Rotax Engines Overheating

[Flintstone]

Minding my own business at the airfield today I was asked if I would mind accompanying a nice chap who had bought himself a Murphy Renegade on which to gain his PPL. I agreed but was warned that the engine had suffered some overheating problems (new aircraft with about 6 hours TT).

Sure enough after a ten minute taxi and climb to 600 feet the water temp gauge was at maximum. CHT and all other indications were normal. I throttled back to 4500rpm and levelled off to get some airflow over the radiator but things only got worse and I headed back to the field. When coolant started spraying back over us I decided enough was enough and flew the tightest circuit I've done for a long time.

Once we were back on the ground I was told that it had displayed similar behaviour before. According to the owner the builder had informed him that it was being flown incorrectly and that it must have been 'hanging on the prop'.

Does anyone have knowledge of Rotax engines (912) ? Please don't tell me this is normal!

[Genghis the Engineer]

Certainly not normal for a 912, which in my experience is the most reliable and user friendly of all the Rotax engines. But I have heard of problems with other engines on the Renegade, and although I've had no dealings with it myself, can't help suspect that there may be something in the engine mounting system that are working against you.

I'd start by looking at the location of the radiators and whether they are getting enough cooling air - both the water and oil coolers, not just the former.

One of many good reasons to read the logbook carefully before flying a new aeroplane, I presume you did, and made a detailed entry afterwards?

G


N.B. If it had only 6hrs TT I imagine it was still on a permit to test, presumably you were also a PFA test pilot, or did somebody talk you into flying illegally against your better judgement?

[Flintstone]

Sorry Genghis,

Didn't make myself clear. It's not registered as a microlight but 'A'. He's had a few people take him for a ride and it seems all have had similar experiences except (seemingly) the pilot who delivered it to it's present home. Either he knew something we all didn't or he chose to ignore it.

The radiators seem to be in a position to catch maximum breeze although there was mention of an optional air scoop which may be fitted if the a/c is built as a microlight. This has not been fitted so maybe it needs to be.

If it were just the water temp gauge reading high I'd suspect it as being faulty but that plus escaping coolant....well.

Owner has gone off to make further enquiries and until it's squared away I think I'll be leaving it alone. Couldn't afford to buy him a new one!

Thanks for the reply.

[Genghis the Engineer]

Can't quite see why the microlight .v. group A definition would make any difference to whether you need an air-scoop for cooling - it's either safe or not! As you say, the combination of high CHT reading and escaping coolant tends to indicate overheating ! Assuming that the EGTs were reading correctly, I can't see that it's going to be anything but a cooling problem.

I think you are probably wise to let others sort the problem out, and there are no shortage of Rotax experts out there. For something like this, my first port of call would be Conrad Beale (Conair). But, just as a general point, I know that both associations tend to suffer from the problem of unauthorised flying whilst on a "permit to test", and it's well worth taking the trouble to ask for sight of the paperwork before getting airborne.

G

[100LL]

Make shure the seals in the pressure cap, on the Coolant vessil are in good order. You could also try running the Water antifreeze mixture at 20%Antifreeze 80% water, I know from experiance that if you mix coolant at recomended levels CHT temps go sky high.

[GoneWest]

Flintstone - check your e-mail for the explanation - but this man (Phil Lockwood) [email protected] is the guru of Rotax engines.

He's been made aware that you have a question and is awaiting your e-mail at that address.

He is in Florida - and this Monday (27th) is a holiday.

Try to give him some actual details...weights, weather, temperatures etc.

[Volume]

An interresting detail is that ROTAX requires no coolant temp gauge for any of the 4 strokes, not even for the certified ones. Only CHT gauge is mandatory and there are two probes aviable at cylinder heads 2 and 3, the hotter one bust be determined and used for the gauge. There are no limitations for coolant temp, neither in the 912 UL nor in the certified 912 A manual.
So maybe it is quite normal that the coolant temp rises more severe as CHT at takeoff power.
Rotax requires 100 % Glycol in the 912 manuals but allows for 80% glycol and 20% water (not the other way !) in the 914 F maintenance manual. This manual also allows up to 50% water if ´no vapour problems after engine shutdown exists´.

So maybe you should check your coolant type first befor thinking of other problems, it might be perfectly normal that the coolant sprays out of the system if it consists of water only and you heat the engine to the allowed maximum 150°C CHT. The expansion valve is set to 0.9 bar, for pure water at 150° this might be the wrong number to prevent spraying, for 80% glycol it might be perfect.
Anyway, I only fly the rotax without coolant temp gauge and never experienced CHT´s above 120 °C or oil temp above 110 °C even on hot days. Low oil temp during winter time was more of a problem. Maybe the engine installation in the Renegade is not optimized for engine cooling...?