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Power-less 503 HELP!


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Would appreciate some advice from those with Rotax experience.


Took a first-time passenger up for a spin in the 503 Redback this morning. Here is how it went:


  • Total time = 112 hours - but is 2003 model. Plugs and all fuel lines replaced at 74 hours.
  • Passenger 96kgs; me 76kgs - but compensated by leaving fuel at 20L mark (Holds 40L).
  • 8:15am - Air temp at ground level 20oC. Clear, light Easterly
  • Pre-flight done; tested both ignition systems at 4,000 RPM
  • Airstrip alt. 3,000'. Took off and climbed to 1500 AGL at 6,000RPM, eased to 5,500 but still in steady climb, THEN after total time in air of 6-7 minutes, engine died to a rough idle; nose down, tried throttle on-off still rough but picked up to 2,500RPM.
  • Did a 180o turn and headed for my farm strip. Did a glide in 'no power' approach (Didnt bother trying throttle again as I figured best to go straight in rather than regain power and fly away from the strip only to find it 'stops' again).
  • Landed OK; taxied to bottom end of strip (700m long), then with passenger still on board did 2 rapid ground runs at varying throttle settings = no problem or misfire.


So then we put it in the hanger and did some checks:


  • Fuel drain cock - no dirt
  • Removed fuel filter - tiny specs of dirt only
  • Removed both float bowls - no dirt
  • Checked plugs (20 hours since last replaced) - no problems
  • Noted that circuit breaker was 'out' - depressed it to reset. But remember that the engine was still idling and had 'normal' power after landing - even with circuit breaker out.


So, where to from here??? I know that the fuel pumps on 503s often give up after 200 - 300 hours or so, so figure I should replace it. But other than that what else can I look at???


Appreciate any advice :;)2:







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What was the weather like? Any chance of carby ice? Would explain the regain once you where on the ground and it had melted. I know Rotax's do get it and there is no real "good" solution to fix it.

Little chance of icing - as was still on 80% power after climbout. The extra weight of heavy passenger was making the old girl work pretty hard. So maybe the long sustained slog loaded up the fuel pump - and it was on the way out anyway - but it recovered under lighter load, once on the ground. All I can think of at this point...



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I pre-mix. No injection.


As for conditions: 20oC at 8:15am; clear skies; Easterly breeze at say 5kts; felt fairly humid.. Air a bit lumpy and felt about 2 - 3oC cooler than ground level temp, at 2,000AGL.



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I've got a couple of hundred hours in front of a 503 and I'd be thinking fuel problems could be the culprit, as in fuel pump, fuel feed, air in the line, etc, etc. Sounds like you could be on the right track.


Assuming your engine is DCDI, then electrical problems are unlikely.


I've only ever had one problem with my 503 that I have seen countless times on other aircraft too and that is when it simply won't rev much past idle and no matter how much you manipulate the throttle, squeeze pump fuel or fiddle with the choke, it just doesn't come good. I never managed to find the problem. It only ever happened on the second start, after the initial warmup.



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do you have oil injection on your 503 or use pre-mixing chris ? if using oil injection more chances of having carby icing even at 80% power settings under certain temp. and humidity conditions

Just studied this chart more closely; it's a gem. Thanks.


What's the easiest way to measure dew point? Although it seems that it does not change the 'intersection' point all that much; and in practice I cant see pilots checking the dew point b4 each flight.... So I guess a more simple assessment of temp and humidity is going to provide a clue to chance of icing..



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Hi Chris, its easier to use humidity scale vs temperature on the chart as opposed to dew point depression. Relative humidity is shown on right hand side (blue tangent lines) on chart. however if u r keen on dew point measurement u can use the following crude method. scrap off the paint of a can of coke using sandpaper, cut the top section and put some water and ice in it. stir it with a thermometer(u can add salt to melt the ice quicker), when u see the condensation start to build on the outside of can, note the temp in thermometer. this is your dew point temp.





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Given the "right" conditions, even the humble Rotax 2-strokes (582 and 503), whether oil-injected or not, can suffer moderate to severe carburettor-icing. The oil-injected varieties suffer greater temperature depression on vapourisation/atomisation of the fuel in the carburettor throat than the pre-mix ones, but no carburetted engine is immune, especially if it lacks an efficient form of carb-heat. Rotax 2-strokes usually have none fitted, relying on a very short induction-tract for some measure of protection. Be assured this affords little or no real protection.


