Fuel Gauges

[blueldr]

Those too!

[ralphyoung]

I prefer the Lowe's paint mixing sticks over the Home Depot sticks.

[GAHorn]

Lowe's sticks read Lower.

...The gauges in all 170s bounce when their new they bounce when they're old. Some bounce more than others ....

Gauges in RED 170's bounce less-per-mile than those in GREEN 170's, especially those Green ones flown by helicopter guys. (However those guys are used to vibration so they likely can't tell the difference.)

On a more serious-note: The "Fuel Hawk" brand (Model M-JAR-172) of clear, acrylic dip-tube made for the 19 GALLON TAnk Cessna 172....is a good product for the A and B-model C-170s with 19 gallon tanks. This tube is designed with the "Zero" mark about one-inch above the bottom of the tube (along with a couple of indices-markings below the Zero mark.)

Fuel Hawk.jpg (19.92 KiB) Viewed 69376 times

Take a tubing-cutter and remove the portion of the tube below the "Zero" and, using a file or a wire-wheel, polish the newly-cut end to match the factory rounded end which previously existed....or don't, your choice.
Now the modified C-172 Fuel Hawk for the 19 gal tanks...can be reliably used. To use accurately, place the tube at the rear of the fuel-cap opening, with the bottom of the tube submerged to the bottom of the tank...and resting alongside the mid-tank baffle. (About mid-tank, immediately aft of the fuel filler cap, is an internal baffle to reduce fuel "sloshing". Place the bottom of the modified Fuel Hawk at the juncture of that baffle and the bottom of the tank.)
YOu will find the modified Fuel Hawk will accurately depict the tank contents.

(Warning: Do NOT buy the C-172 for 26.5 gal. tanks. You want the Fuel Hawk for C-172's with the 19 gal. tanks.)

[170C]

George, regarding the 19 gallon fuel tanks, aren't the tanks actually 21 gallon tanks? I ask because on OLE POKEY my tanks capacity is 21 gallons although the most I can put in them is 19.6 gallons. I have always felt the reason for not getting additional fuel in each tank had to do with the conventional gear configeration and that in the standard 172 trike configeration one could possibly squeeze in additional amounts. Of course due to the filler location there will be some capacity lost to dihedreal.

Of course it might be that with the speed of GREEN airplanes they decided 19.6 would surfice

[Bruce Fenstermacher]

Frank, the capacity is 21 gallans on the A and B models. BUT we are talking dip sticks and the Air Hawk 172 19 gal stick can be modified as George suggest to be accurate on our 21 gallon tanks.

[GAHorn]

Frank, Bruce answered correctly (of course!)
The tanks in A/B models are 21 gal tanks (and why the factory claims these are 42 gal capacity fuel systems.)
However the USEABLE fuel is 18.5 per side (UNuseable fuel being 2.5 gal per side). (Hint: See the AFM)
Therefore the 19 gal Fuel Hawk is correct for the 170 A and B (and your early 172.)

Trivia: The mid-tank fuel baffel mentioned earlier...works well because...the A/B models take fuel from the ...mid-tank fuel strainer. (Fuel which is aft of the mid-tank baffel cannot access the fuel strainer in the 3-point attitude, and that is why we need to remove the 2 index-marks below the zero on the Fuel Hawk. Remember? Unuseable fuel is that fuel which is not useable in ALL normal flight attitudes...such as CLIMB (which is the same as the 3-point stance.)

Reminder: The Fuel Hawk (or ANY other dipstick) is only reliable when the aircraft wings are laterally-level.

[n2582d]

If I recall correctly a fuel gauge in an aircraft only has to be accurate when the tank is empty. In other words a gage can read full when the tanks are not, they can read half full when there is more than half left but when the tank is empty the gage needs to read empty. …

I recalled this discussion when I recently read the following clip from CIES FAQs page. Obviously not an unbiased opinion.

Click to Enlarge

[Bruce Fenstermacher]

I recalled this discussion when I recently read the following clip from CIES FAQs page. Obviously not an unbiased opinion.
IMG_0620.jpeg

Hmmm. Glad I don't want to put CIES probes and gauges in my Vagabond case I don't think they know what they are talking about. CAR 3 3.672 calls for the fuel gauge to read zero when at the unusable fuel level. In other words no more fuel that can be used. There is no other specification under CAR 3 as to how a fuel gauge must read at any other amount of fuel. Yes I concede I stated zero when the tank is empty. Technically there could be unusable fuel in the tank the engine can't use and the engine will get very quiet such as in a 170 with 4 gal. in the 3 tanks or a 170A or B with 2-1/2 gal in each tank. I must have been thinking about my Cub at the time which has a single 12 gal tank and all fuel is usable. When the stick stoped bobbing you'd better be on the ground. My Cub had a very accurate rod on a floating cork, my Vagabond which has the same tank has a modern Ford model A fuel tank gauge but a little less accurate than the Cub.

