Fuel gauge inop

Flyingfanatic

Flyingfanatic

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So a Cessna 172 has one of the fuel gauges not working.

Can you legally fly the airplane?

This came up today and there was significant discussion. I didn't readily know the answer and neither did several others. I have the answer, but I thought it would be good to share with others.
 
How was this a significant discussion. INOP, not legal.
 
They're crap instruments on a good day. A dipstick and a watch are better tools to bet your life on. Besides, you can fly on the dud gage side and reserve the other tank for takeoffs and landings.
 
INOP = No. However, the standard is that the fuel gauge cannot show usable fuel in the tank when the tank is empty (conversely, it may have usable fuel in the tank when the gauge reads zero).
 
My position was that it wasn't legal per 91.205. Then someone brought up that the fuel gauge only has to be accurate when it reads empty, which it did despite the fact that there was fuel in the tank.

Logically, you could stick the tank and knowing you're burn rate, estimate the fuel level.

That logic obviously doesn't agree with the apparent intent of 91.205.

I was confident it wasn't legal, but couldn't immediately justify my argument with facts and regs. I hadn't thought of this scenario before (in recent memory), so I had to think it through.

By the way, the FSDO said they wouldn't give a ferry permit.
 
Flyingfanatic said:
My position was that it wasn't legal per 91.205. Then someone brought up that the fuel gauge only has to be accurate when it reads empty, which it did despite the fact that there was fuel in the tank.

...

By the way, the FSDO said they wouldn't give a ferry permit.
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The whole “only has to be accurate when empty” is an OWT that won’t die, based on old certification details and it isn’t even accurate. We’ve discussed it here many times. If you want to make the person who said that’s head explode, ask them to show you that assertion in the regs.

That works for almost all aviation OWTs really.

As far as the FSDO goes, and the ferry permit, that’s interesting. They sign permits all the time for long range ferry with ferry tanks that you have no gauges whatsoever on and have to pump the damn things into the mains... I guess as long as the mains have a way to see they’re empty, they’re happy.

I could see them being worried about a higher than normal burn rate, and/or a leak sending fuel overboard being un-noticeable with a dead gauge, but it would seem a checklist for reasonable operational checks for such a flight would be possible to write. “Look and see if fuel is streaming overboard behind both wings...”
 
SbestCFII said:
INOP = No. However, the standard is that the fuel gauge cannot show usable fuel in the tank when the tank is empty (conversely, it may have usable fuel in the tank when the gauge reads zero).
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actually, according to 23.1337(b):
Fuel quantity indicator. There must be a means to indicate to the flight crewmembers the quantity of fuel in each tank during flight. An indicator, calibrated in either gallons or pounds, and clearly marked to indicate which scale is being used, may be used....
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The confusion comes in when people read (b)(1) out of context...the context is the quote above, which continues
In addition--
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before it gets to
(1) 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 determined under Sec. 23.959;
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Which simply says that when the usable quantity of fuel is zero, the gauge is only required to read zero in a level flight attitude...it can read something else if the airplane is climbing, diving, or inverted.

CAR3, under which most of our light airplanes were certificated, reads almost identically.

As @Flyingfanatic indicated, 91.205 also requires
(9) Fuel gauge indicating the quantity of fuel in each tank.
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Not "the quantity of usable fuel in the tank when there is no usable fuel in the tank."

So 91.213(d) should show fuel gauges as non-deferrable both for
(i) Part of the VFR-day type certification instruments and equipment prescribed in the applicable airworthiness regulations under which the aircraft was type certificated;
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And
(iii) Required by §91.205 or any other rule of this part for the specific kind of flight operation being conducted;
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denverpilot said:
As far as the FSDO goes, and the ferry permit, that’s interesting. They sign permits all the time for long range ferry with ferry tanks that you have no gauges whatsoever on and have to pump the damn things into the mains... I guess as long as the mains have a way to see they’re empty, they’re happy.
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That's actually just a little further down in 23.1337:
(6) No fuel quantity indicator is required for an auxiliary tank that is used only to transfer fuel to other tanks if the relative size of the tank, the rate of fuel transfer, and operating instructions are adequate to--
(i) Guard against overflow; and
(ii) Give the flight crewmembers prompt warning if transfer is not proceeding as planned.
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Years ago I had a Cherokee with one side inop. I don’t remember having it fixed for a long time. With a known quantity, burn rate, then timing, I did just fine.
 
I should start a poll on how many people have aircraft with working fuel gauges. I bet it would be enlightening.
 
steingar said:
I should start a poll on how many people have aircraft with working fuel gauges. I bet it would be enlightening.
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The problem would be pilots who assume that an airplane fuel gauge should work like a car fuel gauge, and therefore assume it's not working when it actually does work.
 
