You can watch in amazement how each cylinder's EGT gradually creeps up the scale during cruise leaning, along with the associated changes in CHT. It's frightfully easy to determine which of the cylinders reaches peak EGT first and how the remainder stack up if you elect to lean beyond the first's crescendo. And both the Gemini and EDM- 760 have lean-find routines to alert you to the peaking of the first cylinder, which gives quick and handy reference for fine-tuning the mixture.

Insight's Gemini

Insight's Gemini (and the 610) takes off where the old GEM had pretty much maxed out, although many familiar concepts remain. (The Gemini and 610 are also plug-compatible with earlier GEMs, so upgrading is comparatively painless.) Stacked bars represent EGT, and a missing segment denotes the top of the CHT scale, referenced to a central scale on the instrument face. A single numeric display above the bars (one each engine for the twins) gives the value for any of the sampled channels to 1 degree F (or C) of resolution. Boxes around the cylinder numbers at the bottom of the display tell you which parameter is being displayed by the numbers; the missing box is the value in question.

Each EGT stack has above it a trend indicator; the small triangle indicates in which direction the EGT is moving. The Gemini has a lean-find mode; poke a front-panel button and the unit starts searching for the first EGT to peak and begin to fall. The EGT bars of the cylinder that does so begin to flash. You can then use the 25- degree-F bars to tweak the mixture, or select the cylinder's numeric display for 1-degree leaning. Insight also includes a Normalize function that allows you to level all the EGT bars on the display so that any changes will be readily apparent; the normalizing feature does not change the numeric value you see while button-pushing through the channels.

In addition to the basic EGT and CHT displays, the Gemini will support single or dual TIT probes and a digital OAT. None of these secondary functions is given a bar graph, however. Insight intends to have, for a late-summer release, manifold-pressure and engine- speed sensors that will integrate with the Gemini and 610.

JPI EDM-760

To the casual diner, the JPI seems like the same steak as the Gemini. But while they may be from the same part of the cow, the JPI arguably gives you more trimmings. Both the EDM-760 twin and EDM- 700 single models give dual digital displays in addition to the ants- up-a-wall graphics.

You'll also notice that the EDM-760 has one more row of bars, offering a graphical representation of TIT as well as all channels of EGT and CHT. And while it's generally more useful to have a TIT number in mind — just as with the CHT, because you have a maximum limit to be concerned with — the bars offer a useful at-a-glance read on the general state of mixture management.

The JPI box annunciates — by a small dot beneath the cylinder number — the cylinder to which the digital display is linked; other call-outs include temperature units (Celsius or Fahrenheit) and EGT display modes in use, Percent or Normalize. In the JPI, the EGT bars are normally stacked with reference to a user-defined redline value, and the bottom of the display calibrates to half of that value. In the Normalize mode, which acts as does the Gemini's, those bars are leveled so that any change will be hammer-over-head obvious. You can choose to view any of the cylinders' specifics in the digital form by poking the Step button, or let the EDM automatically scan through the choices for you. (Even in this mode, the JPI is watching all the probes in the background for alarm purposes.)

Several forms of warning systems are also embedded in the JPI's brain. These include high or low voltage and oil temperature; EGT spread; CHT, EGT and TIT high limits; and cooling rate for CHT. All of these parameters are factory preset but can be easily altered by the user from the front panel. (In fact, we did so once in flight without losing control of the airplane or hitting anything harder than an air pocket.)

The EDM-760 contains a lean-find feature, but it's implemented differently. Because each EGT bar denotes about 40 degrees F, JPI felt that leaning would be more precise with a numeric reference point. After you've punched up the lean-find mode and the first cylinder to peak shows an EGT drop, the numeric display locks on to that cylinder. You can use the digital readout for leaning reference.

JPI also sails a different tack with the CHT display. Rather than being a scaled representation of the temperatures, the missing segments indicate cylinders that fall into one of four categories — the hottest cylinder, those 20 degrees below the average, those within 20 degrees of average, and those 20 degrees above average. You have no immediate reference to the absolute temperatures, just their relationships.

Commanding data

Maintenance mavens will envy the data-gathering capabilities of these two units. Both spew the contents of their brains in a serial- data format, allowing long-term logging of engine health. JPI goes about it the simplest way in the new 760 and the so-called EDM-700 Basic. A small jack is installed in the cockpit for a laptop or palmtop; the JPI spits out a comma-delimited stream with headers and time markers of all the sampled probes.

