A Dangerous Flaw in Electronic International's FP-5 User Interface

WARNING: A Dangerous Flaw in the Electronic International's FP-5

Electronic International's FP-5 and FP-5(L) are fuel management instruments that employ a calibrated transducer in the main fuel line to calculate the rate of fuel flow. The pilot programs the FP-5 with the quantity of fuel onboard, and the FP-5 derives and displays useful rate-based information such as Time to Empty and Fuel Remaining. I have owned an FP-5 for several years and have found it to accurate to within one or two tenths of a gallon (an error I attribute to by my inability to replenish the fuel tanks to precisely the same level each time). Indeed, using the accurate fuel accounting of the FP-5, I confidently and routinely made long-duration flights that I would not attempt using manual fuel calculation methods.

I recently discovered that the FL-5's user interface contains a subtle, but dangerous flaw that makes it possible for the pilot to inadvertently change the units of measurement used to compute fuel flow--without that change being indicated on the FP-5's display. This leads to under-reporting the rate of fuel burn by more than 20% and, consequently, over-reporting Time to Empty and Fuel Remaining by the same amount.

I do not know whether this problem exists also for the FP-5L, a more advanced version of the unit that interfaces with your GPS to predict the amount of Fuel on Arrival and Miles per Gallon.

To understand the nature of the flaw, let's look at the manual page description of how the FP-5 displays fuel flow:

In the "Flow" Mode, the FP-5 will display fuel flow as follows:

A. When set to display in Gallons the display will read in .1 Gal/Hr increments up to 199.9 Gal/Hr.

B. When set to display in Imperial Gallons the display will read in .1 Gal/Hr increments up to 162.0 Gal/Hr.

C. When set to display in Pounds the display will read in 1 Lb/Hr increments up to 1199 Lbs/Hr.

D. When set to display in Liters the display will read in 1 Ltr/Hr increments up to 749 Ltrs/Hr.

The flaw under discussion lies in items A and B. When the units of measure are set to Gallons or Imperial Gallons, the display reads "Gal." This is exactly how the FP-5 behaves.

Consider the following scenario.

You land with the FP-5 in Flow mode, taxi to fuel, then power-down the engine.
Because you already know that you're going to add fuel to the tabs, you decide to program the FP-5's "Fuel Remaining" to 38 gallons before turning off electrical power.
Following the procedure you use to program the FP-5 after fueling, you simultaneously press both red PRG buttons.
Instead of seeing the familiar word "ADD" on the display, however, you see an unexpected, unlabeled screen--and conclude that you're in the wrong mode.
In an attempt to get out of the wrong mode, you push one or both of the red Prg buttons, or perhaps push the step switch (a toggle switch) a few times--to no avail.
In frustration, you again simultaneously push both red Prg buttons, which just returns you to the step mode screen.
As a last resort, you power cycle the FP-5.
The FP-5 powers up in the familiar "Fuel Remaining" mode with the REM indicator blinking.

Ah, relief! Now you know where you are.

You simultaneously push both red prg buttons to enter the "ADD Fuel" mode.

You now manipulate the STEP switch and red buttons so that the display indicates 38 gallons, then save the new value by simultaneously pushing both red Prg buttons.

You now turn off power to the FP-5, fill the tanks to the tabs, restart the plane, and fly away.

While cruising to your destination, you monitor either the REM or Time to Empty display. You'll also probably use the fuel flow mode for leaning. Based upon the FP-5, you conclude you can easily land with 30 minutes or reserve fuel.

With the LP-5 indicating 48 minutes to empty, your engine stops. Both fuel gauges indicate "Empty."

You are out of fuel.

Here's what happened.

