andrew lott

Remember this?

Date: 19 December 2001

 

The final report on the Whyalla Airlines Piper Chieftain VH-MZK accident on 31 May 2000, in which all eight occupants died, was released today by the Australian Transport Safety Bureau.

 

ATSB Executive Director, Kym Bills, made the following statement: "The VH-MZK accident occurred after mechanical failures involving both engines forced the pilot to ditch the aircraft in Spencer Gulf, about 26km from Whyalla, on a dark, cloudy and moonless night.

 

Based on careful analysis of the engine failures and recorded radar and audio data, it is likely that the left engine failed first as a result of a fatigue crack in the crankshaft. This was initiated about 50 flights before the accident flight due to the breakdown of a connecting rod bearing insert. The combined effects of high combustion gas pressures developed as a result of deposit-induced pre-ignition, and lowered bearing insert retention forces due to an 'anti-galling' lubricating compound used during engine assembly by the manufacturer, led to this breakdown.

 

Lean fuel practices used by the operator increased the likelihood of lead oxybromide deposit-induced pre-ignition but were within the engine operating limits set by the aircraft manufacturer.

 

It is likely that because of the increased power demanded of the right engine after the left engine failed, abnormal combustion (detonation) occurred and rapidly raised the temperature of the pistons and cylinder heads. As a result, a hole melted in the number 6 piston causing loss of engine power and erratic engine operation. The subsequent ditching involved great pilot skill.

 

The ATSB examined components from a further ten similar engines that have failed since January 2000 (including two engines from another manufacturer) in order to better understand the failure mechanisms. Combustion chamber deposits that may create lead oxybromide deposit-induced pre-ignition were found in these engines. The Bureau concluded that engines that were operated at lean fuel-air mixtures during climb, and towards best economy mixtures during cruise flight, were more likely to show signs of such deposit-induced pre-ignition than those engines operated at full rich mixture during climb and at best power mixture during cruise.

 

On 30 October 2000 ATSB released recommendations about the risks of detonation and lean running and in relation to the desirability of life jackets and other life-saving equipment on smaller passenger aircraft flying over water. Today, we release further recommendations to:

 

• the US FAA in relation to engine deposits that may cause pre-ignition;
   
   
  
• the US FAA and the engine manufacturer on the use of anti-galling compounds between connecting rod bearing inserts and housings during engine assembly;
   
   
  
• CASA in relation to high power piston engine reliability more generally; and
   
   
  
• CASA in relation to providing guidance to pilots on ditching.
   
   
  

 

 

While there were deficiencies with the Whyalla Airlines safety culture and gaps with the extent of the regulator's surveillance of the operator, neither were significant accident factors.

 

No-one should be blamed for this accident, but if the lessons from it are learned, both in Australia and internationally, some good will have come from the tragic deaths of eight people."

Seems the same culture still exists in the ATSB

Always bear in mind the two different octane measures used for Avgas and Mogas. Mogas is about 7 lower and subject to being adulterated by who ever wishes to. It's already variable in production in some instances Two stroke enthusiasts shouldn't let the fuel go stale especially after mixing where the life is more likely to be more reduced.A rich mixture will cool the engine by latent heat of evaporation, and increase power because there ia more mass of charge in there. Most engines ( Petrol) that are run at the limit of their power will enrichen the motor(s). Race cars and aero engines. Road cars would not be allowed as much leeway and they hardly ever go there anyway. Nev

The position of peak pressure above the piston is governed by the ignition timing, which can't be changed.Jabiru actually retarded the timing 5 deg to 20 deg some time ago.

The peak pressure can be changed by fuel air mixture ratios eg Ratios that produce EGTs of 50degF LOP

compared to 50degF ROP, result in a lowering of PP and a delaying of the peak several degrees After Top Dead Centre. The peak occurs ATDC anyway. I think it changes from 12 deg ATDC to 18deg ATDC with the mixture change (my figures may not be completely accurate but close)

Have a look at the graph put up by Ultralight on pg. 1 which shows engine parameters relative to mixture control

and the relationship of Internal Combustion Pressure to Cylinder Head Temperature

You are seeing 100-110 deg F difference, we are seeing 100-150 deg C difference, Also theres no way youd run a Jab CHT up to 190 Deg C, they reportably change permanently much over 180 deg."100 - 150 deg Celcius is a huge spread. I have never seen that spread in mine. Something definitely not right there. Don't know what the limits are on th Jab CHT but on the Continental the max should be considered to be 400F. The tensile strength of the head alloy drops off considerably over 400F"

 

Even the best tuned Jab Ive seen is only at the numbers your seeing in terms of EGT spread. Most are far worse. We are targeting differences you have.

