Fuel tankering (including scenario)

[Ledhead27]

Evening all,

My first post here on PPRuNe after meaning to sign-up for a while but never actually remembering to do so. Unfortunately my first post is a bit of a turgid one.

I'm writing an assignment on fuel tankering and part of the brief includes the following:

"Discuss the issues with tanking when operating a return sector to a busy airfield in Germany (e.g. Munich Airport), with a late arrival and early departure the next morning. The flight is to take place in winter with the OAT at a constant -15°C. What issues need attention? What dangers to the aircraft could there be? Would there be a time during inclement weather when tanking would not be prudent?"

I understand the properties of fuel with regards to freezing point and when it starts to wax, and I also understand the concept of tankering. However, I'm not quite sure what to explain for this part other than the possibility of cold soaking on the wing and perhaps not de-icing if wx conditions are clear as the fuel can be pumped into the wing warm.

Any help would be really appreciated.

Dan

[B737900er]

Hi , as you have already correctly mentioned, you will have issues with wing icing top/bottom of the wing, so de-icing will need to happen. You won't be pumping any warm fuel into the wing because you have just tankard the fuel. Fuel freeze temperatures will need to be considered but Jet A1 can go as low as -45c. If your landing heavy because of the extra fuel and the runway is contaminated you could be Landing performance limited.

Your not allowed to tanker fuel when the runway is classed as contaminated (company SOP may differ).

[Jonty]

The main thing I would be thinking about is cold soak, and ice on the wing. 6000kgs is the max to land with to avoid ice on the upper surface of an A320. We can take up to 3mm of ice on the lower surface of the wing. Also with an OAT of -15 the fuel you pump in will not be 'warm', so deicing may be an issue. But again with an OAT of -15 your probably going to de-ice before take off anyway, so is this an issue? if you intend to de-ice before takeoff then you could tank up to max landing mass. As long as there are no performance considerations.

[Ledhead27]

Thanks for the response chaps,

All of what you've said definitely makes sense, increased weight due to tankering would lead to greater braking force required which could increase the risk of skidding on a contaminated runway, depending on weather conditions of course.

Unfortunately the brief didn't make it terribly clear if the fuel has been tankered prior to arriving at Munich or once the aircraft has landed.

Not sure if I'm completely shooting in the dark here but can some airport/handling agents choose not to refuel in cold weather (I know during thunderstorms refuelling isn't really an option)?

[FlightDetent]

It sounds like a good thinking excercise, to learn about the wide variety of factors that may come to play. Kudos to whoever gave you that.

Operationally, at the end of the day, the decision needs to be either YES or NO. Given the narrative, I'd go with YES.
- you are not likely to escape deicing anyhow,
- in case the WX really is favourable that de-ice / anti ice is not required, having the tankered fuel inside the tanks from the evening will not make a difference.

regards,
FD.

[Ledhead27]

Our first assignment that has been set since the Summer break and it's certainly making all of us think. The chap who wrote the assignment is an ex-Swissair 767 captain and has also done quite a bit of work on private jets so he has a wealth of knowledge.

So with all this information in mind, should I go with the assumption that the aircraft has been tankered prior to arriving at Munich?

[kenparry]

So with all this information in mind, should I go with the assumption that the aircraft has been tankered prior to arriving at Munich?

Yes; tankering means taking more than you need for the sector + reserves, so you are discussing landing with possibly lots of fuel

[FE Hoppy]

Yes; tankering means taking more than you need for the sector + reserves, so you are discussing landing with possibly lots of fuel

and lot's of weight.

Winter ops, what's the runway state?

and your instructor would have done very well to be a 767 captain for Swissair ;-)

[Ledhead27]

Thought it made a lot more sense for it to be landing with tankered fuel, cold soaked wings and so forth. Weather conditions are only noted at -15 degrees OAT, although I suppose it gives me scope to discuss what the prevailing weather conditions might have an effect on.

Woops, he appears to have made his own airline up. Peculiar as he always seems to say "When I worked for Swissair on the 767". Unless he was working for BelAir that were a subsidary of Crossair, or I might have got muddled up completely.

[Chris Scott]

Hi Ledhead27:

Always a good discussion topic but, as others have already indicated, you seem to be stuck without much specific information. That may be deliberate on the part of your instructor, so you have to look at a range of cases.

Like kenparry, I'm confident that the term "tanking" in your assignment refers to "tankering" fuel, i.e., an arrival at Munich (your choice) with considerably more than the normal reserves. That's usually only done, of course, if the fuel price (or maybe availability) at the original departure airfield is better than at the first destination (Munich). It may or may not leave enough fuel for the second sector without topping-up.

If it's purely a fuel-price differential, the first criterion in deciding to tanker (or not) will be the length of the sector, on the basis that it costs fuel to carry extra fuel. During the sudden fuel crisis of 1973, my company's (VC10) flight ops dept quickly produced a graph of cost-differential versus flight time, and a list of prices at all the airfields on our network. Problem was: the fuel prices were being frequently renegotiated by our hard-pressed, one-man, fuel-purchasing department!

