Aer Lingus is the latest airline to trumpet its use of sustainable fuels

Airlines, keen to burnish their sustainability credentials, are falling over each other to highlight their intentions to use sustainable aviation fuel (SAF), which emits less carbon than kerosene jet fuel. The latest is IAG, the parent of Aer Lingus and British Airways. On Monday, it announced a seven-year deal with a US SAF supplier to supply fuel to the two airlines for use on flights through San Francisco airport.

It is worth carefully parsing the blizzard of statements from airlines promoting their use of SAF, however. The announcements are not always as significant as they might at first appear.

Aer Lingus always chooses its words carefully on the subject. Echoing the language typically used by its corporate parent, the airline said it has set a target to “power 10 per cent of flights using SAF by 2030″. A casual reader might conclude that this means that one-tenth of Aer Lingus’s total fuel needs would be met by SAF within eight years. But this is not necessarily the case.

SAF is made from biofuels derived from feedstock, or else it is manufactured using materials such as waste cooking oils. It must then be blended with normal kerosene jet fuel, before being pumped into the plane. It is not possible to power a flight with 100 per cent SAF.

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Fuels giant BP says SAF can be blended with traditional fuel at a ratio of up to 50 per cent. Assuming Aer Lingus blends fuel up to the maximum ratio, even if it hits its target to power 10 per cent of its flights with SAF, up to half the fuel used on those flights could still be traditional kerosene jet fuel.

Like Aer Lingus, Ryanair has also set a 2030 SAF target, although it is slightly higher at 12.5 per cent of flights. It uses a 40 per cent SAF blend on many of its Amsterdam flights, according to company announcements.

The thing about SAF is that it is three times as expensive as traditional jet fuel and there is nowhere near enough of it to go around, if all airlines are to hit their targets. Inevitably, governments will end up having to subsidise its production.