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Induction Icing

M20K 231 Induction System System Icing

Just back today from the holidays. Look forward to 2003! Saw two questions were posed over the holidays concerning 1) engine induction system icing in the 231 and 2) in-flight engine restarts at high altitude in the 231. I'll address the icing question here. The restart question will be another posting. Putting both together would be too long and would be delayed. Hope some of this information and some of the suggestions help.

231 Engine Induction System Icing
One of my very first assignments as Mooney's engineering test pilot in November of 1983 was to take our heavily instrumented 231 prototype to Northern Alberta, Canada. Several M20K 231 customers at the time were complaining of mysterious engine power losses flying in level cruise flight with very cold OAT and visible moisture. After two weeks of experimental flying in ice, in very cold air and in all kinds of clouds in Northern Canada, we found the answer - engine induction system icing.

We found the worst kind of condition for induction system icing in the 231 is sustained level cruise flight in very cold air in cirrus type clouds - or in clouds that are composed of frozen ice particles. Snow was also a problem. Simply stated, the ice particles from the cirrus type cloud or dry snow would enter the unprotected opening of the engine air induction duct located in the forward RH engine cowl opening. From there, the ice particles traveled down the duct to the face of the induction air filter. There, they were stopped. After 15 minutes of so, the face on the inlet side of the induction air filter looked like a snow cone - we watched the solid frosting of ice particles form with a mini-cam inside the inlet duct. With the inlet face of the air filter completely blocked, induction air to the engine is choked. The engine loses power.

The first indication the 231 pilot has of of this phenomona is a "not so gradual" loss of manifold pressure (about 3"hg per minute). In the early 231's with the manual alternate air system, if the pilot doesn't open it as the engine (manifold pressure) begins to spool down, the engine will quit. If it's opened in time, the engine will be able to maintain about 70% power on alternate air alone. But pilots were not remembering to open the alternate air system. This answered why several 231 customers had descended without power, luckily into warmer air below that melted the ice on the filter and allowed for a restart before a forced landing was required (whew!).

Obviously, the factory could not allow this potential safety concern to continue. I was very proud of the factory's response to what we found in flight test. A completely new, automatically opening, firewall mounted alternate air door box redesign was offered to all existing 231 owners AT NO COST, INCLUDING LABOR. And all new 231 model airplanes beginning with the mid '84 models ( if I remember correctly) were fitted on the production line with the new firewall mounted, automatically opening alternate air system. We also sent all 231 owners a long letter from me detailing our flight test experiences flying the 231 prototype in ice and our findings concerning induction system icing (I still have a copy of the letter).

All this happened 18 years ago and is history. But please, all 231 owners today - check and make sure you have the automatically opening, firewall mounted engine alternate air door box installed in your airplane. It's the single most important safety improvement made to the airplane in it's history.

While at MAPA, I was horrified to learn there are some 231s out there without the firewall mounted alternate air box mod incorporated. DON'T FLY YOUR 231 AT HIGH ALTITUDE/COLD AIR/VISIBLE MOISTURE OR IN A SNOWSTORM WITOUT IT! Without it, you're potentially a cirrus cloud or a snowstorm away from a power loss. I can't emphasize this enough - if your 231 doesn't have the firewall mounted alternate air box, find one somewhere - from the factory, from one of the service centers or from a salvage yard. But get one. Now is the time of year for the flight conditions to get induction system system icing in your 231. The firewall mounted alternate engine air mod will keep you safe.

Best Regards;

Bob Kromer



Revision: 10/28/2010