Phoenix fuel indications and calibration

[Jim Lee]

Phoenix Fuel Concepts

 

Dynon supplies complete instructions on calibrating the fuel tanks.  This report is to provide additional information for Phoenix owners.  But as my attorney likes to say, anything you read here can get you hurt or killed.

 

Question:  What fuel gauge accuracy is mandated by the FAA?

 

The Westach fuel sender (probe) works by supplying power to the outer aluminum tube of the probe. The amount of  power which is induced into a second conductor inside the tube (and insulated from it) depends upon the dielectric values of the medium separating the two conductors. Electronics in the head of the probe measure the induced potential, amplify it and send it to the Dynon Skyview ( 0- 5V). As the amount of fuel in the probe decreases from burn off, the amount of air in the probe increases, thus continuously changing the amount of power being induced.

 

The electronics are sealed to isolate them from weather, fuel spills, etc. The system will work properly with nearly any hydrocarbon-based compound ranging from kerosene to diesel oil.

 

Turbulence-induced fuel "slosh" has little effect on the system because the fuel being measured is inside the probe and the fuel cannot move fast enough to affect the gauge reading.  (There are 2 small holes in the probe which allow the fuel to enter and exit).

 

The accuracy of the Westach sender combined with the digital fuel readout of the Dynon is very exact.  However, there are issues with the system that introduce inaccuracies, which we will discuss.

 

Fuel sender viewed at wing root:

 

The probes come in two-foot lengths.   They have four wires exiting the end, one of which is a ground wire attached to the fuel plate.  The remaining 3 wires go to the plug inside the plane.  The red wire supplies 12 volts to the probe, the yellow wire returns 0-5 volts to the Dynon, and the black wire is ground.

 

The probe’s aluminum tube bends straight down, and then slowly angles up to touch the top of the tank.  Therefore the probe measures fuel from the bottom of the tank to the top of the tank.  But since the tube is only 2 feet long and has bends, it extends less than 2 feet out from the wing root.  Since the wing has dihedral, the fuel cannot be measured in the last few gallons of the tank.  We find that the voltage readings don’t change when the last 2 or 3 gallons of fuel are added.  This means that when the tanks are full to the top, you can fly for a half hour or so before you see a drop in the fuel level. 

 

Bends in the aluminum tube:

 

 

The Phoenix is generally fueled and calibrated with 93 octane unleaded, non-ethanol fuel.

The type of fuel used for the calibration will not be exactly correct for other fuels.  When the plane is fueled with 100LL, there will be a very small drop in fuel readings.

 

Fuel calibration in progress:

 

 

The Phoenix fuel system is calibrated with the tail propped up in the level flight attitude.  The probe is in the rear of the tank.  When the plane is sitting on the ground with a high angle of attack, the gauge reads high by about 2 gallons per side (if the tanks are not full). 

 

As you climb away at a high angle of attack with full fuel, the fuel reading remains high, and you are burning fuel in the process.  If you climb to a high altitude and then level off, there will be a steady decrease in the fuel reading over the next minute due to burned fuel and the change to level attitude.  This has resulted in more than one pilot thinking that there was a sudden fuel leak overboard!

 

Note:  The 2 carbs don’t burn all of the fuel supplied to them.  The excess fuel (about ½ gallon per hour) is returned to the left tank.  If you fill both tanks to the brim (not recommended unless departing on a long flight) and you do not select the left tank first, that returned fuel will be vented out of the left wingtip.  And you will not only lose fuel, you will make a mess of your wing.  The same thing will happen if you fuel in cool weather and then it warms up.  It will also happen if you bounce along on an uneven surface with full tanks.

 

The Dynon asks for total fuel at the beginning of the calibration process.  We enter 14 gallons.  The AOI states that the fuel capacity is 13.2 gallons.  This is the minimum fuel capacity.  Almost every Phoenix can hold 14 or even 15 gallons per side.  When the wing halves are mated in the factory, one guy places the outer panel of the fuel tank in place.  If he moves it an inch one way or the other, it makes a gallon or more difference.

