Narrative:

This report is being submitted with the intent to increase awareness of the inherent inaccuracies of the MD80 fuel gauges. I have been compiling a database of fuel used (as indicated by the fuel counters) versus fuel remaining (as indicated by the gauges). The good news is that out of the number of aircraft in my database; to date; the majority (73 percent) show a discrepancy of 500 pounds or less. Equally good news is that 85 percent of the errors are that the indicated fuel remaining reads less than what we would expect when subtracting the fuel used (from the fuel counters) from the fuel indicated on the gauges at engine start. I say this is 'good news' (although how can any error on the fuel gauges be good?) because this type of error would have a pilot land short for fuel; based on what the gauges show (as opposed to an error of the opposite nature which could lead to fuel starvation while the gauges show adequate fuel remaining). The bad news is that I have documented 5 percent aircraft with errors in excess of 1000 pounds. Even though these specific errors are of the 'good' type (gauges reading lower than expected); I believe this is unacceptable. In reality; we do not know if the gauges are in error when the tanks are full (meaning we are not getting the prescribed pounds of fuel as per our flight release) or when the tanks are at the low end of their capacity (which could lead to an unnecessary; but safe; fuel diversion based on low fuel gauge indications). I also have documented one aircraft that indicated an error of excess indicated fuel at shutdown of 800 pounds. This error is the most disturbing for it is easy to believe a gauge shows excess fuel remaining. (It makes a pilot proud to under-burn.) the bottom line for us is that we only know exactly what our fuel load is when an empty fuel tank has fuel metered into it. Since this is an operational impossibility; an equally acceptable substitute is to stick each tank before every flight. Since sticking the tanks is an operational nuisance; we need to continue working on methods to correct any discrepancies in the MD80 fuel indicating system to bring them all into an acceptable tolerance that we can once again trust. To that end; I ask that line pilots be given guidance as to what is an 'acceptable discrepancy' in fuel gauge errors so that we can make appropriate entries for maintenance. I would have been happy to write up the fuel gauges on the two aircraft with over 1000 pounds errors; but what criteria would have I addressed? In addition; I would prefer to note an error of that type as an information to maintenance with the hope that the next crew flying that aircraft would monitor the gauge and confirm or dispute the error I claimed. I realize asking line pilots to feed accurate data to maintenance may be asking a lot; but what alternative do we have in re-establishing the credibility of our fuel gauges? Although the fuel confidence checks have brought us a lot closer to fuel indication accuracy than we were 6 months ago; we're not yet where we need to be in addressing this problem. Perhaps we should consider increasing our fuel reserve on this fleet by at least 500 pounds until all probes are replaced; and/or the fuel gauges are calibrated at a major inspection event. As long as fuelers are afraid to fill our wings to the vto shutoff and instead shut off the fill valves according to possibly erroneous capacitance probe readings; we will never know how much fuel we have on board at pushback.callback conversation with reporter revealed the following information: the reporter stated that with the data compiled; the MD80 fuel qty system needs work to bring the system to an acceptable tolerance. The company started a program to replace the fuel tank probes but suddenly found no source of new probes due to being out of production. The company has signed a contract with a manufacturer for replacement probes but it will take some time to get up to speed. The reporter stated the fuel slip alone; when calculating fuel pumped from the truck; the allowable error can be as high as 125 gallons. A consideration would be to board 500 pounds reserve until the fuel probes are replaced and the indicator tolerances are tightened.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN MD80 CAPT COMPILED A DATABASE OF A NUMBER OF MD80 ACFT OF FUEL USED VERSUS FUEL REMAINING USING FUEL QTY INDICATORS. MAJORITY INDICATE DISCREPANCY OF 500 LBS OR LESS.

