Engine Problems Lycoming IO360 - Robin DR 500

As Peter pointed out there are a fairly small number of DR500s about, but still they all seem to work OK, and presumably this one has done so for a good while as the saga cannot have been going on for nearly 20 years.

Given that there are a number of examples which are working OK, surely a logical thing to look at is the difference between your example and the rest. I have not seen a 3 blade propeller on others, I thought they had a 2 blade sensenich.

Have you discussed the matter with the Factory in Darois?

Peter wrote:

I’ve never seen mags being cooled by airflow

Installations are so utterly dependent on what the OEM thinks is important on any given day. IO-540 on an Islander has mag cooling ducted from (edit: air inlets air box), amongst a host of other ancillaries (fuel pumps, diodes, alternators etc). No such thing as common designs in GA – get 5 engineers in a room and get 20 solutions.

As to the original investigation – toughie. Thoughts that immediately spring to mind are if there are any flexi-hoses providing cooling for the mags or fuel distribution/pumps. They can collapse under vacuum (perhaps temp differentials or small holes for wear and tear?) if they are ducting a long way and getting past baffles etc. Would explain how the problems remain but the parts are all new. Any of them with particularly tight bends?

@northerndirk it would help if you could finally post some pictures of the inside of the cowling… ;)
Might help

The point about that oven test is to make sure that the coil and capacitor are in good condition – for a while hopefully. When this test shows up an obvious reduction in max. spark gap all is not well. A mag can cope with 80 to 100 degrees easily as long as all parts inside are fit for the job. So as soon as you find out any temperature-related deterioration in mag performance you better skip that mag and get a heat tested replacement as you will soon be faced with bigger troubles in flight, no matter if you do your mag check before takeoff with a cool mag when defects will not yet show. It is much more helpful to do another test right after landing, we do it on the taxiway , no need to produce noise at the apron for that. So the poorer performance at higher temps is just a telltale that a mag is likely to fail in near future. Cooling ducts to the mags are a good idea and will extend the useful life of coils but cannot prevent failure when something is already on the way out. Coils are simply mortal anywhere in engines that is why there are lots of rewinders – with mixed competences – and for a reason we have two mags on one engine not just because a drive gear may break or anything else, it is mainly the coil and capacitor which is a lot more likely to fail anytime without obvious warning than any other mechanical defect that should not happen due to careful dimensioning on the drawing board. Vic

Strange…something weird is going on there… (surely we all knew that, arent I clever?)

The three basic items that drive power output are airflow, fuel flow and spark.

AIRFLOW
Since MP is the same and the engine is new (so assumed good valve train) and the exhaust is new, then airflow should be the same in the cold and hot tests.

FUEL FLOW
Is slightly reduced at full throttle (to be expected with the lower RPM) but increased for a given RPM (13.2 vs 12.5 GPH @2500, to be expected to compensate for the power loss) . We still do not know whether this is the flowmeter figure in the electronic indicator or the spider pressure figure in the mechanical indicator. Regardless, FF does not seem to be significantly hurt (as if there were bubbles/fuel vapour/some other blocking) since the reported EGT’s are similar. If effective FF were reduced then we should see higher EGT’s. EGT is slow to react and I can imagine these reports are not very stable/trustworthy figures since you dont do a three minute full throttle engine runup, more like 30 secs…however the measurements are consistent also at lower RPM, so I if we are lacking fuel, it is not by much or else EGT’s would be increased. Injector clogging (which can drive wrong FF indications) is also to be discarded since injectors were installed new and it should not be temp dependant anyway.

SPARK
Is the obvious remaining culprit, but a faulty mag should drive increased EGT’s which are not being reported …if a mag coil or capacitor is failing why is EGT unchanged?

Perhaps the runup was too short to show the expected increase in EGT as a result of mag issues?

Can you confirm which FF indicator are you reporting (ie electronic flowmeter or mechanical spider pressure dial indicator)

There has to be something really major going on here. It cannot be a small difference in airflow. Planes would be falling out of the sky all the time.

I reckon it is what happens quite often: the maintenance company says X has been changed, but it hasn’t actually been changed.

And to spend 100k €… not even an aircraft owner could be that stupid, surely. The whole Robin is not likely to be worth that much. It cannot be flown in the present state, with a weird problem in the propulsion which could bring it down anytime.

Or somebody is winding somebody up

Hi folks,
Have you checked the vent lines of the tanks in detail? On DR400 airframes these lines coming out of the fuselage down under approx 50cm aft of trailing edge of the wing and are made of a very small plastic tube. The inner diameter fits perfect to insects and other debris/
The system works perfect until the engine sucks more fuel than is replaced by fresh air. Depending on the remaining diameter of opening of vent lines it will work more or less.

Willi66 wrote:

The system works perfect until the engine sucks more fuel than is replaced by fresh air.

The OP stated, that the engine was tested independent of the aircraft’s fuel system with no change in behaviour.

Hallo, have you checked it personnally? Otherwise check it again. Disconnect breather and vent lines, blow it with pressurised air in opposite direction. Engine run on the ground is not sufficient.