CHT Temp Alarm

[billdom]

I'm a student pilot learning in a CTLS, equipped with dual Dynon FlightDEK-D180. On a few occasions, I've got an alarm for high CHT temp while on the taxiway waiting for clearance to takeoff, it only happened after long period with the engine in idle or near idle (I fly from a very busy airport). The instructors of my school have taught me to silence the alarm, increase RPM a bit and then keep an eye on CHT and make sure it go back to normal after takeoff. It works, the CHT always go back to normal after takeoff. Now, my concern is that if this happen during my checkride, the DPE might not like my decision to continue with the flight. I understand that the Rotax cylinder head is water-cooled and that the engine have been designed to operate without coolant for 2 hours without engine damage. This means that a few minutes with high CHT should not be an issue. But I don't know what the DPE will expect me to do as a part of his evaluation of my ADM, when and if this happens during my checkride. Will he expect me to taxi back to the ramp or will he see this as a non issue? should I discuss this with him prior to the flight?

[Roger Lee]

Hi Bill,

 

You have been misinformed. The Rotax 912 can run for 30 minutes without coolant and then you would have the have the head metal tested for hardness. That type of running will cause overheat and was only designed to get you on the ground safely. You fly 2 hours that way it may seize up completely, but no matter what your throwing away a $20K engine after you land whether you land voluntarily or you're forced to land with a seized engine. People do have overheat sometimes at low rpms and increasing rpms is the right things to do and hope it cools. Fuel is what cools the engine here not increased air flow. The second is what do you consider a high CHT? You can go into the D180 setup and go to the CHT menu and set the temp alarms up a tad. The ones in the D180 may be set a little too low so you keep getting alarms. My high temp alarm is set at 250F and the high yellow at 240F. Rotax doesn't want you to go over 248F with only a 50/50 coolant because it MIGHT form a hot spot or a vapor area and cause a specific area overheat. The plane can handle up to 266F for the 912ULS. 266F is too close to the 270F 50/50 coolant vapor forming point so Rotax reduced the max temp for 50/50 coolant to 248F for safety. As far as your test goes many planes can overheat at idle and it is how you handle this situation that the tester is looking for. If you explain that you are keeping the rpms up to help with cooling that is what he wants to hear so he knows you know your plane and how to handle adverse situations. Same thing in the air. Things happen, it's how you deal with them that makes the difference. If you don't know what to do to handle issues that is when you'll have problems.

[billdom]

Hi Bill,

 

You have been misinformed. The Rotax 912 can run for 30 minutes without coolant and then you would have the have the head metal tested for hardness. That type of running will cause overheat and was only designed to get you on the ground safely. You fly 2 hours that way it may seize up completely, but no matter what your throwing away a $20K engine after you land whether you land voluntarily or you're forced to land with a seized engine. People do have overheat sometimes at low rpms and increasing rpms is the right things to do and hope it cools. Fuel is what cools the engine here not increased air flow. The second is what do you consider a high CHT? You can go into the D180 setup and go to the CHT menu and set the temp alarms up a tad. The ones in the D180 may be set a little too low so you keep getting alarms. My high temp alarm is set at 250F and the high yellow at 240F. Rotax doesn't want you to go over 248F with only a 50/50 coolant because it MIGHT form a hot spot or a vapor area and cause a specific area overheat. The plane can handle up to 266F for the 912ULS. 266F is too close to the 270F 50/50 coolant vapor forming point so Rotax reduced the max temp for 50/50 coolant to 248F for safety. As far as your test goes many planes can overheat at idle and it is how you handle this situation that the tester is looking for. If you explain that you are keeping the rpms up to help with cooling that is what he wants to hear so he knows you know your plane and how to handle adverse situations. Same thing in the air. Things happen, it's how you deal with them that makes the difference. If you don't know what to do to handle issues that is when you'll have problems.

 

Thanks for the clarification. As for 50/50, do you mean 50% water and 50% coolant? I don't know how is mixed in the plane I'm renting for lessons. The RPM have a yellow arc in idle, I got into the habit of not letting the idle drop to the low yellow arc for too long and I haven't got the alarm so far. I don't remember at what temp the alarm goes off but I'm also checking CHT thermometer display in the D180 to see if its getting too close to the red.

[Roger Lee]

50/50 does mean 50% water and 50% anit freeze. You should always warm up an engine at 2200-2400 rpm on the first time of the day. Yes it is okay to idle at 1800 after the engine gets to 120 degrees oil temp, but you are still better off at 2000+ anytime you need to idle. Things are just smoother and less likely to load up.