CIVIL AIR REGULATIONS
——————
PART 3—AIRPLANE
AIRWORTHINESS—NORMAL,
UTILITY, ACROBATIC, AND
RESTRICTED PURPOSE
CATEGORIES
——————
As amended to November 1, 1949

§ 3.672 Fuel quantity indicator. Means
shall be provided to indicate to the flight
personnel the quantity of fuel in each tank during
flight. Tanks, the outlets and air spaces of which
are interconnected, may be considered as one
tank and need not be provided with separate
indicators. Exposed sight gauges shall be so
installed and guarded as to preclude the
possibility of breakage or damage. Fuel quantity
indicators shall be calibrated to read zero during
level flight when the quantity of fuel remaining in
the tank is equal to the unusable fuel supply as
defined by § 3.437.

[n2582d]

Bruce,
I’ve been with you in the “fuel gauges must only be accurate when tanks are empty” camp. On first glance, CAR 3.672 seems to be straight forward in this regard. But after reading several articles on this subject, I think we are incorrect. Google “aircraft fuel gauge myth” and you’ll find numerous articles or discussions debunking the common understanding that the fuel gauges only need to be accurate when there is no usable fuel. Most of the articles are discussing FAR 23.1337 (b)(1):

Fuel quantity indication. There must be a means to indicate to the flight crew members the quantity of usable fuel in each tank during flight. … Each fuel quantity indicator must be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply.

The main difference between this FAR and CAR 3 is that the word “usable” in the first sentence. This FAR was superseded in 2017 by FAR 23.2430

(a) Each fuel system must -
(4) Provide the flightcrew with a means to determine the total useable fuel available … .

CAR 3, not FAR 23, is what applies to the C-170 but, like the rule about latching seatbelts, C-170s must also follow FAR 91:

FAR 91.205 (b) For VFR flight during the day, the following instruments and equipment are required: … (9) Fuel gauge indicating the quantity of fuel in each tank.

The clearest explanation I’ve read regarding the CAR 3.672/FAR 23.1337 rule about calibrating the gauge for zero usable fuel is that this is not saying the gauge doesn’t need to be accurate at other settings, it’s only clarifying that “empty” on the fuel gauge doesn’t mean no fuel in the tank; “empty” should indicate no usable fuel in the tank. The first part of CAR 3.672 requires the fuel gauges “indicate … the quantity of fuel in each tank during flight.” Ironically, in their apparent effort to provide clarity to this rule by defining “empty,” the CAA/FAA caused confusion. This was finally clarified in FAR 23.2430 which, I think, is what they were trying to say all along.

[DaveF]

Why is this even a topic of discussion? It's illogical to read CAR3 as saying the gauges must be accurate at zero usable but are allowed to be arbitrarily inaccurate at any other fuel quantity. What IA would sign off an airplane whose fuel gauges were stuck at E, saying they meet CAR3? If accuracy is only required where specified by FAR, the rest of our engine gauges never have to be accurate.

"Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight." All gauges are presumed to be accurate within the manufacturers' standards, but in the case of fuel gauges, CAA wanted to additionally define "zero" as "zero usable". That's all there is to it; the rest is lore, OWT, and maybe sarcasm.

[GAHorn]

I once had a very respected engineer tell me that the gauges only requirement is that they read “Empty” when the useable fuel is exhausted…and he made (by inference) the implication that would be the Only requirement for accuracy.

I believed him at that time because he was vastly more experienced than I …and it suited me at the time.

However, just because an STC specifies that the propeller approved in that STC is approved with a particular make/model of floats…. does NOT equate to that prop being unsafe with other floats… It only means that prop has …so far…been approved by that particular STC …. with only the floats mentioned.

A similar situation exists in this discussion. YES…when all the useable fuel is exhausted the gauge MUST read “Empty”. It may NOT read any other level of fuel. (Which is why I kick the rudder to see if the fuel gauge will “bob” and move whenever I’ve seen my gauges on “Empty” and I’ve not yet landed.)

In other words, the statement in CAR 3 is not the ONLY requirement of the fuel indication system…but it IS a SPECIFIC one.

CAR3 requires a “means” for indication to the crew of the fuel quantity …. the “means” I personally use is to check the “Full” tank condition on preflight inspection, the gauges do not ready “Empty” but instead read “Full”…and I use the clock and 8 gph….. and I confirm the “empty” gauge indications are telling me the truth by kicking the rudder. (The one time most memorable to me that system is the Best system is the story I told of my flight home from the VT convention…when I failed to check the linemans’ work by visibly confirming full tanks. The gauges indicated “Full”…. the lineman was told to “top the tanks”…. )

So much for CAR3 in any situation other than Empty fuel tanks.

HOWEVER…. I do not believe ANY fuel indication system approved for our airplanes is any better than the properly-maintained and operational mechanical system Originally approved and installed. While there’s nothing “wrong” in my view with those who like higher technology installed in older airplanes…. that higher-technology does Not Equate to “Better”. You still cannot trust any fuel indication system without additional confirmation….such as proper preflight inspection …and a clock.

Spending money on modifying our airplanes does not equate to “better”.

Before I’d spend money “upgrading” the fuel gauges …. there’s a lot of other areas that could benefit ….. YMMV