The Gemini goes about data-logging quite differently. When you want to see what's been happening, simply point the supplied Hewlett-Packard HP200LX palmtop at the Gemini's faceplate and the information will be transferred by infrared link. The information remains encrypted in the HP200LX — it cannot be altered by the user- -so it may be more useful to resolve a warranty dispute or to see how renter-pilots are treating your leaseback bird.

What's the bottom line?

Considering the multifaceted usefulness of these — dare we say? — gadgets, their prices aren't by any means out of line. Insight's prices for the Gemini start at $4,395 for a twin with four-cylinder engines; add $400 for sixes. Turbocharged twins range from $4,595 to $5,195 for a setup with dual turbochargers a side. For singles, there's the GEM 610, starting at $2,500 for a four-cylinder model, to $2,970 for a twin-turbocharged setup. Each system includes all probes, including the OAT and TIT senders, harnesses, and the HP200LX palmtop computer. Special packages requiring longer harnesses or oddball probes — including those to piggyback to the stock CHT thermocouple — are slightly more.

JPI's twin offering starts at $3,995 for four-cylinder twins — add $500 for six-cylinder applications — while the single-engine EDM-700 retails for $1,840 in non-turbocharged, four-cylinder form and $1,995 for sixes. (The nine-cylinder EDM-700 runs $2,349.) Additional probes for $150 for each engine include oil temperature, a second TIT, and induction-air and compressor-discharge temperatures. A piggyback probe for the stock CHT location is $75. A newly released fuel-flow system is a $700 option. Finally, there's the older EDM-700 with data logging and sensors for manifold pressure and engine speed; together these offer a percent-of-power display. This is the model with the remote processor, and a fully loaded six-cylinder model costs $3,845.

These instruments may well pay for themselves in time spent trouble shooting. But what's most impressive is that they give you tools to determine whether that slight hesitation in cruise stems from a hiccup that warrants immediate attention — or from a glitch as benign as a slightly fouled spark plug. Peace of mind is all.

Contact Insight Avionics, Inc., at Post Office Box 194, Buffalo, New York 14205; telephone 716/852-3217, fax 905/871-5460. J. P. Instruments may be reached at Post Office Box 7033, Huntington Beach, California 92615; telephone 714/557-3805, fax 714/557- 9840.

What Do the Readings Mean?

It's one thing to have these whiz-bang instruments on board, but quite another to interpret accurately what they are telling you. Here are some common scenarios to get you thinking about what the temperatures mean.

• High EGT, normal CHT, all cylinders. Overall, the mixture's probably too lean for the given power setting; may be incipient vapor lock. Cross-check with mixture position and fuel flow. Another possibility is that one or both mags have drifted in timing; retarded spark will make the combustion take place later and result in the EGT probe's "seeing" more of the combustion event.
• High EGT, normal CHT, one cylinder. Here, either the mixture is too lean in that cylinder, or something is causing the combustion event to occur later than normal in the cycle. Could be a blocked injector nozzle — which would also show up as increasingly poorer mixture distribution among the cylinders over time — or a fouled spark plug. Try switching magnetos to pinpoint the plug or lead that's causing problems.
• Low EGT and CHT. Assuming the power is set to a normal cruise figure, this could indicate an induction-system blockage; because it affects all cylinders, start looking for items that would be common to all, including the fuel-injection throttle body or carburetor, exhaust system, or turbocharger. May also be the latter stages of an exhaust valve sticking.
• Low EGT and CHT, one cylinder. Could be a broken ring, deflated lifter, or a valve that won't seat properly. All are indicative of lowered compression or failure of the fuel/air mixture to arrive in the cylinder. Excessive back pressure from an exhaust-system restriction could also be indicated.
• Normal EGT, high CHT, all cylinders. A rapid increase in CHT could be the onset of detonation; reduce power, enrich the mixture, and see what happens. Double-check that you're not using more power than you think. Also, you might have left the cowl flaps closed during the go-around.
• One cylinder slightly low EGT, adjacent cylinder high CHT. Here's an interesting problem that's happened to others. An exhaust gasket blows (or the riser somehow otherwise comes adrift of the cylinder) and the escaping gases strike the adjacent cylinder, heating it up.
• High CHT on odd- or even-numbered cylinders only. Indicates a blockage of cooling air on one side of the engine; have the baffling and sealing material checked.
• Dark bars. Apparently dead cylinders. In EGT, it's probably a completely clogged fuel injector or some other induction obstruction limited to that cylinder; verify by noting engine roughness. Remember that from the remaining good cylinders there will still be some EGT indication — even if it's only 800 degrees or so; you'll also have a lower, but on-scale CHT reading. If one or both of these readings drop to zero or some absurdly low number — and there's no engine roughness — chances are good that you've got a probe problem. — MEC