Direct vs 1 stop Long-Haul Flights - Theory vs Reality

Last week’s Project Sunrise announcement revived earlier debates comparing the relative merits of making refuelling stops on long haul flights. The following article addresses this matter in general for a fixed payload. Project Sunrise is more related to the section ' where do refuelling stops make sense'.

Many previous papers from respected institutions and authors conclude that airlines should operate existing >5,000nm flights with a refuelling stop to deliver lower average airline energy intensity, i.e. fuel per revenue tonne-kilometre or RTK.

The theory derives from the shape of aircraft energy intensity (fuel burn to transport 1 tonne of payload 1kilometer = RTK) as a function of flight distance with a constant payload (SAD/Fuel consumed where SAD = still air distance). The figure below shows curves for generic aircraft models using technology from the 1970s (747-200, DC-10-30 etc), the mid-1990s (A340 and 777) and around 2010 (787 and A350). The data derives from RAWAvCon models.

All curves achieve minimum average energy intensity at 2,000-3,000 flight distances. The energy intensity increases substantially as the flight distance reduces from the optimum value; it increases more slowly above the optimum value as the flight distance increases from the optimum distance.

The effect of aircraft efficiency is critical. Improving aircraft efficiency/technology increases the optimum SAD a little. However, the main difference is the substantial reduction in the relative inefficiency of flying further than the optimum; the short-range relative inefficiency increases a little with improved technology. The two effects combine to reduce the penalty of flying 6,000nm from almost 15% to just over 5%.

These percentage increases broadly represent the theoretical penalty of a direct flight relative to a stopping 2 x 3,000nm flights operating at close to the optimum energy intensity SAD.

The current widely held perception that making a refuelling stop saves considerable fuel is largely based on the earlier research performed using aircraft using 1990s or older technology. Modern aircraft gain less from making refuelling stops.

Given that airlines usually fly their most efficient aircraft on their longest routes (economics), even the theoretical benefit of making a fuel stop is now relatively small. Then there are practical considerations that will typically exceed the theoretical benefits.

Routing Efficiencies – GCD based (for now)

Firstly, making a stop introduces various routing inefficiencies that erode the theoretical benefit. Theory assumes an airport situated exactly at the mid-point on a flight’s optimum air routing on every flight.

Let’s consider great circle routings for Shanghai to Los Angeles (SHA-LAX). It is 5650nm and suggests a theoretical 4% benefit for making a refuelling stop (see payload range chart below). However, routing via existing international airports located in Anchorage (ANC) or Honolulu (HNL) adds 2.5% and 15.5% track distance, respectively: 1% extra track distance roughly equates to 1% extra fuel burn.

Image from http://www.gcmap.com/

Further, ANC is not midway between SHA and LAX, requiring about a 65%:35% split in flight SADs (3,750 and 2,000nm). A 50:50 split requires two 2,900nm flights, i.e. close to the 2010 technology optimum SAD. Looking at the curve for 3,750 and 2,000nm SADs shows both introduce a small inefficiency relative to the 2x2,900nm flights resulting in an extra ~0.5% fuel burn inefficiency.

Hence, the theoretical benefit of using ANC reduces from 4% to about 1.0% - it is still a benefit. The considerable extra track distance using HNL consumes more fuel than a direct flight.

This is a single route, representative of Asia-US. Other routes will have a GCD closer to ANC or are longer so might generate a small benefit on some days. Others will have even longer track inefficiencies or are shorter flights reducing generating larger fuel burn penalties, but it illustrates the real-world routing challenges and uncertainties.

Winds

Upper atmosphere winds also influence optimal air routings and they can change substantially over short periods. Adding a refuelling stop anywhere along a route adds an extra constraint to the route planning. If the most favourable tailwinds or headwinds yield an optimal direct air routing offset from the great circle, the stopping flight must deviate away from the optimum route to make its refuelling stop.

Hence, the direct flight gains additional wind benefit (tailwind) or less wind penalty (headwind) thus reducing the SAD required to fly the missions: -1% SAD = -1% fuel (roughly). Defining a generic benefit is impossible as it depends on the specific route, and the upper atmosphere wind characteristics at the time of flight (strength and gradient vs offset from the optimal direct routing).

However, average direct flight benefits is probably worth several % as it is almost impossible for the stopping route to gain a benefit. If the optimum wind routing passes directly over the refuelling airport, the direct flight will fly directly over the stopping airport negating any significant benefit.

Assuming a direct SHA-LAX routing in optimum winds gains a further 2% SAD reduction, i.e. fuel burn, relative to the SHA-ANC-LAX routing, the net effect of stopping is 1.5% penalty. If it is just 0.5%, there is no net fuel burn benefit for stopping.

There are various other small penalties that most theoretical studies overlook. Although all contribute <1% of extra block fuel, they are individually significant against a 3-5% theoretical benefit. When combined with the routing and wind efficiencies described above, they are very significant.

Space precludes detailed discussion on all. Examples include:

i) fuel usage outside of the block fuel, e.g. APU usage and engine start fuel;

a. energy to cater for passengers and refuel the aircraft.

ii) faster engine deterioration due to more major cycles;

a. more frequent component replacement (resources).

iii) greater tyre and brake wear due to more landings;

a. more tyres and brake replacement (resources).

iv) reduced contingency fuel reduces the direct flight penalty;

v) Mid-points for many long haul places are inhospitable or sparsely populated.

vi) terminal manoeuvring at midpoint airport at 20-30nm track distance (average)

Where do refuelling stops make sense?

The benefits of a refuelling stop in an operational environment only become significant when the payload is constrained by the aircraft’s payload/range diagram. Making a refuelling stop enables a higher payload to be carried over the total mission distance. A significant increase in payload causes a relatively minor fuel burn increase (at a given missions distance), hence a significant improvement in fuel consumption per revenue tonne-kilometre should occur.

All the operational benefits of direct flights described still apply, but the substantial fuel burn per RTK should win.

The payload range chart below is public data for a long-range airline with ESADs calculated using RAWAvCon models for the aircraft concerned.

This chart shows that the theoretical benefit from most flights is 3% or less (due to their 4,000-5,500nm ESAD), implying that the penalties of stopping will usually swamp this benefit. The longer flights are trans-Pacific are affected by the previous SHA-LAX discussion regarding track inefficiencies.

It is interesting to note that the total payloads are average so will include significant scatter around the point shown.

CONCLUSIONS

Earlier theoretical studies concluding a fuel burn benefit for making refuelling stops are probably correct when applied to the older aircraft they considered.

However, repeating the exercise for modern aircraft greatly reduces the theoretical benefit available due to their substantial efficiency improvements. The remaining theoretical benefits are now typically swamped by the various operational realities on most long-range flights, and at least greatly reduced on all. Technology cannot reduce geographic or wind effects, indeed modern fuel policies should also reduce the theoretical benefits.

Stopping substantially increases flight operating costs due to additional maintenance and landing fees. If the fuel costs increase too, then stopping is economically and environmentally nuts.

The only remaining significant fuel burn benefit occurs if the refuelling stop enables increased payload, and this is conditional on a route having the necessary traffic.