Calculated and Totalizer Fuel, Pre-Fuelling

After an 8 month hiatus, I returned to flying in August to the Los Angele route. On my first trip, I noticed that during the return sector from LA we had burnt more fuel than I could readily account for, and the FMC Progress Page Calculated Fuel was somewhat lower than the Totalizer fuel (more on this later).

Note : There are two means for determing fuel on board on the 777 – Totalizer Fuel (fuel sensed in the fuel tanks); and Calculated Fuel (calculated as fuel on board since start by the Flight Management Computer FMC)

Over subsequent flights, and based additionally on feedback from other Instructors – it appears we’re seeing some odd behaviour in the Calculated/Totalizer split. From my experience – and that of others – the following is what I would expect to see from these two values. Note I’ve validated my background data with other 777 operators who agree with my expectations.

  • Calculated equal to Totalizer in the very early stages of flight; typically within a couple hundred kg’s at top of climb.
  • Slow variance between steady state Totalizer and Calculated values; trending generally in one direction or another (Calculated exceeding Totalizer or vice-versa).
  • Often a developing Calculated/Totalizer split will reverse it’s trend for a while.
  • Split values of 1000 kg or more are unusual but not unheard of; usually towards the middle or last third of a long haul flight you would see this maximum value.
  • Such larger values have usually reduced to a few hundred kg’s approaching Top of Descent.
  • There has never been an absolute consistency of Calculated Higher or Lower than Totalizer all the time across Aircraft/Sector/Season, etc.

What I’ve been seeing however is the following:

  • At Top of Climb, Calculated is up to 1000 kg below Totalizer.
  • This split tends to increase slowly over the rest of the flight, occasionally reversing back towards but not coming under the original Top of Climb 1000 Kg split.
  • At Top of Descent (3x LAX-Australia sectors so far) – a split of about 1500 kg’s; Calculated always belowTotalizer.

Implications

It should be remembered that all FMC fuel predictions are based on the Calculated value. Based on the expected behaviour of the aircraft systems we have always trained that for most of a long haul flight the FMC Calculated fuel on board (and hence the FMC fuel predictions) are the more accurate figure. At the end of the flight, the Totalizer should be (more) accurate, but pilots are reminded to review the Calculated/Totalizer split – particularly those cases where the Calculated is higher than the Totalizer, since this can lead to false impression of fuel state on descent.

With a 1500 kg lower Calculated value, and a slightly increased fuel burn – we are seeing the FMC/EICAS [] INSUFFICIENT FUEL message, which cautions the crew that based on current FMC predictions, they will not land at destination with enough fuel to conduct a missed approach and proceed to their alternate with statutory reserves intact. This message is not a train smash, but it is an indication that fuel consumption has been in excess of what was planned, and combined with poor weather could lead to a diversion.

However, if this message is coming up based on a falsely low Calculated value – in comparison to the higher Totalizer value – then this is a concern. I should mention that like all airlines there’s an active fuel monitoring program in place which utilizes extensive onboard recorded data to ensure that the aircraft is consuming fuel as it should, resulting in performance decrements that are maintained in both the flight planning system and the aircraft to enable accurate fuel consumption prediction.

Before I go much further, I’d better recap on the systems for those who are still catching up.

Boeing 777 Fuel Quantity Indicating System (FQIS) Totalizer Fuel

The FQIS system in the 777 works by bouncing sound waves around the fuel tank, using 28 sensors in each main tank (20 in the centre tank) to determine the quantity of fuel on board. Based on the reception data returned by the probes (VOS is Speed of Sound in the maintenance page shown right) – the FQIS calculates the volume of fuel in the tank. Built in Densiometers (is that a real word?) provide the fuel specific gravity (SG) to turn the volume into mass.

I’m not kidding – this is how it works.

