Fuel Gage Indication: “Pucker Effect” versus Reality

[drdehave]

Today, flying my 2007 Sting Sport LSA, I tested my tolerance for fuel gage indicator “pucker effect.” Pucker effect occurs when I’m still a good number of miles out from my destination, but the fuel gauge needle is stuck hard against the empty mark and has ceased moving.

Actually, I have prepped for such a day several times. Every 6 months or so, or at minimum once a year during Annual Condition Inspection, I drain the fuel tank. Then, I put back various measured quantities of fuel and check this using my measuring-stick-in-the-tank; but I also go fly, to see what the gauge actually indicates flying straight and level (which is quite different from that whilst sitting, taxiing, climbing or descending.) That way, I can always estimate (either by the gauge itself, the measuring stick, or both) how much gas I have left–-usually to within about a gallon, for my 20-gallon main tank.

Anyway, today I flew another California-drought-video mission with a wonderful colleague who is nonetheless substantially overweight at 220 pounds. Since I would be dealing with extreme heat and take-offs from a short mountain runway, I wanted to be as much under gross weight as possible (i.e., and still take the robust passenger along); So I opted to leave home with minimum weight of fuel (including 1-hr reserve) of 18 gallons (the airplane can hold 32.5 gallons).

Well we completed the mission (which was longer than anticipated), I dropped off the heavy passenger at an intermediate way-point and pointed my baby for home, 88 nmi away. The gauge was already bouncing back-and-forth across the last-quarter mark. Soon, it was well into that last quarter. And at 33 miles out, the needle was stuck firmly against the empty mark and never moved again. (In this airplane, weather by design or chance, the movement of the gauge needle greatly accelerates--I presume [due to shape of the tank] to ensure an added 'reserve'--during the last quarter of the game.)

Nevertheless, from all the previous testing, I was confident I could make it–-unless, God forbid, something with regard to gauge functioning (or my 'stick measurer') had changed! This doesn’t mean I wasn’t puckered up pretty darned good. Also, I did hedge my bet coming in: I climbed to 5,000 feet so I’d have a 10-mile glide if needed; I dropped RPM to a fuel-conserving 4800; I followed a course with plenty of farm-roads, "hard" fields and a couple of "private" airstrips; and to shave off the last couple of miles, I entered left traffic on a long, extended base.

When I finally landed and got to my hangar, the very first thing I did was open the fuel-sump drain valve and stick a gas can under it. By weight, exactly 2.2 gallons came out--almost all “usuable” in this particular airplane. While a 30-minute reserve (the planned 1-hr was reduced by headwinds not in the weather briefing) is the lowest I have ever landed with in almost 1300 hours of flying this machine, it was nice that all the earlier testing had given accurate enough data that I could take her to the “edge,” on fuel weight, to reduce gross weight, as a hedge against high density altitude (DA). The DUC propeller on this airplane really does not pull it off the runway very well in high DA; aloft is where it's performance excels.

Excuse me now. I do need to go put a little Vaseline someplace! And I am going to add this to my list of "I'll never do that again," events.

[snaproll]

Glad you made it back safely.. Great blow by blow of your thoughts and actions, and wise preparation for the unknown. Haven't chased any California-drought conditions here in SoCal - not much left to find except low reservoirs and dry creek beds. Looking forward to your next video.. VR.. Don

[MrMorden]

I have to say it rubs me the wrong way that you call your "wonderful colleague" names like "fatso" and "lard bucket."

I bet you wouldn't write that if this great guy read this forum.

[Wm.Ince]

I have to say it rubs me the wrong way that you call your "wonderful colleague" names like "fatso" and "lard bucket."
I bet you wouldn't write that if this great guy read this forum.

I was thinking the same thing Andy.
For me personally, those characterizations, regardless of the authors intent, detracted from the post.

[dstclair]

Rich,

What did the gauge read with the 2.2 gallons on the ground?

[drdehave]

I bet you wouldn't write that if this great guy read this forum.

Actually, he does read this--and we were joking about this issue yesterday, while planning our flight. But it nevertheless was a totally unnecessary inside joke that wouldn't be "gotten," by some and the wrong thing to do regarding an otherwise serious issue. I do apologize for offending some of you outside of our loop. And I have edited the trash-talk away.

How much did the gauge show back at the hangar, stopped, sitting flat on the ground?

A little bit.

[dstclair]

Rich -- I have to agree that the Sting fuel gauge needle gets a bit jumpy once it his the 1/4 mark which, of course, lights the little red 'low fuel' light. With ~21.5 usable gallons in the main tank and took of on a planned 3hr flight last summer. I'm conservative so assumed a 5.5g burn (which I'm never over 5/hr for my flying) which gave me essentially 1 hr reserve on planned landing. Winds were a bit more than forecast and it took me 3.5 hrs with the last 15 minutes being in the 'Pucket Effect" zone -- needle at just above 1/4 but swinging above/below. Turned out I landed with just about 6 gallons.

