Engine cooling
At the end of every flight the pilot has to sit and twiddle his or her thumbs, waiting for the engine to cool down.

Believe it or not, this is crucial for the safe and reliable operation not only of your engine but also your bank balance. Only the foolhardy will disregard such an important procedure.

Example: Turbine Engine (Allison)

The primary reason for the two-minute cool-down period is to prevent the formation of carbon in the hot oil wetted areas of your engine.

The engine oils used are generally mineral-based and therefore when they reach a critical temperature the oil breaks down, forming carbon. When you bring the engine down to idle, the temperature for example in the power turbine strut is above 680 degrees F.

night flight crash

In the NO.6 bearing the temperature is 680 degrees, in the No 8 bearing the temperature is 630 degrees, and the Gas Producer turbine strut is 530 degrees.

After shutdown, these temperatures are sufficient to cook the residual oil left in these areas, causing a build up of carbon that will reduce the oil flow through the engine and in extreme cases block the oil ways and oil jets feeding bearings.

After the two-minute cooling down period the temperatures have come down to 440, 400 and 320 degrees F respectively. Although there are some third generation oils that have a higher temperature resistance to cooking, it is recommended that you follow the two-minute cool-down period.

Example: Piston Engine (Lycomlng)

Apart from turbo charged engines like the Bell47G3B1, which have a cool-down period of five minutes, there is no fixed period for cooling these engines.

The airframe manufacturer will usually recommend something like "wait for a positive indication of a drop in the cylinder head temperature" before the pilot stops the engine.

So what are the reasons for this cool-down period in a piston engine? Broadly speaking they are the same reasons as those for the turbine engine.

However, it might be advantageous to know the primary means of heat dissipation in the hottest parts of your piston engine. (We are talking about normally-aspirated engines here).

exhaust valve
  • The hottest exhaust valve is usually the one that is in the cylinder furthest away from the cooling system intake, the one on the right here

 

 

 

 

 

 

The exhaust valve is by far the hottest part, and the hottest exhaust valve is usually the one that is in the cylinder furthest away from the cooling system intake.

The exhaust valves are hollow. In the hollow space inside the valve is sodium. Sodium is a great medium for soaking up heat.

This sodium is liquid when hot, so when the exhaust valve is closed the sodium is at the bottom of the valve stem, adjacent to the head of the valve. From this position the sodium absorbs the heat from the valve face.

When the valve opens, the sodium is forced up to the other end of the cavity and in this position transfers the heat to the top of the valve stem, which in turn transfers the heat away through the rest of the valve train and the oil that visits this area.

If the pilot shuts down the engine without a cool-down period, the oil around the exhaust valve face and stem will cook and produce carbon deposits, causing the valve or valves to stick.

Sticking exhaust valves will cause severe damage to the engine and to your bank balance as well as causing a severe power loss, resulting in a limited power forced landing.

To help prevent carbon deposits causing damage to your engine, the engine manufacturer gives recommendations on oil change periods. These recommendations should be adhered to. Oil is cheap compared to the cost of the engine repairs associated with build-up of carbon deposits.

One of the big disincentives for a proper cool-down period we have in the industry is operators charging from the Datcon/Hobbs meter.

The Datcon/Hobbs meter for example in the Robinson R22 is running as long as you have engine oil pressure, so if the pilot switches the engine off as soon as possible, he or she has just saved some money - but ultimately, this will result in more expenditure for the operator/owner.

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