There is only one thing that is non-op in my plane. Oddly enough, it's 6'0", 150# and completely useless. ;)
 
MauleSkinner said:
That's actually just a little further down in 23.1337:
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Actually, unless we're talking about a 172R or later, the pertintent regulation is CAR 3.672, as the pre-R 172s are all CAR3 airplanes. The result is the same, that the fuel gauge has to actually work and read at all levels, not just "zero." Those who claim that the regs, whether CAR 3 or Part 23, only require the gauge to be accurate at empty lack basic reading comprehension skills.

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§ 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.
 
bradg33 said:
Actually, unless we're talking about a 172R or later, the pertintent regulation is CAR 3.672, as the pre-R 172s are all CAR3 airplanes. The result is the same, that the fuel gauge has to actually work and read at all levels, not just "zero." Those who claim that the regs, whether CAR 3 or Part 23, only require the gauge to be accurate at empty lack basic reading comprehension skills.

----------------
§ 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.
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Agreed. The CAR page wasn't working this morning when I was posting this.
 
TommyG said:
How was this a significant discussion. INOP, not legal.
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Cuz one of the five hazardous attitudes covers it nicely. :)
There are MANY people out there who think rules don't apply to them. Some of those are pilots too.

openbinary.asp
 
Ravioli said:
^^^^ who is he calling a dipstick with a watch? :)
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Reminds me of the line from Blue Thunder:

He checks his sanity with a wrist watch!
What do you check yours with, a dipstick?

No you can't fly with an INOP fuel gauge no matter how inaccurate they are.

I always use the most pessimistic of the gauge, my fuel flow meter, and my watch.
 
PilotWhite said:
Agree one hundred percent. Fuel gauges have proved to be unreliable. Another issue is pilots that trust them when they are inaccurate which can lead to a tank running dry.
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Some of us run tanks dry on purpose...
 
PilotWhite said:
Agree one hundred percent. Fuel gauges have proved to be unreliable. Another issue is pilots that trust them when they are inaccurate which can lead to a tank running dry.
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That's solved by fixing the fuel gauge or not flying the airplane.
 
Fuel gauges are like whiskey compasses. They’re there for the FAA. I understand the requirements but I don’t respect the instruments much. In my planes? I add better tools for monitoring fuel.

If you get into an unfamiliar airplane to conduct a flight are you satisfied with the fuel instrument information without otherwise checking the fuel levels in the tanks? I mean, the airplane is airworthy and it has the required instruments, right?

Sight gauges in a Cub are the best fuel quantity instuments I know. Way better than even the digital stuff I’ve owned. What you see is what you have. That said my Cub still has a flow monitor.
 
Stewartb said:
Sight gauges in a Cub are the best fuel quantity instuments I know. Way better than even the digital stuff I’ve owned. What you see is what you have. That said my Cub still has a flow monitor.
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There are no foolproof fuel gauges. Even the Cub's cork-on-a-wire can stick in its guide, sit for a long time in one spot, and drop suddenly in some rough air. Unnerving, that.

The absence of a foolproof gauge is not a legally acceptable excuse for having inop gauges. I know that it's frustrating having a half-million-dollar airplane with gauges less reliable and accurate than the gauges in $15K cars, but there it is. Part of it is the tank shapes in so many airplanes, and another part is the expense of getting certification for a better system. And part of it is the age of our aircraft. Not many 40-year-old cars have accurate gauges either.
 
Cork on a wire? My sight tubes are clear tygon tubing. Not even a little floating ball. Super simple.
 
BigBadLou said:
Cuz one of the five hazardous attitudes covers it nicely. :)
There are MANY people out there who think rules don't apply to them. Some of those are pilots too.
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You missed #6 "Ain't no law against it!"

Antidote: Don't expect the rules to keep you safe.
 
Stewartb said:
Cork on a wire? My sight tubes are clear tygon tubing. Not even a little floating ball. Super simple.
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Simple, and yet I have replaced that tubing because the fuel attacks it and it starts cracking from the inside. Sure wouldn't want fuel start dribbling into the cabin in flight. The Aviat Husky uses that stuff. Certified. Ugh.

Nothing lasts forever. Not even simple stuff.
 
Can you select a tank with an operating gauge, and isolate the tank with a suspect gauge?
 
Flyingfanatic said:
So a Cessna 172 has one of the fuel gauges not working.

Can you legally fly the airplane?

This came up today and there was significant discussion. I didn't readily know the answer and neither did several others. I have the answer, but I thought it would be good to share with others.
Click to expand...

Might just be a corroded lead at the sender.
 
I look at them on preflight, then usually not again - I mean, what possible useful information can you glean from a C-172 fuel gauge, that could be relied upon? If I was switching betepween tanks, I'd probably use my watch. . .
 
SbestCFII said:
INOP = No. However, the standard is that the fuel gauge cannot show usable fuel in the tank when the tank is empty (conversely, it may have usable fuel in the tank when the gdauge reads zero).
Click to expand...

By that standard, if the inop gauge is showing empty, it's ok to fly? :) but if it is stuck halfway, not?
 
Atlee Dodge sight gauges. My favorite fuel quantity indicators!


C4E7A84C-A0C5-48FD-8949-5530BD84DFAB.jpeg
 
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