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Accuracy Of Fuel Tank Stick Checks.  
User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Posted (1 year 4 months 1 week 5 days 8 hours ago) and read 3577 times:

Anyone able to tell me how accurate stick checks are on the 777 fuel tanks?

I ask because I had a flight where our fuel gauges were inaccurate and we had to perform a pre-departure stick check. The stick check gave us 1300kgs less than the figure we asked the refueller for. In the maintenance work-sheet for a stick check it was written that a straight and level aircraft can expect a 2% stick check accuracy. On a longhaul with over 100,000kgs of fuel, this potential shortfall of fuel is not acceptable. The engineer told me that the fuel figure was within 2% of what I asked for and therefore within limits. The way I saw it was that I potentially had 2% less than the stick check figure, giving us potentially 4,000kgs less than what I wanted. Once inflight we would have no real idea of knowing how much fuel was exactly in our tanks.

I have tried to get an answer from my own company but have not gotten anywhere with an answer that makes sense to me. Ideally, if this 2% error is true, it should be written under the DDG procedure for the fuel tank totalizer inaccuracy. This way, pilots can ask to be refuelled to a figure 2% more than what they need for the flight, and this will ensure enough fuel is loaded, rather than realising at a late stage of the pre-flight that we need additional fuel and causing a delay (45mins for us).

Thanks for any insight into this!

28 replies: All unread, showing first 25:
 
User currently offlineTristarsteve From Sweden, joined Nov 2005, 4024 posts, RR: 33
Reply 1, posted (1 year 4 months 1 week 5 days 7 hours ago) and read 3555 times:

Stick checks on large aircraft are inaccurate. I would say that 2pc is good. To do one properly you must measure the pitch and roll of the aircraft (from the INS) and then let the fuel settle for a while. Even then two people will get readings that are different. The sticks 'stick' as they float and you have to flick them to get a reading. Depending on how you do it can easily give you the discrepancy you saw.
They are very useful to show there is fuel in the tank!, but as an accurate quantity check, no.
The B787 has no sticks.


User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 2, posted (1 year 4 months 1 week 5 days 6 hours ago) and read 3548 times:

Thanks for the info. I asked the company why the 2% error is not written in the DDG (MEL) and the reply was that Boeing do not think it is necessary. I have my doubts and have visions of the swiss cheese lining up on a day where you declare a fuel mayday thinking you are short of fuel just as the engines flame out when that 3000kgs reserve you thought you had actually isn't there!

User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 3, posted (1 year 4 months 1 week 5 days 5 hours ago) and read 3527 times:

The Aircraft would be needed to be aligned to the Pitch & Roll ideal position,Normally the spirit level/plumbob in the MWW is used, not sure on the B777s.....If Aligned accurately & Sp Gravity & Temp is know, one can be accurate with the required tolerances.

The DDG should have a limitation of the tolerances.


In this case was all the quantities INOP, then that would be a NOGO.
Only one Ind can be INOP if not mistaken.



Think of the brighter side!
User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 4, posted (1 year 4 months 1 week 5 days 2 hours ago) and read 3491 times:

Quoting HAWK21M (Reply 3):
The Aircraft would be needed to be aligned to the Pitch & Roll ideal position

This isn't necessary. Boeing produce fuel measuring stick manuals which provide the correct quantity for a given pitch or roll.

Personally,I've not had to use this document however, it should be available to you via your company's MyBoeingFleet account.


User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 5, posted (1 year 4 months 1 week 5 days 2 hours ago) and read 3480 times:

Quoting HAWK21M (Reply 3):
The DDG should have a limitation of the tolerances.


In this case was all the quantities INOP, then that would be a NOGO.
Only one Ind can be INOP if not mistaken.

Our DDG makes no reference to tolerances so as far as flight crew are concerned, the stick check is accurate. This sort of contradicts the engineers worksheet stating a 2% accuracy.