Because the FP-5 was still in Flow mode after landing, pushing both Prg buttons puts the FP-5 in "Pilot Programmable Flow" mode, which presents (if you're not expecting it) a screen of perplexing choices of units of measure: Gal, br Gal, Lbs , and Ltr. "Gal" is a US gallon. ("br Gal" [sic] is a "British" gallon, referred to throughout the documentation as an "Imperial" gallon); "Lbs" is pounds; and "Ltr" is liter. Once in this mode, one cycles through the selections by moving the step switch. Simultaneously pushing both red Prg buttons again saves the new selection to non-volatile memory and returns to flow mode.

In step #5 above, pushing the STEP switch only once changes the current selection from "Gal" (i.e., US gallons) to "br Gal" (i.e., Imperial gallons). If, as in step #6, you push both red Prg buttons, the newly-selected unit of measurement is permanently stored in the FP-5's non-volatile memory. The only way to get out of this mode without making permanent changes is to power-cycle the FP-5--an important fact that is not explained in the manual.

Now you're set for disaster: you've inadvertently programmed the FP-5 to measure and display all rates and quantities in Imperial gallons (one Imperial gallon equals 1.202 US gallons). This is bad, but it gets worse: the display does not indicate "br Gal/hr," but continues to indicate "Gal/hr." So, due to an insidious interface design error, you've just redefined a gallon upwards by more than 20%--and the FP-5's display gives no hint that anything has changed¹.

Let's assume (to keep the arithmetic simple) that your plane burns 9.1 US gallons per hour in cruise. At this rate, your 38 US gallons will be exhausted in 4.2 hours. Because it is measuring fuel flow in Imperial gallons, however, the FP-5 indicates fuel exhaustion in 5 hours (38 divided by 7.6 Imperial gallons per hour). Your engine will therefore run out of fuel 48 minutes sooner than indicated. (In reality it will be exhausted sooner because your engine burns substantially more than 9.1 gallons per hour during take-off and climb to cruising altitude--but you get the idea.)

The only indication that something is wrong comes when the FP-5 is in Flow mode, where it indicates about a 20% lower-than-expected fuel burn--7.6 instead of 9.1 gallons per hour. An alert pilot would certainly notice this discrepancy and would immediately start looking for an explanation in the airplane's performance (i.e., airspeed), which, of course, would be normal. The astute pilot would quickly reason that because the airplane's performance is normal, the flow indication must be erroneous, and would revert to manual timing to ascertain remaining fuel on board and time-to-empty.

Although there are three ways to inadvertently misprogram the FP-5's flow mode, only the case of Imperial gallons is pathological. Consider when either "Ltrs" or "Lbs" is selected. First, in both cases, the numbers shown on the display over-report the rate fuel consumption. Second, the display correctly indicates the units, so there's a chance that the pilot will notice the error. Third, if the wrong units on the display don't catch the pilot's eye, then the absurdly high numbers in the rate-based display modes certainly will. Again, assuming a flow rate of 9.1 US gal/hr, misprogramming for Liters would display a flow rate of 34.44; misprogramming for Lbs would produce a flow rate of 56.4. Other display modes (e.g., Flow, Fuel Remaining, Time-to-Empty) would have equally exciting numbers. In short, such egregiously out-of-bounds numbers would instantly catch the attention of the pilot, who would conclude that the instrument was malfunctioning and either cease relying on it, or get back on the ground immediately to troubleshoot the problem.

Although I have not personally been bitten by the FP-5's misrepresentation of flow units, I know two pilots who have.

In the first case, the pilot, Steve Williams, doesn't recall the amount of fuel indicated by his FP-5, but does remember his astonishment when he pumped 22 gallons into his Grumman Yankee (which holds 22 useable), then subsequently discovered that his FP-5 had been measuring fuel flow in Imperial gallons.

In the second case, my co-owner Justin Hecht landed with the FP-5 indicating 17.5 gallons when less than 9 gallons were aboard. I discovered the error a day or so later when I refueled the plane prior to its next flight². After learning what caused the error, I asked Justin whether he had noticed reduced fuel burn in cruise. He replied, "I topped the tanks off in Portland, then programmed 52 gallons into the FP-5. At 10,000 feet, I adjusted the mixture both aurally and with the EGT meter. The fuel flow was about 7gph, which seemed really low. I carefully monitored all the engine instruments and sounds, saw no indication of trouble, and then kept watching." Had the encountered a stronger-than-forecast headwind or been forced to divert around weather, his flight could easily have had an unhappy outcome.

