Fuel sender replacement + gauge calibration.

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plynkus
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Fuel sender replacement + gauge calibration.

Post by plynkus »

Greetings, all. (Been a while!)

My 'C1 moved from KSQL -> KBVS late last year. It had been receiving very good service from an Authorized shop @ KPAO (Aero Works). Initially tried to have an annual scheduled with another Authorized shop @ KBLI (Command Aviation, no response). Shop on the field @ KBVS (Vertex Aviation) handled the annual instead, and my read after the fact is that they have relatively little experience with Diamond aircraft.

Anyhow, the fuel tank sender was replaced for condition (signs of past minor gasket leakage or staining, flange a little beat up due to prior attempts at tightening). No attempt whatsoever was done to calibrate the gauge to match, and of course the gauge now reads pretty well off (full when full, but very rapidly degrades to readings a full quarter tank below actual for much of the range). I haven't pushed it below half a tank actual in ops.

Is this not as simple as the shop emptying the tank, taking a screw driver to a set screw or something on the VDO gauge and making it read empty with the tank empty? The shop owner is super defensive and not-my-problem about the issue, and it is a little baffling---I'd think this is a relatively common matter to have to deal with on *any* aircraft, if either a sender or gauge requires replacement for any reason.

Any similar experiences? Was I just lucky my prior gauge was in nearly total agreement with the physical tank dip stick?

Thanks, and greetings to those of you in the Pac NW!

Brian
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Re: Fuel sender replacement + gauge calibration.

Post by Spinner »

For calibration the manual says to drain all but unusable (.5 gallon). Check the resistance of the wire to the sender per the manual. Zero the gage (with a tiny screwdriver, be careful and gentle). Then fill the tank with measured full fuel from a pump and make sure it indicates properly. Make sure the plane is level when you start this. There is no in-between measurements it just says measure empty and measure full.

We usually fill by quarter tanks at a time when you get to half and your gage is out to lunch there is no sense in going further. Unfortunately we have also been able to install new senders that were garbage out of the box.

They have a new SB that installs better gage and sender which seem to work well. (DAC1-77-01) The SB includes replacements for oil pressure, oil temperature, fuel pressure and fuel quantity as well as the CHT gage.

hope it helps a little.

Paul
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Re: Fuel sender replacement + gauge calibration.

Post by plynkus »

Quite helpful, Paul---thank you. (Just a little puzzled that this wouldn't have been a standard practice ops check after replacing the sender, before sign off, in the first place!)
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Re: Fuel sender replacement + gauge calibration.

Post by Rick »

Isn't a working fuel gauge an airworthiness requirement? If so, then the shop signed off and returned to you an un-airworthy aircraft, which I would think the FAA would be concerned about. Seems like you might have some leverage...
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Re: Fuel sender replacement + gauge calibration.

Post by Spinner »

The screen shot I have is straight out of the DA20 maintenance manual so it would make it a requirement to do the calibration after replacing a sender or gage. If they were following the manuals then they should have done the calibration.
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Re: Fuel sender replacement + gauge calibration.

Post by Colin »

It is a requirement that it show full and empty. Oddly, it doesn't have to be accurately in between the two.
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Re: Fuel sender replacement + gauge calibration.

Post by plynkus »

I originally reported the discrepancies verbally after a quick test flight---but mentioned I'd get a little more time on it, burn some gas from full and confirm suspicions. I did so, confirmed it with them in email---and as I said before, the reply was somewhat defensive:
I see the replacement VDO fuel quantity sending unit we installed, must not be calibrated to your gage, or vice-versa. I would be happy to reinstall your original sending unit that was given to you. It was removed due to evidence of fuel stains from the gasket area only. It also exhibited warped flange from prior attempts to tighten it down. If the replacement sending unit that was purchased, which is the same part number and manufacturer fails to perform, I will gladly take it back for full refund. We have no way of confirming if your fuel gage/indiation was accurate or not prior to arrival.
I also explained that we did NOT perform any calibration tests on your system after replacement of the sending unit, other than connection of one wire and operating the float manually to see if the gage responded as expected. Which it did. Further testing would have required draining of the tank, and refilling in increments to check for accuracy.
The statements above make it pretty clear the choice to not check was explicit. Operating the float manually to see if the gauge responded as expected sounds like they just looked for basic needle movement. To the point that they had "no way of confirming" accuracy before, I mentioned politely in response that the aircraft come with a physical, calibrated dipstick which of course could have been used as a basic reading sanity check both before and after. The original sender was also not given to me.