The other thing affected by the flight time, of course, is the temperature of the fuel on arrival. During the cruise, the fuel temperatures fall steadily, and the smaller tanks tend to get colder quicker. On my last fleet, the A320, we had a rule of thumb that if the flight time exceeded a certain value (might have been 1:15, but I forget), AND high humidity with freezing temperature was expected at the first destination, we would normally not tanker fuel.

High humidity at the first destination during the turnround can be a spoiler if you are not going to need to de-ice the whole airframe for other reasons (sleet, snow, or air frost). However, as Jonty says, on some types like the A320 you may be able to accept 3 mm of frost on the BOTTOM of the wing tanks (not the top) for departure. So provided none of the wing tanks is full, you may get away with it. The problem on many types (including the A320) is that the small outboard tanks will normally be scheduled as full (for wing-bending relief) until the inboard tanks are at a low state (which is unlikely in this case). So in high humidity the wing skin above the outers may develop frost even on the landing and taxiing in.

(In the early days on the A320, as wing-bending relief was not an issue at lower weights, we used to preempt that top-skin frost forming by transferring the fuel from the outers to the inners at top-of-descent. But it involved tripping and resetting CBs in the cockpit, so it may no longer be acceptable as an SOP. )

Finally, speaking generally, the effect of the extra weight on landing performance must be taken into account, as B737900er has already said. In your case, it seems unlikely to be a problem on Munich2's long runways.

My choice? Don't live in Bavaria, but a constant temperature of -15C sounds like anti-cyclonic gloom, and snow seems unlikely. High humidity first thing in the morning is normally associated with clear skies that have caused a considerable fall in temperature. So you might be lucky in this case; otherwise be prepared to join the queue for the (eco-friendly) de-icing facility on departure. You can't win 'em all!

[Ledhead27]

Many thanks for a very comprehensive answer Chris and everyone else, certainly helping me get a better general understanding of this concept. I'd say, given the conditions, de-ice prior to the early morning departure is almost a certainty.

My only other question is one of personal interest. I believe I'm right in thinking that tanking can be done on multiple sectors, however I got completely shunned by a fellow student who said its an economic no no to complete tanking on multi-sectors, however I disagree.

Anyone's views would be great

[Meikleour]

Leadhead27:
Imagine a flight from A - B - C - B

Relative fuel cost A =100
B =200
C =150

Tell your classmate that in this situ it is still worthwhile to tanker on the first TWO legs!
Hope this makes it clear. I have exaggerated the price differential for effect.

[JammedStab]

I am not quite sure that I understand any of the responses except consideration for runway conditions at a higher weight but this is Munich. Doesn't it have 4000m runways. Perhaps on a very few types this could come into play or in very poor conditions. I suspect that LH is landing at Max Landing Weight here on most winter days.

As an interesting question, ask the person who assigned this project, how much fuel would they typically tanker to this airport and if they would bump it for contaminated runway conditions even if the performance figures show it is within limits. If the answer is....they would not take the extra fuel, ask if they would still take in an equivalent amount of extra payload.

Someone talked about the requirement to de-ice, but at -15, won't most aircraft get frost anyways during an overnight stay.

On the plane I have flown, frost on the bottom of the wings from fuel is not de-iced for most aircraft unless it is significant(more than 3mm). If you are going to get frost on the bottom of the wings from cold soaked fuel, won't it happen even if not tankering as there is always some fuel in the tanks.

Fuel freeze temperature does not need to be considered because the example says that the OAT is a constant -15 degrees Celsius.

[Skyjob]

Many various limitations and recommendation for different types of aircraft, but in a nutshell, I think OP needs to consider the following generic advice:

Payload burns the equivalent of 2.5% to 5% of its weight in fuel per flight hour depending on the aircraft and the flight conditions. 4% of fuel burn / flight hour is a convenient reference. The payload % fuel burn is therefore purely a function of time.
• To carry 1,000 lb (or kg) of fuel for 2.0 hr. flight, it will cost (4% / hr * 2.0 hr) = 8% in carried fuel, or 80 lb (or kg).
• Therefore, the fuel at the arrival airport must less than 8% cheaper than the price of the payload fuel to justify tankering in the fuel.
• Per the above formula, tankering favours shorter legs as opposed to longer ones.
• The facility and other fees must also be taken into consideration in the price analysis.

• The PIC has the ultimate responsibility for deciding to tanker fuel or not based on the specific requirements and conditions of the flight.
• All things being equal, tankered fuel is payload and thus increases the aircraft take-off and landing weights, which in turn impacts performance e.g. the take-off balanced field length and landing distance. External factors such as altitude, temperature, wind, runway gradient, runway condition (dry, wet or contaminated) must also be considered when deciding to carry additional weight.
• If engine thrust needs to be increased due to higher payload weight to safely take-off, there will be a more engine wear and therefore higher engine maintenance costs.
• Finally additional fuel in the tanks can contribute to the extent of non- environmental icing i.e. wing frost at the destination airport depending on the cold soak, as well as ground temperature and dew point.
• Anti- or De-icing may already be required due actual or forecast weather conditions, thus any additional costs incurred with the requirement for this to be completed may already be operationally accounted for thus not a penalty for tankering cost calculations.