While the tanks are baffled (the other baffled, the FQIS is not confused; well mostly not) and compartmentalised over and above basic the Left/Right/Center nomenclature, the measurement can be subject to a number of errors including dynamic movement in flight (pitch/roll/yaw changes). Certification accuracy is based on ground readings or level flight and even then the potential error is designed to be less than 1% – which could be +/- 1470 Kg with full tanks. That said, in my experience, the system is far more accurate than this.

The system is pretty good, but it can be seen that with an error rate proportional to the quantity being measured – it is likely to be far more accurate at the end of a long flight than at the beginning. On top of this is the impact of flight dynamic effects such as reading the Totalizer as the aircraft turns over waypoints. As mentioned previously, the recommendation is to use the FMC Calculated over the Totalizer value for most of a long haul flight – keeping in mind the Calculated/Totalizer split – and evaluate both Calculated and Totalizer values when preparing for the arrival.

Moving on the the FMC Calculated value …

Boeing 777 FMC Calculated Fuel On Board

The FMC calculated value is set to match the FQIS Totalizer value until engine start. At this point the Totalizer value is taken as a base reference, and from this point on the FMC Calculated value is equal to the original Totalizer value at engine start, minus the fuel consumed by the engines. Fuel Flow transmitters in the engine are converted to mass flow by the EEC’s and sent (per engine) through the Aircraft Information Management System (AIMS) to the FMCs. The FMC displays the fuel consumed by each (LEFT / RIGHT) engine, the addition of these two values (FUEL USED), and the value Calculated to be left on board.

The accuracy of the fuel flow figures range from 12% (at idle thrust) to 0.5% at cruise thrust. Since the cruise error rate is based on Fuel Flow (0.5% of ~8000 Kg/Hour) which therefore produces a potential error of 400 Kg/hour, less than the possible error of the Totalizer value. While nothing is certain, my experience is that the accuracy of the calculated value is much better than this 400 kg/hour certification limit.

These values are shown on the FMC Progress Page as Fuel Used Left/Total/Right, and are used in the subtraction to calculate FMC Calculated Fuel. This value is used throughout the FMC to predict fuel ahead at waypoints throughout the flight to Destination, and by implication the weight of the aircraft throughout the flight. Predicted weight impacts the forecast step climb points, which therefore impacts fuel and time predictions at destination.

As mentioned before – over the course of a long flight my experience is the relationship between the two values can be characterized as:

  • Calculated equal to Totalizer in the very early stages of flight; typically within a couple hundred Kg’s at top of climb.
  • Slow variance between steady state Totalizer and Calculated values; trending generally in one direction or another (Calculated exceeding Totalizer or vice-versa).
  • Often a developing split will reverse itself for a while.
  • Split values of 1000 kg or more are unusual but not unheard of; usually towards the middle or last third of a long haul flight you would see this maximum value.
  • Such larger values have usually reduced to a few hundred Kg’s approach Top of Descent.
  • In my experience it is unusual for one value to always be below/above the other across different aircraft (or routes, seasons, etc).

So why are we seeing what we are seeing?

Investigations

While I chased down opinions and information from other 777 operators, we also asked for data from our instructors. My three flights notwithstanding it became clear that while the Calculated/Totalizer anomaly was quite apparent (LAX – Australia). On many flight fuel consumption seemed higher as well. A fuel consumption anomaly is an old story and there are a number of avenues to explore; including excess cabin baggage, standard weights, aircraft weight, Jet A fuel calorific value, and others. However the Calculated/Totalizer was a pearler.

Then I sat down this week with Captain Bill Hunt, Chief Technical Pilot 777, from Boeing. With some input from a Virgin Technical Pilot, we hit on a scenario which just might address most of both issues.

