Reduced Design Range Aircraft - Practicalities

Transport aircraft initially designed for shorter ranges than competitors often spend much of their post-EIS programme and considerable investment trying to claw back the range discarded during their initial design once the market speaks.

Historical examples (mostly European aircraft designed for European markets) include DH Trident, A300, Mercure and A310-200. Even the highly successful A330 only achieved market success when the smaller but longer-range A330-200 launched to outcompete the 767-300ER.

When the original but larger A330-300 belatedly had its centre fuel tank activated and MTOW increased to extend its range, its sales then eclipsed the A330-200.

Despite this, various researchers previously concluded that replacing long-haul aircraft with medium-range alternatives can substantially reduce long-range air travel emissions. Substantial fuel burn improvements of about 20% are sometimes quoted, derived roughly equally between the refuelling stop and redesigning the aircraft or medium range.

A previous blog showed that modern long-range aircraft gain no significant benefit for making a refuelling stop relative to a direct flight on most missions; indeed, they will incur fuel burn penalties on some flights. The inefficiencies of refuelling stops apply to all aircraft, irrespective of their design range. https://www.rawaviationconsulting.com/single-post/direct-vs-1-stop-long-haul-flights-theory-vs-reality

Another noted that aerial refuelling of civil transport is not environmentally beneficial due to the tanker fuel consumption and an economic basket case. https://www.rawaviationconsulting.com/single-post/aerial-refuelling-why-not

Hence, various researchers claim a 10% fuel burn benefits for reducing long widebody aircraft design range because of substantially smaller wings and less powerful engines. The following shows this advantage as <4% due to a design attribute never mentioned in the various reports I have seen from various eminent institutions.

However, firstly, I discuss the generally valid reasons why many airlines currently use their long-range aircraft on short-medium flights, while accepting that some airlines do use widebody aircraft exclusively for short-medium range operation

SOME FACTS ABOUT THE MISUSE OF LONG HAUL AIRCRAFT

‘Misuse’ = aircraft operated on missions well below their certified capability.

Some researchers justify the need for reduced design range aircraft by observing that a substantial proportion of widebody flights connect airports <3,000-4,000nm apart and that 6,500nm is rarely exceeded - all distances are great circle distances (GCD = measured across the Earth's surface). Further, aircraft design studies tend to use 3,000-4,000nm flights for economic analysis. These observations are based on data so are correct. Hence, some ask, ‘why do the aircraft need an 8,000nm nominal range?’ and ‘can we design a short-range widebody to reduce fuel burn?’.

Firstly, the analysis should consider the operational Take-Off Weights (TOW) distribution relative to the Maximum Take-Off Weight (MTOW), although this information is not generally available outside the airlines. Operating at or close to MTOW typically indicates a flight using the aircraft’s full design capability i.e. payload/range, between two airports – not the great circle distance with a nominal payload.

Long-Range aircraft designed for 8,000nm nominal range typically need their full MTOW to reliably connect two airports 6,000-6,500 nm apart with profitable payload. Routes with 5,500-6,500nm GCDs represent a significant/substantial proportion of flights for many international airlines in all global regions. The difference is due mainly to winds at cruise altitude (possibly >20% of cruise speed in winter months), performance deterioration, as well as heavy cabin equipment/supplies and payloads relative to that used to define the nominal 8,000nm range capability.

However, many of the long-range airlines also ‘misuse’ their aircraft to some extent for valid reasons. These need understanding.

Others (mostly in Asia) use widebody aircraft mainly for their large cabin, and perhaps their freight capability, exclusively on short/medium-range flights. The latter would benefit a little from a reduced range aircraft but the market is not currently large enough and there are some commercial issues (see early obsolescence discussion)

Why do airlines ‘misuse’ their aircraft’s range capability?

Is some misuse justifiable? Yes! Most of it.

Long-range operators ‘misuse’ of long-range aircraft is justified for various economic and even environmental reasons to enhance airline economics by increasing aircraft utilisation, hence the revenue generated by their high-value assets.

Some airlines alternate long haul and short-medium flights to meet their schedule constraints. It is impossible for many to efficiently operate long haul flights exclusively without parking the aircraft for long periods between long flights – aircraft only generate revenue when flying. These constraints, imposed by geography and time zones, are often challenging to design away.

Others, such as European charter operators, vary the balance between long haul and high volume short-haul destinations on a seasonal basis, e.g. winter sun in the Caribbean and summer holiday flights to the Med.

In both cases, using bespoke aircraft for the short missions requires the design and production of a new aircraft type (resources and emissions) and aircraft parked up for extended times (daily or seasonally).