The above points are not to discourage fuel tankering, but rather to be considered by both dispatch and the crew in the flight planning process.

[Chris Scott]

Quote from Ledhead27:
"I'd say, given the conditions, de-ice prior to the early morning departure is almost a certainty."

Not necessarily: it depends on the dewpoint, which we don't have. Not much moisture in the air at -15C even if relative humidity reaches the nineties. But if on the day the whole airframe has to be de-iced anyway, I think you've taken the point that the whole issue of the wintry conditions becomes less relevant to your decision (to tanker or not). As JammedStab and I have said, Munich2's runway lengths (LDAs) make the landing weight irrelevant. (Other German airfields with only one long runway might be a different story, e.g., if it was closed or partly closed for WIP.)

Perhaps I should have commented previously that the combination of the very cold conditions and the overnight parking of the a/c represent a very different scenario from a quick turn-round when the airfield temperature is around zero. If the first sector was only an hour or so, and particularly if it started from a much milder place (such as London and Hurn often are), the fuel temperature on arrival might still be higher than your -15C. In that case, frost formation on arrival would not happen on top or bottom of the tanks. Overnight, the fuel temperature would fall to ambient, so its presence would be irrelevant to the skin temperature top or bottom. OTOH, if the a/c had flown a long way from a very cold airfield, particularly from one which uses bowsers (rather than underground hydrants) for refuelling, the fuel temp might have been lower than -15C even before departure. On arrival at Munich, if frost forms on the top of any full tank immediately after landing (see my previous post), it is likely still to be present in the morning - even if the rest of the airframe is clear.

Too many variables, I'm afraid... De-icing is an expensive business on a large a/c. But it is possible to mitigate the cost by ordering wings only, or wings and tailplane.

BTW, I now see that I missed another important criterion for whether you can carry enough fuel into Munich to operate the next sector (or maybe two, as Meikleour proposes) without refuelling: the payload-dependent ZFW (zero-fuel weight) on the first sector subtracted from the RLW (regulated max landing weight) for the conditions. And of course the payload for the next sector may shoot up overnight, giving a higher-than-expected ZFW that would increase the fuel requirement. It may be possible to allow for that possibility by loading a bit extra. But you can't cater for every eventuality!

[FE Hoppy]

We have a policy of no tankering if the landing runway is expected to be contaminated or if icing conditions are forecast for the approach. So regardless of length wouldn't do it. Remember that landing heavier than you need to for economic reasons is fine if you're not impacting safety margins. We also have to take into account next leg so as not to arrive with too much fuel. Imagine the next sector is very short to a short runway with a full traffic load. You may end up limiting yourself due to the next landing weight if you arrive with too much fuel.

[TheiC]

Your first objective is always to get the airframe and payload from gate to gate in one piece and on time.

When there are additional pressures such as contaminated runways, de-icing, etc, it may be wise to focus on that first objective.

If you choose to address other objectives, such as strategies which address your airline's inability to do equitable fuel deals across its network (don't forget that tanking is ALWAYS bad news for the environment) and then you screw up, you may find yourself wishing you'd stuck with objective one.

Oh, and always be wary of people whose stories don't stack up.

[Ledhead27]

I think the problem with my question is that the question is incredibly vague i.e with regard to lack of weather information from the departure point and further prevailing conditions. It isn't even clear where the aircraft if returning from, so might be worth me asking my instructor so that I can give a more defined answer.

Conversely, he might want me to discuss the difference in outcome between a long a short return sector. SO MANY VARIABLES and it's certainly ambiguous whether the aircraft should be tankered or not. Also, I presume company SOPs may or may not allow for tankering into a contaminated runway as Hoppy has said.

Finally, the question states that the flight takes place in temperature's at a constant -15 degrees OAT. Do I assume that includes being at cruise altitude, it seems quite warm when the aircraft might be operating at an altitude of 33,000ft.

[Piltdown Man]

With the OAT at a constant -15°C, I don't think cold soak is your problem. This temperature will also give rise to relatively dry air, even if it is misty. Also, the fuel left in the aircraft after a night-stop won't change the need for de-icing - it will either need it or it won't. This will not change if you have to uplift fuel. Therefore two issues remain. 1. Does the aircraft's AOM preclude this sort of operation and 2. Is it economically worthwhile to do so?

Answer those and you have your answer.

PM

[Chris Scott]

Quote from FE Hoppy:
"We have a policy of no tankering if the landing runway is expected to be contaminated or if icing conditions are forecast for the approach. So regardless of length wouldn't do it."

Yes, IIRC that was the policy in my last airline, and may apply to most.

Unless I'm misunderstanding the OP, we are having a generic, academic discussion here for a student; neither airline-specific nor type-specific. The runway at the first destination (suggested to be Munich by the OP) may or may not be contaminated. But I think Piltdown Man and I agree that, speaking generally, contamination of runways is more likely to affect operations when the temperature is just below zero than at -15C. And, although one must always be super-vigilant, the same applies to airframes.