Fuel Quantity Indicator Question

[SmitteyB]

I seem to be in a little disagreement with my roomate here about the highly touted-

14 CFR 91.205 Powered Civil Aircraft with stantard category U.S. airworthiness certificates: Instrument and equipment requirements.

Particularly 91.205(b)(9) - Fuel Quantity Indicator(s)

The regulation says- "Fuel Gauge indicating the quantity of fuel in each tank"

What is considered airworthy? Those of us who fly older equipment, know that most of our FQI's are not accurate. If the FQI is considered airworthy only if it indicates accuarately, then I'm sure there are a number of us here who have broken this regulation.

My take-

I was under the impression that to be considered an airworthy FQI the only time is needed to read accurately is when the tanks were indeed empty.

My roomates take-

Unresolved.

All responses are appreciated.

 

[ZWflyer]

To be a bit more specific with the situation........we have had a fuel quantity indicator that is obviously broken and reads zero fuel in tank at all times. SmitteyB is trying to sell me on the fact that since this indicator indicates zero fuel when the tank is empty (even though it also indicates zero fuel when the tank is full), then it is considered airworthy. My opinion differs.

 

[tgrayson]

To be a bit more specific with the situation........we have had a fuel quantity indicator that is obviously broken and reads zero fuel in tank at all times. SmitteyB is trying to sell me on the fact that since this indicator indicates zero fuel when the tank is empty (even though it also indicates zero fuel when the tank is full), then it is considered airworthy. My opinion differs.

Part 23.1337 reads:

(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:
(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 §23.959(a);
(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;
(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;
(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);

I interpret the above to say that the indicator must be reasonably correct in flight, and exactly correct when the tanks are empty.

 

[SmitteyB]

Ok, I stand corrected.

Thanks Gray.

 

[MidlifeFlyer]

I think you're right, but this is a real question.

If you look at the certification regs on fuel gauges you'll generally find language such as this language in Part 23:

==============================
23.1337(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:

(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 § 23.959(a);

(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;

(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;

(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);

(5) Tanks with interconnected outlets and airspaces may be considered as one tank and need not have separate indicators; and

(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.
==============================

Somehow the requirement that the gauges must read empty when there is no usable fuel (instead of say, showing 5 gallons when you have nothing you can use) has somehow gotten read in to meaning that this is the =only= time in needs to have any accuracy whatesoever. Which is sort of like saying that, so long as your bathroom scale reads zero when you're not on it, it doesn't matter what it reads when you step on it.

Unfortunatelty some of this silliness has found its way into some official FAA publications. I can't locate it right now, but I remember being show on eplace where some AC or something indeed suggests that this is the case.

 

[falconvalley]

As previously stated there are rules on how those gauges must be calibrated and how often they are supposed to be checked.

Then there is reality. Certain types of fuel measuring equipment cannot be trusted for one reason or another. Sometimes it's how the equipment is maintained. Sometimes it's the fact that those who fly it don't note discrepencies. I've found that many guages can be trusted, but why take a chance on an aircraft that you don't fly often or haven't flown before?