JPI’s FS-450: An End to an Aviator’s Avgas Anxiety?

Obtaining the maximum range from our aircraft can often be guesswork: "Did I lean this correctly? How much fuel was consumed in that climb to escape icing?" A digital fuel totalizer can help ease the anxiety of these and other questions. AVweb Special Projects Editor Dave Higdon recently installed a J.P. Instruments FS-450 fuel totalizer in his Comanche. Was it worth it?


Somewhere north of Cape Girardeau, Missouri …

Normally, by this point on a trip like today’s my fuel situation never generates a thought. We’ve logged around 700 hours in the Comanche in the four years we’ve owned it, and we’ve flown this trip more times than any other; about 15 or 20 times in the past six years and nine or 10 of them in this airplane. We know it by heart: Four hours or less en route generally; usually direct; with seldom a fuel-related concern, especially when starting with a full load of fuel.

But conditions today generated some avgas anxiety, as usually happens when so little goes according to plan and deviates so much from past practice: A couple of significant altitude changes totaling 6,000 feet; numerous power increases to counter strong sink; and enough weather, traffic and ATC vectors to add more than 100 nm to what typically spans fewer than 600.

And, though we know their patterns well, there’s no way we trust the original-equipment fuel gauges with their needles that bounce and bob in synch with the floats on the senders down in the two wing tanks. So generally, with scattered IMC, big cumulus build-ups and heavier-than-usual IFR traffic contending with these inclement conditions, normally I would have worried.

Today, however, and for the first time ever, none of the above made me sweat our fuel situation. Indeed, little should in the future, thanks to our new FS-450 fuel totalizer from J.P. Instruments (JPI). JPI’s newest piece of panel wizardry relegates fuel-situation questions to relics of the past. From its new home between the top row of flight instruments and the top of avionics stack, the FS-450 blinked and winked the numbers to answer all my fuel-situation questions.

Connected to our Honeywell Skymap IIIC, the FS-450 happily winked and blinked its way through its information inventory and left me as relaxed as any fat-dumb-and-happy aviator should get – minus the “dumb” part of the equation, of course. Heavy-duty information and a light-duty price; could you want more? Sure, easy installation would be nice, something JPI also covered.

Curious? Read on and learn how it all came together.

Confidence At A Glance

Some Of The Basics…

At its basic level, the FS-450 delivers plenty of utility even without the groundspeed and distance data from a GPS. For one thing, the FS-450 constantly displays the current fuel-consumption rate on the top of two bright LED displays. So, checking fuel flow takes little more than a glance.

Kit contents, JPI’s FS-450 fuel totalizer.

In one of those glances, for example, the FS-450 told me how much fuel our Comanche has burned since engine start, what remained of the 60 gallons we started with, even how long it would last at our current power setting. Furthering my fuel-situation awareness, the FS-450 also showed the fuel needed to reach our next waypoint, what would be left at that waypoint, and, interestingly, the fuel efficiency of our bird. And in case you’re curious, at the moment we crossed the Mississippi River, the Comanche was getting 14.8 nmpg – nautical miles per gallon. For the record, the fuel flow varied slightly between 8.8 and 9.2 gallons per hour.

What a treat to spend less than a grand for this peace of mind. Curiously, this newly realized sense of confidence cost no more than a top-end monochrome GPS portable. At Sun ‘n Fun 2001, where the totalizer first came to my attention, JPI listed the FS-450 at less than $700 and vendors sold it for less than $600. JPI’s owner, Joe Polizzotto – the “JP” of JPI – recognized that the heavy sales he experienced meant he had a winner and the list price went up to under $800. But many outlets continue to advertise the FS-450 at under $700. Installation for our Comanche didn’t get the total price up to the $1,000 mark, including some $4.00 in extra hardware from Radio Shack that I used to make a plug-in connection between the FS-450 and our Skymap.

And this may be one of the biggest appeals the FS-450 offers: In addition to its high utility and low cost, it works with most portable GPS navigators. When connected to a GPS system, portable or not, the FS-450 can calculate the last three items in the unit’s display sequence: fuel to waypoint, reserve at waypoint and nmpg. With the price of the FS-450 comparable to many high-end monochrome portable navigators, those who manage to afford a handheld GPS can probably manage the bucks for this totalizer and enjoy functionality similar to that available with higher-end – and higher-cost – equipment.