In our case we had CWT totalizer inaccuracy. The wing tank indications were working normally.


User currently offlinefr8mech From United States of America, joined Sep 2005, 5490 posts, RR: 14
Reply 6, posted (1 year 4 months 1 week 5 days 2 hours ago) and read 3476 times:

A 2 % error isn't bad at all. I recall that on older aircraft you could expect an overall error in the gauging system of 4%.

Why Boeing has chosen not to add it to the DDG...who knows. Apparently they felt it was not significant.



When seconds count...the police are minutes away.
User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 7, posted (1 year 4 months 1 week 5 days 1 hour ago) and read 3470 times:

Quoting fr8mech (Reply 6):
A 2 % error isn't bad at all. I recall that on older aircraft you could expect an overall error in the gauging system of 4%.

Why Boeing has chosen not to add it to the DDG...who knows. Apparently they felt it was not significant.

A 2% error on a longhaul flight means 2000-3000kgs of fuel difference. When your reserves are only 3000kgs anyway, it means the difference between diverting to to your alternate with reserve fuel intact, or having your engines flame out on short finals. This is VERY significant!


User currently offlinefr8mech From United States of America, joined Sep 2005, 5490 posts, RR: 14
Reply 8, posted (1 year 4 months 1 week 4 days 23 hours ago) and read 3428 times:

Quoting CX Flyboy (Reply 7):
This is VERY significant!

Yes, It's significant across the whole aircraft. What about in the one tank with the inoperative fuel tank?

I'm not familiar with the tank lay-out, nor the total qty of each tank or the aircraft. How many tanks and what are their respective capacities?

And, I don't have access to a B777 manual, but I assume the 2% allowable limit is within the tank and not across the whole aircraft. Then again, I may be wrong.



When seconds count...the police are minutes away.
User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 9, posted (1 year 4 months 1 week 4 days 16 hours ago) and read 3305 times:

Quoting fr8mech (Reply 8):
Quoting CX Flyboy (Reply 7):
This is VERY significant!

Yes, It's significant across the whole aircraft. What about in the one tank with the inoperative fuel tank?

I'm not familiar with the tank lay-out, nor the total qty of each tank or the aircraft. How many tanks and what are their respective capacities?

And, I don't have access to a B777 manual, but I assume the 2% allowable limit is within the tank and not across the whole aircraft. Then again, I may be wrong.

No actually you are right, it would be in the one affected tank only but it is still the difference potentially of a tonne. There are days when we can only make our destination because of higher than expected fuel burn where we disregard our alternate and start eating into that. There are a bunch of conditions which must be prevalent in order to throw away our alternate, basically landing must be ensured at destination. This is how tight airlines operate these days. To have a tonne less than you think in these situations is still not acceptable.

If the CDL can mention the need to increase fuel burn by 0.5% for missing seals etc, you would think that a 2% shortfall in fuel would also be written up.


User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 10, posted (1 year 4 months 1 week 4 days 16 hours ago) and read 3292 times:

Leaving aside dripstick fuel measurements, what do your company procedures allow for the difference between the calculated fuel uplift and the actual fuel uplift.

User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 11, posted (1 year 4 months 1 week 4 days 13 hours ago) and read 3248 times:

Quoting Mender (Reply 10):
Leaving aside dripstick fuel measurements, what do your company procedures allow for the difference between the calculated fuel uplift and the actual fuel uplift.

For a longhaul flight with fuel in the CWT, fuel must be within a discrepancy of +2000kgs or -1000kgs on the 77W. Anything outside of this and we have to make sure we know what has happened to the fuel and can only then depart when we are satisfied that there is an adequate reason and that we have enough fuel on board.

However, if the stick check shows less than the fuel we ask for, that is a different issue.


