... what can happen with a defective wastegate flap servo

… the TCU warning lights indicate a possible defect in the wastegate flap servo and what the consequences can be.

The input says: When the power supply is switched on, an automatic function check of the two lamps is carried out.
What most pilots should also have noticed is that the servo performs a „check run“ from stop to stop.
This is clearly audible when the ignition is switched on.

If there is a problem with the potentiometer in the servo, the orange light will flash.
If it is also determined that the current required by the servomotor to keep the wastegate closed is too high, the orange light will also flash.\

I was asked by Gustav¹⁾ to have a look at his wastegate flap servo because he thought it was not working properly.

He described that the orange indicator light was flashing on his second-hand aircraft, which he had not yet flown after taking delivery, and that he did not want to operate the engine in this way, as stated in the user manual.

According to his information and the statement of the previous owner, he once had an aborted start because the engine „ran away“.
The previous owner then discovered a fault with the servo, dismantled it and „repaired“ it.

I was presented with the following picture:

• the wastegate flap cable was set correctly
• the orange light was flashing
• the servo did not perform a correct check drive

I had a servo with me for testing. We plugged it in and it worked perfectly.

Now I first read out the operating parameters from the TCU with the old DOS program and saved them as a PDF.
The result²⁾ looked like this:

• The engine has an operating time of 269:02 h³⁾
• At 238:26 h we have an overspeed of 5957 rpm
• At 238:26 h we have an overboost of 2366 mbar

From this we can conclude the following:
The event occurred before 30:76 h\\. The wastegate flap was not open wide enough according to the specified parameters, so we had an overboost of 2366 mbar.
At the same time, either the slipping clutch slipped or the engine revved up to 5957 rpm due to the high power output.
The latter does not yet require any action - this speed does not bother the engine.

Overboost, on the other hand, is a cause for concern.
Experience has shown that a boost pressure of over 1800 mbar is dangerous for the interference fit of the crankshaft.

This can then twist or elongate.

If the crankshaft elongates, the crank webs in the area of the main bearings grind against the walls of the crankcase, causing the crankshaft to jam or become stiff.

We now checked the rotation of the crankshaft naccording to Chapter 72-00-00 CRANKSHAFT DISTORTION INSPECTION of the Rotax Maintenance Manual Heavy 912/914 Edition 2 Revision 0, dated 28.10.2022, and found the following:

Piston turned to the piston stopper at cylinder 1 and the degree disk set to 0°.

Turn the piston to the piston stopper at cylinder 2 and read the position on the degree disk.

Here you can clearly see that the crankshaft is rotated by approx. 6° between the two measured cylinders, in this case between cylinders 1 and 2. The measured value for cylinders 3 and 4 was also 6°.

This means that the crankshaft is only twisted between cylinders 1 and 2.
For cylinder 1, the piston is therefore advancing by 6°.

As you can imagine, there was a hard lump in Gustav's face - the crankshaft needs to be replaced.

… I may tell you later how the story continues