…Installation Consideration…

Installation is easy and simple, but you’ll need help on the transducer. No biggie since you need professional help to sign off the paperwork, anyway. At less than a half-pound, the weight of the FS-450 borders on the negligible, particularly when you consider that the total weight covers the totalizer in the panel, the transducer in the fuel line, the wiring connecting the two and sundry other pieces of hardware. In other words, the installation’s weight is under a half-pound and spread out in three ounces here, under three there, another ounce elsewhere.

The panel of the author’s Comanche, about to receive the FS-450.

Space is basically another non-issue. At a diameter of 2.25 inches and an equal depth requirement with its connector installed, even the tightest homebuilt and antique airframes should have enough space. Thankfully, the panel of our classic Comanche provides lots of real estate for gadgets, dials and instruments, including a vacant 3.25-inch opening just to the left of the avionics stack and above our number-one CDI. Of course, the opening required a step-down adapter which, amazingly, proved to be the most-involved part of the panel work: We popped off the glareshield, removed the panel overlay, removed a blank covering the opening, laid out the pattern for a circle and the screw pattern of the FS-450, and modified the blank to hold the totalizer. You can also buy ready-made adapters or use a smaller opening to carry the unit.

Modifying the fuel line required more thought and skill than the panel work, so that chore fell to my favorite AI, Earl “The Leprechaun” Long at Dead Cow International Airport in Wichita, a.k.a. Westport (71K). JPI supplied a set of fittings for the transducer but, in the end, The Leprechaun needed others. The transducer has to be in the line between the fuel pumps and the carburetor – in other words, in the last portion of the fuel line before the carburetor. The Leprechaun fussed and fooled with the line, cut and flared some parts, poured over fittings in his parts catalog, and came up with the right combination relatively quickly.

With the transducer installed in the fuel line, it was time to thread the line and the transducer wires into the large-diameter piece of firesleeve JPI supplies with the kit. A hole drilled in the firesleeve provided a path for the wires; a pair of clamps secured the sleeve to the line and a daub of high-temp silicone closed the hole for the wires.

…And Connections

From here, the ball was back in my court and, despite a little confusion on routing the wires, connecting the FS-450 to the ship’s main electrical buss and then to the transducer proved the easiest part of the whole job. JPI’s wire harness is very well-labeled, so there was never any doubt about which wire went where. That clarity extended to the two wires supplied to make a connection to a suitable GPS, also.

The FS-450’s transducer, installed and fire-sleeved.

The only question not covered concerned the “how” of physically connecting the FS-450’s GPS wires to a portable GPS, although the wiring diagram shows what goes to where. Polizzotto later told me that such instructions never came up because he expected technicians to do most of the wiring, whether to panel-mount or portable navigators. So he expressed some surprise and delight when informed of my solution for connecting a portable navigator to a panel-mounted device: A tiny, 1/8-inch stereo headphone jack set from Radio Shack.

Stereo headphone jacks are usually three-conductor connectors with two positive leads sharing a common ground. In my case, the goal was to insulate the connection from ground and use only two positive leads. For my needs, this tiny male-and-female connector was perfect. The input line from the GPS to the FS-450 went on the center post connector and the output line from the FS-450 back to the GPS went on the second connector. The ground connects to nothing.

The female connector went into the sub-panel adjacent to the Comanche’s cigarette-lighter plug, another female connector, while the male connector went onto the two wires in the Skymap’s multi-conductor cable that connects to the data feed from the GPS. Now, removing the portable GPS means simply unplugging the male jacks from the power and totalizer plugs or plugging them in if the GPS is being installed. If we owned a second GPS that was capable, I could even wire it to use the FS-450 connection; if we rented the Comanche, we could help renters adapt their portable navigators to work with the FS-450, as well.

Configuring And Using It

The Honeywell Skyforce Skymap IIIc.

