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Aviation is a safety-driven business – starting with the fuel in the tank. While SAF is designed as a “drop-in” fuel – with the aircraft seeing no difference between it and fossil JET A/A-1 – using it isn’t as simple as proving it’s an equivalent set of hydrocarbons. The job of approving a SAF for use falls to the USA-based ASTM International.

Anyone wanting to sell sustainable aviation fuel needs to be familiar with both the relevant standards and the process for approval. Accordingly, let’s look next at what both involve.

Let's start with some definitions. Conventional jet fuel is not “certified”35 as such; a fuel is approved as meeting a standard, and aircraft are certified to that standard, with ASTM D165536 the key specification for JET A/A-1.

Approval for SAF, however, is not a case of applying for D1655 approval. There are a few more documents in play: ASTM D756637 and ASTM D405438.

Let's take ASTM D7566 first. This document describes the quality standards required for each SAF production pathway – currently seven of them, each with its own annex, with more being added as they’re developed. Basically, it defines what feedstocks must be used, the processes that act on those inputs, and the properties of the outputs for each pathway. ASTM 7566 also specifies blend requirements, including a maximum blend ratio with JET A/A-1.

The upside: once a SAF is tested and blended according to the requirements of its D7566 annex, it’s automatically considered to meet ASTM D1655, and can be safely put to work as drop-in jet fuel. Meeting those needs, of course, isn’t easy.
The process for approval is set out in a third document, ASTM D4054. This is a set of guidelines for engaging with ASTM International and what the organization will expect from a SAF producer – not a task-by-task instruction book. There are two pathways to approval: “traditional” (normal) and a Fast Track39.

The traditional approach is in four parts. If a six-month initial examination of the fuel’s specification and properties (called Tier 1 and Tier 2) finds the fuel fit for purpose – carrying costs of around USD 50k and needing 200L of neat product – it goes into testing with aviation partners, including Airbus, Safran, and Rolls-Royce, with costs of around USD 350k over another six months.

Positive results lead to 2-3 years of further testing (Tiers 3 and 4) with a multi-million-dollar price tag and a minimum 100,000L of neat fuel. Success leads to FAA approval and balloting of the ASTM’s panel of experts, all of whom must agree before the fuel can be offered for sale.

The Fast Track approach sounds easier: a set of screening criteria and testing, and an accelerated test schedule that bypasses the normal schedule’s four tiers. But needless to say, the standards are no less stringent – essentially, it’s a means to prove early viability so FAA approval and the ASTM ballot can happen sooner. What’s more, a Fast Tracked SAF can only be blended to a maximum 10%, against 50% for a traditionally approved one. And obviously there’s no chance of Fast Track if your production pathway isn’t covered by a D7566 annex already.

Note that meeting the D7566 standard does not mean the fuel is sustainable. In fact, the documents don’t describe SAF as SAF, but as “synthetic hydrocarbons” and similar. This is reasonable, given it’s a quality standard for aviation fuel, not a guarantee that the feedstock or the energy to process it came from sustainable sources.

Once a SAF blend meets ASTM D1655, it becomes plain JET A/A-1, interchangeable with any other source of jet fuel. And since SAF’s main value proposition is that it’s a drop-in replacement for fossil, there’s no reason to name it differently.

Because D4054 isn’t prescriptive, there are a variety of ways to approach ASTM when looking for approval. So, let’s end with three factors that contribute to a successful approval process.

First, prescreen your product for its composition and properties and check your production process against the approved pathways noted in the annexes to ASTM D7566. If it’s a good fit, your journey starts here. And the volumes needed are small: less than a liter.

Second, engage ASTM committee members personally – an approach recommended by the ASTM itself. Present them with initial data, a description of your pathway, and your plans for commercializing the product. (This last one seems unusual, but it’s vital: the approval process costs money, and the ASTM wants to know you’ll take the product to market at the end of it.)

Third, establish a team within your organization as an ASTM Task Force! This will focus your efforts on meeting each milestone on the way to your initial goal: entering the D4054 process itself, after which you’ll submit up to 300L of product for further research and testing. And the real work, of course, starts there.