Pre-Fuelling, Fuel Stratification, FQIS Accuracy

All our flights to LA arrive in the morning; all leave in the evening. All are pre-fuelled for the evening departure up to about 90 tons of fuel (against an eventual departure fuel of 110-120 tons). The aircraft are then towed away from the stand, parked in the sun and left there, the fuel heating up, before being towed back onto stand just before refuelling for departure late in the evening. It’s a known phenomenon that such a process, particularly with large diurnal variations in temperature, can result in “fuel stratification” in the Centre Tank. Now don’t bother Googling this, because I can’t find a reference to it – but we’re theorizing that a centre tank with 40 tons of warm fuel, and 20 tons of colder fuel on top could result in an FQIS error. Imagine the following scenario:

  • The aircraft arrives in from Australia. Passengers and crew disembark and the aircraft is pre-fuelled with 90 tons. Since the mains hold about 60 tons, the mains are full and the centre tank takes the remaining 30.
  • The aircraft is now towed away to stand for the day. It’s now 9am. Any APU fuel consumption comes from the Left Main Tank. On a warm day you could expect the fuel in the tanks to warm appropriately.
  • At 9pm (or later) the aircraft is towed back to a stand. At about an hour before departure the last 10 to 20 tons of fuel is finalised and fuelled. This additional fuel comes from ground tanks which could be expected to be at a lower temperature than the fuel on the aircraft.
  • Now let’s suggest (and I have no evidence for suggesting this – it’s a hypothesis only) that the FQIS at this point is under-reading the fuel in tanks by about 1000 kg. As such the aircraft has 1000 kg (in the center tank) more than the pilots believe there is.
  • With the first engine start, the Calculated Fuel is set to the low-reading FQIS Totalizer value, and the FMC starts to subtract the fuel flow value.
  • During taxi, unless Nigel Mansel is at the tiller, there’s unlikely to be much mixing to restore accuracy to the FQIS reading. Hence the Totalizer at takeoff (and FMC Calculated) will be similar – and still reading Low.
  • Takeoff is a different story and in combination with the pitch attitudes of climb on a centre tank with at least 20 tons of empty space, you would expect mixing to take place. This would likely reduce the FQIS error, if not eliminate it all together.
  • Would the pilots notice? Probably not given they tend to be busy at this stage. Fuel is something of a low priority after takeoff – and the engines consume fuel at about 26 tons an hour during takeoff anyway. You wouldn’t see an increase, just a slightly lower decrease in fuel with the normal consumption associated with Takeoff.
  • The aircraft reaches Top of Climb. At this point the Center Tank fuel is mixed, the FQIS reading accurately. However the FMC Calculated value – divorced as it was from the Totalizer since Engine Start – is now committed to the higher value. The most obvious evidence of this “Fuel Stratification – FQIS over-read phenomenon” (if it exists) – would be a split at Top of Climb, with Calculated below Totalizer by at least the parked on stand under-read.
  • Another impact of this would be the aircraft is at least 1000 kg heavier than planned. As such you would expect the aircraft to burn 300-500 kg more over the course of the flight leg.
  • At the Destination, this scenario manifests as the FMC showing the aircraft short 1500 kg of fuel at Destination – 1000 kg due the under-read; 500 for the increased weight of the aircraft.
  • Any additional (Calculated less than Totalizer) worsens the perception of the situation as the FMC shows you arriving without Alternate Diversion fuel – even as the FQIS shows 1500 kg in the tanks the Calculated can’t see.

We’ve started looking at this in conjunction with Boeing technical. I can see a couple of in flight short term solutions such as resetting calculated to totalizer at top of climb (having verified the split is due to an on-stand FQIS under-read)… but the first priority is to confirm the diagnosis – or develop another one.

In Australia, our aircraft do relatively short turns between sectors. In Abu Dhabi (where the temperatures soars) – we don’t pre-fuel. This goes part way to explaining why we might see this on LAX – Australia (if this is actually what we’re seeing) and not anywhere else. It would also make this a seasonal phenomenon, with the anomaly reducing or disappearing during the Northern Hemisphere Winter. My contacts with other airlines has failed to identify any airlines that pre-fuel and sit the aircraft on the ground all day like we do in LA, which means I haven’t been able to compare. Most agree the theory sound plausible.