 

You certainly wouldnt want to run an engine LOP with 150 degrees C between richest and leanest.

 

"I don't think you could. I have not seen those differences in any stage of flight."

 

To say rough running due to mixture problems "doesnt matter" is a big call seeing the number of possible detonation related problems out there with Jabiru engines

 

"That's not what I am saying. You don't let the engine run rough. You take steps to smoothen it up which I do by mixture adjustment, throttle position and carby heat. Smooth running shows that all cylinders are producing about the same power each. The differences in EGT when the engine is smooth really don't matter. I have seen EGT spreads of 40degF and still had rough running. I have also had it smooth at 40 degF and the same at the large spreads of 100-110degF. It can vary from flight to flight."

 

Id go further and say older design engines are simply stronger and more resistance to what you call rough running. Penalty is weight.

The problem you are overlooking is that this is not a continental its a jabiru. Now that might sound flippant but this is actually very significant. Jabs are a very light engine with tolerances for heat dissipation running right on the edge. Continentals are massive heat sinks and tolerate heat much better by Comparison.In jabs Individual cylinder cooling is often varied and limited by the cowel set up. Fuel delivery to individual cylinders is always varied. Egt readings reflect heat generation while cht reflect the summed effect of both generation and cooling.

In jabs since they have limitations in all directions have to have all the parameters considered and addressed.

A Jab engine is really pretty advanced in its power output. If you scaled its volume up to the equivalent of the Continental O-470 it would produce 50 more horsepower . This applies to both the 2200 and the 3300. Quite remarkable really.

I don't agree with the Continental being a big heat sink. While it is larger, it has to get rid of much more heat than the smaller engine and once it is up to temp, it runs at the same temps as any other air cooled engine, be it a Jab, a Lycoming 540 or an 1820 Wright. CHTs result from a number of factors including also, the cooling arrangement, the mixture setting, the position of Peak Pressure in the cylinder (refer to APS info re that one.)

Yes. Have a JPI 700 with CHT & EGT on all cylinders, fuel flow, carb inlet temp, EGT difference,, oil temp, volts, and other fuel management (linked to GPS) functions. The unit is needed to run LOP which I do all the time in cruise. At max. power in the climb EGTs range from high 1200f/700c to 1470f/800c with very rich mixture.

CHT is what I watch for. In cruise and leaned out to LOP, the EGTs could range between the 1400f and 1500f range. Because of the awful inlet system I have, it is not uncommon to have a difference in EGTs of 100 -110*f, however provided the engine is running smoothly, it does not matter. Carb inlet temps of 50*f/10*c is the optimum temp for mixture distribution between the cylinders. I use carb heat to get that temp if needed. If the engine is rough after leaning, applying the correct amount will smooth it up again.

Remember that you have four or six single cylinder engines bolted to a common crankshaft. Unless it has tuned Gami style injectors, you are bound to get mixture distribution differences and hence differences in EGT readings.

Any roughness or vibration is because one cylinder is not producing the same power as the others. When it is running smoothly, all the cylinders are producing about the same power. You can still have EGT differences with a perfectly smooth running engine. There always differences between the cylinders performance and indeed each cylinder. Personally I consider that as long as my engine is running smoothly, the CHTs are below 380f and the EGTs are close enough, then Ops are normal.

The best info on engine operation is at the Advanced Pilot Seminars and there is one graph from them in an earlier post. Look up their website of the same name and there is a section that is open to the public. Also look up John Deakin, Pelicans Perch, as well as The savvy Aviator- Mike Busch. Those three sites have some remarkable reading on engine matters.

Seems like a lot of effort being made to even up some EGTs which in my humble opinion may not have any beneficial effect on the engine with the fuel management systems that you are stuck with.

What is critical is the CHT which directly affects engine health and is not directly related to EGT. EGT has many variables so is useful mainly for establishing peak EGT for the purpose of leaning or richening your mixture.

After over 3000hrs of running a big bore Continental with the worst induction system (carby) devised, I have found that EGTs are all over the place but have no measurable effect on the running etc.

Mine regularly exhibits temps (and higher) to those you are concerned about but no bad effect so far.

I religiously keep CHTs below 380f (193c) and the cylinders last. Cooling is critical.