With the wiring done, the connectors in place, and the transducer mounted and checked, putting the panel overlay and glareshield back in place completed our work. Total time? About six hours, and an hour of that could come off with a better approach to modifying the fuel line to take the transducer, according to The Leprechaun. By the way, he checked all my work twice and I checked his once before we closed up the panel and pulled the airplane out for a test run.

The first time we applied power to it, the FS-450 lit up and started its self-test sequence; on energizing the fuel boost pump the fuel-flow numbers started lighting up on the top display line. And on engine start, the FS-450 started winking and blinking away at me, eager to go to work, sequencing through the first three data points on the bottom line of the display. And once I fired up the GPS and entered a waypoint, the FS-450 started sequencing through the entire spectrum – – all six data points took their turn. Success – so far.

Programming Options…

With the Comanche signed off and ready to fly with its new FS-450 fuel totalizer, the time had come to program the unit for this particular installation. You push both the “Step” and “Auto” buttons simultaneously to get into program mode, and then again to go into the deepest settings. For example, you need to select the fuel capacity and the units – gallons, liters or pounds – as well as your engine’s fuel-system type, as in injected or carbureted. You then tell the unit whether you have aux tanks and their capacities. And you need to program the type of electronic signal your GPS uses, if connected. These menus also allow you to select the dimming level of the FS-450’s display; bring a couple of pieces of black electrical or vinyl tape to cover the light sensor for this one.

The Many Faces Of The FS-450

Click any image to view a larger version.

FS-450 in automatic scan mode, showing GPH on the top display and gallons used on the lower one.

Now, the FS-450 is showing 1/10 of a gallon less fuel flow in the top display and the total fuel remaining in the lower one.

Now, the fuel flow is down to 8.1 gph and the lower display is showing hours and minutes ( remaining.

Fuel flow has gone back up to 8.3 gph. The lower display shows the fuel required, in gallons, to reach the next waypoint, which is very close.

Fuel flow has risen to 8.8 gph on the upper display while the lower display shows reserve fuel on arrival at the next waypoint.

Fuel flow is up to 9.0 gph on the upper display. Lower display shows a fuel efficiency of 12.4 nautical miles per gallon.
(The gear must be down… – Ed.)

You can also control how quickly the second-line display cycles through its various readings, anywhere from one to nine seconds. Anytime you want to see one number constantly, you simply tap the “Step” button and the scan stops; tap “Step” more times and the lower display cycles through the various readings at your pleasure. And you simply push the “Auto” button to resume the scanning-display function.

The FS-450’s memory comes programmed with a default “K-factor,” the correction value the unit’s computer uses to decipher input from the transducer. JPI provides you with the specific K-factor shown to be accurate in tests of the transducer, to get your unit close to pinpoint accuracy. And the FS-450 also self-corrects its K-factor on command by its user to get the best accuracy level. The FS-450 performs this self-correction procedure when you tell it to arm the program and then plug in the actual fuel used when you topped off the last time. The FS-450 remembers what it thought the plane used, compares that to the actual number you provided, and corrects the K-factor accordingly. According to Polizzotto, two or three times through the self-correction factor are needed to get the FS-450 to its highest accuracy level.

In the 40-plus hours of use logged on the Comanche since we installed the FS-450, it consistently has run about .4 gph “fast” – that is, it thinks it’s using fuel faster than it really is. Programming the transducer’s specific K-factor number brought the accuracy level up to within .4 gph from about .7 gph using the generic K-factor. I haven’t yet run the self-correction program, but I will on an upcoming trip to the east coast.


The FS-450 also allows you to program in two fuel alerts: One is a low-capacity warning triggered at a value you select. If you want the low-fuel alert to flash at 10 gallons, that’s what you set; want it at 25, set it at 25. We set our low-fuel alert trigger at 10 gallons, so the FS-450 will start to flash the “Rem” readout when the totalizer calculates our remaining fuel at 10 gallons.

The second alert is a low-time warning based on your current fuel-flow rate. Want a low-fuel warning with 45 minutes left? That’s what you program; want an hour, set the FS-450 for 60 minutes. We set ours at 45 minutes, so we’ll get a warning when we have the amount of fuel equivalent to 45 minutes based on our current consumption rate. If we’re burning 10 gph, we’ll get the warning at 7.5 gallons on board.