We’re going to tactically eliminate the pre-fills on warmer days to see if the effect reduces, and gather more comprehensive data on fuel consumption from crew over the coming weeks. Boeing will no doubt be involved, and we all look forward to learning something new about the aircraft.

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9 comments

  1. CK777

    Hi Ken,

    I really enjoyed the article and look forward to hearing more about your research. I came to your website from avsim and your postings on the PMDG 777. Please continue to contribute, it’s very valuable.

    Cheers

    • Ken

      Thanks Antoni

      Flight Ops conducted a trial comparing pre-fuelled with non pre-fuelled departures. I believe the result was not conclusive, but my feeling is that since the trial took place in January (Winter) – the temperatures aren’t enough to provoke the effect. We might need to try again in August

  2. Karan

    Good to read such a descriptive explanation of these situations, on the b777 flying in command we do come across the same problems as mentioned above, we fly non stop BOM EWR / DEL JFK
    I have seen a totalizer and calculated difference of 1.9 T

    Regarding other articles , you have mentioned very amazing to read.
    The performance restricted takeoff is also a very usual problem we face for these ULR flights.

    Would love to share more info on email

    Regards
    Karan

  3. Karl Montens

    Good article on a subject that can be approached from several angles.

    i) Why rely on calculated fuel?

    Since a couple of decades various “tools” have come on the flight deck to make our job easier.
    Calculated fuel is one of them. It is a tool like any other and has its restrictions. You do not have to use it. One important issue with CAL FUEL is that the DRAG/FF correction for the particular airframe needs to be correct. Same reasoning for the DRAG/FF correction the FPL (OFP) is using.

    ii) When the FQIS is installed or gets some attention from maintenance (calibration) there are tolerances that are present/accepted. If you take 100000 kg of fuel you actually take 100000 +/- a certain value. Suppose the centre tank is overreading (calibration error) by 300 kg then that error will disappear when the tank is empty. An empty tank cannot show any error since there is no fuel in it.

    iii) Even if pre-fueled one has the means to make sure the correct fuel (within the tolerances) is uplifted. Liters from the bowser * bowser-density (whatever it may be compared to the density in FQIS) added to the fuel that was already there. You will have to accept that the reading before topping up was correct. And the difference between what you have calculated and what has been indicated will give you an idea of the diff you will see between CALC and IND fuel.

    vi) Then there is the issue of the legal aspect. Indicated fuel and available fuel are two different issues. This is where contingency fuel comes into the picture. It is used to cover anything you do not know beforehand. Simply because if you know it you have to take fuel for it. In fact, the minimum landing fuel of 30 min should be increased with the known tolerance of the FQIS. On the B747-400 this is up to 1100 kg. It is ver very big bucket and at the end of a flight only the bottom of it contains some fuel. This is the moment the indicated and usable should be as close together as possible.

    vii) On the B747-400 we accepted a tolerance of 3% during fuelling. That could be up to +/- 4000 kg so a read of 8000 kg depending on the fact if your indicators were under or overreading.

    viii) If you work for an airline that likes to go minimum fuel with a reduced contingency i.e. a contingency only from the decision point till destination then a good operating practice (GOP) would be to do a drip stick after every arrival.

    ix) Rule of thumb. 0,8 for ISA (15 C) and for ever 15 C on top 0,01 less.

  4. Sigvard Wahlin

    Hello,

    I enjoyed reading your write-up on 777 FQIS. You mentioned that you worked with Boeing Technical… Have we met? I am the Boeing Design Engineer and Equipment Manager for the 777 FQIS system and I work in Propulsion Engineering.

    I have taken an interest in this phenomenon myself over the last decade across the Fleet. I really appreciate your experiences with this TOT/CALC issue… thanks for sharing! I have also seen some interesting data from multiple operators and would genuinely like to share information with you to help try and understand this phenomenon if you are interested. You can contact me at Boeing.

    Kind Regards

    Kind Regards,

    Sigvard Wahlin

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