User currently offlinevc10 From United Kingdom, joined Nov 2001, 1412 posts, RR: 16
Reply 12, posted (1 year 4 months 1 week 4 days 7 hours ago) and read 3147 times:

Can I just ask what the tolerance is on the aircraft's fuel gauging . If this is also 2% then on a 100 tonne fuel lift you could be up to 2 000 kgs short without knowing about it

User currently offlinehorstroad From Germany, joined Apr 2010, 272 posts, RR: 0
Reply 13, posted (1 year 4 months 1 week 4 days 6 hours ago) and read 3141 times:

2% is quite good. with the MD11 you have a tolerance of 5%.
keep in mind that this is not the difference to the actual fuel on board, but the difference between the cockpit indication and the sticks... cockpit indication might be inaccurate as well... that's why you pull the sticks in the first place.

in the end you should trust the sticks, that's what they are there for.


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (1 year 4 months 1 week 4 days 5 hours ago) and read 3129 times:

Quoting horstroad (Reply 13):
cockpit indication might be inaccurate as well... that's why you pull the sticks in the first place.

Comparing the two...The Sticks would be more accurate and reliable.



Think of the brighter side!
User currently offlinevc10 From United Kingdom, joined Nov 2001, 1412 posts, RR: 16
Reply 15, posted (1 year 4 months 1 week 4 days 4 hours ago) and read 3112 times:

Quoting horstroad (Reply 13):
2% is quite good. with the MD11 you have a tolerance of 5%.
keep in mind that this is not the difference to the actual fuel on board, but the difference between the cockpit indication and the sticks... cockpit indication might be inaccurate as well... that's why you pull the sticks in the first place.

in the end you should trust the sticks, that's what they are there for.

I would agree with you on this point , but I am getting a bit out of date now so was not quite sure what the tolerances were on modern aircraft


User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 16, posted (1 year 4 months 1 week 4 days 2 hours ago) and read 3092 times:

Quoting CX Flyboy (Reply 11):
For a longhaul flight with fuel in the CWT, fuel must be within a discrepancy of +2000kgs or -1000kgs on the 77W. Anything outside of this and we have to make sure we know what has happened to the fuel and can only then depart when we are satisfied that there is an adequate reason and that we have enough fuel on board.

However, if the stick check shows less than the fuel we ask for, that is a different issue.

IIRC the company I work for allow for the difference between the calculated fuel uplift and the actual fuel uplift to be 500 kgs or 2% whichever is the greater.

Quoting CX Flyboy (Reply 7):
A 2% error on a longhaul flight means 2000-3000kgs of fuel difference. When your reserves are only 3000kgs anyway, it means the difference between diverting to to your alternate with reserve fuel intact, or having your engines flame out on short finals. This is VERY significant!

I see your point but from a technical point of view a 2% error is very good. It sounds to me like the real issue is that the reserve fuel policy of your company was written for aircraft that hold no more than 50,000 kgs but has been applied to an aircraft that holds getting on for three times that amount.


User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 17, posted (1 year 4 months 1 week 4 days ago) and read 3066 times:

Quoting Mender (Reply 16):
IIRC the company I work for allow for the difference between the calculated fuel uplift and the actual fuel uplift to be 500 kgs or 2% whichever is the greater.

Is that for a 777?

Quoting Mender (Reply 16):
I see your point but from a technical point of view a 2% error is very good. It sounds to me like the real issue is that the reserve fuel policy of your company was written for aircraft that hold no more than 50,000 kgs but has been applied to an aircraft that holds getting on for three times that amount.

Reserves are calculated at 30mins of flying tme at low altitude at the planned landing weight of the aircraft concerned. The total fuel capacity has no real bearing on this.

Quoting horstroad (Reply 13):
keep in mind that this is not the difference to the actual fuel on board, but the difference between the cockpit indication and the sticks... cockpit indication might be inaccurate as well... that's why you pull the sticks in the first place.

in the end you should trust the sticks, that's what they are there for.

Interesting, but the cockpit indication on days when a stick check is required is considered u/s anyway, so what is the 2% being referenced to then? The engineer tried to sell me the idea that the stick check was within 2% of what I asked for so it was fine and within his 2% tolerance.