…And More…

Once installed and programmed, operating the FS-450 really boils down to the ultimate in simplicity. When you have engine start, the FS-450 blinks a message – “Fill? n?” – which gives you a chance to update its memory with current fuel information. If you haven’t topped off, you simply tap the “Step” button and the unit retains its readings from the last time you told it you added fuel.

But if you have topped, you touch the “Auto” button and the icon for “Yes” comes up, then the fuel capacity, and you hit “Step” to complete the update. And if you only added fuel but didn’t top, you can cycle through the settings with the “Auto” button until the Add screen is on the display; hit “Step” and you get a screen that lets you cycle through numbers with the “Auto” button until you see the amount you added. It sounds more complicated than it really is. The FS-450 is the only totalizer available that supports two different fill levels.

One of my favorite “tricks” of the FS-450 is its ability to simultaneously remember and use information from two separate segments; the last one flown and the one you’re flying today. Here’s what that means to you, the user/pilot: You depart your home field without updating the FS-450 with your current fuel information for whatever reason and realize your mistake about 90 minutes into the trip. You have no idea what you’ve burned since engine start on this flight and no clue of what to make of the numbers the FS-450 displays, which are based on your last input. How do you fix this?

Elementary: You press the “Auto” button until you get an “Add” screen and tell the unit you topped off or how much you added. The FS-450’s brain updates your fuel numbers, then “remembers” what the plane has used since this engine start and subtracts that number from the fuel number you just supplied, whether it was a topoff or a short-of-topping fuel addition. Then, simply return the unit to its normal function with information current for the leg you’re currently flying. It’s one of the cleverer features JPI provided in the FS-450. And the FS-450 is the only totalizer that does this.

Another clever feature is a flashing warning anytime your fuel consumption rate and distance to a waypoint are out of synch. Let me explain: You’ve programmed your GPS for a direct flight of 589 nautical miles – a number well within your cruise-range fuel capacity. But the higher fuel flow of the takeoff and climb segments exceeds that needed to make your destination, so the numbers flash when the lower display cycles to the “Req” and “Rem” displays, the ones that tell you how much fuel you’ll need to reach that GPS waypoint and how much fuel will remain at that waypoint. This is the FS-450’s way of telling you that your fuel capacity is inadequate for reaching that point at your existing fuel-flow rate. Once you level and reduce power, the FS-450 recalculates and the blinking stops, presuming your available fuel will let you get there from here.

Where this function should be particularly usefull is on flights where a deviation or alternate destination is required for whatever reason. So, when you program in the new waypoint or destination and your existing fuel is insufficient at the current consumption rate, the FS-450 will instantly flash the “Req” and “Rem” displays to let you know. Clever, useful and independent of pilot action.

…And Versatility

There’s not much Joe Polizzotto didn’t think of when he designed and created the FS-450. For example, since the totalizer works with an industry-standard, off-the-shelf transducer, the FS-450 can serve as a substitute for failed totalizers made by other companies. That means a failed Hoskins, Shadin or Electronics International instrument can be replaced by the less-costly FS-450 without the cost or trouble of swapping out a transducer. At their relative size and weights, any space the older models fit into should also fit the FS-450. Since the FS-450 supports the use of two transducers within its existing electronics and wiring – that is, without an add-on device – owners of injected or pressure carburetor-equipped aircraft will be able to remove the extra box that computes fuel returned to the tanks and save a bit more weight while reducing the complexity of their installation. In the bargain, you’ll gain the many capabilities I outlined above, such as the ability to update fuel retroactively after launching on a flight, the ability to update fuel information with your choice of information (topped or fuel added) and automatic K-factor calculation, without losing any information points, such as fuel remaining, time-to-dry-tanks and the like.

About the only fuel-system parameter missing from the FS-450 is fuel pressure, a function also absent from most other totalizers in their standard forms.