I also offered that perhaps the gauge may also be suspect. Exerting some force on the gauge glass can cause needle movement, but this could also have been true before. The hands-off values are stable, however, and consistently lower by the quarter to a third that I reported. I told them I'd be happy to pay for a new gauge and installation, even, but the pair need to indicate properly.

They accepted the keys and logs back this morning, and I walked in with videos of me sticking the tank and powering up the ship to catch a reading at half a tank physical (indicates in the yellow, just below 1/4).

I'll follow up here with whatever happens, of course.

Brian
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Re: Fuel sender replacement + gauge calibration.

Post by plynkus »

@Colin: I thought I'd remembered, too the bit about reading empty being the requirement. But in my looking around today I found that this seems to have been cleaned up in the FARs in recent years.

E.g., from https://pilotworkshop.com/tips/fuel-gauge-accuracy/:
"The fuel gauges in the airplane I rent are nearly useless. Often, they show empty even when there is gas in the tank. Or when they do seem to be working, they don't accurately reflect how much gas is in the tanks. I’ve been told that legally, fuel gauges only need to be accurate when the tanks are empty. Is that true?" - Greg M.

Common sense would suggest this isn’t true, and, in fact, it isn’t. FAR 91.205 says that a fuel gauge indicating the quantity of fuel in each tank is required equipment. If the tanks are half full, a gauge that reads empty is not doing its FAR 91.205 mandated job. So where did this idea come from?

The common misconception originates from the pre-2017 version of FAR 23 for aircraft certification. FAR 23.1337(b)(1) stated that 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. In other words, fuel gauges should be calibrated in terms of remaining usable fuel. But zero usable fuel isn’t the only time they’re required to be accurate. This immediately followed FAR 23.1337(b), which stated that there must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator that’s only accurate when the tanks are empty doesn’t satisfy that requirement. In 2017, FAR 23 was rewritten, eliminating that wording. The new FAR 23.2430 says that fuel systems must provide the flightcrew with a means to determine the total usable fuel available.
So, since 2017, we may now be in a regime where the system may simply need to be accurate---period---though degree of accuracy is of course not specified.
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Re: Fuel sender replacement + gauge calibration.

Post by plynkus »

Latest update. Went to the field to grab the logs (for other purposes) and inquire about status. Shop mechanic asked if I'd flown the aircraft yet---confused, I said no, not until resolution. She then indicated that they had already replaced the new sender with the original one that they said they'd given to me (they hadn't---guess they found it there). They hadn't communicated to me that any of this had occurred, and the revert operation was not logged in the airframe log (which I complained about). I asked if they'd even tested it against themselves against the stick reading and they had not. Furthermore, it appears that the gauge still reads inaccurately (checked after leaving the shop with logs). This has all the appearance of the shop simply washing their hands of it by the cheapest method possible (dropping the old part back in). Still as inop as it was after it left the annual.

Q for those who know better: Is it a legal requirement that maintenance be performed as per published manufacturer's procedures, or is that just an obvious best practice? The airframe log entry for the annual just indicates the sender was replaced---no specific procedure indicated.

This has been incredibly frustrating. If Diamond's procedure wasn't followed, what else is no good about the install (and backward re-install)? I didn't receive a plane back after either the annual or now with full fuel, charged for a top off, etc. Which indicates to me that no leak check was performed, given placement on the tank and the documented procedure.

(What other shortcuts were taken on the annual as a whole?...)

Next steps are likely to get someone else to simply replace the gauge and sender pair and write off the original shop entirely. Probably go the UMA gauge/sender route, too, per Paul's comments regarding their improved reliability. Get it into a known good configuration---by the book: leak checks, accuracy checks, everything.

And perhaps talk to a lawyer and/or the FAA. (Anyone have recommendations?)
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Re: Fuel sender replacement + gauge calibration.

Post by plynkus »

Seems pretty straight out of https://www.law.cornell.edu/cfr/text/14/43.13, no?

§ 43.13 Performance rules (general).

(a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in § 43.16. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. If special equipment or test apparatus is recommended by the manufacturer involved, he must use that equipment or apparatus or its equivalent acceptable to the Administrator.

(emphasis mine)
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