By Vincent Wigmans

Introduction Avgas

The aviation industry has long relied on Avgas 100LL (Low Lead) as the primary fuel for piston-powered aircraft. However, concerns about environmental impact and health risks associated with leaded fuels, have led to the development of unleaded alternatives. Two of these alternatives are Avgas UL91 and UL94, designed to offer similar performance while being environmentally friendlier and safer. In this blog, we will explain the key differences between Avgas 100LL and Avgas UL91/UL94 and discuss the need for a Supplemental Type Certificate (STC) or Service Bulletin (SB) for this transition. 

Avgas 100LL: The Old Standard

Avgas 100LL, the most common aviation gasoline in use today, contains tetraethyllead (TEL), a substance added to boost octane levels and prevent engine knocking in high-performance aircraft engines. This leaded fuel has been in use for decades, but concerns about the environmental and health impacts of lead have prompted the search for safer alternatives.

Key Differences:

  1. Leaded vs. Unleaded: The most obvious difference is the presence of lead in 100LL, while UL91 and UL94 are lead-free. This makes the unleaded Avgas a more environmentally responsible choice.
  2. Octane Rating: Both fuels offer similar octane ratings (around 94-100), ensuring that aircraft engines can run smoothly without knocking or detonation.
  3. Color: Avgas 100LL is dyed blue while UL91 and UL94 are clear to Yellow (no dye).
  4. Storage and Handling: Avgas 100LL requires special handling and storage considerations due to its lead content. Leaded fuels can contaminate storage tanks and pipelines, which makes transitioning to unleaded fuels a cleaner and more straightforward process.

Avgas UL 91 / UL94: The Unleaded Alternative

Avgas UL91/UL94 represents a significant step forward in aviation fuel technology. It meets the need for a cleaner, safer, and more sustainable option while ensuring that aircraft engines perform at their best.

Key Advantages:

  1. Environmental Impact: The absence of lead in UL91/UL94 reduces harmful emissions, making it a greener choice. This is particularly important as the aviation industry aims to reduce its carbon footprint.
  2. Health Benefits:  Eliminating lead from aviation fuel is a crucial step in protecting the health of pilots, ground crew, and anyone involved in aviation fuel handling.
  3. Infrastructure Compatibility: Avgas UL91/UL94 can often be used in aircraft designed for Avgas 100LL without any engine modifications, thanks to its similar octane rating and performance characteristics.

Supplemental Type Certificate (STC) / Service Bulletin (SB) Requirement

To transition from Avgas 100LL to Avgas UL91/UL94, aircraft owners and operators should first contact their maintenance facility, to see what needs to be done to perform this transition. Most airframe and engine manufacturers will deliver a Service Instruction (Lycoming for instance) with the Fuel Grades that are approved in their airframe/engine. Other manufacturers are delivering this information in their Type Certificate Data Sheet (Continental). Typically, you need an STC or a SB. An STC/SB is a formal authorization, to modify an aircraft’s type design. In this case, the STC/SB is required to ensure that the aircraft’s engine and fuel system are compatible with Avgas UL91/UL94. Most modifications consist out of the implementation of an addition to the flight manual, and the attachment of new placards to the filler necks of the fuel tanks. Although some other aircraft need a technical change to the fuel system itself.


The shift from leaded 100LL to unleaded UL91/UL94 is a positive development for the aviation industry. It addresses environmental concerns, reduces health risks, and promotes sustainability in aviation. While the transition may require an STC or SB, it is mostly a small price to pay for the benefits of a cleaner and safer aviation fuel. As aircraft owners and operators embrace this change, they contribute to a more sustainable and responsible aviation future.