Specific events such as regional sports tournaments, e.g. Euros football, or the Hajj need large cabin aircraft for relatively short flights in a narrow time window. The rest of the year involves long and short flights as the customers require where flexibility is key.

There are others, but leasing companies that own about half the global airliner fleet also want range flexibility to ensure broad market appeal when trying to place their aircraft and would need a compelling fuel advantage to order reduced range widebody aircraft.

THEORY OF REDUCED DESIGN RANGE BENEFITS

The fuel fraction of a long-range aircraft is about 40-50% of the MTOW. Reducing the range by half enables an initial ~20-25% MTOW reduction. However, operating a long-range aircraft with a refuelling stop reduces its mission TOW by 20-25% without changing the MTOW, removing this advantage.

However, many researchers assume the reduced design range aircraft’s 25% lower MTOW enables a 20-25% smaller wing and engines for constant take-off wing loading and thrust/weight. These provide direct aircraft empty weight and drag benefits, delivering a further ~5% fuel burn benefits and even lower MTOWs and less wetted area (less drag). Further, empennage, undercarriage and some aircraft systems weight reductions combine to deliver overall fuel benefits approaching 10% once the full ‘snowball’ design iteration converges. The process assumes constant aerodynamic, propulsive and weight efficiencies.

10% reduced fuel burn is a compelling theoretical fuel burn benefit.

REALITY WITH CONSTANT TLARs – EXCEPT DESIGN RANGE

Reducing aircraft nominal design range from about 8,000nm to 4,000nm struggles to deliver block fuel benefits exceeding 3-4% at all mission lengths not requiring a refuelling stop, i.e. less than half the often reported values. It is mainly due to the approach and landing wing sizing case constraining reductions to the wing area and engine sizing. Once refuelling stops are necessary, the associated fuel and time inefficiencies will often increase over mission fuel usage.

Previous research papers on reduced design range claiming ~10% benefit appear to focus solely on MTOW. Indeed, I found none that mention landing performance at MLW; a consideration equally important on most long-range aircraft. Typically, an aircraft’s wing area balances MTOW and MLW sizing cases. Reducing either one independently leaves the other sizing the wing.

Removing the considerable fuel mass due to reduced design range does not significantly affect MLW as it contains little fuel (<5%). The 3-4% block fuel benefit when including MLW wing sizing is due mainly to minor wing mass savings due to reduced structural stress (from lower MTOW) decreasing the wing mass, i.e. OEW. The ensuing design iteration rolls the lower OEW into a reduced MLW that reduces the wing and engine sizing (as well as other components and systems) that iterate to convergence at the 3-4% level.

The engine's take-off thrust rating can reduce, but a well-designed engine balances the take-off thrust requirement with delivering the optimum efficiency at cruise flight conditions (cruise defines fan diameter). The Maximum Climb thrust is closely linked with the cruise thrust and consumes as much engine life as the take-off thrust (less throttle but longer dwell times). Hence, just reducing the take-off thrust will not reduce engine sizing unless the climb/cruise thrust also reduces.

The climb/cruise thrust requirement is primarily linked to wing area (plus wing aero design), not aircraft weight - a 1% bigger wing requires about 1% more thermodynamically powerful engine. Changing aircraft weight with a constant wing area requires a higher cruise altitude to maintain its optimum performance. The absolute cruise thrust required reduces but the engine must continue to work at about its optimum throttle setting to produce the lower thrust due to decreased atmospheric air density. Note: how long-range flights climb as they burn off fuel - it keeps both engine and wing operating efficiently.

Hence, the MLW wing sizing case limits the reduced design range benefit to 3-4%

Real World Range Capability of reduced design range aircraft

The various factors linking an 8,000nm nominal design range to real-world payload/range limits at 5,500nm-6,500nm also apply at 4,000nm nominal design range. The shorter nominal design range equates to about 3,000nm real-world capability, i.e. just enough for London-New York but not enough for any airports further into Europe or the US.

Pushing the nominal design range beyond 4,000nm increasingly improves the aircraft’s market coverage (good), and requires fewer refuelling stops, but also erodes the already small fuel burn benefits relative to a long-range aircraft (not so good) making the potential benefit for reducing design range less appealing.

The economics also suck! Maintaining short and long-range aircraft doubles the administrative and spares inventory costs.

Early Obsolescence

Further, early obsolescence of a reduced design range aircraft is a huge programme risk. If a competitor aircraft launches with 3-5 years later technology, it should achieve more range, covering more market sectors with similar or better efficiency at all mission lengths. The resulting outlook for the original reduced range aircraft will become bleak (reduced sales and depressed market values).

The economics look even less credible!

