The use of Avgas UL 91, when this fuel grade has been approved for the particular engine types, is allowed even if the airframe TCDS specifies a higher minimum number of octane (e.g. Avgas 100LL). In this case, the Avgas UL 91 can be used with no approval required for the aeroplane, provided the aeroplane is already approved for operation with Avgas or Mogas and the engine is already approved to use unleaded Avgas UL 91. Avgas UL 91 may also be used in all engines and aeroplane types approved by the Type Certificate Holder for use with Mogas RON 95 (MON 85) in accordance with Standard EN228:2008.
The background of this special situation is that in the last decades many aeroplanes have been approved for 100LL only (because no other avgas grade was available) despite the fact that the engine is approved also for lower avgas grades (like 80/87).
SIB 2011-01R2 has been published to inform all owners and operators of aeroplanes powered by spark-ignited piston engines about the use of unleaded Avgas UL 91. The SIB clarifies the actions to be done to allow use of Avgas UL 91.
IMPORTANT: Use of unleaded Avgas UL 91 in engines that have not been approved for the use of these fuels may cause extensive damage or lead to in flight failure, due to the lower Motor Octane Number (MON) of the fuel, compared to Avgas 100LL.