It's important to note that neither of the pilots recalls when he might have misprogrammed the FP-5. Indeed, in the second case, I might have been the culprit. This is not surprising. Highly modal interfaces such as that of the FP-5 (and similar instruments from a variety of manufacturers) are susceptible to inadvertent programming errors, because the operator seldom has any indication either of the mode he's currently "in," or how to get "out" of it. (Recall that the only way to get out of the "Pilot Programmable Flow" mode without programming it is to power cycle the instrument.)

The FP-5 manual, of course, contains a sensible caveat against relying solely on the FP-5:

"While in flight the FP-5 readings should only be used to crosscheck the fuel level gauges, and calculations of the fuel on board from flow rates that you measured from previous flights. The use of the FP-5 does not eliminate or reduce the necessity for the pilot to use good flight planning, preflight and in-flight techniques for managing fuel."

Although a call for caution is always sound advice, in this case it's irrelevant, especially when you consider that the FP-5 is behaving precisely as designed and documented. The simple fact is that when programmed for Imperial gallons, the FP-5's display indicates the same the units-of-measurements as when it's programmed for US gallons.

When I discovered the error in my FP-5, I called Electronics International about the problem. I described the problem to the support person as my "FP-5 under-reporting fuel use by about 20%." Without asking another question, he diagnosed the problem as "Flow mode misprogrammed to British gallons." This suggests that my call on this problem was not his first. When I pointed out that this was a very dangerous flaw, he mentioned that in more recent revisions, the interface has been changed to make it "almost impossible" for the pilot to misprogram this constant accidentally.

Indeed, the Rev H. manual does include a new, safer procedure for actually changing the units. Recall that in earlier revisions, after entering Flow mode in my revision of the FP-5, the pilot could cycle through the various settings simply by pressing the step switch. The REV H manual, documents a significant improvement::

To change the display to "Gal", "br Gal", "Lbs" or "Ltr" - Moving the Mode Select Switch to the left while pushing the left program button will alternate the display between "Gal", "br Gal", "Lbs" and "Ltr".

This change is a significant improvement because it creates an effective interlock to prevent the pilot from accidentally or casually changing the value.

I recently upgraded to a newer version of the FP-5. My new unit, serial # 077760, has the two-button interlock described above; however, my new unit still incorrectly displays Gals when programmed to measure Imperial gallons. It's difficult for me to understand why the company didn't also correct the underlying problem of misreporting the units of measure.

Electronics International has a well deserved reputation for excellent products, customer service, and technical support. In fact, I find EI the best company in this regard I've ever dealt with--in or out of aviation. This is exactly the type of company we in general aviation need to support. It's therefore perplexing that such an excellent company hasn't moved to correct this potentially fatal problem. Indeed, I have tried several times to reach the president of the company, both via email and telephone, but he hasn't responded to my queries.

I therefore encourage all FP-5 and FP-5L owners to contact the company to determine whether your model/serial number possess the flaw described, and to negotiate a price for upgrading to the most recent version. Prospective owners ought to contact the company to make certain that this problem is fixed in the model they're intending to purchase.

Caveat Aviator.

Click to email comments, corrections, or similar experiences to the author.

Joe Campbell
Berkeley, CA
N4524P '79 Tiger

¹Well, if you're on your toes, there is a single clue that something has changed: when you get back to the "Add" screen, the number of gallons remaining indicated will have been revised downwards. But since the premise is that you landed in Flow mode, it's not very likely that you'll remember how many gallons of fuel were remaining. ² I became suspicious after visual inspection of the fuel levels during preflight.

Last revision: 12Nov2005