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Horstroad
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Posts: 494
Joined: Thu Apr 08, 2010 8:19 pm

Fuel Quantity indication and unusable fuel

Fri Jul 19, 2019 8:57 pm

Today I read about an incident when an A320 arrived at its destination with only 260kg of fuel remaining.

http://avherald.com/h?article=4ca8397f&opt=0

I was wondering how much of that was actually usable fuel. One user commented that all of that is usable fuel as only usable fuel is indicated on the FQI, not the total fuel quantity including unusable fuel.

I'm not familiar with the A320, maybe someone can confirm, that indeed only usable fuel is indicated and when the boost pumps run dry, the indication actually reads zero.

I'm familiar with the MD-11 though and that aircraft shows the total fuel quantity, including unusable fuel, on the cockpit indications. I have defueled and drained an aircraft for weighing just recently, so there is no doubt about that. The MD-11 has about 500-600kg of unusable fuel after defueling and still about 150-200kg of undrainable fuel after draining.

What's puzzling me is that the weight of ununsable fuel is already included in the ZFW. Now when you add the total fuel (including unusable fuel) indicated on your instruments, to the ZFW, the unusable fuel would be accounted for twice.
Additionally there is usable fuel in the manifolds that is not indicated on the cockpit instruments. I'm not talking about the engine feed lines, but in case of the MD-11, the fill/crossfeed manifold that contains about 270kg of fuel which can be drained into tank #2 if the quantity of that tank drops below 900kg.


So now I have about 500kg of unusable fuel that are counted twice and 270kg of usable fuel unaccounted for for TOW calculation. What do the regulations actually say about that?

All I could find regarding unusable fuel was this:

Sec. 25.959 — Unusable fuel supply.
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank. Fuel system component failures need not be considered.




Also how is the amount of unusable fuel determined? According AMM, when weighing, all (unusable) fuel is removed, only the undrainable amount of fuel may remain in the tanks. As it is included in the basic empty weight of the aircraft it has to be added to the result of the weighing process somehow.

Empty weight. The weight of the airframe, engines,
all permanently installed equipment, and unusable fuel.
Depending upon the part of the federal regulations under
which the aircraft was certificated, either the undrainable oil
or full reservoir of oil is included.
 
strfyr51
Posts: 3935
Joined: Tue Apr 10, 2012 5:04 pm

Re: Fuel Quantity indication and unusable fuel

Fri Jul 19, 2019 11:25 pm

Horstroad wrote:
Today I read about an incident when an A320 arrived at its destination with only 260kg of fuel remaining.

http://avherald.com/h?article=4ca8397f&opt=0

I was wondering how much of that was actually usable fuel. One user commented that all of that is usable fuel as only usable fuel is indicated on the FQI, not the total fuel quantity including unusable fuel.

I'm not familiar with the A320, maybe someone can confirm, that indeed only usable fuel is indicated and when the boost pumps run dry, the indication actually reads zero.

I'm familiar with the MD-11 though and that aircraft shows the total fuel quantity, including unusable fuel, on the cockpit indications. I have defueled and drained an aircraft for weighing just recently, so there is no doubt about that. The MD-11 has about 500-600kg of unusable fuel after defueling and still about 150-200kg of undrainable fuel after draining.

What's puzzling me is that the weight of ununsable fuel is already included in the ZFW. Now when you add the total fuel (including unusable fuel) indicated on your instruments, to the ZFW, the unusable fuel would be accounted for twice.
Additionally there is usable fuel in the manifolds that is not indicated on the cockpit instruments. I'm not talking about the engine feed lines, but in case of the MD-11, the fill/crossfeed manifold that contains about 270kg of fuel which can be drained into tank #2 if the quantity of that tank drops below 900kg.


So now I have about 500kg of unusable fuel that are counted twice and 270kg of usable fuel unaccounted for for TOW calculation. What do the regulations actually say about that?

All I could find regarding unusable fuel was this:

Sec. 25.959 — Unusable fuel supply.
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank. Fuel system component failures need not be considered.




Also how is the amount of unusable fuel determined? According AMM, when weighing, all (unusable) fuel is removed, only the undrainable amount of fuel may remain in the tanks. As it is included in the basic empty weight of the aircraft it has to be added to the result of the weighing process somehow.

