How much fuel?

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Fuel.  It is an ongoing issue.  We all want to ensure that we arrive with enough fuel at the destination so we have options.  The best way to accomplish that is to have more fuel on departure, right?   Well, maybe not.  It turns out that is really depends on the type of aircraft we fly, and if we are flying big airplanes the issue is a bit more complex than just how much gas we have when we takeoff.

Now, we all agree that there is no question that we must have enough fuel to be safe.  We also agree that there is no question that the Captain is the final authority as to how much fuel that is necessary to meet that required.  No question.  This article is not about telling you how much to carry, but rather to help you make an informed decision.

How much fuel is enough to be safe?  Is it a number in pounds or time?  Ah, now we get to a real issue.  As a Captain, the pounds of fuel (aside from limitations for your aircraft or possible company specific requirements requirements) is actually less important than time.  The concern here is that picking a number of pounds as “safe” may, or may not be safe.

A performance analysis of fuel required for airplanes of B-757 size and larger showed that the amount of “time in our tanks” is about 50% more at a lighter weight as compared to max landing weight.  I did not run the numbers for “smaller” jets, but I would expect a similar outcome.  Weight makes a huge difference in the burn rate, so if we are just using a pound or kilogram value for how we assess our fuel quantity we can be wildly off of what we think we have, either more or less than what we are attempting to accomplish for safety of flight.

What can be seen is that when we are at a light weight we actually have about 50% more time in our tanks than we do if we are heavy.  The other thing that becomes clear in all this is that adding 2,000 pounds if it is only 4 minutes may not be enough to be helpful, particularly when one considers that we are burning perhaps 10% of that amount just to carry the extra weight.  As pilots we have safety as our first priority, but we do directly benefit from making the airline more efficient also.  Having more fuel is also not always more safe as it can adversely impact performance during takeoff and cruise.  It can make the difference between cruising over the top of the weather or slugging through it and deviating constantly thus burning more fuel than we added to “be safe”.  These are the decisions we are paid for and knowing how much fuel you have, in time, is your best path to ensuring you are carrying the amount you need.

One way to frame the thought of how much fuel over destination might be to consider how much time do you want to give ATC with a Minimum Fuel advisory call?  Recognizing this is a dynamic number and would be different for EWR, HKG or PVG on a bad weather day compared to TUL or PEN on a pleasant afternoon.  Let’s say we choose 30 minutes of fuel remaining as when we declare and emergency to get on the ground NOW (I’m just using this for a demonstration, I might very well declare an emergency sooner depending on what’s going on!).  Using typical performance values for the larger jets, 5 minutes of fuel equates to 17% (but for easier mental math, round it to 20%) of the amount of fuel it takes to hold at 30 minutes.  What ever values of fuel weight as compared to time you have, you can use that as a baseline to calculate what you are really adding in terms of safety margin in minutes.

If we use these values or something close to it and add fuel for a go around we may be giving ourselves a reasonable margin for safety and allowing for contingencies without creating a crisis in the middle of a busy arrival sequence, but remember again the potential for burning more of it enroute due to the weight of carrying it plus the higher fuel burn when stuck at lower altitudes.  The tops of the clouds, the winds and the potential for direct routing as we get over the bulk of the traffic are all factors to consider.  It is entirely possible to have more fuel at the destination by starting out with less on departure in many scenarios.

A few years ago I was headed from MEM to MIA.  There was a wide area of weather with imbedded thunderstorms enroute.  As we all have seen, when flying through the weather in an area like this it can be a challenge to pick out the storms from the general rain in the area.  Dispatch had put on plenty of fuel so we could weave as necessary on our way to MIA.  As a result of the amount of fuel we were limited to around FL310 due to the weight.  I noticed that the tops were not above FL350 with few exceptions so I did something that might seem counter-intuitive. I asked for less fuel.  Enough less that we were able to climb to FL370 and sail over the top with just some very minor visual deviations along the way.  Do you think I landed with more or less fuel than I would have if we had stayed on the original plan?  Comparing the flight plans our burn was significantly less even without deviating.  A similar outcome might happen on an Atlantic crossing, where adding more fuel might mean that we are unable to get a higher altitude on the crossing resulting in burning through all the fuel we added and more, and actually landing with less than if we had not added any fuel at all!

Again, it is a complex scenario that must include considering many factors. Weather, tops, winds, aircraft performance, the effect on traffic based on the time of arrival (which in turn is impacted by the other decisions).  Each flight is different and it is up to us to consider all the aspects.

Regardless of the decision made, remember to always to keep the flight dispatcher in the loop.  They share legal authority and regular communication is not only smart but is also required by regulation.

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About Shem Malmquist FRAeS

B-777 Captain. Air Safety and Accident Investigator. Previous experience includes Flight Operations Duty Officer, Assistant Chief Pilot. Line Check Airman, ALPA Aircraft Technical and Engineering Chairman, Aircraft Performance and Designs Committee MEC Chair, Charting and Instrument Procedures Committee, Group Leader-Commercial Aviation Safety Team-Joint Safety Implementation Team (CAST)-Loss of Control-Human Factors and Automation, CAST-JSIT- Aircraft State Awareness. Fellow of the Royal Aeronautical Society, full Member of ISASI, AIAA, IEEE, HFES, FSF, AFA and the Resilience Engineering Association.
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