No morale to the story other than after more than 20 years of flying I still rely on my watch as the primary fuel gauge which I cross-reference to the physical gauge.

[CharlieTango]

Gotta love the simplicity of a gravity fed high wing with sight tubes.

I was fuel critical once but could manage to keep visible fuel at one wing root / sight tube so I knew I had remaining fuel available.

[drdehave]

Sting fuel gauge needle gets a bit jumpy once it his the 1/4 mark which,...

Yes, I've stopped a few times to to put Av-gas in, because of that, only to find 6 or 7 gallons still sloshing around in there. That's when I started all the measuring.

I presume this to be due to the integral main tank, which conforms to the airplane's hull shape.

[CTLSi]

......

[snaproll]

Sting fuel gauge needle gets a bit jumpy once it his the 1/4 mark which,...

Yes, I've stopped a few times to to put Av-gas in, because of that, only to find 6 or 7 gallons still sloshing around in there. That's when I started all the measuring.

I presume this to be due to the integral main tank, which conforms to the airplane's hull shape.

All of us have had the “Pucker Effect” once or twice though many will not admit it. Mine was on my first solo cross country (mid-60’s) from Flabob (Riverside CA) to Deer Valley Airport (Phoenix AZ), a nice 302 mile flight. I had checked weather and winds aloft, planned based on the predicted 5 MPH headwind, marked up my sectional with course and checkpoints (landmarks), and had my trusty Timex and E-6B. After the first hour, my estimated groundspeed of 95 MPH turned out to be 80 MPH with a stronger headwind (FAA Weather was wrong – nothing new). Started receiving Phoenix Sky Harbor Airports VOR about 100 miles out, running low on fuel, so switched to my left tank and timed it until run out – about 30 minutes and just reached the outskirts of Phoenix. Left with 30 minutes of fuel in the right tank, set the OMNI on Sky Harbor’s outbound radial lining up with Deer Valley Airport, and landed 15 minutes later. After landing, fueled the bird which took 22 gallons with a 24 gallon capacity. Hence – “Pucker Effect” was in full force, felt like an idiot, and never repeated the same mistake again. Was flying the SA-9A Skycoupe, O-200 Continental powered, with two 12 gallon wing tanks. Flew home the next day after checking weather.

[Merlinspop]

I do apologize for offending some of you outside of our loop.

I don't believe people who are offended on behalf of someone else deserve an apology.

[drdehave]

This morning, being that the gas tank was still dry, I arrived with my jugs, each with exactly 5 gallons from the gas station (plus the leftover 2.2 from yesterday) and slowly re-filled, whilst checking my various 'marks.'

I discovered that I actually departed yesterday with about 17.5 gallons, not the 18.0 I thought (yesterday involved about half in the tank, plus half new added). So on yesterday's trip, I used 15.3 gallons, with three take-off climbs of ~4,500 feet at 450-500 lbs load and headwinds. Hobbs-hours total for the trip was 3.7, which equates to 4.1 gallons per hour. This shows what can be done using throttle position to conserve fuel and that, for me at least, planning for 5.0 gallons per hour should provide a margin of safety.

Oh, by the way: You Sting Sport/Sting owners, take a close look at your gas gauge. Each of the 1/4th-tank intervals is quite different--I presume a necessity due to the variable vertical shape of our integral tanks!

[FastEddieB]

I have twice run uncomfortably low on fuel.

Once in a Grumman Traveller - ancient history.

But once in my Sky Arrow. Headwind's effect on planned range can start as subtle and slide effortlessly into critical. It's insidious.

Though the potential drawbacks of fuselage tanks have been discussed, they do make plumbing easier than two separate wing tanks.

Below you can see my tank, over my head as I torque my main gear bolts:



The tank is translucent, and in it's lower capacities is marked in both liters and gallons.

In any case, I know from experience that 1/8 showing on my gauge is just over 2 gals remaining, and not enough. By planning legs not much longer than 2 hours, I now nearly always land with at least 1/4 tank showing which should be between 4 and 5 gallons. Bear in mind the tank only holds 17.8 usable.

[MrMorden]

I do apologize for offending some of you outside of our loop.

I don't believe people who are offended on behalf of someone else deserve an apology.

I agree. And I was not offended, as I said it just "rubbed me the wrong way", as in it seemed unnecessarily pointed and not required to tell the story. I'm sure this guy can take care of himself, and I don't expect anybody to change their behavior on my behalf...but I'll usually point out something that annoys me, rather than let it sit and fester.

Clearly there was some inside joking and ribbing here between these two guys I was not privy to -- no harm, no foul.