The Skyview asks for 1 gallon to be added at a time.  Then a button is pushed, and another gallon is added, all the way to 14 gallons.  After that we punch in “Full”.  We might be able to get another gallon into the tank, but it doesn’t matter because of the dihedral, remember?

 

The voltages corresponding to each particular gallon amount can be seen on the Skyview, but it is of no value to us during the process unless something is going wrong.  We have had a couple of bad Westach senders and a couple of sender plugs which were bad and needed to be replaced.  If a sudden fuel indication problem is noted, looking at the connector plug would be the first place to look.  You can pull the plug out of the outer seat belt slot on either side to check, without removing the wing.  It can also be accessed by removing the seat pan.

 

Back in the good ‘ole days, we had analog gauges which read full, ¾ , ½ , ¼, and empty.  That used to satisfy us, even though they probably were not even that precise.  Today, with the advent of glass panels and digital gauges, if the gauge reads 7 gallons and we think it should read 8, we freak out! 

 

I have consistently flight planned 4gph and flown 6 hours.  It is always a good idea to cross check the fuel readings with the clock, and your pre-launch visual check of the fuel in the tank, and go with the most conservative.  I can tell you from experience though that if you fly for 6 hours and then decide to head for the dirt from 14,000’, you won’t like it a bit when the fuel levels go from 2 and 1 gallon remaining to zero on both sides when the angle of attack goes negative!  Which brings up an interesting point.  Say you have really overstepped your bounds and are about to run out of fuel with some way to go to your destination.  You can burn every last drop of fuel (well, ok ½ cup unusable) at a climbing angle of attack.  Maybe you would want to climb all the way to your destination to use all of the fuel.  Maybe you would want to reduce power to 4000rpm, and speed to 70kts and fly a shallow climb.  After all, you are flying a glider and can land engine off, right?  What are the legal requirements for minimum fuel, VFR day in a glider?  Yep, none.  But do you want to go there?  Of course not.

 

So how accurate is that Westach and Dynon anyway?  I have run a tank dry several times in different planes just to see.  At 4000rpm, after the Dynon goes from 1 gallon to zero, there is about 2 minutes of fuel left. That is pretty damned accurate.  (Running out of fuel at a high rpm is not good for the engine.  That is partly why we shut down by slowly switching from both mags to one to achieve minimum rpm prior to shut down which also reduces kick back.  And why we slow to 60kts for an in-air shut down.  But that’s a whole nuther story).  Switching tanks, turning on the fuel pump with the prop windmilling, the engine starts back up immediately. 

 

So how about them FAA regs anyway?  They state that the gauge must read empty when there is zero usable fuel in the tank.  That’s it!

[Eric Greenwell]

9 hours ago, Jim Lee said:

You can burn every last drop of fuel (well, ok ½ cup unusable) at a climbing angle of attack.

Is the fuel pickup near the rear? I'd assumed the pickup was at the low point while in cruise attitude. Is it actually near the fuel sensor?

9 hours ago, Jim Lee said:

At 4000rpm, after the Dynon goes from 1 gallon to zero, there is about 2 minutes of fuel left

On my Dynon, the gallons are displayed as truncated integers; for example, 2.9, 2.4, and 2.1 gallons are all displayed as 2. So, when mine displays "0", initially there is still 0.9 gallons available. Of course, that will continue to decrease, and it will still be reading "0" when the engine stops! Practically speaking, if I have 4 gallons or less, I'm into my reserves on a cross-country flight.

[Jim Lee]

Yes the fuel pickup is in the rear, directly over the fuel drain, right next to the lowest part of the fuel sender.

[Eric Greenwell]

When I was determining the amount of fuel that could be used, I did it in cruise attitude. I now wish I had then put it in 3 point attitude to see how much more was available. Well, something to try after the other dozen things I'd like to do