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My opinion before reading this report was that one of the external systems had failed and it is not surprising that it was the exhaust system because this has happened REPEATEDLY to many rotary powered aircraft. It is surprising that it happened to Mistral given their technical ability. They certainly do not need advice from me. However, I recently tried to tame the mighty rotary exhaust by placing a small tangential muffler ahead of the regular muffler. This seemed to work based on exhaust note on a RotaMax single rotor. No instrumentation was available at the time so I am reporting my subjective evaluation. It was quite amazing how tame the exhaust immediately became and without the creation of any back pressure. Rotamax kept the prototype for further experimentation. I have not heard more.

The system is very compact and light weight. It will fit inside the cowl. I can't prove it but I think this system would protect the internals of down stream mufflers provided they were well made out of SS. For the test a FlowMaster (I think) muffler was added to the tangential. It was very quiet. The FlowMaster by itself was not quiet at all. A smaller all stainless steel muffler such as the Magna flow could be used in final configuration. Burns Stainless has some small and expensive mufflers that look to be perfect for this application. We could easily make our own. In addition to cooling the exhaust gas and lessening the exhaust pulse another advantage of this system is that no muffler is hanging out in the breeze. Jerry

First photo. exhaust system is installed upside down to clear fixture. engine is running at about 6000 rpm. You can see how calm it was working next to the engine. The only difference for a two rotor would be a longer tangential can. When the tangential system is installed in its normal orientation, there is room for a small muffler to hang down along side the engine.

On Jun 7, 2008, at 10:37 AM, Giffen Marr wrote:

The following was posted on the Mistral-Engines web site. It is another data point on the very harsh environment of the exhaust system, particularly in the NA engines.
Giff Marr
LIV-P/Mistral 70%

More fear than harm
07 June 2008
On Tuesday morning, 3rd of June, our test pilot, Mr Stephen Roth, took off for a first one-hour flight that went flawlessly. He landed at Palatka’s Kay Larkin Field Airport,Florida, to refuel. Pre-take-off checks were normal and so was take-off. On initial climb, however, the engine started to lose power while still running perfectly smoothly. The pilot performed a 180° turn to return to the airport. The loss of power increased, and the pilot was unable to maintain altitude.
He landed the plane under control and at minimal airspeed in a wooded area to avoid buildings and houses. The wings were sheared off, one of them staying in a tree, and the fuselage fell sideways onto the ground. Coming to his rescue, two courageous citizens helped him out of the cockpit conscious and unharmed. There was no post-crash fire, only smoke from engine coolant leaking onto the heat exhaust.
MISTRAL Engines is extremely happy and most relieved to announce that Stephen Roth suffers only from minor cuts and bruises. After a thorough check in the hospital, which proved to be short, he was released and able to go back to the site to contribute to the beginning FAA enquiry.
MISTRAL Engines immediately dispatched to the site Mr Gordon Anderson, its Chief Operating Officer. On Thursday 5th of June, Mr Anderson, Mr Roth and the FAA investigator inspected the plane’s wreckage as well as the whole engine data log which was retrieved entirely uncompromised from MISTRAL’s proprietary Digital Engine Management (DEM) system.
It was quickly evident that all engine parameters were normal from the moment of take-off to the moment of the accident, with the exception of abnormally low propeller RPM. The cause of the power loss was subsequently found in the failure of the internals of the exhaust system’s muffler, obstructing the free flow of exhaust gases from the engine.
This particular muffler was a temporary and experimental solution developed and manufactured by a third party, which was planned to be replaced this coming summer by a certifiable unit, developed in-house. The FAA, acting on behalf of the NTSB1, retained the muffler in order to conduct flow tests.
A preliminary report was produced by MISTRAL Engines. It will be posted on this website shortly. A final company report will be issued after further analysis of the engine is conducted at MISTRAL’s facility in Geneva.
After its pilot emerging unscarred from this mishap, MISTRAL is extremely relieved that its technology is proven uninvolved in the cause of the accident and that, through the innovative data logging capability of its DEM system, it was able to significantly contribute to a prompt and thorough determination of the causes.
MISTRAL Engines is in the process of acquiring a new airframe to resume its flight testing programme as soon as possible, and the type certification programme of its G-300 engine model continues unhindered.
1 NTSB : National Transportation Safety Board

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