Empty weight. The weight of the airframe, engines,
all permanently installed equipment, and unusable fuel.
Depending upon the part of the federal regulations under
which the aircraft was certificated, either the undrainable oil
or full reservoir of oil is included.

periodically the OEW of the Aircraft has to be re-calculated with the tanks Empty and all the galley's loaded with empty food carts. we usually did it after a C-check or an Annual visit for mod work. at United.
 
thepinkmachine
Posts: 368
Joined: Tue Apr 28, 2015 4:43 pm

Re: Fuel Quantity indication and unusable fuel

Sat Jul 20, 2019 8:55 am

As far as I remember, the FQ indicators are corrected for unusable fuel - ie. all indicated fuel is usable...
"Tell my wife I am trawling Atlantis - and I still have my hands on the wheel…"
 
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747classic
Posts: 2675
Joined: Sat Aug 15, 2009 9:13 am

Re: Fuel Quantity indication and unusable fuel

Sat Jul 20, 2019 9:49 am

A320 unusable fuel
3 tank configuration : 65,7 kg (82,1 liter)
4 tank configuration : 79,3 kg (99,1 liter)
5 tank configuration : 92,9 kg (116,1 liter)
See page 22/97 : https://www.easa.europa.eu/sites/defaul ... 140714.pdf

Unusable fuel is NOT indicated by FQ indicators, all indicated fuel is usable.
If (due a fuel system failure) the indicated fuel is (partly) unusable, the effected fuel Qty indication(s) will be boxed amber.
See : http://www.a320dp.com/A320_DP/fuel/sys-9.1.2.html
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
User avatar
Horstroad
Topic Author
Posts: 494
Joined: Thu Apr 08, 2010 8:19 pm

Re: Fuel Quantity indication and unusable fuel

Sat Jul 20, 2019 10:36 am

Thanks a lot for your replies.

Is there an actual FAA or EASA regulation? As said above all I could find was the definition of unusable fuel, nothing about the fact that it must not be indicated.
 
trijetsonly
Posts: 689
Joined: Tue Jul 21, 2009 10:38 pm

Re: Fuel Quantity indication and unusable fuel

Tue Jul 23, 2019 3:00 pm

Opposed to Airbus, Boeing is quite detailed about the definition of all the different fuel quantities:

Unusable Fuel:
Fuel remaining after a fuel runout test has been completed in accordance
with government regulations. (It includes drainable unusable
fuel plus unusable portion of trapped fuel.)

Drainable Unusable Fuel:
Unusable fuel minus unusable portion of trapped fuel.

Trapped Unusable Fuel:
Unusable fuel remaining when aircraft is defueled by normal means
using the procedures and attitudes specified for draining the tanks.

Usable Fuel:
Fuel available for aircraft propulsion.

Drainable Usable Fuel:
Usable fuel that can be drained from the aircraft by normal means
using the procedures and attitudes specified for draining the tanks.

Trapped Usable Fuel:
Usable fuel remaining in the fuel feed and engine lines after standard
tank defueling.

In addition all fuel quantities that are usable and not within the tanks (i.e. in manifolds, feed lines and engines) are not gauged.

The operator gets all the different quantities that I've mentioned above from the Weight and Balance Manual and has to figure out how to calculate them best to get a correct Takeoff Weight on the loadsheet in the end.

When we weigh an airplane at my airline first we have to differ if the fuel has been drained beforehand or the fuel system has been dried completely (tanks opened, ventilated and wiped clean, all fuel lines dried).
Next we want to achieve a dry situation. Either the airplane has been weighed like that or all undrainable fuel quantities have to be removed by calculation after the weighing.
To achieve the Basic Empty Weight, all unusable fuel has to be added by calculation now. Additionally, all ungauged usable fuel quantities have to be added now or as part of the operational items while calculation the OEW. Otherwise that weight could not be accounted for when calculating the takeoff weight.
In theory the fuel quantity indicators should still show 0 now and the weight calculation is as precise as possible to that point.

In Airbus, Embraer and Bombardier airplanes things are a bit more simple as they have fewer different fuel terms.
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