Aspen2013 Posted January 4, 2021 Report Share Posted January 4, 2021 (edited) ALT AIR Light illuminates when full power is applied for takeoff. The alternate air door is held closed by a magnet. I takeoff mostly from home airport at 6800’. Most airports around that I fly to are above 4000’. I went to Ft Worth Texas couple weeks ago and I didn’t see it happen at lower altitude. Probably turbo not working as hard with waste gate more open. I believe that the induction air to the turbos is so great that it is pulling that door open. Once power is retarded below 28 MP then the light extinguishes. I can then advance the power to full throttle and it never goes on again. Only takeoff. So possibly the nose high position has something to do with air inlet not allowing enough air to enter. Air filter is Donaldson and is new. Maybe magnet is weak? Thoughts? Edited January 4, 2021 by Aspen2013 Quote Link to comment Share on other sites More sharing options...
buddy Posted January 4, 2021 Report Share Posted January 4, 2021 I have the same problem with my M20R 310 hp and use a K&N air filter. The Alt Air light comes on about 70% of the time on takeoff using 2700 rpm and once I pull the RPM back to cruise climb I push the knob back in (pulls out about a 1/4 in) the light stays off. Quote Link to comment Share on other sites More sharing options...
Aspen2013 Posted January 4, 2021 Author Report Share Posted January 4, 2021 Exactly what you don’t want when producing high power is warm air coming into engine. What we don’t know is how much the door opens. I am going to have someone watch the panel while I push the door open to see how much the door has to move before light comes on. Obviously there is enough suction to overcome the magnet. All this may be normal since it is an automatic system designed to open in the event of blockage. If the magnet were stronger it might not open early enough if primary air inlet blocked. Thanks for sharing. Quote Link to comment Share on other sites More sharing options...
exM20K Posted January 4, 2021 Report Share Posted January 4, 2021 Had the same issue with 310 HP takeoff high and hot. Checked the pressure to open alt air, and it was in spec. cleaned the magnet and its landing. An exaggeratedly gradual application of power in high/hot conditions seems to help. -dan 1 Quote Link to comment Share on other sites More sharing options...
LANCECASPER Posted January 5, 2021 Report Share Posted January 5, 2021 On the K models (231, 252) the magnets used to loosen up as they aged, something to look for on annual. 1 Quote Link to comment Share on other sites More sharing options...
Mooney217RN Posted January 6, 2021 Report Share Posted January 6, 2021 Read carefully Aspen 2013 - I had this exact problem on my Ovation 3. the solution is not what you might think. The alt air opens when there is insufficient airflow going to the engine, which occurs on departure. It ends up the problem is not with the magnets, but rather, your air filter. When I took my plane in to annual last year, they noted that the air filter was filthy, lots of dirt, dust and dead insects. they cleaned, didn't replace, the air filter. The problem resolved and never returned since then. Clean your air filter. 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted January 7, 2021 Report Share Posted January 7, 2021 Delta P... if the filter gets dirty, or iced over... the engine still asks for as much air as it needs... when the Delta P gets too high, the door opens and allows unfiltered air into the system... If the magnets are allowing this to happen at a lower Delta P... that is a maintenance item... check the maintenance manual for proper details... PP thoughts only, not a mechanic... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
MatthiasArnold Posted January 7, 2021 Report Share Posted January 7, 2021 (edited) Dear all, I had a similar problem with my O2 when I bought it last summer: Whenever I applied full power (e.g. takeoff run) the alternate-air annunciator got engaged. Because I experienced some problems with the flap micro-switches just before, I focussed on the associated micro-switch in first place - however it worked fine. Inspecting the air-filter came next. Looking at its date of manufacturing (12'2005) it still seemed to be the original install from Kerrville - 15years old. Intensive cleaning only gave little improve, if any. So I ordered & installed a K&N filter (and filed the 337 form, of course). However - the symptom of flashing alternate-air annunciator at full power (caused by opening of the alternate-air door) resumed to exist. At slightly higher MAP/RPM settings than before, though. The resolution was quite simple in the end: There was a significant air gap between the magnet and the alternate-air door. This air gap reduced the magnetic flux and thus the magnetic force significantly (compared to the value given in the service manual). Adjusting the position of the magnet in relation to the door easily solved the problem (there are two screws holding the magnet in place). As a result I have a brand new air-filter with low flow resistance installed I have slightly better power during take-off (ram air instead of warm air) and up in flight levels (lower flow resistance) I got rid of an irritating indication during take-off while still being able to open the alternate door manually if necessary.. Best regards, Matthias Edited January 7, 2021 by MatthiasArnold typos 3 Quote Link to comment Share on other sites More sharing options...
Aspen2013 Posted January 8, 2021 Author Report Share Posted January 8, 2021 Matthias, thank you for that input. As I mentioned earlier it has NEW Donaldson filter. No bugs and no dirt. Unless some brand of filters cause it?? Supposed to one of the best ones. I think the culprit is the magnet. Anthony describes best how and why that door opens. # 1 trouble shooting item should be filter. After that it’s micro switch or magnet. My switch seems fine. 1 Quote Link to comment Share on other sites More sharing options...
Guest Posted January 8, 2021 Report Share Posted January 8, 2021 A Donaldson filter is a better choice than a K & N, they breather better because they’re junk. Cylinders are worth $3000, the engine is worth more than $60,000 and the filter $100. Cheap maintenance and insurance for your engine. Clarence Quote Link to comment Share on other sites More sharing options...
MatthiasArnold Posted January 8, 2021 Report Share Posted January 8, 2021 4 hours ago, Aspen2013 said: think the culprit is the magnet. Exactly - as stated in my post it was the air gap between the magnet und the door itself, reducing magnetic flux and thus magnetic force. Simple readjustment helped.. Quote Link to comment Share on other sites More sharing options...
carusoam Posted January 10, 2021 Report Share Posted January 10, 2021 Great detailed pirep, Mathias! Thanks for delivering all the detail... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
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