PT6 Engine Failure (caravan) after take off.

[lilflyboy262]

First of all, Im not sure if this is the right section, if not, please send me in the right direction.

In the last 18 months that I have been in Botswana, we have had two caravans go down shortly after take off from seperate companies. Before you automatically think africa = poor maintenence, in this part of the world it is not so.


One was just out of a heavily contaminated airstrip, where shortly after getting airborne, a loud bang was heard and the plane carried out a forced landing, where it overturned in the soft field and the aircraft was damaged beyond repair. Thankfully with no fire nor fatalities.

After pulling the engine apart, nothing was found to be wrong with the engine and the thought was perhaps a water induced flame out.
There was no rain at the time and the aircraft took off with the inertial seperator was open. After it was closed the "bang occured"

The second accident occured a few days ago where shortly after getting airborne, there was a loud bang heard and the plane went down. Unfortunately this time the plane clipped trees and came to a halt against some other trees approximately 50m futher on. There was a fire post impact. This time we were not so lucky and 8 people died, including the pilot, 4 luckily escaped.
This time there was no weather at all. Temperature was around 25-30 degrees (same as the first accident).
Company proceedure at this company is to also close the seperator shortly after takeoff.

The accident hasn't been throughly investigated as it only just happened and I can only speculate as to the cause of the accident. But meanwhile my friends and co-workers continue to fly while this is investigated and I'm looking for answers while I am on leave.

My concerns are that is there a possibility that at a certain airspeed, temperature, and angle of attack, whereby closing the seperator could lead to a surge/stall situation with disrupted airflow into the engine.
Could this be a situation unique to operating these aircraft here? Could there be other accidents out there like this one?

[Doc]

Kind of disappointed some maintenance folks aren't taking a swing at this. Ideas?

[Troubleshot]

I am not sure but going with the info provided I would be looking closely at the bleed valve. Normally the bleed valve is in use for low power settings but if one were not closed/seated properly due to contamination (dirt, FOD, Etc..) the sudden reduction of air from the separator position change could cause the compressor to stall/surge.

Just a guess.

TS

[lilflyboy262]

Thank you for that insight, i will do a bit of research in that avenue.

[2R]

Take a look in the fuel tanks ,in some hot climates funghi can bloom in hours if no anti-fungals/biocides are used in the fuel.
More likely a fuel contamination issue than an air flow issue.Jet fuel can hold a lot of water as well so check for water as well.
MAKE SURE YOU HAVE ANTI-ICING INHIBITOR IN THE FUEL.Even in hot countries as the biocides in that crap will keep the fuel system clean.
Get a large clean white bucket and drain at least 10 litres from the inboard lowest point sump drains,open the drains up so you get a good flow that will wash and carry any crap out of the tanks.Keep draining until you get clean fuel.
The fuel filter may also need an internal inspection if the filter warning device did not pop up and indicate a blockage or that it was operating in by-pass mode.

Was the emergency fuel lever used ?
Was the ignition turned on ?

[robertw]

The "bang" that was heard might possibly be a compressor stall. I've heard them before and they are loud. Almost sounds like a baseball bat hitting the fuselage. Having it happen on two diferent aircraft at the same location is a bit out of the ordinary unless there are some environmental conditions (extreme dust and airborne dirt and lack of adequete engine cleaning measures) that are contributing to lack of engine compressor performance.