The 12% margins are only determined during certification tests. You would never enter these margins unless you purposely operated your engine in excess of its certified maximum and minimum fuel flow operating limitations.
I know you have the concept, but you’re not expressing it quite correctly. Let’s talk about never below…
if your mixture control at full rich equated to 100% and 60% at the minimum operating lean fuel flow, in order to pass the detonation margin test you would have to increase the mixture to 112% and reduce it to 48% before any limiting detonation was detected.
Nah, that’s not how that works! Let’s use your 100% full rich example. For my turbo-Cardinal, that’s 24 GPH at takeoff power, 31” manifold pressure. The FAA detonation test would require me to not exceed limiting detonation at full power, 31” manifold pressure, and 12% *less* fuel flow, or in other works, at 21.1 GPH.
Remember, at takeoff full rich, your engine is set up to flow *extra* fuel to suppress detonation. If, instead, you flow less fuel, you don’t have that extra-rich mixture to suppress detonation, and it can occur.
My airplane doesn’t have a “minimum operating lean fuel flow” specification. But in cruise, at 31” and 2500 RPM, I can come back to 11 GPH, about 60 degrees lean of peak, and the engine is very happy operating there for hours on end… cool CHTs, reasonable oil temperature, smooth power. If instead I decided (foolishly) to operate at 13.5 GPH, that would be about peak EGT. The CHTs are going to get very warm, and on an ISA+15 day, I might even see evidence of detonation. But only good operating practice (and CHT limits) influence that. There’s no FAA red line fuel flow to protect me. I have to think about and understand my engine’s operation.
>> Paul wrote: If whatever is happening is not operationally significant, then why do I as pilot, or you as mechanic, care if it's incipient or limiting?
>> Bell206 wrote: Operationally insignificant per an FAA acceptable reference or…?
Sure! Start with the FAA approved red line CHT for the engine. If you run at peak EGT on my turbo’d engine, you’re not going to stay below red line CHT under all circumstances, and certainly not on the FAA engine-cooling-limiting-case 100F OAT day. An even better FAA acceptable reference is Lycoming’s service instruction that advises that for longest cylinder life, keep CHT below 420F. That’s great guidance there, and Lycoming says that statements with airworthiness implications have been vetted by the FAA.
in the case of detonation, the conformity guidance in the CAR or FAR points to “no detonation.” So it doesn’t conform to the type design and is not airworthy.
Again, the problem is what is “no detonation?” It’s not the engineering definition of no detonation. As we’ve just discussed, the FAA says both incipient and limiting detonation are just fine. So apparently, the airworthiness definition of no detonation means limiting detonation or less. I find that definitionally confusing and unhelpful. Apparently, with the FAA, one’s understanding of English mileage may vary.
it is only limiting detonation that is the trigger, provided you can differentiate between incipient and limiting detonation.
Doesn’t it bother you at some level that observable detonation is defined as “no” detonation. Words don’t mean what we think they mean! It’s all useful from an engineering perspective, but it certainly is more than confusing enough to confound pilots and others trying to puzzle out just what’s being defined.
>> Paul wrote: What GAMI’s novel work demonstrated, to the FAA’s satisfaction, is that not all detonation events are alike,
>> Bell206 wrote: But they still had to pass the detonation margin requirements, same as the PAFI fuels have to with the AC. What I’m curious about is if they had to follow the 12% benchmark or if they came up with their own margin value? Same with the limiting detonation definition. Would be interesting to read the approval document listing for the STC.
There’s a white paper (special FAA terminology for policy publications) that explains all that… but it may be considered GAMI confidential. In participating in the FAA’s GAMI certification review as the fuels’ expert, I got to see some parts of that white paper. GAMI did use the same 12% IIRC reduction of fuel flow to demonstrate adequate detonation margin. What GAMI disagreed with the FAA about is how to translate/interpret the observed fiber optic pressure events into incipient and limiting detonation as defined by the Sperry water-cooled accelerometers. GAMI’s white paper described what the FAA eventually deemed an acceptable alternative method that did NOT include the math errors of the FAA’s adopted method. Instead, I’d say GAMI’s method is superior to the FAA’s ASTM-blessed method… but that’s just me.
Bell 206: You would never enter these margins unless you purposely operated your engine in excess of its certified maximum and minimum fuel flow operating limitations.
There could well be times that is a good idea. Let's say you're doing a maximum performance takeoff from a mountain strip, and an exhaust valve sticks. I've had this happen to me. With 25% less power on a four banger, you may not clear the trees at the end of the runway, or have sufficient room to land and stop before the tree line. By understanding how the engine is setup, you will realize that you can reduce fuel flow roughly 10%, increasing power output, though eating away at the detonation margin. In the 60 seconds or so it will take you to clear the trees, the thermal mass of the three hard-working cylinders will protect you from destructive effect... you can then return to full rich. That beats eating branches all to heck.
Paul