I would have thought the 777 FQIS was pretty accurate. The maintenance page shows a vast number of readouts for each fuel tank with various readings for when the tank is operative. Certainly gives the impression of extreme accuracy of the FQIS.


User currently offlinefr8mech From United States of America, joined Sep 2005, 5490 posts, RR: 14
Reply 18, posted (1 year 4 months 1 week 3 days 23 hours ago) and read 3034 times:

Quoting CX Flyboy (Reply 17):
so what is the 2% being referenced to then?

The 2% should be off the calculated fuel. Fuel remaining, plus the calculated uplift (gal * density weight) should provide you with the calculated weight in that tank. The stick reading should be within 2% of that calculation.

While stick readings are quicker, when the turn time allowed, I would always pump the tank dry and have the fueler tell me how many gallons he put in the affected tank. Sometimes, this would require some playing around with the distribution in the other tanks to maintain some semblance of balance.

Quoting CX Flyboy (Reply 17):
but the cockpit indication on days when a stick check is required is considered u/s anyway

If the cockpit indication is unserviceable, it shouldn't be referenced for anything.



When seconds count...the police are minutes away.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 19, posted (1 year 4 months 1 week 3 days 20 hours ago) and read 3003 times:

On small aircraft:

If the tanks are fixed volume, a stick check is very accurate.

If the tanks are of the rubber bladder type, all bets are out the window unless you've taken the wings apart recently and you know what kind of condition the bladders are in  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineCX Flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 20, posted (1 year 4 months 1 week 3 days 15 hours ago) and read 2946 times:

Quoting fr8mech (Reply 18):
The 2% should be off the calculated fuel. Fuel remaining, plus the calculated uplift (gal * density weight) should provide you with the calculated weight in that tank. The stick reading should be within 2% of that calculation.

Thanks for that. So what you're saying is that the mechanic's 2% is the same as our +2000/ -1000kgs. So if stick checks are accurate, and the discrepancy is for calculations only and not the actual amount in tanks, then the only issue here is that they short fueled us by 1300kgs and tried to fob it off as being within limits?


User currently offlinefr8mech From United States of America, joined Sep 2005, 5490 posts, RR: 14
Reply 21, posted (1 year 4 months 1 week 3 days 2 hours ago) and read 2854 times:

Quoting CX Flyboy (Reply 20):
Thanks for that. So what you're saying is that the mechanic's 2% is the same as our +2000/ -1000kgs. So if stick checks are accurate, and the discrepancy is for calculations only and not the actual amount in tanks, then the only issue here is that they short fueled us by 1300kgs and tried to fob it off as being within limits?


It's early and I may not be understanding what you're saying.  

An example, maybe (the following does not reflect an actual aircraft):

Aircraft lands with 4000 lbs of fuel in a tank with an inoperative indication system. This quantity is verified by stick reading (if possible), otherwise it is a calculated number. This is a point where an error can be introduced into your calculation.

Fueler puts 4000 gallons in the tank. This works out to 26800 lbs of fuel uplifted for a total of 30800 lbs in the tank. The calculation is: gallons uplifted * density (we use 6.7lbs/gal as a standard). Not sure if you're operator uses a standard density, density for the day, or density at the time of fueling, But, you can see where an error can be introduced at this point.

Now, maintenance pulls the sticks. The sticks are calibrated .2 inch. So, mx gets a stick measurement of 12.4" (I used to pull the stick 3 times and take an approx average) and that comes in at 30400lbs. This will be considered the "actual" fuel on board. If I'm doing the math correctly, this 400 lb error gives us a 1.3% error, in this tank.

Again, we see 2 places where errors are introduced: FOB before fueling and the density. Enter into the equation that the sticks are calibrated to a "perfect" attitude listed in the book, i.e. 1.5 deg nose down and .5 right wing down. Add to that that 2 people pulling the same stick may not read it exactly the same and you can see where the errors begin to compound on themselves.