Narrative: THIS RPT IS BEING SUBMITTED WITH THE INTENT TO INCREASE AWARENESS OF THE INHERENT INACCURACIES OF THE MD80 FUEL GAUGES. I HAVE BEEN COMPILING A DATABASE OF FUEL USED (AS INDICATED BY THE FUEL COUNTERS) VERSUS FUEL REMAINING (AS INDICATED BY THE GAUGES). THE GOOD NEWS IS THAT OUT OF THE NUMBER OF ACFT IN MY DATABASE; TO DATE; THE MAJORITY (73 PERCENT) SHOW A DISCREPANCY OF 500 LBS OR LESS. EQUALLY GOOD NEWS IS THAT 85 PERCENT OF THE ERRORS ARE THAT THE INDICATED FUEL REMAINING READS LESS THAN WHAT WE WOULD EXPECT WHEN SUBTRACTING THE FUEL USED (FROM THE FUEL COUNTERS) FROM THE FUEL INDICATED ON THE GAUGES AT ENG START. I SAY THIS IS 'GOOD NEWS' (ALTHOUGH HOW CAN ANY ERROR ON THE FUEL GAUGES BE GOOD?) BECAUSE THIS TYPE OF ERROR WOULD HAVE A PLT LAND SHORT FOR FUEL; BASED ON WHAT THE GAUGES SHOW (AS OPPOSED TO AN ERROR OF THE OPPOSITE NATURE WHICH COULD LEAD TO FUEL STARVATION WHILE THE GAUGES SHOW ADEQUATE FUEL REMAINING). THE BAD NEWS IS THAT I HAVE DOCUMENTED 5 PERCENT ACFT WITH ERRORS IN EXCESS OF 1000 LBS. EVEN THOUGH THESE SPECIFIC ERRORS ARE OF THE 'GOOD' TYPE (GAUGES READING LOWER THAN EXPECTED); I BELIEVE THIS IS UNACCEPTABLE. IN REALITY; WE DO NOT KNOW IF THE GAUGES ARE IN ERROR WHEN THE TANKS ARE FULL (MEANING WE ARE NOT GETTING THE PRESCRIBED POUNDS OF FUEL AS PER OUR FLT RELEASE) OR WHEN THE TANKS ARE AT THE LOW END OF THEIR CAPACITY (WHICH COULD LEAD TO AN UNNECESSARY; BUT SAFE; FUEL DIVERSION BASED ON LOW FUEL GAUGE INDICATIONS). I ALSO HAVE DOCUMENTED ONE ACFT THAT INDICATED AN ERROR OF EXCESS INDICATED FUEL AT SHUTDOWN OF 800 LBS. THIS ERROR IS THE MOST DISTURBING FOR IT IS EASY TO BELIEVE A GAUGE SHOWS EXCESS FUEL REMAINING. (IT MAKES A PLT PROUD TO UNDER-BURN.) THE BOTTOM LINE FOR US IS THAT WE ONLY KNOW EXACTLY WHAT OUR FUEL LOAD IS WHEN AN EMPTY FUEL TANK HAS FUEL METERED INTO IT. SINCE THIS IS AN OPERATIONAL IMPOSSIBILITY; AN EQUALLY ACCEPTABLE SUBSTITUTE IS TO STICK EACH TANK BEFORE EVERY FLT. SINCE STICKING THE TANKS IS AN OPERATIONAL NUISANCE; WE NEED TO CONTINUE WORKING ON METHODS TO CORRECT ANY DISCREPANCIES IN THE MD80 FUEL INDICATING SYSTEM TO BRING THEM ALL INTO AN ACCEPTABLE TOLERANCE THAT WE CAN ONCE AGAIN TRUST. TO THAT END; I ASK THAT LINE PLTS BE GIVEN GUIDANCE AS TO WHAT IS AN 'ACCEPTABLE DISCREPANCY' IN FUEL GAUGE ERRORS SO THAT WE CAN MAKE APPROPRIATE ENTRIES FOR MAINT. I WOULD HAVE BEEN HAPPY TO WRITE UP THE FUEL GAUGES ON THE TWO ACFT WITH OVER 1000 LBS ERRORS; BUT WHAT CRITERIA WOULD HAVE I ADDRESSED? IN ADDITION; I WOULD PREFER TO NOTE AN ERROR OF THAT TYPE AS AN INFO TO MAINT WITH THE HOPE THAT THE NEXT CREW FLYING THAT ACFT WOULD MONITOR THE GAUGE AND CONFIRM OR DISPUTE THE ERROR I CLAIMED. I REALIZE ASKING LINE PLTS TO FEED ACCURATE DATA TO MAINT MAY BE ASKING A LOT; BUT WHAT ALTERNATIVE DO WE HAVE IN RE-ESTABLISHING THE CREDIBILITY OF OUR FUEL GAUGES? ALTHOUGH THE FUEL CONFIDENCE CHKS HAVE BROUGHT US A LOT CLOSER TO FUEL INDICATION ACCURACY THAN WE WERE 6 MONTHS AGO; WE'RE NOT YET WHERE WE NEED TO BE IN ADDRESSING THIS PROBLEM. PERHAPS WE SHOULD CONSIDER INCREASING OUR FUEL RESERVE ON THIS FLEET BY AT LEAST 500 LBS UNTIL ALL PROBES ARE REPLACED; AND/OR THE FUEL GAUGES ARE CALIBRATED AT A MAJOR INSPECTION EVENT. AS LONG AS FUELERS ARE AFRAID TO FILL OUR WINGS TO THE VTO SHUTOFF AND INSTEAD SHUT OFF THE FILL VALVES ACCORDING TO POSSIBLY ERRONEOUS CAPACITANCE PROBE READINGS; WE WILL NEVER KNOW HOW MUCH FUEL WE HAVE ON BOARD AT PUSHBACK.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT WITH THE DATA COMPILED; THE MD80 FUEL QTY SYSTEM NEEDS WORK TO BRING THE SYSTEM TO AN ACCEPTABLE TOLERANCE. THE COMPANY STARTED A PROGRAM TO REPLACE THE FUEL TANK PROBES BUT SUDDENLY FOUND NO SOURCE OF NEW PROBES DUE TO BEING OUT OF PRODUCTION. THE COMPANY HAS SIGNED A CONTRACT WITH A MANUFACTURER FOR REPLACEMENT PROBES BUT IT WILL TAKE SOME TIME TO GET UP TO SPEED. THE RPTR STATED THE FUEL SLIP ALONE; WHEN CALCULATING FUEL PUMPED FROM THE TRUCK; THE ALLOWABLE ERROR CAN BE AS HIGH AS 125 GALLONS. A CONSIDERATION WOULD BE TO BOARD 500 LBS RESERVE UNTIL THE FUEL PROBES ARE REPLACED AND THE INDICATOR TOLERANCES ARE TIGHTENED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.