To be effective, a carb-heater must raise the temperature of incoming air by 50 degrees C - which explains why using hot water from the radiator via a heat-exchanger somewhere in the induction-tract is inefficient for such a purpose. The incoming air must be rapidly heated by a considerable amount, and the only way to do this in most aircraft engine appplications is via direct convective and radiant transfer heating in a chamber fitted closely around the hot engine exhaust-pipe. Remember, this air must be raised by 50 degrees C prior to the carburettor, because it can lose 35 degrees or more as the fuel vapourises/atomises. If enough moisture is present and the fuel-air mixture drops significantly below dew-point, icing is almost a Don Chipp "rolled gold" certainty.


With high levels of relative humidity, and a throttle setting for cruise power, I have recently experienced two occurrences of severe carby icing on a Rotax 582 oil-injected engine whilst flying in coastal air.


Random variations of 600RPM, poor or non-existent response to varying thrtottle settings, and the feeling of the engine "surging" or "choking" were the symptoms. Maintaining altitude was problematical, but the aircraft was safely brought back to the airfield and the symptoms vanished as the warmer air of lower levels was encountered during descent for the circuit.


Thorough checks of the carburettor needles, jets, floats, fuel-pump and fuel-filter disclosed no anomalies or technical maladjustments to which the symptoms Another possibility, cracking in the carburettor-sockets between the output side of the carby and the intake side of the engine, disclosed no cracking, and the intermittent nature of the loss of power seemed to rule out variations in fuel-air mixture from that cause in any event. EGTs remained normal during this behaviour.


So by process of elimination, it would appear that carburettor-icing was the cause, and the lesson is salutary. If flying in very moist air, expect icing in any carburetted engine. If you have an engine fitted with an effective carburettor-heat system, be prepared to use it during cruise, but not during high power flight such as during takeoff or climb as it can cause detonation and engine-damage.


If, on the other hand, you fly a Rotax 2-stroke which is devoid of any form of carby heat, be prepared to deal with carburettor icing by avoiding cruising at altitudes where the temperature and moisture content make the engine predisposed to carby-icing or, regularly vary the throttle setting to shear off any ice that is forming or, climb above the level of condensing cloud where the moisture content is significantly lower which may greatly reduce the chance of ice forming in the first place.


The problems described relating to the 503 engine in the first post would therefore appear to be carburettor-ice related.


The chart on carby-ice formation is much appreciated and very useful.



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Thanks for all of the observations and suggestions! Some real helpful lessons here.


I spoke to Wal at Bert Floods today and as insurance I've ordered a new fuel pump. Will likely add a fuel pressure gauge as well - to keep an eye on what is going on..


Anyone out there retro-fitted a fuel pressure gauge? Just wondering about type and brand.. I guess any 'Repco' supplied diaphragm type will do; no wiring required.



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Guest MundooTriker

Hello Chaps,


I just had a very similar experience to Chris on Christmas Eve. Had a 1st time passenger, and we just did a lap of the town etc and were heading back to the aerodrome. It was overcast and very humid. Clouds were approx 2000 and I had started slowly descending from 1500 and was at 1200. I was just going to do an orbit when as I applied throttle the engine felt starved for fuel. Maximum revs were 4000 rpm. I told the passenger everything will be just fine, but we have lost power and and were going down in a paddock. He said no problem, he trusts me and promptly went back to sight seeing. (Ignorance is bliss is a very true saying!!) . I put out a call and was tossing a coin as between a cane or cow paddock. I wasn't terribly fussed on either. I then realised that at approximately 2nm to the strip at 1200ft, 4000 rpm would probably get me pretty close. I must admit to relief when at 700ft the motor come roaring to life. It was shaping up as a stinking hot muggy day, and I didn't fancy explaining to a farmer, chasing off cows, packing up the trike etc. (Lazy boy)


In less than 2minutes, I was able to pull up at the hanger and give the "clear of all runways". The LAME who had just serviced it heard the call and came over. The prognosis was carby icing. The outside of the carbs were dripping like a shower.


I put some fresh fuel in, climbed in solo and went for a test fly. The little 503 ran like clockwork.


I suppose the major difference to Chris was that I was on a slow descent rather than climbing out.


Like Chris however I'm grateful for a very uneventful landing, not to mention a very calm, trusting(!!) passenger.


One for the log book. will make a bit of a difference to the usual Touch and Go.





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