No More “Fuelishness”

And No More Avgas Anxiety…

If knowledge is power, what’s more powerful than knowing your fuel status? Revisiting our IMC flight between Augusta, Kan. (3AU), and Clark County, Ind. (JVY), a few months back – the much-modified IFR trip the week after installing our FS-450 – we approached our Mississippi River crossing near the Festus NDB, about 45 nm south-southeast of St. Louis’ Lambert International. When our Skymap IIIC showed us flying into southern Illinois, its data readouts revealed that we remained 80 minutes out of JVY and my watch showed our flight time to that point at 3:45. Meanwhile, the FS-450 simultaneously revealed our fuel use as 27.5 gallons with 32.5 gallons remaining, enough to cruise 2.7 hours more at the 9.1 gph fuel-flow rate of our cruise setting. And the FS-450 also informed us that we needed only 11 gallons more to reach our destination at our current burn rate and groundspeed. The totalizer also revealed that we’d have 21.5 gallons left when we arrived at JVY. Interestingly, at that point the FS-450 showed out fuel efficiency as 16.7 nmpg.

All of which is comforting – very comforting. Particularly comforting was getting my top-off numbers after we landed at JVY. In the end analysis, our Comanche actually used only another 10.1 gallons during our last 80 minutes aloft. What the FS-450 could not anticipate or calculate in advance was the lower fuel-flow rate during our descent and visual approach. And as the prop wound down on the Clark County ramp, the last numbers to cycle through the bottom display showed our fuel use at 37.6 gallons for the trip and that the tanks still held 22.4 gallons. My final number-crunching for the trip revealed the FS-450 to be a bit on the pessimistic side. In reality, the Comanche used about two gallons less – right at 35.7 gallons.

Disparity aside, my ultimate final analysis concluded the obvious: A totalizer beats clock-based fuel-situation calculations hands-down – even before running the self-calculating K-factor program a couple of times.

…It’ll Change The Way You Fly

Like most pilots, my previous methods for tracking my fuel situation involved timing, an informed, experience-based set of numbers for fuel consumption at different power setting, and calculations based on those fuel-flow numbers, all of which were derived using experimentation and top-off figures. This was the same method I used to check fuel-consumption changes as a result of installing Unison’s LASAR ignition system in our bird last year. Imagine my surprise at learning that my calculations were actually fairly good, accurate to less than a half-gallon an hour.

Even with comfortable fuel-flow numbers, I always further hedged my fuel-condition analysis by limiting flights to assure me of landing with at least 90 minutes of fuel remaining. After all, my numbers would pretty much go out the window if I made an errant mistake in leaning, delayed leaning for any period of time, or flew unusual power settings or an unexpectedly large number of altitude changes or route deviations on a flight. Such concerns are now largely abandoned.

Of course, nothing can replace good judgment. There are entirely too many (read: “more than none”) fuel exhaustion accidents each year. Can improved, digital-accuracy awareness of fuel flows, fuel-remaining and time-remaining-to-dry-tanks information help eliminate these kinds of accidents? It surely can’t hurt.

Living With The FS-450

Thanks to the FS-450’s ability to deliver so much accurate information, left-seat sweats should be a thing of the past, barring, of course, some mistake in judgment or awareness that keeps me airborne long enough to trigger a low-fuel or low-time alarm. After more than 40 hours of time using the FS-450, there’s no reason other than malfunction or misjudgment for me to ever face another fuel-related confidence-challenging situation again.

The final installation in the author’s instrument panel.

It’s fun and interesting flying with the FS-450 in the panel. It makes tackling the longest flight legs comfortable and preserves my comfort zone when dealing with deviations that, in the past, generated worry enough to sap the fun out of the flight. The FS-450’s ability to talk to GPS navigators – including handhelds and portables – also enhanced the worth of our portable navigator.

Worth the time, effort and money? You better believe it. For about $700, plus installation, the FS-450 provides a degree of confidence and comfort I’ll miss any time I fly without that capability. Would I do it again? You betcha; as quickly as Joe Polizzotto could get me another one and The Leprechaun could get our Comanche’s nose in his hangar.

Once you’ve experienced that level of peace-of-mind, it’s hard to go without it.


  1. I’d like to know how you determined 125% of takeoff FF per the instructions. And how you checked it. A little confused on that part of the instal.I can’t even find what my T/O fuel flow is supposed to be in my manuals. Any help would be appreciated. Thanks