Again, when time allowed or te DDG required, I would have emptied the tank and removed one potential error source, but a DDG may not require that.

Now, understand, in the aircraft have recently dealt with, we did not fuel by stick, we always fueled by transfer from a "good" tank or by fueling the tank individually and calculating fuel qty derived from the gallons pumped. Sticks were used to verify the qty in the tank.

I hope I haven't muddled this any further. But, you can see where there a couple of points where an error can be introduced into the manual fuel quantity determination.



When seconds count...the police are minutes away.
User currently offlinecx flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 22, posted (1 year 4 months 1 week 3 days 1 hour ago) and read 2838 times:

Thanks for the explanation. I can see how there can be many errors introduced into the entire process. In the example I gave of my flight, I believe there was no stick check prior to refueling. The "known" amount was the totalizer amount left over in the tanks but since it was our centre tank indication that was faulty, the wing tank totalizers would have been pretty accurate. We had two refueling trucks in attendance since the aircraft was late on stand and we needed over 120,000kgs ASAP. The aircraft registered SG was the same as the one the mechanic used for fuel calculations.

The part where I was confused about was the 2% tolerance as written on his check sheet. I wondered what this 2% referred to and why as pilots we did not need to be told about it in the DDG. In your example where you worked out an error of 1.3%, would you say that it is within the 2% and therefore acceptable?

As a pilot any time I have less fuel than what I asked for, it makes me pause and think about whether it is enough for my flight ahead. Now if the stick check is accurate, I can simply ask for more fuel if I need it. If the stick check is not accurate then I takeoff not knowing exactly how much fuel I have and that is a scary thought.


User currently offlinefr8mech From United States of America, joined Sep 2005, 5490 posts, RR: 14
Reply 23, posted (1 year 4 months 1 week 3 days ago) and read 2827 times:

Quoting cx flyboy (Reply 22):
In your example where you worked out an error of 1.3%, would you say that it is within the 2% and therefore acceptable?

Absolutely.

Quoting cx flyboy (Reply 22):
. If the stick check is not accurate then I takeoff not knowing exactly how much fuel I have and that is a scary thought.

Something I failed to relate:

Our overall "calculated to indicated" tolerance is 4% (across the aircraft). If we come up with an error > 4%, and the incorrect fuel tank is not readily evident, we will pull the sticks, in all the tank, to help determine the faulty indication system.

The sticks are accurate, so long as they are used correctly.



When seconds count...the police are minutes away.
User currently offlinecx flyboy From Hong Kong, joined Dec 1999, 6616 posts, RR: 55
Reply 24, posted (1 year 4 months 1 week 2 days 17 hours ago) and read 2768 times:

Quoting fr8mech (Reply 23):
Quoting cx flyboy (Reply 22):
In your example where you worked out an error of 1.3%, would you say that it is within the 2% and therefore acceptable?

Interesting! So at what point would you consider that the flight has been short fuelled? See, 4% may be the acceptable discrepancy in working out the sums of how much fuel you expected to uplift but 4% on a long haul flight equals several tons and from my point of view I would be asking for more fuel if the shortfall came to even only a few hundred KGs. So if you did a stick check and the amount came to less than what the flight crew asked for, wouldn't you put more in? ...especially if the stick check was accurate.


25 vc10 : Cxflyboy I agree with you on this point . If you are going to all the trouble to do a stick check [for whatever reason] then you have to believe what
26 cx flyboy : Thankfully our management still trust in the pilots and I am free to load as much fuel as I want within reason. I don't believe the mechanics in Hong
27 fr8mech : As far as I'm concerned, if a flight crew asks for more fuel, that's between him and dispatch. It is not a maintenance issue. They load more fuel, we
28 cx flyboy : Maybe things are done differently over there. In my airline we do not have accredited dispatchers. We have staff who are trained to input parameters
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