Mass Fraction Aircraft Design Methods

Some aircraft design methods use MTOW-based mass fractions for weight forecasting. If you use these methods for reduced design range, ask yourself how reducing an aircraft’s design range, and hence MTOW, significantly reduces the mass of a fuselage designed to hold 300 passengers? It does not!

It is similar for the payload, cabin furnishings/equipment for 300 passengers. A shorter design range might reduce catering supplies mass, but the effect is secondary.

What about the MLW wing sizing case?

Mass fraction methods are invalid for this exercise as the underlying data from existing aircraft includes no reduced design range aircraft – all fly as far as they can to balance the MTOW and MLW wing sizing cases.

CHANGING OTHER TLARs TO MITIGATE MLW CONSTRAINT

Changing other TLARs for the reduced range aircraft might improve its fuel burn, but those same changes should also be applied to the long-range aircraft for consistency

Mitigating the MLW constraint wil provide the greatest benefit for the reduced range aircraft. It is theoretically possible by either increasing the maximum usable approach lift coefficient (CLmax) or reducing the payload included in the MLW will improve the potential fuel savings.

Increasing approach CLmax should enable a greater decrease in wing area and engine cruise thrust sizing with no change to approach speed, thus reducing MLW. However, certified Landing Field Lengths (LFLS) at MLW tend to be significantly shorter than the associated MTOW for various reasons.

Critically the aircraft must have a reasonable LFL following a high lift system failure, i.e. very little lift increment from the slats and flaps. I experienced such an event sitting as an observer on a 757 flight deck; 30knots added to the flapped approach speed required a significantly longer LFL that used almost all of the available 10,000ft runway! ATC offered the flight crew a fire tender at the end of the runway (to cover brake fire risks).

Increasing the flapped CLmax and reducing the wing area will increase the failure case LFL – does this remain acceptable? It can also be applied to the long-range aircraft although the benefit might be smaller.

Alternatively, reducing the sizeable freight payload capacity of many widebody aircraft above the design passenger payload reduces the MLW. It also allows a smaller wing (with the baseline CLmax) and engine without compromising the failure case LFL.

The requirement for air freight and its effect on fuel consumption per energy intensity per revenue tonne-kilometre is another philosophical discussion. However, the A380’s limited market success is partly ascribed by some to its lack of freight capacity – a relatively small freight hold mainly filled with passenger baggage meant less freight capacity than smaller aircraft. Hence, it requires a market requirements debate and a compelling fuel burn advantage with the lower payload.

The A321XLR ~4,000nm nominal design range matches the 'reduced design range' discussed previously. However, its MTOW and MLW wing sizing cases are balanced, as are its engine take-off and cruise/climb cases.

CONCLUSIONS

Many current short/medium-range flights have valid operational and economic justification. Any future assessment of aircraft ‘misuse’ must consider why airlines fly these shorter flights with their long-range aircraft and the impact of not doing so on their economics for not doing so. How will the flight schedules be changed and economics be affected?

For constant TLARs, reducing design range from 8,000 to 4,000nm for a modern aircraft deliver up to 4% fuel burn benefit at all flight distance not requiring a refuelling stop. Adding a refuelling stop may erode this benefit on longer flights.

Designing and producing an additional reduced design range aircraft will consume considerable resources and generate emissions if used alongside long-range aircraft. Any overall environmental and economic assessment of the reduced design range benefits should include these effects. If they replace long-range aircraft, the inefficiencies of making refuelling stops must be considered

Think about early obsolescence risks. If a newer longer-range competitor uses technology to offset the minimal benefits of a slightly older reduced design range aircraft, the reduced range model offers no advantage unless its value is reduced.

All reduced design range studies must include the MLW-related wing sizing case and its associated impact on engine sizing. If using an existing aircraft as a baseline, the reduced range aircraft should achieve the same approach speed, a proxy for LFL.

Do not use aircraft design methods using MTOW-related mass fractions for reduced range aircraft as the underlying data includes no reduced design range aircraft.

One final thought: the A321XLR designed for about 4,000nm nominal range is not a reduced design range aircraft despite having a 4,000nm nominal range. It achieves its maximum range possible with balanced MTOW and MLW. Similarly, its engine take-off and cruise/climb cases also balance.

The main reason for the wing and engine’s design cases balancing is the A321XLR’s lack of a freight payload requirement and considerably smaller cabin floor area per passenger. These are very different TLARs to a long haul widebody.

The A321XLR might be more efficient for routes with little freight traffic and where passengers will accept a lower comfort level than in the widebody cabin. However, assuming common comfort cabins on the two aircraft and adding a significant freight payload to the widebody might make the A321XLR the more energy-intensive per RTK of the two, despite also flying slower – a potential future study and post.